Basic suspension setup for downhill riding
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[edit] Introduction
This guide is intended to help with the fundamentals of suspension setup for downhill riding (and similar things). It assumes you have read http://www.farkin.net/wiki/index.php/The_basics_of_suspension_-_terms_and_general_info and understood it. It does not intend to cover specific models of fork or shock, as the range of such products available on the market today is far too large to do so. At times, however, specific (common) models may be referred to. It is important to note that all suspension tuning is just that: tuning. There isn't any one "best" setup, and the only way you'll get the feel you want is by actually playing with your adjusters and setup in general. It's pretty well impossible to get any kind of off-the-shelf reccomended setup unless you're comparing two of the same bike with similar weight/style riders - you have to actually work it out yourself; these are only guidelines intended to help you work out what you need.
[edit] Fork setup
Since forks are usually simpler to set up than rear shocks, we will cover this first.
[edit] Fork spring rate
First of all you need to work out what spring rates you will need in your fork. This is determined by a few things: - your weight - obviously heavier riders need stiffer springs, and lighter riders need softer springs - the amount of travel your fork has - more travel generally means you can run softer springs - your personal preference for suspension feel
Most/all fork manufacturers (Fox/Marzocchi/Rockshox/Manitou among others) should either have a list of appropriate springs for rider weight (for each fork they make) on their websites, or be able to provide them to you via email. This is usually the best way to work out what spring you will need. Please take note that a "medium" spring for one fork is not necessarily the same as a "medium" spring for another fork. Notably, the RS Boxxer stock springs (which are labeled medium/firm) are actually the correct rate for riders weighing approx 95kg, and as such, a lot of RS recommendations give an oversprung rate. On a similar note, Marzocchi spring rates (and recommendations) are typically softer than average - a "medium" spring is a fairly soft setup.
Generally there are three main factors to take into account when setting up your fork's spring rate to be stiff or soft (relative to your weight); these are bump absorption, bottoming resistance, and fork attitude/ride height. A softer spring will usually absorb bumps better than a stiffer spring, up to a point - if a fork bottoms too easily (or rides too far down in the stroke where it ramps up suddenly or has a lot of compression damping) it will perform worse than a stiffer spring. Bottoming resistance is pretty simple, a stiffer spring is harder to bottom out (however, spring rate is not the only thing that determines a fork's resistance to bottom-out). Fork attitude is a tricky one - a stiffer spring will sit a fork up higher in its travel and generally tend to corner/jump better than a soft/undersprung setup (to an extent - if a fork is too stiff it will not give the front wheel enough traction in corners). It will also help reduce brake dive, and the tendency of a fork to dive when being ridden aggressively. This is an important factor to balance against bump absorption. A well set up fork will usually yield approximately 20-35% sag (when in a normal riding position) depending on whether you prefer to run it soft or stiff - this should generally be incidental to all other factors, but it may be a useful figure to help gauge your setup if you are not particularly familiar with suspension settings. In most cases it is best to run as little preload as possible, as this works the best with the rebound damping. Preload does not significantly affect the overall spring rate, it is usually only used to adjust sag. If you feel that your fork is too soft, then you will normally gain more from going to a stiffer spring than by preloading the existing spring.
[edit] Fork rebound damping
Rebound damping is critical in any suspension, as it is what effectively absorbs the energy stored in the spring from a bump, but it also determines how quickly the fork is able to return from a bump and thus keep traction. For forks it is important to consider the handling effects of rebound settings as well as the bump absorption effects. Whilst running the rebound somewhat slow will generally give a more stable and to a point, a smoother ride (too slow will make the fork harsher though), it also causes the fork to ride lower in its travel in rough sections, which can mean that depending on the type of fork, it may be in a region of travel where the compression damping is increased substantially. On any fork, obviously the further into the travel it sits/rides, the stiffer the spring will be at that point. This is an unavoidable tradeoff. If the rebound is run too fast, the fork will bounce around (giving the front wheel little traction), and more shock will be transferred to the rider. It may also top out harshly, which is a sign of a fork which definitely does not have enough rebound damping. If the rebound is run too slow (especially if the rebound on the rear of the bike is not run as slow), the bike will ride lower at the front, "bog down" too much when pushed hard in corners and technical sections, have poor traction in corners and in the rough, and not spring out of corners/off lips. It is very important that a fork's rebound speed is NOT slower than that of the rear suspension - if the rear end rebounds faster than the fork, the bike will try to rotate forwards through corners, rough sections and off lips. Generally a reasonable way to estimate a good rebound speed for the fork is to bounce the fork whilst straddling the bike and see if the fork's rebound speed appears to be artificially slow (compared to a fork with very little/no rebound damping in the same situation). If the fork is noticeably slowed down (compared to said undamped setup) in this situation whilst straddling it, it will feel fairly slow when you are actually riding it (as the rider's weight helps resist extension of the fork). This is definitely a setting worth tinkering with, a good rebound setting can make a bike feel so much better to ride. To achieve the desired rebound setting may require playing with oil weights in the rebound damper in some forks - usually it is best to read the manufacturer's recommendations about oil weights as they are generally on the money.
