jump geo

[ajay]

Hi all, ive got a wierd one here, i want to build some jumps in my backyard, but my folks arent keen on me diging it all out so i thought that i could build them out of wood - kind of like a skate park but with dirt jumps in mind. the construction process doesnt faze me as i have the tools and supplies to actually build them.

but as you all know when shapping a tranny it doesnt just appear - i mean you actualy have kind of 'carve' it out. So - i have one shot to get the tranny right other wise i will have tyo start again (being built out of wood).

I guess it would be like building a 1/4 pipe to some extent.

does anyone know how i should go a bout getting the right shaped tranny? :?

[roasted]

use a big sheet of ply as sideboards. to get the curves, take a string and a pencil, mail the string down at the center of the sheet, or corner, then measure a circle. cut it off at the height you want (i.e. 266cm radius, 122cm tall)

sorrry, that sounded retarted.

search for ramp plans on google, and get the high res version of the launch ramp, from heckler.

[ajay]

Thanks Mate! :D

[Joel O]

also make sure the landing ramp has lots of cross braces because i've put my wheels through plenty of crappy ramps :oops:

[ajay]

yeah i thought of that. thanks for the reminder! :D

is this common - wooden jumps?

[roasted]

well.....

a box jump made of wood is commonplace at most indoor parks. so not really commonplace.
most people just build a kick and take it around to natural traanys, or build a dirt landing for it.

[Shintaro]

use a big sheet of ply as sideboards. to get the curves,

I basically use this method, but when I trace out the curve onto the big rectangle of marine ply (really strong ply)

But with string and a pencil I seem to remember the fixed point for the string is way off the board somewhere. Someone holds the fixed pivot point for the string, and you need to work out the curve (make it go from corner to corner, but the curve should be steeper near the lip, and not so steep at the base) then draw it on. Basically 1 board makes 2 sideboard transitions, leaving a (surfboard shaped) bit of ply to chuck. The second curve should be traced off the first curve.

To finish the job, I would simply nail fence pailings (probably about 15, you can leave space between them, but no more than the width of a fence pailing) accross the two supports from the base of the jump to the lip of jump, then nail a layer or 2 of thinner ply wood as the surface (thin enough so it bends but do not compromise on strength). Brace the back of the launch ramp (with cross supports) and you are set.

For the landing ramp I would not use a curve, but simply cut the marine ply from corner to corner, lay the fence plaings accross it, then attach ply. Because it does not need to be shaped for a curve, and due to the fact it is a landing ramp, you should use a marine ply for the downramp.

Good luck~

[Joel O]

if its going to be a decent sized jump it might be a good idea to have some braces at the back to stop it moving every time you hit it, this also stops the back from flexing so the ramp feels more solid.

[roasted]

DONT MAKE IT MORE THAN 8 FOOT WIDE, without an extra sideboard shaped thingo in the middle.

Friend of mine has two quarters, one 4 foot wide, one 8. the 8 is that much flexier, and scarier too ride. Its starting to crack down the middle.

[S.]

To get a decent launch out of it, the kicker has to have a parabolic shape (ie NOT a constant radius (circular shape)!). You know when you go to the skatepark, how hard it is to get air out of a quarterpipe compared to off a dirtjump? It's because the curvature of the quarterpipe is constant, and you're still accelerating inwards (and more importantly, having outwards momentum - relative to the centre of the arc obviously). So it's important that you have a tighter curvature at the base of the jump, and have it opening up as you get further up the lip - but it should never be dead flat at any point on the kicker (it should still be very slightly curved right where you take off), otherwise it gets a really dead feeling because you're not being pressed into the lip. Think of a basic y=x^2 shape (but altered to suit your needs).

The landing can and probably should just be flat (by flat, I don't mean horizontal ;)).

[Joel O]

are you going to be having more than one jump because if there'll be more you also need to think about carrying speed ie. maybe not having a flat landing.

[ajay]

Yeah just the one.

Thanks heaps for the feedback - it has been extemely helpful!!! :D

Cheers

Andrew

[Joel O]

one more thing, it might be a good idea to chop a big bit of pvc in half and put it over the top of the downramp so if you case it wont smash the wood.

[ajay]

:idea:

thats a good one bikefreak!

Im just about finished the design stage - kicker will be about 4ft
is it a bad idea to put a fine carpet mat across the kicker and landing?
my reasons being reduced noise and more grip - please let me know if this is a bad idea!

[Joel O]

shouldn't really be a problem as long as it's not left out in the rain, coz if the carpet gets wet it'll just be holding water on the ramp and shitting it up.

[Ty]

http://www.bmxtrix.com/

then - HOW TO

then - BUILD RAMPS





good place to get started

[ajay]

Cheers guys :D

[Kammy]

you should build the frame out of wood and then use plywood as the actuall up ramp