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Just wondering. Is there such a thing as specifications for dirt jumps? Size, height, angle, lip2lip distance, etc.

I'm trying to figure the area and amount of dirt I would need for a serie of 3 to 4 intermediate/advanced jumps.

I'm more into trails than jumps :madman:
 

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there are no official specs/rules of dirt jumps or "trails" as they are known in the BMX world. but there is a wealth of experience that BMX and MTB-DJ builders have. the best BMX / MTBMX trails have a mixture of steepnesses and gap lengths with rollers and berms and hipped landings thrown in for good measure. even better MTBMX trails have mixtures of terrain--natural inclines and declines in the landscape which adds to the overall experience. different trail builders have different styles as well. guys who race bmx or 4-cross tend to want longer, bigger/taller jumps but with somewhat mellower transitions, whereas riders who are focused on tricks or blasting height will want steep lips and landings but shorter gaps. do you want to flow around casually, or do you want to blast straight up and nose dive down?

DIMENSIONS

indoor wood BMX bikepark box jumps are more "standard" than trails and the dimensions are a good starting point. typically you'll see bikepark box jumps between 3 and 6 feet tall, with most being 4 'or 5'. the lip to landing gap is between 8' and 12', with most being 10'. launch transition radius is usually between 45 and 65 degrees of an arc, with most about 50-55 degrees.

to recap how "radius" works . . . . if you use an 8 foot radius and draw a 90 degree arc, you end up with a classic 8 foot tall 8 foot tall quarter pipe that goes right up to vert. cut that down in height by half, and you have a 4 foot tall launch with an 8 foot radius. (that's where the green line is in my picture). most bikepark bmx'ers would consider this mellow . . . but bmx racers are used to tall jumps with much flatter/mellower transitions for high speed and distance... their launch might be only 30 degrees of arc but still 4 or 5 feet tall. for a little more "pop" or "boost" a lot of ramp builders will go with 7'6" (53 degree arc) transitions cut to 4 feet tall.


landings are just as, if not more, important than launches to have jumps with good flow. a lot of trail builders build landings taller than launches ("step-ups") (typically 1 to 3 feet taller) for the reason that you get a smoother landing if you can get more wheelbase/bike lengths on the surface of the landing. the only way to make the landing mirror the launch as far as steepness, but also make room for more wheelbase, is to make the landing taller. Short steep landings can be bad news because they are really easy to overclear into flat ground.

this "parabolic trajectory" diagram is good to think about--but it's not an exact science for the reason that a rider can use their body to affect their trajectory by 'boosting' or by 'sucking up' or 'racering' a lip. still though it's good to keep in mind if you are thinking about some variation in your line. even a mellow jump of 30 degrees can be very effective as a fun setup to a steeper jump. note that going steeper than 45 degrees does not naturally make a person go farther. having a line of jumps with varying steepnesses and varying gaps can either be fun and challenging or it can totally suck--you see riders constantly over-clearing or casing. so, the first time around it's better to build all your jumps in the line the same gap and roughly the same steepness, then go for variation later.

also note that the gap you clear has mostly to do with your speed and NOT with the height of the jump. rate X time = distance. you could have a 2 foot tall mini setup jump with 10 foot of gap followed by a 5 foot tall set also with 10 feet of gap. the setup jump will set your speed for the rest of the line. classic beginner mistake is making the first set either too short of a gap or too long, so that when you hit the second set you have to do weird body english to land in the right spot.



SO...... here are some rough categories, keeping in mind that a good trail designer can and should mix things up......:

BEGINNER:. 3 foot tall, 30 degree arc, 8-10 feet of gap. this is great for beginners to learn. don't go shorter than 8 feet, cuz it will be pointless. 10 is still very doable for beginners with this mellowness. if it's going down a hill and there's some decent speed you can even go longer, like 12-15 and beginners can still clear it (look at BMX tracks)

INTERMEDIATE: 4 feet tall, 45 degree arc, 10-12 feet gap

INTERMEDIATE PLUS : 4 feet tall, 50-55 degree arc, 10-12 feet gap

ADVANCED: 5-6 feet tall, 45 degree arc, 16-20 feet of gap (mellower for farther distance--see trajectory illustration below... mtb'ers tend to like long distance floaters).... OR Steep-AND-Deep: 5-6 feet tall, 55 degree arc, 12-16 feet gap.