Summary: faster rebound (within reason) generally gives better overall bump absorption and traction. Slowing the rebound down tends to deaden the bike somewhat, which increases stability to some extent.
[edit] Fork compression damping
A lot of high-end forks now come with compression damping adjustments (usually external, sometimes internal). Some forks only have one compression adjuster (such as pre-2006 888s, and Dorados), but others may have separate high-speed and low-speed compression adjusters (Boxxers, 2006 888s, Fox 40RC2s etc). Generally the low-speed adjuster will have a small effect on the high speed damping, but it may not be enough to be significant or noticeable. The most important of the compression settings is the low-speed compression setting. Low-speed compression damping (LSC) is an important factor in determining how your bike maintains its attitude whenever you move your weight around on the bike (such as during braking, cornering, jumping, and down steep chutes etc). Running no LSC will give the fork a tendency to dive when braking, and cause the bike to try and "push" the front wheel too much when cornering (it has a similar effect to running a spring that is far too soft). Exactly how much LSC you want is almost entirely a personal preference; generally the best way to tune it (as always with compression damping) is to start with it backed right off and then increase it as you see fit (ie if you feel that the fork is diving too much in corners or under braking). Forks with lots of LSC damping (such as many Boxxers, and 40s if you crank up the LSC) tend to be very stable and predictable, it's generally only the eventual loss of high-speed suppleness that affects how much low-speed damping you use.
High speed damping affects how fast the fork can move when it takes fast hits (such as braking bumps). There is no real reason to use anything but the minimum of this unless you're bottoming out on fast stutter bumps and such - it will help prevent bottoming from that kind of input, but it will also make the fork harsher and less sensitive if too much is used. Again, ideally the minimum possible amount of HSC should be used.
[edit] Fork progression
This is the easiest part of fork setup. Increasing the progressivity of your forks (in terms of either effective spring rate, or by ramping up the compression damping at the end of the stroke) should be done if you are satisfied with all other facets of your setup, but find that it still bottoms out too much/too easily on the big hits. In many open-bath forks this can be done by simply increasing the amount of oil in the damper legs. Some forks now have external bottoming-resistance adjusters (such as the Manitou Travis and the 2006 888 [in the case of the 888, the preload adjuster affects bottoming resistance as well]). Other forks may need to have the damper's internals altered (such as Dorados and 40s) in order to increase the progressivity. In any case, simply increase the progressivity until you find it satisfactorily hard to bottom your fork - keep in mind that you should be able to bottom the fork out if you really try, otherwise you're not using all the available travel and thus rendering it a waste.
[edit] Rear shock setup
Rear shock setup is very dependent on the type of shock and the frame. Some frames use high leverage ratios (a lot of wheel travel compared to the shock stroke) and others use low leverage ratios (not as much wheel travel compared to the shock stroke). Most bikes sit somewhere around the 3:1 mark - that is, for every millimetre the shock compresses, the axle moves 3 millimetres (on average). Some frames have falling rates (these are relatively uncommon and the degree to which the rate changes is usually fairly minimal), others are linear, and others have a rising (progressive) rate. The amount of travel any bike has also affects how it can be set up, obviously a bike with more travel can be run softer than a short-travel bike without bottoming out excessively. Always refer to the factory setup manuals for shocks (to start with at least), as they are typically quite useful, and will explain what each adjustment does.
[edit] Shock spring rate
This is not too dissimilar to the fork spring rate setup, except that you generally run a bit more sag, and running a softer spring will cause the bike to rake out more in corners rather than dive forwards. The specific spring rate you need is a factor of the frame's leverage ratio and the suspension rate (whether it "ramps up" towards the end of the travel or not), as well as obviously your weight. It is generally recommended to run between 30-45% sag, as most of the time this yields a fairly appropriate spring rate. A rough spring rate calculator is available at www.tftuned.com . A soft spring rate will usually provide better bump absorption but be easier to bottom out, and a stiffer one will be the opposite. The handling effects are harder to quantify with rear shocks - it can be useful for a bike's rear end to squat a bit into turns (so you don't want the spring rate too stiff) but too much of this will make the bike rake out a lot in the corners and feel like it's unweighting the front wheel too much. It should also be noted that running a softer spring will give more sag which also slackens the head angle and lowers the bottom bracket - it is up to the rider to decide whether this is desirable. Try to run as little preload as possible - compensating for a too-soft spring by preloading the shock is not as good a solution as using a heavier spring. Preload should only be used to adjust sag, but even then sag should really be incidental to other factors rather than a determining factor on its own (as it doesn't have a very direct effect on anything except the bike's geometry when sagged).
