"Brake jack" - an explanation.

[S.]

Warning: very, very long posts.

Since there’s been a lot of incorrect explanations and theories on brake induced suspension interference (BISI), here’s a full explanation with diagrams. If you’re just interested in what happens but not so much “why” then skip to the end, most of this is just a reiteration of commonly-understood physics and its application to bicycles in relation to braking.

Disclaimer: this article is purely intended for demonstration of the basic concepts of BISI. It is not a full quantitative method of calculation, nor should it be used as such. Calculations shown deliberately omit various less-significant/more complex factors such as moments of inertia, weight shift, centres of mass etc. The article remains a fair approximation of the basics of BISI, but if you wish to calculate the precise reaction to braking that your bicycle has then you must be aware of the other factors, and know how to incorporate them. Keep in mind too, that motion characteristic generation (ie designing for a characteristic) is much more difficult than simply analysing an existing system.

The physics:
Imagine you have a random rigid structure/body attached to another, fixed rigid structure/body by means of a simple pivot, so that when the fixed (stationary) body is taken as a point of reference, the other one is free to rotate around the pivot that joins them. (Fig 1)
http://img.photobucket.com/albums/v260/snms/Fig1.jpg


Now imagine if you had the same thing, but you applied a force at any random point on the rotating body. The force can be broken up into its vector components relative to the pivot, which are called the normal (directly towards/away from the pivot) and tangential (perpendicular to the normal) components, as shown. (Fig 2)
http://img.photobucket.com/albums/v260/snms/Fig2.jpg


The concept of a moment is fairly basic – it is the product of a force and its perpendicular distance from another reaction force (which prevents the body from translating, ie moving in a straight line) at a fixed point (in this case the pivot), and measures their coupled tendency to cause a body to rotate. In other words, the tangential component of force F in figure 2 (since it is perpendicular to the normal force at point A), multiplied by the distance between point A and point B (the pivot), gives the moment acting on the rotating body, about point B. The tangential component of force is given by F x cos(q). So the moment about point B is Fcos(q) x dist(AB). Not too complicated. This is pretty easy to visualise – imagine lifting your front wheel off the ground (brakes not being applied) and pushing against the tyre with your hand. If you push directly in at the axle (perpendicular to the tyre/rim at that point; this is a purely “normal” force) then the wheel will not spin, and you will cause an equal and opposite reaction force on the wheel from the axle. However, if you push parallel to the tyre’s surface at that point (tangent to its path) then you will cause the wheel to spin. Anywhere in between will give a combination of the two – pretty obvious.

Newton’s second law of motion: SF = m x Sa
(the “S” means “sum of”, or “net”). That is to say, if there is a net force (that is not zero), there will be a net acceleration (in the same direction as the force is acting, obviously), directly proportional to the force applied and inversely proportional to the mass (due to the mass’s inertia resisting movement). The same applies to moments – the sum of all moments is equal to the body’s mass moment of inertia (the rotational equivalent of normal inertia) multiplied by the angular (rotational) acceleration. For the moment (no pun intended) you only need to understand the basic concept of this, not the technicalities (which are fairly complex).

Newton’s third law: For every action (force) there is an equal and opposite reaction (force). This gives us the concept of static equilibrium – if a body is supported in such a way that it cannot move (relative to your reference frame) then any force acting on it will generate reaction forces at its supports. Depending on the supports holding a body, it will be considered either statically determinate or statically indeterminate. If the body is statically determinate you can work out reaction forces based on basic equilibrium equations (forces acting in the X direction, forces in the Y direction, and moments about any given point), in other words it’s fairly simple to deal with. If the body is statically indeterminate, you will burn it at the stake and write several books specifically concentrating on denying that it ever existed (it’s much easier than doing the calculations). Fortunately, bicycle suspension components are (in the plane which they are intended to move, ie discounting lateral flex etc) nearly always statically determinate (and those which technically are not, can be approximated as being determinate anyway). If this sounds complicated, that’s because you’re not reading my mind well enough.

First we will look at a basic bicycle (single pivot) swingarm, (initially) having three points of attachment to the outside world. These points are point A (the axle), point B (the pivot), and point C (the shock mount). Note specifically that all three points are pivots and are free to rotate relative to the bodies to which they are attached (the front triangle, the shock, and the wheel). If you’re wondering why the axle is a pivot, it’s because the rear wheel is attached via bearings, and cannot exert a moment on the axle itself (because if you spin the wheel, it will simply rotate freely around the axle – leaving braking forces and chain interference out of the picture, and assuming that there is negligible friction in the wheel bearings). So the wheel and the swingarm can rotate independently of each other at this time. Assume the bicycle is on flat ground.

At sag, when the suspension is not cycling (moving), the bike’s swingarm is in a state of static equilibrium (all forces and moments sum to zero). See fig 3 – blue lines are forces acting at the various points, pink lines and letters are to show the notation used for the distances from C to B (vertically), and A to B (horizontally). Note that the force at A (the axle) can ONLY be vertical because any non-vertical (ie tangential) force at the tyre will simply rotate the wheel around the axle, transferring no force to the axle other than the vertical (normal) component. Also note that the force at C is only horizontal (axial to the shock) as shocks can only transfer load ALONG their axis (quite obviously – any force on the shock that is not along its axis will simply rotate it about its other mounting point). For ease of calculation, we will neglect the shock’s rotation relative to the front triangle, and assume it stays horizontal at all times.
http://img.photobucket.com/albums/v260/snms/Fig3.jpg

[S.]

Ok, so now into some basic calculations. Assume that A(y) is a given force on the axle. Since the swingarm is in equilibrium, moments about ANY point on the swingarm must sum to zero. For ease of calculation we will start with point B. Positive moments are taken to be clockwise:
SM(B) = 0
= [A(y) * -AB(x)] + [C(x) * CB(y)] Note that AB(x) has a negative sign in front of it
[A(y) * -AB(x)] + [C(x) * CB(y)] = 0
[A(y) * AB(x)] = [C(x) * CB(y)]

If we let AB(x) = 0.45 metres (realistic measurement), A(y) = 500N (again not unrealistic, this is equivalent to about 50kg), and CB(y) = 0.15 metres (we will assume that this distance does not change significantly over the stroke of the suspension, for ease of calculation), then we get:
[500 * 0.45] = [C(x) * 0.15] Divide both sides by 0.15 to calculate C(x)
[500 * 0.45]/0.15 = C(x)
C(x) = 1500N (this is roughly equivalent to 330lb – if you had a 330lb/inch spring you would get 1 inch shock stroke sag, which given the 3:1 shock leverage ratio would be 3” of sag at the axle – again, a realistic example).

So now we have calculated C(x) using basic equilibrium formulae. The only unknowns left are B(x) and B(y). Note that in Fig. 3 these forces are drawn in the opposite directions to C(x) and A(y) respectively. To calculate these we have two choices – balancing moments about either A or C, or balancing forces in the X and Y directions. Balancing the forces is easier since in this example all the forces are either horizontal or vertical, and we have no measurements for some of the distances necessary to sum moments about A or C.

SF(x) = 0
= C(x) – B(x) Minus B(x) because it is acting in the negative direction
\ C(x) = B(x)
B(x) = C(x) = 1500N

SF(y) = 0
= A(y) – B(y)
\ A(y) = B(y) = 500N

Hence we now have the reaction forces (Fig. 4)

http://img.photobucket.com/albums/v260/snms/Fig4.jpg

[S.]

Now consider a free body diagram of a wheel with a disc brake rotor attached, under braking forces. Assume the caliper is directly above the axle for ease of calculation. (Fig. 5)
http://img.photobucket.com/albums/v260/snms/Fig5.jpg


Assume that the bike, despite braking, is maintaining a constant speed (as though it’s rolling down a hill with the brakes applied to a level where constant speed is held). Due to the wheel’s axisymmetric structure (it is identical all the way around, ie everything is concentric about the axle – brake rotor, rim/tyre etc), if we are to neglect the wheel’s own rotational momentum (because relative to the other forces/momentums at play, it can be considered insignificant) then the wheel can be modeled as though it is in static equilibrium (since we’re not worried about loads internal to the wheel, only the external forces and reactions) – all (external) forces and moments sum to zero. Assume the radius of the whole wheel (to the outside of the tyre) is 0.33m (13”) and the radius of the disc rotor is 100mm (4”). Let us choose an arbitrary force for T(x), which we will say is 200N. Take positive rotation to be clockwise.
Thus:
SM(A) = 0 summing moments about the axle
= [T(x) * -0.33] + [D(x) * 0.1]
= [200 * -0.33] + [D(x) * 0.1]
200 * 0.33 = D(x) * 0.1
\ D(x) = 660N

SF(x) = 0
= D(x) + T(x) – A(x)
= 660 + 200 – A(x)
\ A(x) = 860N (pointing to the left in the diagram)

SF(y) = 0
= T(y) – A(y)
\ A(y) = T(y)
If we take axle force to be 500N once again, then
A(y) = T(y) = 500N


Now we will look at the reaction forces acting on the swingarm. Note that the forces are identical in magnitude at all points of attachment/contact with the wheel (that is, the axle and the caliper) but act on the swingarm in the opposite direction to the directions they act on the wheel (Newton’s action/reaction pairs). (Fig. 6)
http://img.photobucket.com/albums/v260/snms/Fig6.jpg


Note that the force acting horizontally on the axle (with regards to the frame, not the wheel) is the sum of the frictional force on the tyre, and the frictional force on the brake rotor, and that it is always higher in magnitude than the braking frictional force between the caliper/rotor – if it wasn’t, braking would actually accelerate the bike forwards due to a net forwards force acting on it (which obviously is impossible).

From here we have two options to analyse the effect of this braking force on the swingarm. Firstly and most obviously, we can simply use each point force (X and Y components acting on the axle, and the X component on the brake caliper) and their respective normal distances from the main pivot (at B) to calculate the moment about B and thus the reaction force C(x) (as well as the reactions B(x) and B(y) if we so desire). If the reaction force C(x) increases, then that means that the shock’s resistance/spring force has to increase (and the opposite is obviously also true). Given the nature of springs, we know that to increase the shock’s reaction force, it has to sit further into its stroke in order to compress the spring to suit.

