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.
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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
