[samtaleterapi]

i have been thinking about arcs
say i have a 2 foot jump that endt in 75 degrees - or a 6 foot jump that ends in 40 degrees
wont the first one feel like a really big curb because its so short and steep - and the second not require any thinking on my part at all but scrub a good part of my speed?
what i mean is i have to pull up very aggressively on the first one - and the second one is so long that i aligns my bike perfectly on its own
so what i was thinking is this: if i want jumps that will do some of the aligning for me so i can relax and concentrate on doing interesting things - what is the ratio then?
is it 2 feet no more than 45 degrees, 4 feet no more than 60 and 6 no more than 75? but would it also be an advantage to have jumps as short as possible to maintain speed? or is it the other way around?
i thought further that the ideal arc would be different for 20 inch and 26 inch wheels - and speed: if i go fast enough the steep jumps will require more action on my part however big they are
does this make any sense?
i am of course interested, because i would like to build my jumps well from the beginning
thanks
rasmus

 

[cmc]

For speed and distance jumping, look at BMX race tracks as your model. The jumps are not steep. The face tends to be almost flat, or with very mellow transition. Almost like jumping out of a flat bank ditch.

For air, tricks, boosting, etc etc, the dirt jump will have more like skatepark style transitions. The face has an "arc" as you put it. In the diagram below you can imagine drawing a full quarter pipe, with 6,' 8', 10', or 12' radius and cutting the top off at the desired height. You pick the radius you want depending on how tall the jump is going to be.

Cutting the jump off at the red line, makes it the mellowest. Blue is getting steeper. Green mark is ideal in my opinion for having a jump you can go long and high on ... Yellow and above you've got a serious booster.

The typical beginner mistake is making a 2 foot tall launch with the shape up to the yellow line. Short jumps have to be mellower, simply because you get less wheelbase of your bike on the face as you are riding over it.

Then again if you want a rad jump for downhill MTB, go with something with the red shape but make it 6 feet tall !




Typical BMX track:

Check out the shape of these Freestyle Motocross launch ramps:

Box jump plans that were posted here:

 

[samtaleterapi]

In the diagram below you can imagine drawing a full quarter pipe, with 6,' 8', 10', or 12' radius and cutting the top off at the desired height. You pick the radius you want depending on how tall the jump is going to be.

You seem to almost get my idea. The way i understand your answer is i decide how tall i want the jump and what color/steepness i want for the transition - and then i dont actually have any choice as to the radius of the arc.

My question is really: what radius is ideal?
Meaning if i want a 60 degree lip i need a certain height actually determined by the radius. Or if i want a 4 foot tall jump, the steepness would be the dependent variable - because i start out with a choice of arc radius.

I really like to hear your preference for the green transition. What i was originally looking for are prefences as to the radius of the arc.

So, any preferences?

Thanks
Rasmus

 

[Cru Jones]

exactly like this is ideal...

 

[sixgun_sound]

The best way to build jumps is to throw the dirt in a pile, pack it with a flat shovel, and then ride your bike off of it.

 

[w00dy]

8' is pretty standard. You won't go wrong with that.

 

[TheTruth]

If you wanna be a jackass and do math go ahead. You have to feel out the vert of a jump and how much speed you have to work with. the more speed the taller the jump must be. it is up to you how high or low you want to go. You just have to give it your best shot of placing the landing. When you become experienced builder it just comes naturally.

 

[demo 9]

regardless of how steep or tall you make it, remember that it has to be nicely transitioned into. the flat ground. this is not good _/

 

[sittingduck]

i thought further that the ideal arc would be different for 20 inch and 26 inch wheels - and speed: if i go fast enough the steep jumps will require more action on my part however big they are
does this make any sense?
i am of course interested, because i would like to build my jumps well from the beginning

8 or 9 foot radius is good for 26", 7 or 8 foot radius is good for 20".
8' works pretty well for dual use.....
If you are building long and low jumps (launch ramps), you'll want to flatten the radius out a bit.

 

[cmc]

You seem to almost get my idea. The way i understand your answer is i decide how tall i want the jump and what color/steepness i want for the transition - and then i dont actually have any choice as to the radius of the arc.

My question is really: what radius is ideal?
Meaning if i want a 60 degree lip i need a certain height actually determined by the radius. Or if i want a 4 foot tall jump, the steepness would be the dependent variable - because i start out with a choice of arc radius.

I really like to hear your preference for the green transition. What i was originally looking for are prefences as to the radius of the arc.

So, any preferences?

Thanks
Rasmus

have you started building yet?

Savage Animal puttin words to action ! :
http://www.ridemonkey.com/forums/showthread.php?t=213478

 

[stino]

i don't think there's an 'ideal' radius for a jump, unless it's standalone maybe.. i'm an engineer so normally i calculate stuff before even thinking of building it, but in my experience this does not work for dirts.
the thing is, if you lay down a set of say 3 dirts that are right after each other, and the terrain has a slight slope for example, it is just impossible to know on beforehand what your speed will be when facing each jump but the first one. as a consequence, you cannot know exactly how steep each kicker should be, nor how long the gap should be.
the only thing you can do is estimate, then ride the line, then adjust, then ride some more and adjust shapes some more and eventually it'll be near perfect.
also by the time you've done that, you'll have acquired experience and that's what it is all about: practice enough and you won't be measuring stuff anymore, but shape everything by eye.. these days I can easily see what's wrong with a shape just by looking at it, same for the example above: I got better at estimating shape/height/length of jumps/berms/... now which saves me tons of work, but only because I already spent hours and hours with a shovel to fix my beginner mistakes.

 

[cmc]

You seem to almost get my idea. The way i understand your answer is i decide how tall i want the jump and what color/steepness i want for the transition - and then i dont actually have any choice as to the radius of the arc.

My question is really: what radius is ideal?
Meaning if i want a 60 degree lip i need a certain height actually determined by the radius. Or if i want a 4 foot tall jump, the steepness would be the dependent variable - because i start out with a choice of arc radius.

I really like to hear your preference for the green transition. What i was originally looking for are prefences as to the radius of the arc.

So, any preferences?

Thanks
Rasmus

I understand your question.... You could have a 6 foot tall box jump launch ramp with a 9 foot radius and it would be rad. But if you had a 2 foot tall launch ramp with a 3 foot radius, it would be terrible. But in both cases, you have a 60 degree arc. It's a "quarter pipe" (90 degrees) cut down by 1/3. In my drawing, that means going up to the about the yellow line.

Like SittingDuck said, long and low jumps (launch ramps), like 2 or 3 feet tall, have to be mellower (have a shorter arc) because the of the size of your bike's wheels and the wheel base. Your bike and body need time to "transition" from being on flat ground to being pointed at the sky. That's also why the classic quarter pipe size, 8 foot tall with 8 foot radius.... doesn't work at 4 foot tall with 4 foot radius. It's too "quick."

A launch ramp should not go up to yellow. It should be cut off at the red, maybe blue, line.

At the same time, if you want to jump 25 toyota tundras, you would also want the "red" shape, to go for distance, not straight up.

If you want to go for the "green" steepness--you could make your ramp 4 feet tall with 8 foot transition, 5' with 10' radius, 6' with 12' radius etc. It doesn't make sense to have a "favorite" or "ideal" radius without also talking about the height of the jump.