The second method, and one which will prove more useful later on when considering linkage bikes/floating brakes, is to consider the forces acting on the swingarm from the caliper and the axle as a single couple moment (generated by the caliper force pushing forwards, and an equal force pushing backwards at the axle so as to have no net force in any direction) plus a horizontal force at the axle instead of two separate forces. In the diagrams shown, this couple moment would be anti-clockwise. This method has the advantage of being able to separate these two components so that we can recognise the following things:

- Regardless of the orientation of the caliper or its distance from the axle, the couple moment will remain the same for any given frictional force on the tyre; this is because that frictional force multiplied by the radius of the wheel is what gives the magnitude of the moment and it is independent of rotor size (note though that the caliper/axle forces change inversely proportional to rotor radius however).
- There are two elements which can semi-independently cause BISI, and thus are separable: these are axle path/horizontal axle force and the couple moment. Couple moments can easily be removed from the equation of by parallel linkages (including floating brakes) which will be covered later, and the horizontal force acting on the axle needs a moment arm (perpendicular to the line of force, so in other words a vertical distance) between the axle and the swingarm pivot, in order to generate a moment about the pivot. As such, both factors need to be taken into account when considering brake interference. It is possible (but not necessarily desirable) to create a setup where these two factors cancel each other out (this will be elaborated on later).
- What determines the amount of squat (or jack, but that will be dealt with later) is determined by two things: the height of the pivot (or instant centre, explanation of instant centre to follow shortly), and the length of the swingarm. The height of the pivot/IC is what determines the vertical distance (which is the moment arm as mentioned before) between the pivot and the axle, and so a higher pivot = longer moment arm = more squat generated by horizontal axle forces. The length of the swingarm changes the effects of the couple moment (which is effectively a constant for whatever calculation you’re doing) by altering the moments generated by the vertical force on the axle and the force of the shock on the swingarm, about the main pivot. For example, if, as above, the axle was 0.45m from the main pivot and the vertical force on the axle was 500N, the moment generated by that (which has to be equaled by the shock’s force multiplied by its normal distance [0.15m] from the pivot) would be 225N.m. A couple moment of say 22.5N.m would be an increase of 10% in this case, which if the shock rate was perfectly linear, would make the bike sit 10% further into its travel in order to regain equilibrium. Now consider that the swingarm was twice the size in every dimension; ie axle to pivot is 0.9m, and pivot to shock mount is 0.3m. The shock ratio is still the same so the shock doesn’t have to supply any additional force at equilibrium. However, the moments (note that these are not “net” moments and as such we are not concerned with acceleration) are now doubled also due to the doubled moment arms – that is to say, the axle force/swingarm length would yield a moment of (0.9 * 500 = 450N.m) that the shock would obviously balance out in the opposite direction. However, an addition of the 22.5N.m couple moment via braking is only a 5% increase now, half of what it was before. From this we can see that swingarm length is inversely proportional to squat generated by the couple moment alone.

[S.]

Instant centre theory:
In a true 4-bar linkage (this includes FSR, VPP, DW-link, Lawwill etc, as well as floating brake linkages), ie not linkage-driven singlepivots, the instant centre (IC) of zero velocity is a point about which everything in the “isolated” link (the rear triangle/seatstays) is moving tangent to. A good explanation can be found here http://www.mtbcomprador.com/pa/english/chapter2_4.htm#InstantCenter, I highly recommend reading it. Please note the mention of a “constant centre” also.

With respect to braking performance, forces/reactions on a 4-bar linkage bike can be calculated at any instant in the same way as a singlepivot (as demonstrated above) by using the distance from axle to IC as the length of the swingarm when trying to determine the effect of the couple moment. If the linkage is a parallelogram (as many floating brakes are) or the links are parallel at that point, then the IC will be at an infinite distance away, and as such the couple moment will have no (direct) effect on the suspension’s state (it will not attempt to compress or extend the suspension). If the linkage is a parallelogram, then you will note that the axle’s tangent path is parallel to the tangent path of each of the two “arms” of the linkage. From this we can see that it must have a centre of curvature that is fixed, which can be placed relative to the forward pivots, identically to how the axle is placed relative to the two rear pivots. (Fig. 7)
http://img.photobucket.com/albums/v260/snms/Fig.jpg


In order to calculate the moment acting on the suspension due to the horizontal force on the axle, the vertical displacement between axle and main pivot (on a singlepivot) can be substituted for the vertical distance between the axle and the centre of curvature here. Calculating the reaction forces at the shock and pivots however, is more complex due to the linkage setup. If you are proficient with basic vector calculations and trigonometry (or instant centre theory in its entirety), you should be able to calculate these reactions. However, the calculations are too variable and numerous to demonstrate here.

A few notes:
- The vast majority of bikes have some amount of brake squat. Very few bikes actually have brake “jack”, but apparently that doesn’t stop every man and his dog misusing the term “brake jack”.
- Generally the instant centre of any bike is in front of the axle. Some bikes have ICs that are behind the axle (such as Lawwills) which due to the brake’s couple moment effect may tend to develop a next extension force under braking – but, and this is a big but, having an IC behind the axle does not automatically necessitate that a bike will “jack” (extend), due to the horizontal axle force giving an equal or greater moment trying to create squat.
- All demonstrations of calculations above are for conceptual explanation only – they make assumptions for the sake of simplicity which make them mathematically inaccurate (by which I mean not exact, not that they give a “wrong” answer/idea). They are used only to give the reader some basic understanding of how the main calculations are performed. The inertia of the wheel, brake, linkage/swingarm etc have been ignored thus far, and all calculations have been assumed to be at equilibrium – this is not precisely the case in the real world, but it will suffice purely for the purpose of conceptual demonstration.
- It is possible, although not necessarily desirable, to have a suspension setup which does not have a squatting or extending tendency under brakes (at a given instant or instances). This, I believe, should be kept separate in terminology, from suspension extension due to weight shift. It is fairly easy to understand that if the brake torque/axle reaction force doesn’t exhibit a compressive or extensive force/moment on the suspension AT ALL, then under any braking the suspension will extend due to weight shifting forwards. For this reason, it may be useful to have some amount of pro-squat (tendency to compress). I know of no situations where it is helpful to have anti-squat (a net extension force, in other words actual “brake jack”) under brakes, as this only exaggerates the forwards weight transfer.
- The “couple moment” and “horizontal force on axle” effects can be treated as independently as above in all situations – note that in some cases, one can be negative whilst the other is positive, reducing either the pro- or anti-squat reaction under braking (depending which one has a greater effect).
- All calculations above work for all situations that I know of. That is, to my understanding, this explanation is universally applicable.
- There are other factors which I have not bothered to mention due to their relative consistency between bicycles, such as centre of mass, shock rate etc. These do have an effect on braking, believe it or not, but their contribution is not significant enough (as well as being much more complex) to be worth mentioning in this article.
- Parallelogram linkages and floating brakes do remove the couple moment component of BISI, but they do not (necessarily) make any change to the effect of the horizontal force on the axle. On many (singlepivot) bikes a floating brake makes a considerable difference to the braking characteristics because of said removal of couple moment component, however this does not completely eliminate all brake-induced effects on the suspension (for better or worse).
- There are other ways to explain braking characteristics, as far as I know these are all in agreement with what is written above. A common way is, with acceptance of the internal forces within the wheel/swingarm, to simply consider the rear wheel contact patch (and the frictional force acting on the wheel at that point) with relation to the IC, and nothing else. This is a perfectly viable method of calculation/comprehension, but I have chosen to explain it differently because separation of the two main components of brake interaction shows more clearly how the forces internal to the suspension can separately affect the suspension and how they can be manipulated to give the desired effect.
- The claim that bikes will “brake jack” and/or “lock up/out under brakes” should always be treated with suspicion. This is nearly always untrue or inaccurate – braking does not “lock out” any bikes, and in no circumstances that I am aware of is braking capable of bottoming out a properly set-up bike. True brake “jack” (extension) can give the rider the impression that the bike has stiffened suddenly, but unlike a rigid/locked out bike, the braking extension tendency is not a reaction to a vertical (bump) force, and as such will not be immovable or rigid, as those forces can be overcome.


The end result: what does it mean?
Most bikes squat to some degree. This is not necessarily a bad thing, as mentioned above. This includes most FSR bikes, despite Specialized’s “fully active in all circumstances” claim (with the understanding that "fully active" means "shock absorption completely unaffected by braking"). Some bikes do actually “jack” but these are few and far between, and it’s not always bad enough to even be noticeable, let alone a problem. There is a definite placebo effect surrounding brake systems, and it is not unlikely that this is due mainly to lack of education/understanding on the subject. Some people will swear black and blue that singlepivots are nearly unrideable due to perceived “brake jack”, others will simply state that they’ve never even noticed it. From this we can make a logical conclusion: BISI does exist, and that it is not necessarily a problem – in fact in some forms and to some degrees it can even be useful. However it is hard to believe that any common amount of brake squat can make a bike unrideable, or anything to that end. Notably, the bike on which Fabien Barel won the 2004 world DH championships on had a brake linkage designed specifically to increase the level of (pro-)squat far beyond what normal bikes generate. Riding the production version of this bike, you can feel a huge tendency for the rear end to dive when the rear brake is applied. Given that no owners of those bikes seem to have any problem with the extreme brake setup, one might logically assume that it’s not actually that bad, and that other bikes with considerably less brake induced squat can hardly be any worse off, and thus are perfectly fine to ride – although not necessarily as comfortable as they could be.

The moral of the story is that almost any pro-squatting braking setup is usable. That is not to say that there is no reason to dislike certain degrees of brake interaction – that varies with riding style, terrain and personal preferences. Another important point to note is that true “neutrality” under any acceleration (positive [pedalling] or negative ) is not necessarily an optimum setup – certain reaction forces under braking/pedalling can help stabilise the bike as well as offer greater comfort and traction. It is also useful to know that it’s not hard to make stuff perform worse, so be wary of playing with your bike’s braking characteristics unless you know what you’re doing – that incorporates more than is written in this article.

[b]All content remains copyright of myself (and Kenneth Sasaki, to whose work I linked regarding instant centres). Feel free to reproduce it as long as full credit is given.

[Ride_Guy]

im sorry but it has to be said... that is one freakin huge post.

*nothing to contribute possibly after i have read it all.

[andy73]

Awesome explanation - thanks heaps. haven't started it yet but I've got a 12 hour plane ride tomorrow. Figure I'll tackle it then.

[donthucktoflat]

did you do the pretty pictures in paint?

[lotec]

holy shit steve, you need to get a life :eek:

[S.]

did you do the pretty pictures in paint?

No, um, solidworks 2d version... for children aged 5 and under.

[Wattsy]

so you did 4 unit maths i guess? just curious but have you done any courses, ifso, what and at what level.

[S.]

so you did 4 unit maths i guess? just curious but have you done any courses, ifso, what and at what level.

I don't know what 4 unit maths is (is that high school or uni stuff?), but basically I'm halfway through a mech eng degree right now, if that answers your question. The maths in that article is actually very basic, anyone who finished year 10 maths should be able to understand the actual calculations - it's the physics concepts and being able to consciously separate the various components that is more difficult.