PRO: 6-7 feet tall lips, 60-70 degree arc, 16+ feet gap.


SPACING

In the old days of BMX dirt jumping, jumps used to be spaced out a little more like bmx tracks. You hit one, cruzed, then hit another one. Then when the BMX trails revolution happened during the 90's, "rhythm"sections became the norm. These are back to back jumps. You land at the top of the landing smoothly, basically go into a nice bowl/swoosh shape then blast up the next lip.... Spacing is important and is a stylistic choice by the designer. One rule of thumb is the distance from the top of the landing to the top of the next lip should be about double the gap you just cleared. So.... figure, 10 foot gap, 20 feet of bowl, 10 foot gap, 20 feet of bowl, etc. Whether you dig down to create bowled pits versus keeping everything above grade on flat ground can impact your spacing. What you don't want is too much flat ground between jumps, so that riders have to crank in between--yuck. But if you're on a downgradient, it may not be a big deal to have some cruising time in between.

That's enough for now.



LINKS:

DJ , Pump Track plans
https://forums.mtbr.com/showthread.php?t=402237

Ideal Lip Radius
https://www.ridemonkey.com/forums/showthread.php?t=224301

Ideal Arc
https://www.ridemonkey.com/forums/showthread.php?t=221473

18 Steps to Building a Dirt Jump or Freeride Park
https://www.imba.com/resources/trail_building/build_freeride_park.html
 

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If Im understanding it right, if you use a 45 degree arc/launch and say you wanted the jump to 3 1/2 feet tall, you would use a 7 ft arc; 5 ft tall, use 10 ft arc, etc?
 

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Pwnage24 said:
If Im understanding it right, if you use a 45 degree arc/launch and say you wanted the jump to 3 1/2 feet tall, you would use a 7 ft arc; 5 ft tall, use 10 ft arc, etc?
WAIT . . . . I'm glad you asked me this cuz as I was trying to re-explain what "arc" is, I realized that I have been applying the term wrong this whole time.

Yes, if you want a 4 foot tall jump, you can use an 8 foot radius. That will be mellow by some bmx standards, but will be a great starting point. A lot of BMX park box jumps are 5 foot tall with an 8 foot radius.

Where I screwed up is that I assumed that because 4 is half of 8 then it's a 45 degree take-off and a "45 degree arc" (45 being half of 90). This is wrong. In my drawing below, the launch ramp clearly uses more than 45 degrees or arc. Probably like 58 or something. . . .I wasn't factoring in the width of the "deck" at the top of the lip. The medium steepness launch that I had been talking about is supposed to look like the shaded area.

So ignore my talk about 45,, 50 -55 etc. So bump all those numbers way up.. Damn, I hate being wrong, but when I am I admit it.

 

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OK Thanks a lot. I wanted to make a jump 3 1/2 feet tall that is just scaled down and had the same arc from your 4 foot one but I didnt know for sure if it was right. I got it now.
 
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It is all a bunch of trigonometry, possibly straying into calculus. That sketch of the 8 pack is a sine wave that increases in amplitude and periodicity. Math in action. Now it's time to dig.
 

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imaorobbie said:
It is all a bunch of trigonometry, possibly straying into calculus. That sketch of the 8 pack is a sine wave that increases in amplitude and periodicity. Math in action. Now it's time to dig.
Funny you mention that my math teacher who enjoys biking, knows i enjoy DJ and told me to pay attention in class as this would help build dirt jumps :D
 

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hazdxb said:
Funny you mention that my math teacher who enjoys biking, knows i enjoy DJ and told me to pay attention in class as this would help build dirt jumps :D


He'll be stoked if you bring him this problem and tell him you want to learn how to solve it.




The motorcyclist attempts to jump over a series of cars and trucks and lands smoothly on the other ramp, i.e., such that his velocity is tangent to the ramp at B. Determine the launch speed vA necessary to make the jump.

