Roll ins and dirt jumps

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Roll ins and dirt jumps
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Posted: Mar 22, 2021 at 6:50 Quote
So, the topic of roll-ins always comes up when people want to have jumps in a flat area or in a small area. Does it have to be super tall? Does it have to be super steep? What angle is good? Just wood or dirt and wood?

Both the Whistler Air Dome and the Highland Bike Park indoor park have this exact style of roll-in. Top notch. Note how tall it is. That's gotta be around 15 or 16 feet. To drop in from a standstill and hit a full-size jump that you can do big tricks on, this is around the height you need. 12 minimum up to 16.... Note how big the transition of the ramp is at the bottom. You're rolling in to an actual quarter pipe.

Many people don't want to build something that big. They say, well, I'm thinking 8' or 10' feet tall, don't want to bother the neighbors.

1) Does it have to be super tall? Well yes and no. Let's say you plan on dropping in to hit a standard big BMX box jump or freeride/slopestyle jump. The "standard" gap is anywhere from 10 feet (standard indoor BMX box jump) to maybe 16 feet for freeride/slopestyle.
(a) In order to drop in from a standstill and use only gravity to gain all the speed, then the roll-in has to be tall (in the 12'-16' range) and steep with good transition at the bottom.
(b) BUT, an alternative to dropping in from a standstill is having a long platform that you can get in 2-3 full cranks on, before dropping in; in that situation you do not need the roll-in to be super tall. Think about how most skateparks work--there is a perimeter of elevation (quarter pipes, wedges etc.) creating a basin--you get a few cranks across the deck and drop in on a regular 6-8' tall quarter. That's plenty.
(c) A third alternative is a super long (but not that tall) mellow-angle drop-in enabling the rider to crank down it--this is how BMX race tracks work. When you crank down a slant you gain speed fast, maybe you even end up with just as much if not more speed than a super-tall super-steep roll-in.

2) Does it have to be super steep? What angle is good?
First bookmark this Right Triangle Calculator: https://www.calculator.net/right-triangle-calculator.html If your roll-in is 8 feet tall, with 16 feet of surface to ride down, that's a 30 degree angle.
Most people over-estimate how steep a roll-in should be. Remember that with a mellow slant you can walk your bike up it, whereas a steep slant, you'll have to build a staircase of some kind to haul your bike up top. If your roll-in is steep, you must have a good transition at the bottom. Remember, the goal is to create horizontal speed; if your roll-in is 60 degrees, all that force is pointing you directly into the ground; something has to "transition" your momentum to be at 0 degrees. Watch this video, and note that with a 60 degree roll-in with not enough transition at the bottom, the rider's force goes too much into the ground: (Not to mention the struggle to climb up it)[L=https://www.pinkbike.com/video/475986/[/L]

This is my friend DK's ramp which is 8 feet tall and 16 feet of roll-down, which is 30 degrees. It's a roll-in for a smaller pump track line, so the speed worked out great. It's also mellow enough to walk up, without having to build a separate staircase or ladder.

This ramp appears to be more like 65 or 70 degrees, but because it's so tall, and there's more transition at the bottom (I'd estimate a 9 or 10 foot radius of transition going maybe 5 feet up), it becomes doable. Joe Prisel told me the roll-in felt very smooth and generated a lot of energy for the big jump after it.


On this one, notice how they built up dirt at the bottom then put wood on top of the dirt. This is probably a work-around or after-the-fact fix. It's better to go ahead and build out the bottom transition out of wood.

The geometry on this one looks excellent. Curved radius at the top to roll in smoothly, plenty of transition at the bottom. This might be the kind of roll-in that you actually crank down. Cranking down a roll-in is actually a great way to generate speed when you don't want or can't have a roll in be too tall or too steep. Therefore there's an interesting trade-off between a long mellower roll in that you crank down and a super steep one that you don't.

This roll-in has too much kink at the bottom (i.e. needs more transition).

It might seem clever to re-use pallets, and it definitely saves money because they're usually free... but the ridiculous thing about this roll-in is the difficulty of climbing up there. You gotta keep in mind, in a session, you're going to want to drop in 20 or more times. Maybe 40? Do you really want to climb a beast like that?

This is a video-still from the 1980's BMX movie "Rad." The Helltrack roll-in was super steep, but didn't have enough transition at the bottom.

When the Helltrack tribute track was built for an event in Dallas a few years ago, they put more dirt transition at the bottom of the roll-in:

In my opinion, this one looks excellent. Since it's part of a trail above it, you'll be coming in with some speed already, then get to pump down this. Top and bottom curves will make it feel smooth. The only thing I wonder is how much speed you'd already have at the top from the previous wood feature.



Similar ramp extension style roll-in. Note how the radius of the ramp transition blends seamlessly into the straight part. I would guess the flat-bank part is around 70 degrees.

This one is overly-steep and that little bit of dirt isn't enough to make it work.

A platform with a down transition gap to a dirt landing is another way to think about roll-ins.

Another example of a platform drop-in with step down gap to dirt landing. This one looks pretty hardcore. One thing to note is that the ladder to climb up with your bike looks overly steep--looks like it'd be annoying to climb that 30 times in a day (as mentioned before)--the simple fix is just make a much mellower longer staircase, like 15-20 degrees instead of 45.

Another platform to gap off of. My friend Abel and crew built this one outside of Austin TX. Video here: https://www.youtube.com/watch?v=yaent8tb_5E

I took a photo of this dirt roll-in at Steamboat a few years back. The double pump-down is pretty interesting, and it worked well. The hill itself is probably a good 10'-12' high.

--CMC

Posted: Apr 1, 2021 at 17:28 Quote
As usual this is incredible information. Would you say that in summary your point is, similar to jumps, roll ins require transition radius that is proportionate to their scale and gradient. I recently built a roll in and definitely made a few mistakes but was limited due to somewhat unrealistic space requirements. Specifically my roll in requires you to pop in like a quarter pipe, you can't just roll over it because you'll smack your BB. The other detail is that believe the transition radius is a bit to sharp for the steepness and speed.

Posted: Aug 25, 2021 at 10:43 Quote
I thought this was interesting for comparison regarding the Starting Hill.

It's from a UCI Track Guide book. https://www.uci.org/docs/default-source/publications/uci-bmx-track-guide---developing-bmx.pdf 3.5 meters tall and 12 meters of ride-down length is a 16.9 degree angle.


I don't know the exact dimensions of the starting hill in this photo, but if you think about how a long mellower-angle allows you to crank down it, then it may end up being just as fast or faster than the steep roll-ins above.

Posted: Nov 9, 2021 at 7:27 Quote
This start hill is interesting because you'll notice that the riders who crank down thill obviously go way faster than the riders who barely crank. The interesting question would be whether riders cranking down a hill like this get more speed than a Whistler Air Dome style drop-in of a similar height.

5m Start hill at Manchester Indoor BMX Track
https://www.youtube.com/watch?v=2uS7IrmgX40

Posted: Nov 15, 2021 at 8:58 Quote
Example of starting hill in Bentonville. It was good that it had plenty of room on the top to get in a crank or two, and room for riders to wait, but at this height (I would guess maybe 5'6" or 6' tall??), the feature was not quite as speed-producing as other designs. Overall impression very good. The rock work looks awesome.


Posted: Nov 15, 2021 at 9:10 Quote
This is another example of a mellow slant up the back of the roll-in. That's a lot better than some kind of steep ladder or staircase you have to climb with your bike. And you don't have people climbing up the face of the roll-in.


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