[Elbo]

I'm doing Physics (Yr 11) and we're actually learning this sorta stuff at the moment, just not in as much detail. Our Physics questions/examples are always related to bikes/forces on bikes etc :cool:... our teacher is a roadie and also races XC.

[Tomas]

Year 11 physics is pretty limited to vector addition. And pretty limited to same axis ones. Steve, for someone doing engineering studies at school, and with a little education in moments and X and Y axis vector addition, it wasnt that bad to follow. Thankyou, very informative and much appreciated. Applying school to bikes! Ripper.

(4Unit maths is NSW/ACT HSC(higher School Certificate) Mathematics. A level you can only do in year 12. You do twice the hours of a normal subject (say, geography or physics, or standard english). It is equivilant to 1st year Advanced Maths at uni)

[S.]

Year 11 physics is pretty limited to vector addition. And pretty limited to same axis ones. Steve, for someone doing engineering studies at school, and with a little education in moments and X and Y axis vector addition, it wasnt that bad to follow. Thankyou, very informative and much appreciated. Applying school to bikes! Ripper.

(4Unit maths is NSW/ACT HSC(higher School Certificate) Mathematics. A level you can only do in year 12. You do twice the hours of a normal subject (say, geography or physics, or standard english). It is equivilant to 1st year Advanced Maths at uni)

Good to hear.

We didn't have a maths subject like that (that was double the hours anyway), we basically had Specialist (roughly like 1st semester maths at uni), Methods (all the basics of calculus, statistics, all that sort of stuff - a relatively hard subject too), and Further (basic statistics and surveying type stuff I think). I only did methods in high school (didn't want to do all the rediculous amounts of work associated with Spec), but did another 3 semesters of maths at uni. Pure theoretical maths honestly is not my strong point though. If it can be applied to a physical situation I usually find it easier though.

[rhyno]

nah steve you got it all wrong! that's break jack! :P

[S.]

nah steve you got it all wrong! that's break jack! :P

I'll break you with a jack in a minute... :p

[timmo]

And the "25 words or less" explanation is...? :)

[Binaural]

Hehehe, I recognize all that math - I did mechatronics engineering at uni :D and I know a paraphrased report for a mechanics class when I see one. Still, nice work on an interesting topic, especially one that is benighted with a million contraditory opinions.

By the way, I reckon producing a simple simulation of brake jack/squat in a 3D CAD package like SolidWorks or SolidEdge would be fairly easy and would look really cool! I did something similar when I was modelling a mobile robot steering linkage (Ackerman, btw), and it's amazing how the visualization helps you understand the system better.

[Cave Dweller]

I did something similar when I was modelling a mobile robot steering linkage (Ackerman, btw).

You did the ackerman simulations?? :confused:

Thats not how i remember it punk....... :D

[RaID]

By the way, I reckon producing a simple simulation of brake jack/squat in a 3D CAD package like SolidWorks or SolidEdge would be fairly easy and would look really cool! I did something similar when I was modelling a mobile robot steering linkage (Ackerman, btw), and it's amazing how the visualization helps you understand the system better.


So for what uni did you do FSAE for?

[Binaural]

You did the ackerman simulations?? :confused:

Thats not how i remember it punk....... :D

Hah, you did the details, but we both did the initial calculations, in case your brain has been rotted a bit lately :p

No, mostly what I was referring to was that model I built up in Solid Edge - I definitely showed it to you - remember, you could export it to an AVI file?

[Cave Dweller]

So for what uni did you do FSAE for?

Whats FSAE?

[Cave Dweller]

And the "25 words or less" explanation is...? :)

Brake squat in a bike is not necessarily a bad trait.

[Cave Dweller]

Hah, you did the details, but we both did the initial calculations, in case your brain has been rotted a bit lately :p

No, mostly what I was referring to was that model I built up in Solid Edge - I definitely showed it to you - remember, you could export it to an AVI file?

Don't remember that, i remeber doing the simulations in working model (what a POS bit of software).

On the other hand, just started playing around with ADAMS, now there is some freaking cool software.

[Daver]

I have a floating brake. My bike goes down hill. I can brake and get traction in the corner, something i couldn't ever do on my bullit.

Edit:

People started designing those [floating brakes] because one guy, Richard Cunningham of some US magazine [MBA], started using the term "brake jack", the rear end jacks up when you are braking going downhill, so they designed a whole swathe of things to stop that. And what someone should have done is to go over there and say to him "stop! Don't brake with the rear wheel when there is no weight on it". A whole swathe of MTBs were ruined because of this guy. Most designers tried to design out brake jack and if you think about what they've done its completelty wrong. Motorcycles all had floating brakes once- do you see it now? Do you see it on Carmichael's bike? No. If there is no weight on the rear wheel you can't brake, it will skip and hop. Someone should have clipped that guy around the head and said "stop", but they won't...

[Cave Dweller]

I have a floating brake. My bike goes down hill. I can brake and get traction in the corner, something i couldn't ever do on my bullit.

Shouldn't be braking in corners anyway........ :p

[Binaural]

I am looking for the AVI file now, but I have a nasty feeling I did it on that old server computer I gave to Shaun. Will show you when I get it, it's pretty cool - you create an assembly, then you apply a motion to the connections. Depending on the degrees of freedom you assign in the object alignments, you can see how the linkage moves in real time. Looked awesome :D

Does ADAMS do exports to movie formats? Would be cool to see some of that :)

[Daver]

Shouldn't be braking in corners anyway........ :p

Dicky has been teaching me. I swear.

[Cave Dweller]

There are other factors which I have not bothered to mention due to their relative consistency between bicycles, such as centre of mass, shock rate etc. These do have an effect on braking, believe it or not, but their contribution is not significant enough (as well as being much more complex) to be worth mentioning in this article

hmmm........ I understand your model is very simplified but........

I would have thought that center of gravity was important? Especially seeing the rider is around 4 times the weight of the complete bike. Shifting 80kg 0.3meters each way must have a huge effect.

In vehicle dynamics the COG is very important, especially its location in relation to the roll center/instant center as this also changes the forces on the suspension systems (springs/dampers), changes the value of the inertia at the pivot points (resistance to motion, parallel axis theorem) yada yada yada. Surely the riders location will have a measurable effect on squatting.

Example, If the rider is right over the handlebars all the weight is shifted forward so the rear suspension will stiffen, reducing the tendancy to squat.

Im probably completly wrong, bike suspension is not my area of expertise.....

[Cave Dweller]

Also, for a more advanced model you would need to perform a vibration analysis, the force Cx is actually a force from the spring/damper unit so you need to take into account the velocity and accelerations of impacts / weight shifts etc etc. So you would start getting into the relm of non-linear differantial equations which get very messy.

[S.]

hmmm........ I understand your model is very simplified but........

I would have thought that center of gravity was important? Especially seeing the rider is around 4 times the weight of the complete bike. Shifting 80kg 0.3meters each way must have a huge effect.

In vehicle dynamics the COG is very important, especially its location in relation to the roll center/instant center as this also changes the forces on the suspension systems (springs/dampers), changes the value of the inertia at the pivot points (resistance to motion, parallel axis theorem) yada yada yada. Surely the riders location will have a measurable effect on squatting.

Example, If the rider is right over the handlebars all the weight is shifted forward so the rear suspension will stiffen, reducing the tendancy to squat.

Im probably completly wrong, bike suspension is not my area of expertise.....

You are correct in that CoG is VERY important (not just in braking, in every single facet of suspension performance), however what I meant by consistent between bikes is that the rider's weight shift is fairly consistent under braking (and that being a human variable in itself, the rider's own movements/riding style can change that effect considerably). In that context, braking systems can be modeled comparatively (in a simplistic context such as this) with a reasonable degree of accuracy if your goal is to understand the system (not to model it precisely). Reading/understanding this article will, as I said before, not give you the ability to take a quantitative measure of braking systems - only a basic understanding. The calculations shown were only for explanation's sake, and if you were to accurately perform them quantitatively you would have to take into account that weight shift. As I said above about it being a human variable, the only way IMO to measure the weight shift and somewhat accurately compensate for/integrate it with a bike's design would be to experimentally record how riders react to the forward weight shift under braking. I imagine some people/companies have conducted experiments aimed directly at this, but I have never seen published results (if someone was to spend the time to gather that kind of information, I doubt they would be willing to share it with their competitors).

"Example, If the rider is right over the handlebars all the weight is shifted forward so the rear suspension will stiffen, reducing the tendancy to squat."

I'm not sure exactly how you mean this - but the rear suspension will extend (relative to a normal sag point) if you shift your weight to increase the front bias, but this means that the bike will be in a softer region of its travel. Ultimately the amount of squat/jack possible in any given bike is limited by the traction (all else being equal). If you shift your weight forwards you will have less traction in the rear (so it can't generate as much squat at maximum traction, ie just before skidding) but the bike will also be in a softer region of its travel as mentioned above. This is where shock rate (among other things) comes into play. Also have to consider equations of friction coefficients (since with pneumatic rubber tyres, particularly on dirt, they don't increase linearly - it's all over the place), riding surface etc. If you had some serious time and money to spend experimenting with all those factors you could probably get a somewhat accurate general response for weight shift and its effects (taking into account all the stuff I mentioned before), but without doing that I can't imagine a theoretical analysis being a great deal of use. Hence why I pretty much left weight shift out altogether (along with the fact that the calculations would be much, much more complex). In fact I imagine that for any reasonable experimental scope, the conclusion may well be the same: "there is no conclusive answer because there are too many variables".

But thanks for bringing that up all the same.

[S.]

Also, for a more advanced model you would need to perform a vibration analysis, the force Cx is actually a force from the spring/damper unit so you need to take into account the velocity and accelerations of impacts / weight shifts etc etc. So you would start getting into the relm of non-linear differantial equations which get very messy.

Yep, you'd need to take into account a lot of things which would require months, possibly years, of modelling and calculation - particularly with the current crop of SPV type shocks, there are a lot of adjustments which can make a huge difference (they affect damping curves [in 3d since they are both position and speed sensitive] as well as spring rate). Even the action of the fork, width of the bars, length of the stem, and a lot of the general riding geometry of the bike would start to play large parts in analyses that became that detailed. In the end it may prove completely unfeasible to do that, and be easier/cheaper to get a rider to test your bike without you telling them what adjustments you've made. Even then, the test rider will get tired/get used to the bike/get used to the track(s), take different lines, etc etc. Because of the nature of pushbikes (like you said, being roughly 1/4 the weight of the rider) it's very hard to control all/many of those variables in a scientific manner. In this respect any testing might be almost best off using a trial-and-error setup.

[Cave Dweller]

What i meant was if you place more weight over the front you unweighted the back so the spring rate becomes too stiff and the rear of the bike would stiffen. From the riders perspective is would appear that the rear end has stiffened up.