(A) vA = 16.83 m/s
(B) vA = 15.66 m/s
(C) vA = 11.07 m/s
(D) vA = 11.90 m/s


https://wps.prenhall.com/esm_hibbeler_engmech_10/14/3722/952844.cw/index.html


 

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cmc4130 said:
He'll be stoked if you bring him this problem and tell him you want to learn how to solve it.




The motorcyclist attempts to jump over a series of cars and trucks and lands smoothly on the other ramp, i.e., such that his velocity is tangent to the ramp at B. Determine the launch speed vA necessary to make the jump.

(A) vA = 16.83 m/s
(B) vA = 15.66 m/s
(C) vA = 11.07 m/s
(D) vA = 11.90 m/s


https://wps.prenhall.com/esm_hibbeler_engmech_10/14/3722/952844.cw/index.html


Well i took me a little while and i figured it out, the asnwer's A!.....no i didn't i just cheated and found the answer on the website you gave me :p but ima ask him because that definitely looks like it would help build a lot of jumps hehe
p.s that pic is awesome :thumbsup:
 

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Here's a table of the ramp length (x) vs height (y) and the launch angle (phi) for a 8' radius
ex: if you make it 4' high it will be 6.9' long and have a launch angle of 60 degrees

x y phi
8.0 8.0 90
7.9 6.6 80
7.5 5.3 70
6.9 4.0 60
6.1 2.9 50
5.1 1.9 40
4.0 1.1 30
2.7 0.5 20
1.4 0.1 10
0.0 0.0 0

Here are the formulas to generate a table if you want a different radius (R):

x = R sin(phi)
y = R (1 - cos(phi))
launch angle = phi

So if you wanted a 10' radius ramp, with a 45 degree launch angle, you would draw your 10' radius on the plywood and lop it off at 2.9' [y = 10 (1 - cos(45))]
it would be 7.1' long [x = 10 sin(45)]
 

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so i'm getting ready, w/ a mini excavator this weekend to build some stuff. i got done writing all the stuff down then continued reading...are there any other posts/forums that cover this in greater (and correct) detail? help appreciated.
 

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so i'm getting ready, w/ a mini excavator this weekend to build some stuff. i got done writing all the stuff down then continued reading...are there any other posts/forums that cover this in greater (and correct) detail? help appreciated.
Heh that's pretty funny--"greater and correct detail"... What do you want to know? Did you already read through the "DJ, Pump Track plans" thread ? I have posted a lot of things there. What are you planning to build ?
 

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Farther down in this thread all this was wrong somehow? How then could I correct/change/scale this to work correctly?

"BEGINNER:. 3 foot tall, 30 degree arc, 8-10 feet of gap. this is great for beginners to learn. don't go shorter than 8 feet, cuz it will be pointless. 10 is still very doable for beginners with this mellowness. if it's going down a hill and there's some decent speed you can even go longer, like 12-15 and beginners can still clear it (look at BMX tracks)

INTERMEDIATE: 4 feet tall, 45 degree arc, 10-12 feet gap

INTERMEDIATE PLUS : 4 feet tall, 50-55 degree arc, 10-12 feet gap

ADVANCED: 5-6 feet tall, 45 degree arc, 16-20 feet of gap (mellower for farther distance--see trajectory illustration below... mtb'ers tend to like long distance floaters).... OR Steep-AND-Deep: 5-6 feet tall, 55 degree arc, 12-16 feet gap.

PRO: 6-7 feet tall lips, 60-70 degree arc, 16+ feet gap. "

I'll search for the other thread, link it?
 

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ive seen levels with a 45 degree bubble in them but how exactly are these angles being measured? i think this is where i get confused a bit too, the 45 degree looks to be from the top of that imaginary right angle (shown top left in the illustration)...is the angle then not relative to the riding surface of the take off? some of this i can tell already is just going to be trial and error and "feel" do most people just bulid, eyeball it, ride it and see what happens to develop a feel for it? :) id like to be precise as possible where ever I can....
 
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