[edit] Shock rebound damping
The rebound rate of the rear shock is something that you should really spend time playing with - it can make a night-and-day difference to how the bike rides. As with fork rebound, if you run it too fast it will tend to bounce the bike around more, as well as make the bike less stable (particularly in corners) and prone to kicking the back wheel up whenever the bike gets airborne. If you slow it right down, it will pack down in the stroke and not allow for proper bump absorption, and it will make the bike very sluggish to bunnyhop or jump. The shock's rebound should be set up to be either on par with the fork, or slightly slower. Running the fork's rebound slower than the rear shock can destabilise the bike, as it will constantly be trying to tip the bike/rider forward - not a good combination. A reasonable way of testing the rebound rate is to push down hard on the seat (whilst not on the bike) and see if it returns visibly slowly compared to a completely undamped setup - in other words, try it with no rebound damping first, then crank up the rebound adjuster until you can see that the suspension is extending noticeably slower. This should give you a reasonable middle ground, slightly on the slow side if anything. However, definitely experiment with this adjustment - nearly all shocks have easily accessible, tool-free rebound adjusters that let you stop mid-run and alter it.
[edit] Shock compression damping and progression
Most current-model shocks (SPV-style ones and the Fox DHX) have a reasonable amount of "platform"/LSC damping inherent to their design, as well as progressive dampers to help prevent bottom-out (that is, their progressivity and their dampers are usually related). For this reason, it's usually best to back the compression adjusters right off to start with. On SPV-style shocks (Manitou Swingers and Progressive 5th Elements), there may be two compression adjusters - one for high-speed compression (HSC) and one for low-speed (LSC); these may also be labeled "beginning stroke" and "ending stroke" as they are position sensitive as well as speed sensitive. The Fox DHX (depending on model) may have a "pro-pedal" knob, which is effectively a LSC adjuster, as well as a bottom-out resistance knob (which is an end-stroke damping adjuster). If you feel that the bike is raking out too much or blowing through its travel too easily in turns and g-outs, start winding on a bit of beginning stroke/LSC damping; generally very little of this is necessary. The high-speed/end stroke adjusters (on SPV shocks) should usually be left alone unless you're bottoming on fast rough terrain - use the air chamber volume adjuster to resist bottoming where possible, as it seems to have less effect on bump absorption early in the stroke. Similarly with the DHX - if the bike is bottoming out easily, simply crank up the bottom-out-resistance knob until you're happy.
With other shocks that only have a single compression adjuster (Fox RC, Romic etc) that adjust both high and low speed compression simultaneously, start with the adjuster fully open and only wind it in if you really feel that the bike is lacking in high speed compression damping, as if you crank it up to gain more LSC then you will also increase the HSC significantly, which will give you a harsher ride; however gaining more LSC as a side-effect of increasing the HSC is less likely to have any detrimental effects. Most of these shocks are linear/speed sensitive only however, so the only easy way to increase bottom-out resistance is to either crank up the compression or use a stiffer spring, both of which give a harsher ride over rough terrain. The only alternative to this is to either find a progressive-rate spring, or get the shock's internals revalved to suit you (which can be fairly expensive).
[edit] Notes
The best way to tune your suspension is to get out there and test out different setups - none of this is rocket surgery, it's simply somewhat-educated trial-and-error. Try slower/faster rebound, more/less compression, heavier/lighter fork oil, stiffer/softer springs - and if you like a certain setup, keep using it. Listening to anyone else tell you how to set your bike up specifically is most likely going to end up with a setup that you don't like using. There is a lot of room for personal preference with suspension, if you like it soft and fast or stiff and slow or whatever it doesn't really matter as long as you feel that it works for you. Of all the things to play with however, the rebound is the most important - try it faster and slower until you decide on what you like; nearly all suspension components have easily accessible external rebound adjusters that will let you play around with it quickly and easily.
[edit] Oil weights
Oil is crucial to suspension in both forks and rear shock units (although it is much less common to bother altering the oil weight in a rear shock as the adjustment range is typically far greater). Changes to your oil affect your suspension greatly and apart from springs are the element that have the greatest affect on suspension "feel". The two important factors are oil weight and volume. Typical oil weights used vary from 2.5wt (very thin) up as high as 15wt or 20wt (which are quite thick).
Oil weight is essentially a measurement of viscosity. A quantity of oil is released through an aperture and the time it takes for a quantity to flow is registered as the weight. A low weight oil flows faster and is less viscous than a high weight oil. Without requiring a degree in fluid dynamics, oil weight does not have a linear relationship with damping speed. Shim resistance, flow curves, and static tension variables all conspire to make suspension tuning a complex field. Suffice to say, lower oil weights speed up suspension motion and lower the moment of inertia. As a matter of opinion, it is better to run a thin oil with extra damping adjustment wound in, than vice versa. This seems to allow a greater difference between LSC and HSC damping.
Oil volume is only a factor in forks, not rear shocks (which should maintain a certain volume). Certain forks (open-bath forks usually, such as Boxxers and Marzocchi forks) use oil volume (or oil height) to add progressiveness to suspension and provide some degree of bottoming resistance. Essentially, by increasing the volume of oil in a fork ( a non-compressible fluid) you're reducing the volume of air (a compressable fluid). This means you can adjust the progressiveness and resistance to bottoming by adjusting how high the oil sits in your fork.
In rear shocks, be careful to use a specified rear shock fluid. These are rated to the high temperatures that rear shocks endure, and as such reduce the inconsistency produced by hydrocarbon chains breaking down in heat affected regular fork oil.