Pneumatic tires...... arg......seriously, don't start messing with tires. People have spent years and years, written whole PhD's based around tire models and none of them can accurately predict what will happen, the system is just non-linear with too many variables.

If you had some serious time and money to spend experimenting with all those factors you could probably get a somewhat accurate general response for weight shift and its effects (taking into account all the stuff I mentioned before), but without doing that I can't imagine a theoretical analysis being a great deal of use

Yep, thats why David Weagle instrumented the DW link bike during the design phase. I can guarantee that he had gyroscopes, accelerometers and displacement measurement devices everywhere.

The best models in the world are useless if they can not be validated by real life data.

Yep, you'd need to take into account a lot of things which would require months, possibly years, of modelling and calculation

Welcome to my life...... :(

[S.]

What i meant was if you place more weight over the front you unweighted the back so the spring rate becomes too stiff and the rear of the bike would stiffen. From the riders perspective is would appear that the rear end has stiffened up.

Pneumatic tires...... arg......seriously, don't start messing with tires. People have spent years and years, written whole PhD's based around tire models and none of them can accurately predict what will happen, the system is just non-linear with too many variables.



Yep, thats why David Weagle instrumented the DW link bike during the design phase. I can guarantee that he had gyroscopes, accelerometers and displacement measurement devices everywhere.

The best models in the world are useless if they can not be validated by real life data.



Welcome to my life...... :(

Yep, pretty much. So for all that we have concluded (again) that this is a simplistic, conceptual explanation and nothing more :)

[dazz]

Note: for most of this post I am assuming that front & rear brakes are being applied.

Why I like brake squat:
- While braking the front dive's & the rear gets unweighted to some extent, this upsets the geometry of the bike, a bike with a lot of brake 'squat' compresses the rear suspension keeping the geometry somewhat normal. (both ends compressed a similar amount) This gives more stable straight line braking.

Why I think brake squat is bad:
- Because the shock is pushed into its stroke (getting stiffer) during braking, it is less responsive to small bumps. This means that traction while cornering with the rear brake applied is less than with a rear brake that does not affect suspension action. eg. a floating brake.

I think brake jack is bad, because it accentuates brake dive. Couldn't think of anything worse - steep downhill, hit the brakes, the front compresses, the rear extends & your bike try's to tip you over the handlebars!!

If you want to see if your bike has much brake squat, try this:

1 (not critical) - Set the compression (& platform) damping on your shock to the softest setting. Make sure you know the settings so you can reset it.
2 - Flip your bike upside down, put it in the tallest gear & get the rear wheel spinning like crazy. Get someone to hold the bike steady for you.
3 - Hit the rear brake .... HARD.

If your rear suspension compresses momentarily, you have brake squat. (induced by the inertia of the rear wheel only, the effect is bigger due to friction between the tire & the ground during braking & it also stays compressed the whole time you are braking. ie, the harder you brake, the more squat you get)

[Wattsy]

oh S. you dont live in NSW. if not, completly ignore my post. and yes, 4 unit is like the hardest maths you can study in the yr 11 / 12 HSC course. but still, keep up the good work and for god sakes, get a girlfriend. and if you do, go spend more time with her!

[wombat]

for god sakes, get a girlfriend. and if you do, go spend more time with her!
No, then we'd just have to read articles about her weight shift.

[S.]

I have a floating brake. My bike goes down hill. I can brake and get traction in the corner, something i couldn't ever do on my bullit.

Edit:

I read that interview with Chris Porter and immediately decided that I'm never ever going to buy anything off him or let him touch my stuff. His obsession with copying the motorbike world is pretty misguided IMO.

[S.]

Why I think brake squat is bad:
- Because the shock is pushed into its stroke (getting stiffer) during braking, it is less responsive to small bumps. This means that traction while cornering with the rear brake applied is less than with a rear brake that does not affect suspension action. eg. a floating brake.

Yeah this is a tricky one. I don't really know just how much squat you have to have in order to actually cause a net compression of the suspension under braking, or whether any amount can fully eliminate the forward weight shift under braking (being that bikes have very short wheelbases and are very easy to do nosewheelies on purely with front brake force).

Also, floating brakes don't necessarily alleviate all BISI (particularly on high-pivot bikes), as I mentioned in the original post(s), although on certain bikes (relatively low pivots) I have no doubt that they reduce it to an insignificant level.

[BooyahXTC3]

Geez all that takes me back to my Statics and Dynamics classes when I was doing Mech Eng.

[dazz]

Geez all that takes me back to my Statics and Dynamics classes when I was doing Mech Eng.

I've done the same course, did my time at Ballarat SMB. Actually quite liked those classes.

[fat_tyre_nick]

yere damn that is huge, how long did it take you? It was definatly worth a sticky

Great Job

[S.]

yere damn that is huge, how long did it take you? It was definatly worth a sticky

Great Job

Didn't take me long at all, I just cut + pasted from a word document that took me 3 and a half weeks to write.

[nicklouse]

S. good reading!

you remember my Tank?

well i will be designing a floater for it!

now a question for you. would you go for a completly neutral set up or one that reduces some of the " jack" or reverses the "jack"


on a side note the bike was too fast for me! but the brakeing was just like a single pivot.

[S.]

S. good reading!

you remember my Tank?

well i will be designing a floater for it!

now a question for you. would you go for a completly neutral set up or one that reduces some of the " jack" or reverses the "jack"


on a side note the bike was too fast for me! but the brakeing was just like a single pivot.

Yeah I remember The Tank :)

As for completely neutral or squatting, that really depends. I'd probably go for some minor amount of squat personally, but I'm not sure how easy it would be to design that with a Lawwill design.

[nicklouse]

no probs look at the schwinn straight 8.

[S.]

no probs look at the schwinn straight 8.

Yeah, those Lawwills with floaters generally had em mounted to the chainstay IIRC, which would give a braking characteristic somewhat similar to a low singlepivot in my estimation. You could give that a go, at least it'd be a fairly safe bet.

[nicklouse]

will be doing some calcs.

but must be neat, light weight and not be in a position to get wrecked.

[S.]

will be doing some calcs.

but must be neat, light weight and not be in a position to get wrecked.

Yeah, I think the conventional lawwill style one would work best then. The others are a bit of a bastard to play with.

[cornflake]

good lord......

Do you have a life? Gawd that must've taken you ages.....

[Beena]

Well done Steve, now just apply that level of commitment to your studies!!

For who ever was asking about FSAE - I did FSAE for Melbourne Uni.

(for those who don't know, it's Formula SAE - a competition where Uni's design race cars from scratch which undergo a range of performance and design tests.)

Also, Binaural, did your Avatar happen to come from the road on the way up the mountain (can't remember the name) in Tucson Arizona?

[nicklouse]

Beena:

the formula student thing is great.

i advised for one uni in the uk and will be advising for another in sweden!

[Pete W]

Guys. Three things
. Beer
. women
. Freeride bikes
Choose any two ( or all three ), and spend a shit load more time with them, much......... much........ more time!!!!!!!!!!!!!!!!!!!!

Ps great thread though, me, my girlfriend and my craftworks drank almost two slabs while reading it!!!!!!!!!!

[kona_boy]

If you want to see if your bike has much brake squat, try this:...snipped



another way is to just sit on a bike without a floating brake.
-bouce on the rear suspension
-apply rear brake and bounce on rear
-note difference feel

sit on similar or same bike WITH a floating a brake
-repeat above steps

that 'different feel' doesnt not appear when applying the rear brake on a Floating system

[S.]

another way is to just sit on a bike without a floating brake.
-bouce on the rear suspension
-apply rear brake and bounce on rear
-note difference feel

sit on similar or same bike WITH a floating a brake
-repeat above steps

that 'different feel' doesnt not appear when applying the rear brake on a Floating system

No, brake squat or jack will ONLY show up when the wheel is actually rotating. If you'd read the thread you would see why.

[kona_boy]

what do you call that then ^

[toodles]

what do you call that then ^

All that would indicate is the extent the wheelbase changes during suspension compression. Brake jack/squat/whatever is caused by the grab of the brake transmitting the centrifigal forces/rotational forces of the wheel into the swingarm and affecting the suspension. I think. Ask S. - he's the guru on this shit.

[S.]

what do you call that then ^

Any combination or permutation of the following:
1. Extending wheelbase
2. Difference in axle paths
3. Allowance for the difference in wheel rotation (which is very small during stationary compression except on some high-pivot bikes) causing the bike to move forward a bit (directly related to axle paths as mentioned above)
4. Placebo.

If you chuck a floater on a given bike and bounce on it with the brake locked, there will be negligible difference to that same bike without a floater. The whole brake jack/squat thing is caused by acceleration (deceleration is still acceleration), and does absolutely shit all at a standstill.

[kona_boy]

hmm...ok. im not gonna argue.

and yes i have expreienced 'real' brake jack just as a point of interest.
(stinkys are the king of it :( )

[S.]

hmm...ok. im not gonna argue.

and yes i have expreienced 'real' brake jack just as a point of interest.
(stinkys are the king of it :( )

Stinkys squat, not jack (jacking is the rear suspension trying to extend, squatting is compression), and there are far worse bikes (in that regard) than the Stinky. Generally speaking, the higher/more rearward the pivot (or IC) is, the more squat it will have, and while Stinkys have relatively short swingarms (rearwards pivot), the pivot is also quite low. Obviously if the pivot was behind the axle in any way then it starts to get more complicated than that, but that's a fairly rare scenario (Lawwills, notably, have the IC behind the axle).

[kona_boy]

dammit...you are worse than SLATYE over at overclockers.com.au :p

[residentslacker]

I've done enough Newtonian mechanics to understand all that, but I've got to say it wasn't easy reading.

Anyway, good work S. Thanks for putting the time in.

:)

[S.]

I've done enough Newtonian mechanics to understand all that, but I've got to say it wasn't easy reading.

Anyway, good work S. Thanks for putting the time in.

:)

Cheers. What parts did you have trouble with? If any particular bits are hard to understand, I may re-word them at some point.

[Sov]

I have only skim read your posts, so I might have missed something here:) Great write-up tho, but I will have to print it and read it properly one day as i hate reading stuff on a monitor...

I notice on most 'faux-bar' link bikes that the rear brake mounts are on the chainstay link (like on Stinkys and Banshees.) In my mind, this means they are governed by the same physics as single pivot systems, as the rest of the links only serve to actuate the shock.

So what happens if the brake mounts are on the seatstay link?

eg. if you take a single pivot bike with a floating rear brake and compare the path of the caliper to that of the wheel axle under compression, the caliper moves away from the axle's path. Same thing will occur if the caliper is mounted on the seatstay above the chainstay pivot (although the dynamics will differ depending on the link lengths)

[udi]

If you are suggesting running the brakemount on a faux-bar bike on the seatstays instead of the chainstays, it simply won't work.

The brakemount has to either be on the same link as the dropout/axle, or on a plate that rotates around the axis of the dropout/axle. Following your suggestion, the calliper will at some point in the travel move inwards or outwards in relation to the rotor, resulting in the universe imploding.

[Sov]

Good point, didn't think of that lol:o And here I was thinking that I had solved one of the great mysteries of the universe!

[S.]

I have only skim read your posts, so I might have missed something here:) Great write-up tho, but I will have to print it and read it properly one day as i hate reading stuff on a monitor...

I notice on most 'faux-bar' link bikes that the rear brake mounts are on the chainstay link (like on Stinkys and Banshees.) In my mind, this means they are governed by the same physics as single pivot systems, as the rest of the links only serve to actuate the shock.

So what happens if the brake mounts are on the seatstay link?

eg. if you take a single pivot bike with a floating rear brake and compare the path of the caliper to that of the wheel axle under compression, the caliper moves away from the axle's path. Same thing will occur if the caliper is mounted on the seatstay above the chainstay pivot (although the dynamics will differ depending on the link lengths)

You're completely correct about them behaving as singlepivots in all respects except shock rate. Unfortunately a lot of people have trouble recognising this, but good to see you don't!

Ff the seatstay link was concentric to the axle (ie the pivot was at the axle) then you could run the brake caliper on that and it'd be like one big floating brake... not particularly likely to have "optimal" braking characteristics but it'd be pretty close to a lot of FSR bikes really.

[Sov]

You're completely correct about them behaving as singlepivots in all respects except shock rate. Unfortunately a lot of people have trouble recognising this, but good to see you don't!
LOL, I knew the $25,000 I owe in HECS fees for my engineering degree would pay off one day:p

[MORRK_DH]

That was very Hard reading, im only doing 1 and 2 Methods, Physics and calc and that was very hard to read. but Fark S. IQ of 170 yeh? haha thanks for giving em something to do for 2 hours :P

[fleshbone]

hehehe,i already know all this,we did last year in engineering studies,so bloody hard.

[Bodin]

Ff the seatstay link was concentric to the axle (ie the pivot was at the axle) then you could run the brake caliper on that and it'd be like one big floating brake... not particularly likely to have "optimal" braking characteristics but it'd be pretty close to a lot of FSR bikes really.

For some reason, I've always loved the whole concept of the pivot-around-the-bottom-bracket-a-la-Rotec design. Then more recently, I've also been wondering if it was possible to design a rear chainstay/seatstay pivot that centred around the axle.

Now that you've explained that it would effectively be a floating brake, I'm interested to try it (not that I have the engineering talents or resources)... However, why did you say that the braking characteristics wouldn't be optimal?

I'm only wondering because I'm suggesting that the mount is on the seat stay and therefore technically identical to an FSR, but without the Horst Link...

[S.]

For some reason, I've always loved the whole concept of the pivot-around-the-bottom-bracket-a-la-Rotec design. Then more recently, I've also been wondering if it was possible to design a rear chainstay/seatstay pivot that centred around the axle.

Now that you've explained that it would effectively be a floating brake, I'm interested to try it (not that I have the engineering talents or resources)... However, why did you say that the braking characteristics wouldn't be optimal?

I'm only wondering because I'm suggesting that the mount is on the seat stay and therefore technically identical to an FSR, but without the Horst Link...

For all intents and purposes, they would be nearly identical to an FSR bike. That's not optimal, in my eyes :)

Basically if you did that, it would be like using a singlepivot + floating brake (since one of their pivots is always axle-centric too), with the floating brake also used to drive the shock in some way. Draw from that what you will.

[Martin.au]

Hey S. What's your opinion on the Freedrive and how it's suspension reacts under braking?

I have a hunch that under braking the braking force attempts to extend the suspension but the riders body weight pushes forward on the pedals as the rider decelerates. This lowers the amount of extension of the rear suspension but still allows it to react to bumps.

Any thoughts?

[projectsplat]

completely OT : Damn, I thought this was an offshoot of the recent "attempted bike jacking thread" where some punk comes up to you at a skate park and says - "hi, nice hope 6 pots. I hope you don't expect to be walking away with those bad boys...."

sorry, somewhere in a very small disturbed part of my brain (pre-coffee) i though that was funny.

On T : nup, i've got nothing.

[Lahardesign]

using a forward rotation of the brake caliper undersupension compression is a direct elimination of the positive effect that floating brakes are used for.
Floating brakes enable you to brake smoothly over braking bumps because they don't transfer the rotating mass of the wheel into suspension compression when the wheel comes off a bump and locks up over empty space.
Any system that tries to overcome brake jack caused by the brake force acting below the CoM like this will result in your wheel driving down into braking bumps and hopping up over hollows in a nasty resonant skipping motion. Actually you don't even nned bumps for this to happen- Its the reason braking bumps form in the first place and the principle is equally valid on racetracks.
The only way to get proper brake performance is with steep enough a swingarm to eliminate the inertia stinkbug and a floating brake that is set up not to rotate the caliper forward under susp compr.:cool:

[Lahardesign]

if we are to neglect the wheel’s own rotational momentum (because relative to the other forces/momentums at play, it can be considered insignificant)

Sorry steve, major false assumption here.
I also take issue with the "not many bikes brake jack" thing. Its an inherent part of any conv bikes momentum physics and attempts to mask it create the worst feedback effects as I've mentioned above.

[S.]

using a forward rotation of the brake caliper undersupension compression is a direct elimination of the positive effect that floating brakes are used for.
Floating brakes enable you to brake smoothly over braking bumps because they don't transfer the rotating mass of the wheel into suspension compression when the wheel comes off a bump and locks up over empty space.
Any system that tries to overcome brake jack caused by the brake force acting below the CoM like this will result in your wheel driving down into braking bumps and hopping up over hollows in a nasty resonant skipping motion. Actually you don't even nned bumps for this to happen- Its the reason braking bumps form in the first place and the principle is equally valid on racetracks.
The only way to get proper brake performance is with steep enough a swingarm to eliminate the inertia stinkbug and a floating brake that is set up not to rotate the caliper forward under susp compr.:cool:

Ah -this is an interesting criticism, as you correctly imply that over bumps, the forces and reactions are transient (not steady-state). However (and in reference to your following post) it all relies on whether the rotational momentum of the wheel is great enough to cause any significant suspension compression. Do you happen to have any figures for the approximate mass moment of inertia of a 26" wheel? Otherwise I'll try and find a way of roughly gauging it.

I just went and tried a little experiment with my own bike (04 IH SGS, FSR suspension which has a small but visible amount of caliper rotation under compression. What I did was mount my bike in the workstand, with the bottom-out bumper pushed all the way up against the shock body (the shock is a 2004 Swinger 6 way, with absolute minimum preload to keep the spring in place, and 70psi in the chamber, spring is 400lb/in). I then pedalled the bike in 8th gear (because my derailleur is bent - haha you'd love that - and thus 9th doesn't work) as fast as I could by hand, which I estimate is probably roughly equivalent to what I could achieve by actually pedalling it flat out in that gear. I think it would give me just over 50km/h anyway. With the bike in the stand, wheel spinning as fast as I could get it to, and me bracing the bike to try and hold it as rigidly as possible, I jammed on the rear brake as hard/fast as I could. The shock did not compress whatsoever. This is in the softest portion of the spring's stroke, and arguably with somewhat similar conditions restraining the front triangle from rotating as would do so in real life. This leads me to believe, at this stage, that the rotational momentum of the wheel is, for braking analyses, pretty well insignificant. However, if you can provide figures to the contrary then I'll accept your argument.

Your theory of how braking bumps are formed is interesting though, and may well be accurate regardless of whether the wheel's rotational momentum is significant in the suspension's response. On a somewhat-related note, do you happen to know what causes corrugations in dirt roads? I've been trying to work it out for ages (because it's not only in areas of acceleration/braking), and I still haven't managed to.

As far as the "inertia stinkbug" goes, again my own observation has been that this is not necessarily a large problem at all. Under braking, the mass transfer forwards is not necessarily geometrically directly related to the position of the front and rear suspensions in their respective travel (obviously - since if it was, having lots of brake squat would actually cause a rearwards mass transfer). Therefore, having the suspension in a softer region of the travel (for a given linear spring rate), without the addition of the other compressive forces effectively preloading the suspension (which for a net input should not matter, as spring response is dictated only by spring rate rather than its displacement due to initial/constant conditions) should give a very similar result. This assumes a linear shock rate though, for a progressive shock rate it should actually be better without the compressive force acting to eliminate the "inertia stinkbug" as the effective spring rate would be lower due to the higher leverage ratio.

BTW - "brake jack" as I refer to it (using Dave Weagle's terminology) does not include suspension extension due solely to weight transfer. The reason for this is as I outlined above, as well as the fact that forced extension (as opposed to unloading) exaggerates the geometric shifts (obviously not what people want - I'm sure you'd agree here since you have evidently gone to some effort to try and reduce these shifts on your own bikes), can cause top-out (which means that dynamic responses utilising spring rates become much more complex since the suspension is not actually activated), and is unable to really be considered anything like a steady-state input due to its inherent ability to continue to maintain an extensive force even at top-out. Simply unloading the suspension does not have these same characteristics, and this is why my personal opinion is that it is not "brake jack", and not as problematic by any means.

[Lahardesign]

Ah -this is an interesting criticism, as you correctly imply that over bumps, the forces and reactions are transient (not steady-state). However (and in reference to your following post) it all relies on whether the rotational momentum of the wheel is great enough to cause any significant suspension compression. Do you happen to have any figures for the approximate mass moment of inertia of a 26" wheel? Otherwise I'll try and find a way of roughly gauging it.
The total rim and rubber weight of a dh wheel is around 2.5 kg. At 50kph or 14 m/s it would have about 250 joules of rotational energy which doesn't sound much. If the wheel goes 50 kph- 0 in a tenth of a second which may be a lengthy estimate your looking at 2.5kw - a fair bit of power looking for somewhere to go. when this is a cyclic action several times a second it quick builds up to some high amplitude harmonics.

I just went and tried a little experiment with my own bike (04 IH SGS, FSR suspension which has a small but visible amount of caliper rotation under compression. What I did was mount my bike in the workstand, with the bottom-out bumper pushed all the way up against the shock body (the shock is a 2004 Swinger 6 way, with absolute minimum preload to keep the spring in place, and 70psi in the chamber, spring is 400lb/in). I then pedalled the bike in 8th gear (because my derailleur is bent - haha you'd love that - and thus 9th doesn't work) as fast as I could by hand, which I estimate is probably roughly equivalent to what I could achieve by actually pedalling it flat out in that gear. I think it would give me just over 50km/h anyway. With the bike in the stand, wheel spinning as fast as I could get it to, and me bracing the bike to try and hold it as rigidly as possible, I jammed on the rear brake as hard/fast as I could. The shock did not compress whatsoever. This is in the softest portion of the spring's stroke, and arguably with somewhat similar conditions restraining the front triangle from rotating as would do so in real life. This leads me to believe, at this stage, that the rotational momentum of the wheel is, for braking analyses, pretty well insignificant. However, if you can provide figures to the contrary then I'll accept your argument.
Thats a test I've tried too. The m9 makes a sound like a sharp drum beat as it dissipates the energy and no susp move. Maybe the 4bar link is giving you a pretty good independant brake and platform shocks could disguise things by transfering the momentum to the whole bike. Try a fast jump and lock your brakes in mid air if you want to see if wheel momentum can be significant.
(please don't)

Your theory of how braking bumps are formed is interesting though, and may well be accurate regardless of whether the wheel's rotational momentum is significant in the suspension's response. On a somewhat-related note, do you happen to know what causes corrugations in dirt roads? I've been trying to work it out for ages (because it's not only in areas of acceleration/braking), and I still haven't managed to.

Not my theory, you can find it in chassis design texts. Understood by many pro dh'ers tho. you can often observe it by wheeling your bike on pavement with the back wheel locked. If it judders you've got a resonant feedback going on.
Motor-racing on pavement requires independant brakes, admitedly wheel masses are bigger and V squared gives you lots of energy. Eliminating susp compression under lockup is the goal.

As far as the "inertia stinkbug" goes, again my own observation has been that this is not necessarily a large problem at all. Under braking, the mass transfer forwards is not necessarily geometrically directly related to the position of the front and rear suspensions in their respective travel (obviously - since if it was, having lots of brake squat would actually cause a rearwards mass transfer). Therefore, having the suspension in a softer region of the travel (for a given linear spring rate), without the addition of the other compressive forces effectively preloading the suspension (which for a net input should not matter, as spring response is dictated only by spring rate rather than its displacement due to initial/constant conditions) should give a very similar result. This assumes a linear shock rate though, for a progressive shock rate it should actually be better without the compressive force acting to eliminate the "inertia stinkbug" as the effective spring rate would be lower due to the higher leverage ratio.
This is explored in chassis texts and softer rear susp when its unweighted under braking is considered a bonus, however the high CoM of bikes makes the increased forward weight transfer and steepened steering geometry far more serious issues.


BTW - "brake jack" as I refer to it (using Dave Weagle's terminology) does not include suspension extension due solely to weight transfer. The reason for this is as I outlined above, as well as the fact that forced extension (as opposed to unloading) exaggerates the geometric shifts (obviously not what people want - I'm sure you'd agree here since you have evidently gone to some effort to try and reduce these shifts on your own bikes), can cause top-out (which means that dynamic responses utilising spring rates become much more complex since the suspension is not actually activated), and is unable to really be considered anything like a steady-state input due to its inherent ability to continue to maintain an extensive force even at top-out. Simply unloading the suspension does not have these same characteristics, and this is why my personal opinion is that it is not "brake jack", and not as problematic by any means.

The large moving ballast of a bike-rider chassis allows inertia stinkbug to be less of a prob than theory suggests it should be with the low pivots of conventional bikes. If your bike stays level under brakes ballast movement allows tyre weighting to be better maintained and eliminates the theoretic advantage to softening the rear that unwinding suspension produces.

[S.]

The total rim and rubber weight of a dh wheel is around 2.5 kg. At 50kph or 14 m/s it would have about 250 joules of rotational energy which doesn't sound much. If the wheel goes 50 kph- 0 in a tenth of a second which may be a lengthy estimate your looking at 2.5kw - a fair bit of power looking for somewhere to go. when this is a cyclic action several times a second it quick builds up to some high amplitude harmonics.

Your maths is fair enough, however my opinion remains that based on the mini-experiment I mentioned before, this rotational energy is relatively insignificant compared to the other forces at play, if it can't actually push past the SPV threshold in the shock (or even make the stand look like it's going to tip over when I'm not bracing it). As you say though, FSRs are reasonably unreactive to couple moments; when I get the chance I will try and replicate this experiment with a few other bikes (I think a D8 with a Vanilla RC, ie no platform threshold, might be more useful). If you're interested I'll post the results up here (if there are any that aren't simply zero). If I had the money I'd make an instrumented testbed for this.

Thats a test I've tried too. The m9 makes a sound like a sharp drum beat as it dissipates the energy and no susp move. Maybe the 4bar link is giving you a pretty good independant brake and platform shocks could disguise things by transfering the momentum to the whole bike. Try a fast jump and lock your brakes in mid air if you want to see if wheel momentum can be significant.
(please don't)

Yeah I'm aware of the braking mid-air trick (works especially well with MX bikes and their extremely heavy wheels), but again my opinion differs based on the fact that stability of a bike (or perception thereof) when airborne is balanced on a knife-edge to begin with, and any change in the system's conditions will have effects that can be seriously detrimental (ie eating shit at 60km/h) despite being relatively minor physical effects. Rotating a bike/rider mass even a couple of degrees at mid-air can be not only worrying but extremely dangerous, and it doesn't take a lot to do this.

Not my theory, you can find it in chassis design texts. Understood by many pro dh'ers tho. you can often observe it by wheeling your bike on pavement with the back wheel locked. If it judders you've got a resonant feedback going on.
Motor-racing on pavement requires independant brakes, admitedly wheel masses are bigger and V squared gives you lots of energy. Eliminating susp compression under lockup is the goal.

When riding and skidding on smooth surfaces (asphalt/concrete) I have not noticed enough shuddering to overcome the tyre's natural damping, on any kind of bike (including BB7s, with no floating brake). However, if you wheel the bike upright and jam on the rear brake, yes they all shudder, due to the fact that you now have a force input in the relative vertical direction (ie a component of force in the direction of suspension movement). In this respect you could argue that a higher pivot is more likely to resonate over cyclical inputs due to the fact that a larger component of the braking force is able to activate the suspension (better bump absorption notwithstanding). If you were talking about wheeling the bike with no rider on it, with both wheels on the ground, you will get resonation from the displacement/deformation of the tyre, not the suspension, and I have observed no resonance of the suspension whilst trying this.


This is explored in chassis texts and softer rear susp when its unweighted under braking is considered a bonus, however the high CoM of bikes makes the increased forward weight transfer and steepened steering geometry far more serious issues.

I agree. there is no significant, outright advantage to be had by the suspension sitting in a "softer" region of its travel (since particular dynamic response is dependent only on damping ratio and the natural frequency blah blah, not initial conditions) - all I was trying to point out was that it is, in and of itself, not necessarily a disadvantage.

The large moving ballast of a bike-rider chassis allows inertia stinkbug to be less of a prob than theory suggests it should be with the low pivots of conventional bikes. If your bike stays level under brakes, ballast movement allows tyre weighting to be better maintained and eliminates the theoretic advantage to softening the rear that unwinding suspension produces.

Without a quantitative analysis it is hard to clearly label one setup to be better than the other (in this specific sense), but one point I will make is that as I said before, geometric changes (front suspension compressing/rear extending) do not necessitate weighting changes at the tyres, and so I am not convinced that there is any significant advantage (in terms of traction or bump absorption) in keeping the bike level as such. For the record, I don't believe that there is any such advantage in having the rear end extend either. With that in mind, I dislike excessive suspension movement under hard braking for the handling reasons you mentioned above.

If circumstances ever allow it, it'd actually be interesting to instrument all this stuff (and double-blind test it too).

[Lahardesign]

We just had a government grant approved for independant testing and publication in peer reviewed journals by the Waikato Inst of technology.
we'll be testing braking perf, rolling resistance and power transmission over rough ground. Accelerometer readings on various parts of the bike will be interesting to compare with standard race bikes.
Feel free to suggest any tests that you think will enlighten.:D

[S.]

Your bike, with and without the floating brake :) I'd really like to see just how much difference the angular momentum of the wheel can actually make.

[Ivan]

Man, I love this.


You guys know your shit. Have you both studied some sort of mech eng to do with cars or something?

And S. , when can we expect you to produce a frame?:D

[S.]

Man, I love this.


You guys know your shit. Have you both studied some sort of mech eng to do with cars or something?

And S. , when can we expect you to produce a frame?:D

I'm studying automotive engineering at the moment, yeah.

As for producing a frame, I'd like to but at this stage I'm leaning towards "never".

[Turner_rider]

I'm studying automotive engineering at the moment, yeah.

As for producing a frame, I'd like to but at this stage I'm leaning towards "never".

From a fellow engi it looks like you are well on the way to a good career :)

Off topic but maybe of interest, a few years back I had a look at some of the FSAE cars under test at a race track and it was interesting to note that despite fairly open rules (in some areas) they were completely outclassed lap time wise by Formula Vees (even with same driver). Which probably goes to show that in some cases low tech can suit a track better than commonly regarded higher tech.

[DeSloth]

My understanding was that the FSAE cars really are not designed for speed, more their handling. In comps. they are limited in the gear ratios.

Not sure if this had been rectified in the above situation, but FV cars get up to around 200?

But yes, S. you are crazy. I wouldn't let it *bother you if people don't understand all that. It's not because you did a bad job - just that the whole thing is such a big amount of info all at once.

Even from mech eng. background it took me a loooong time to read that, and even longer to digest it.

[S.]

From a fellow engi it looks like you are well on the way to a good career :)

Off topic but maybe of interest, a few years back I had a look at some of the FSAE cars under test at a race track and it was interesting to note that despite fairly open rules (in some areas) they were completely outclassed lap time wise by Formula Vees (even with same driver). Which probably goes to show that in some cases low tech can suit a track better than commonly regarded higher tech.

Cheers.

FSAE cars are only allowed engines up to 600cc (vs the 1200-1600cc of the Formula Vees), and the air intake has to pass through a 20mm diameter inlet restrictor... if they removed that single restriction, the cars would go easily twice as fast (as they stand at the moment, I believe they rarely top 120km/h - the Formula Vees hit 200km/h). But IMO FSAE is all about teaching people about compromise.

[NCR600]

I haven't really read this thread properly, but I have an observation to make about brake jack and floating callipers.

Back in the mid 80's and early 90's flating rear callipers were all the rage on road racing motorcycles, and found their way in varying degrees of looks cool but probably does nothingness onto road bikes. I fell under the spell and spent many hours designing and manufacturing one for a racing motorbicycle I was involved with. I couldn't tell you if it worked or not, I couldn't tell the difference with or without it, and neither could anyone who rode the bike.

They've all but dissapeared from racebikes now (although they could have come back into fashion without me noticing in the last few years, been out of it for a while now)

Maybe it's a case of the floating calliper not being worth the effort, maybe they spent hours designing complex linkages and then simplified it until it didn't exist anymore, or just the latest engineering fashion, I don't know.

There seems to have always been a movement in MTB engineering that likes to make stuff more complicated than it needs to be.

[Lahardesign]

I put floating brakes on my bikes in 2001 at the request of 2x NZ champ Glen Sisarich and 2 others on my team. The stuttering they were experiencing from the back in heavy braking was completely eliminated!

[Turner_rider]

Cheers.

FSAE cars are only allowed engines up to 600cc (vs the 1200-1600cc of the Formula Vees), and the air intake has to pass through a 20mm diameter inlet restrictor... if they removed that single restriction, the cars would go easily twice as fast (as they stand at the moment, I believe they rarely top 120km/h - the Formula Vees hit 200km/h). But IMO FSAE is all about teaching people about compromise.

All very true, I'm not sure how the rules have evolved over time but from memory the faster cars were running 600cc turbos and gearing/power wise were very quick in a straight line. Cornering speed was where they all fell over - combination of short wheelbase, light weight, too much tyre grip etc etc I can only therorise. The track in question isn't a speed track so outright speed wouldn't be a big question in my mind.

Anyway this is all very off topic so I'll leave it at that :)

[custard]

some people have to much time on their hands....


i use a floating brake...

i dont care if i had one or not... as long as i can stop.]


and RIP SKIDS

[S.]

I haven't really read this thread properly, but I have an observation to make about brake jack and floating callipers.

Back in the mid 80's and early 90's flating rear callipers were all the rage on road racing motorcycles, and found their way in varying degrees of looks cool but probably does nothingness onto road bikes. I fell under the spell and spent many hours designing and manufacturing one for a racing motorbicycle I was involved with. I couldn't tell you if it worked or not, I couldn't tell the difference with or without it, and neither could anyone who rode the bike.

They've all but dissapeared from racebikes now (although they could have come back into fashion without me noticing in the last few years, been out of it for a while now)

Maybe it's a case of the floating calliper not being worth the effort, maybe they spent hours designing complex linkages and then simplified it until it didn't exist anymore, or just the latest engineering fashion, I don't know.

There seems to have always been a movement in MTB engineering that likes to make stuff more complicated than it needs to be.

Motorbikes have a much lower centre of mass than pushbikes. Racebikes (I am assuming you mean road bikes) also don't have the high amplitude bump absorption issues that mountain bikes do. They also have much stiffer suspension and longer wheelbases than MTBs, and so are less affected geometrically by the rear end trying to squat somewhat. And on top of that, they probably gain somewhat from having some amount of brake squat to reduce the forwards rotation of the sprung weight under brakes. You can most definitely notice the difference in braking characteristics on some MTBs, for example, go and try one of those Stab Supremes with the crossed-over floating brake. Huge amounts of squat, it does actually rake out under brakes (or at least feels that way because you're used to bikes steepening up when you brake).

some people have to much time on their hands....


i use a floating brake...

i dont care if i had one or not... as long as i can stop.]


and RIP SKIDS

Thanks for that insightful input. Stop clogging up threads with your mindless shit.

[wombat]

Feel free to suggest any tests that you think will enlighten.:D
Impact tests? I'm sure running them into walls could be fun.

In all seriousness though, what are the "standard race bikes" you'll be using? I'll be very interested in reading the paper when it comes out, but I'm curious as to how you'd establish a base line with such wide variations amongst currently available designs.

[toodles]

Lahar and S. - do you nerds give merit to the idea that a certain degree of brake squat is, in fact, beneficial? Most of us are familiar with the oft-touted trivia that Barel, and some of the other WC riders prefer a degree of squat from their bikes. I'd be interested to know what braking situations benefit from neutral braking as compared to squatting.

While I'm sure you'll both point out that I've got this all screwy, in my experience with riding various types of suspension bike (with and without floating brake system on the same bikes), some corners felt more stable on bikes without the floating brake. Now some floating brakes are so ill-designed as to be of little benefit at all anyway, but on the V10 I prefer the "sinking" feeling and slingshot exit that the non floater model provides. Thoughts?

[S.]

Lahar and S. - do you nerds give merit to the idea that a certain degree of brake squat is, in fact, beneficial? Most of us are familiar with the oft-touted trivia that Barel, and some of the other WC riders prefer a degree of squat from their bikes. I'd be interested to know what braking situations benefit from neutral braking as compared to squatting.

While I'm sure you'll both point out that I've got this all screwy, in my experience with riding various types of suspension bike (with and without floating brake system on the same bikes), some corners felt more stable on bikes without the floating brake. Now some floating brakes are so ill-designed as to be of little benefit at all anyway, but on the V10 I prefer the "sinking" feeling and slingshot exit that the non floater model provides. Thoughts?

Personally I feel that a reasonably small amount doesn't hurt. If I may speak for LD for a second, he has said that the Lahar was designed to have a sufficient amount of squat under brakes that it doesn't actually extend at all due to weight shift (presumably due solely to the rear brake, because you can't do anything with regards to the front brake) in order to keep the geometry from changing too much, and the bike low. So yes, I think it's fair to say that LD believes there is some merit in that idea too.

[NCR600]

Motorbikes have a much lower centre of mass than pushbikes. Racebikes (I am assuming you mean road bikes) also don't have the high amplitude bump absorption issues that mountain bikes do. They also have much stiffer suspension and longer wheelbases than MTBs, and so are less affected geometrically by the rear end trying to squat somewhat. And on top of that, they probably gain somewhat from having some amount of brake squat to reduce the forwards rotation of the sprung weight under brakes. You can most definitely notice the difference in braking characteristics on some MTBs, for example, go and try one of those Stab Supremes with the crossed-over floating brake. Huge amounts of squat, it does actually rake out under brakes (or at least feels that way because you're used to bikes steepening up when you brake).





Well if you want something that rakes out under brakes, why not follow another 80's roadracing motorcycle trend, the mechanical (and sometimes hydraulic) anti-dive is for you!

Again abandoned by everyone after about 1986, I actually made one for a bloke's CBX750 trying to be a Honda RS1000 endurance racer replica in about 1993. Basically following the concept of a floating rear calliper, it fed braking forces back into the sprung portion of the motorcycle via a sysem of linkages (or in the simplest form a single linkage)

My version had a lever in between the calliper and the clamp, which sat under the bottom triple clamp (there was enough room, so why not?)

Didn't work, felt weird under brakes, and made suspension set-up a nightmare. I spent hours on it, and when the stupid thing blew up he just took it to the wreckers, and my very cool looking, but ultimately bloody useless (although it DID stop the front from diving under brakes) anti-dive was lost forever.

I've never ridden a MTB with a floating calliper, so I'm using my only reference point here, but personally I don't think the added unsprung weight and complexity is worth the effort. I'd probably be more inclined to spend the time and money on better suspension set up, or in the case of designing a frame from scratch, getting the linkages and geometry right. Admittedly there are a whole bunch of reasons why the concept of floating callipers would be better suited to DH rather than roadracing motorcycles, not the least is the radically different riding styles!

Ultimately though, riding fast (and it matters not one jot what sort of riding, MTB, MotoX or roadracing motorcycles) is a confidence trick, and if a rider believes that something is going to give him an advantage to make him faster, then in all likelyhood it probably WILL make him faster.

BTW, this gets my vote for the best thread of the year. Thanks Lahar & S.

[Lahardesign]

Personally I feel that a reasonably small amount doesn't hurt. If I may speak for LD for a second, he has said that the Lahar was designed to have a sufficient amount of squat under brakes that it doesn't actually extend at all due to weight shift (presumably due solely to the rear brake, because you can't do anything with regards to the front brake) in order to keep the geometry from changing too much, and the bike low. So yes, I think it's fair to say that LD believes there is some merit in that idea too.

Thanks S, dead right. The suspension geometry of the mark9 produces a neutral characteristic with no movement of the rear up or down if you don't move your weight. As its normal for a rider to move their weight back under heavy braking the rear tends to sink due to this about as much as the fork dives. I use no forces from the floating brake to assist in this so it is an "Independant floating brake". Unlike non-Independant floating brakes the suspension is left free to keep the wheel on the ground and "forced harmonic motion"- stuttering is eliminated by preventing caliper forces from actuating the suspension.:cool:

[Lahardesign]

Well if you want something that rakes out under brakes, why not follow another 80's roadracing motorcycle trend, the mechanical (and sometimes hydraulic) anti-dive is for you!

Again abandoned by everyone after about 1986, I actually made one for a bloke's CBX750 trying to be a Honda RS1000 endurance racer replica in about 1993. Basically following the concept of a floating rear calliper, it fed braking forces back into the sprung portion of the motorcycle via a sysem of linkages (or in the simplest form a single linkage)

My version had a lever in between the calliper and the clamp, which sat under the bottom triple clamp (there was enough room, so why not?)

Didn't work, felt weird under brakes, and made suspension set-up a nightmare. I spent hours on it, and when the stupid thing blew up he just took it to the wreckers, and my very cool looking, but ultimately bloody useless (although it DID stop the front from diving under brakes) anti-dive was lost forever.

I've never ridden a MTB with a floating calliper, so I'm using my only reference point here, but personally I don't think the added unsprung weight and complexity is worth the effort. I'd probably be more inclined to spend the time and money on better suspension set up, or in the case of designing a frame from scratch, getting the linkages and geometry right. Admittedly there are a whole bunch of reasons why the concept of floating callipers would be better suited to DH rather than roadracing motorcycles, not the least is the radically different riding styles!

Ultimately though, riding fast (and it matters not one jot what sort of riding, MTB, MotoX or roadracing motorcycles) is a confidence trick, and if a rider believes that something is going to give him an advantage to make him faster, then in all likelyhood it probably WILL make him faster.

BTW, this gets my vote for the best thread of the year. Thanks Lahar & S.
Durn those anti dive fork gismos were bad! I helped a couple of people remove them in the early 90's!

Unfortunately floating brakes on MTB's have been hijacked as a means of patching the nasty stinkbug that independant brakes will always produce on designs with high CoG and rearwheelpaths near vertical. Their usage to prevent this on conventional bikes gives an identical dynamic to fixed calipers on swingarms that rotate. Therefore conventional multi link bikes, already more complex than single pivot ones, are eshewing the fixed independant brakes that 4 bar suspension linkages provide to add even more complexity with floating brakes that are really just anti jack gimicks that stuff up suspension performance in the way that you've described re ADFork gismos.
It was well known by the 1920's that teleforks with their inertial brake dive were wrong for smooth roads, hence the many Linkgage forks on motocycles of the 1920's and 30's (and the more recent Brittens, BMW's, Yamaha's, Bimoto's. These fork types use the same "directing forces thru vehicle Centre of mass" principle that is used in the rear of high pivot mountainbikes, and front and rear of all (except some badly designed ones-mainly American) cars, and rear of motorcycles basically since the early 1900's.
Good, textbook suspension geometry requires no gimicks and so has none of the bad side effects they produce.;)

[Bodin]

BTW, this gets my vote for the best thread of the year. Thanks Lahar & S.

Damn straight! Lahardesign only has ten posts to his credit, but has probably written more intelligent and educational content than most of the rest of the forum membership put together...:rolleyes:

[rbx]

Hey S.

For calculating braking i use the force applied on the rear wheel and friction of tire patch contact.
So basically i get a resulatante force which is usualy above 45 degre and up toward the cyclist.
Im intersted in this mongoose by analysing brake forces i come to the conclusion that this bike should be pretty neutral under braking.


What do you think

green arrow is force appiled on the rear wheel, blue arrow is the frictional force and red is the resultante of these two force:

[hyperkona]

<yawn/stretch>

Think i'll go for a ride now........ maybe thats what you should have done instead of typing all this shizzy. :D

[WolfCreekPsycho]

I would have thought there would also be an addtional force not in your picture.... i have added the Yellow and Blue arrows.

Using the rear wheel to stop the bike (halt momentum) means that the rider and rest of the bike will want to keep moving forward. However that cannot take place as the bike and rider are all attached to the rear wheel and swing arm. What can happen is that the force of rider and front of bike will try to actuate on the pivot point between the rider and the item doing the braking, causeing force through the swingarm pivot. You have another pivot point in this equation and that is the front wheel on the ground. When you apply the rear brake it attempts to turn the frame holding the rear wheel in the same direction as the tyre is rotating ie towards the ground (shown by blue arrow). As the front wheel on the ground prevents the swingarm and frame from rotating around it has to put that force through the swingarm linkage causing it to flex.


Just my 2 cents in a great thead

[S.]

Hey S.

For calculating braking i use the force applied on the rear wheel and friction of tire patch contact.
So basically i get a resulatante force which is usualy above 45 degre and up toward the cyclist.
Im intersted in this mongoose by analysing brake forces i come to the conclusion that this bike should be pretty neutral under braking.


What do you think

green arrow is force appiled on the rear wheel, blue arrow is the frictional force and red is the resultante of these two force:

Not really sure where you're getting that vertical force from, unless you're trying to calculate the CHANGE in the normal reaction force at the tyre. If that's the case, you're trying to perform a quantitative analysis using qualitative techniques (ie get a specific number as an answer, using only conceptual understanding. This really can't be done, it's not accurate at all. If you want to be able to calculate just how neutral a bike is (and keeping in mind that "neutral" is not always optimal) then you need to take into account a whole lot of factors, such as position of the centre of mass (using cartesian coordinates, ie not just the height), the shock rates front and rear, the wheelbase, the influence of varying geometry under weight shift, the gradient of the slope the bike is on, etc. As I mentioned about the neutral/optimal thing, you need to also work out whether you want to analyse the bike relative to a net input (ie including rider weight shift forwards and the fact that rear suspension inherently extends here), or whether you want to assume that whatever changes occur SOLELY from weight shift (ie not considering other compressive/extensive forces acting on the suspension under brakes) as the benchmark, and concentrate on the forces/moments directly applied to the bike by the braking system. Either method is valid, but you have to be aware of what the difference in outcomes means.

What you also need to consider is that on that Mongoose, although the AXLE moves relative to the front triangle like a singlepivot, the BB (and thus the rider/CoM) also move relative to the front triangle.

I would have thought there would also be an addtional force not in your picture.... i have added the Yellow and Blue arrows.

Using the rear wheel to stop the bike (halt momentum) means that the rider and rest of the bike will want to keep moving forward. However that cannot take place as the bike and rider are all attached to the rear wheel and swing arm. What can happen is that the force of rider and front of bike will try to actuate on the pivot point between the rider and the item doing the braking, causeing force through the swingarm pivot. You have another pivot point in this equation and that is the front wheel on the ground. When you apply the rear brake it attempts to turn the frame holding the rear wheel in the same direction as the tyre is rotating ie towards the ground (shown by blue arrow). As the front wheel on the ground prevents the swingarm and frame from rotating around it has to put that force through the swingarm linkage causing it to flex.


Just my 2 cents in a great thead

Whilst there is a moment applied directly to the rear wheel (by means of the brake caliper/axle couple), if you're going to calculate that on its own (rather than implicitly, from the tyre's contact patch) then you have to resolve the horizontal force on the tyre contact patch as though it was acting through the axle. Otherwise you effectively have that moment on the swingarm twice - once as force x wheel radius, and a second time as a couple moment about the axle. NFI where you've pulled that yellow arrow from though - there is no force, resultant or otherwise, acting like that. I get the feeling you may be forgetting that this is essentially a dynamic analysis, and that reaction forces can be caused by acceleration/deceleration of masses as well as static loads.

[sygote]

Instant centre theory:
Notably, the bike on which Fabien Barel won the 2004 world DH championships on had a brake linkage designed specifically to increase the level of (pro-)squat far beyond what normal bikes generate. Riding the production version of this bike, you can feel a huge tendency for the rear end to dive when the rear brake is applied. Given that no owners of those bikes seem to have any problem with the extreme brake setup, one might logically assume that it’s not actually that bad, and that other bikes with considerably less brake induced squat can hardly be any worse off, and thus are perfectly fine to ride – although not necessarily as comfortable as they could be..[/b]


this is not true there are to points where the floating rear brake can be attached and the top one which is what i ride with does dot squat but when attched to the lower point the bike squates excessively. sorry jus thought i would point that out

[S.]

this is not true there are to points where the floating rear brake can be attached and the top one which is what i ride with does dot squat but when attched to the lower point the bike squates excessively. sorry jus thought i would point that out

Uh, so then it WOULD be true. He runs the floater in the lower point... it's because of him that that point is there in the first place.

[alpinestar12]

have you ridden it in the squat- inducing hole much sygote? or anyone else with a D.O.P.E system on their Kona? I think it would be good, seeing that your forks would be diving and it would keep the bike level and the geometry stable. Maybe it would'nt be as good in really bumpy braking sections, as the suspension would pack down. anyway, tell us how the brake linked kona'a ride.

[rbx]

Not really sure where you're getting that vertical force from, unless you're trying to calculate the CHANGE in the normal reaction force at the tyre. If that's the case, you're trying to perform a quantitative analysis using qualitative techniques (ie get a specific number as an answer, using only conceptual understanding. This really can't be done, it's not accurate at all. If you want to be able to calculate just how neutral a bike is (and keeping in mind that "neutral" is not always optimal) then you need to take into account a whole lot of factors, such as position of the centre of mass (using cartesian coordinates, ie not just the height), the shock rates front and rear, the wheelbase, the influence of varying geometry under weight shift, the gradient of the slope the bike is on, etc. As I mentioned about the neutral/optimal thing, you need to also work out whether you want to analyse the bike relative to a net input (ie including rider weight shift forwards and the fact that rear suspension inherently extends here), or whether you want to assume that whatever changes occur SOLELY from weight shift (ie not considering other compressive/extensive forces acting on the suspension under brakes) as the benchmark, and concentrate on the forces/moments directly applied to the bike by the braking system. Either method is valid, but you have to be aware of what the difference in outcomes means.

What you also need to consider is that on that Mongoose, although the AXLE moves relative to the front triangle like a singlepivot, the BB (and thus the rider/CoM) also move relative to the front triangle.



Whilst there is a moment applied directly to the rear wheel (by means of the brake caliper/axle couple), if you're going to calculate that on its own (rather than implicitly, from the tyre's contact patch) then you have to resolve the horizontal force on the tyre contact patch as though it was acting through the axle. Otherwise you effectively have that moment on the swingarm twice - once as force x wheel radius, and a second time as a couple moment about the axle. NFI where you've pulled that yellow arrow from though - there is no force, resultant or otherwise, acting like that. I get the feeling you may be forgetting that this is essentially a dynamic analysis, and that reaction forces can be caused by acceleration/deceleration of masses as well as static loads.

So the mongoose (with its floating BB)should have more or less brake squat then a normal single-pivot?

[S.]

So the mongoose (with its floating BB)should have more or less brake squat then a normal single-pivot?

If by "normal single pivot" you mean one with the pivot in the same location as the Mongoose (which is VERY high), I'd be inclined to say the Mongoose would have less squat, but such a bike would have a much more rearwards axle path relative to the centre of mass blah blah... they're a relatively complicated system to analyse though, and I've never bothered to measure one up and see.

[Northern Hemi]

Barels fork of choice may have something to do with his need for a rear that lowers into its travel as he brakes.
The 888, when compared to a Boxxer W/C, dives quite a bit.
Unless the new 888 has sorted this issue out, I would say that the whole high-squat design may be in place to make up for the inefficiencies of the fork.

[rbx]

If by "normal single pivot" you mean one with the pivot in the same location as the Mongoose (which is VERY high), I'd be inclined to say the Mongoose would have less squat, but such a bike would have a much more rearwards axle path relative to the centre of mass blah blah... they're a relatively complicated system to analyse though, and I've never bothered to measure one up and see.

Sorry i meant a low position single pivot ala kona.

[angrygecko]

I thoroughly concur that reading the post has made me so smart that I am now dumber than I was 5 minutes ago..

Aaand my head hurts!

Can't we just go ride now?? :D

[Northern Hemi]

I'd like to just go ride now, but it's -40 out...

[S.]

Sorry i meant a low position single pivot ala kona.

You'd have to do a full numerical analysis to find out - the situations are significantly different in this case, so you'd have to work out a value for the percentage pro-squat in each case and compare em side by side, rather than being able to look at the pivot position and say "yeah that has more" like you can easily enough do with a pair of conventional singlepivot bikes.

[toodles]

Barels fork of choice may have something to do with his need for a rear that lowers into its travel as he brakes.
The 888, when compared to a Boxxer W/C, dives quite a bit.
Unless the new 888 has sorted this issue out, I would say that the whole high-squat design may be in place to make up for the inefficiencies of the fork.


Barel runs the BOSS 888s which use a different damper. Apparently they're pretty heavily compression damped.

[fatass]

You're using BIG words now! (very fking big words at that)

[bromontryder]

- What determines the amount of squat (or jack, but that will be dealt with later) is determined by two things: the height of the pivot (or instant centre, explanation of instant centre to follow shortly), and the length of the swingarm. The height of the pivot/IC is what determines the vertical distance (which is the moment arm as mentioned before) between the pivot and the axle, and so a higher pivot = longer moment arm = more squat generated by horizontal axle forces. The length of the swingarm changes the effects of the couple moment (which is effecti