Computing Roller Spacing from Rider Speed
As an upcoming project we are considering adding a section of rollers to an existing flowy descending trail. The rollers will be 12-18" (0.3-0.5 m) in height, 2-3 bike lengths long (10' / 3m), and rider speeds will be approaching 20 mph / 30 kph / 9 m/s.
What spacing have you found work well for similar parameters? I am thinking spacing can be estimated by determining a "pump frequency" which feels natural to the rider and then doing some basic math to come up with a distance given an input speed? Intuitively, 2 seconds between them seems about right but I don't have much experience building rollers.
Time Between Rollers x Velocity = Spacing
Example: 2 sec x 9 m/s = 18 m (60')
If there is a formula, it would have to allow for bikes with and without suspension, riders who can pump and jump and manual etc. I don't have your answer, but we apply the use of a crash test dummy and get him to ride and re-ride until we finesse things correctly.
What I would say is you have to allow for faster riders wanting to double them and also for erosion. 30-50cm is not a large roller and may either wear away or expose any rock used to fill it. Obviously you can just remove material from the dip and add to the rollers if that happens, but unless the trail is closed for a while they would be vulnerable without heavy compaction.
No rock where we are building so gradual exposure shouldn't be an issue. Mostly clay/silt loam soils that are pretty tough when compacted and slow to erode (except during droughts). These trails are more geared towards XC riding (hardtails, short travel full suspension, and a few all mountain bikes) so we aren't trying to make the rollers very large. Large rollers would also clash with the terrain in this particular area and we are aiming for a more natural look.
There will definitly be a small group of people who will want to get a bit of airtime and we want to keep that option available; probably not doubling them though since they will be relatively small. I was thinking more along the lines of a high speed pump track.
Builder of Trails
First, 20 MPH is fast for a roller section but very doable. We built rollers for speeds between 10 and 17 MPH at a height of about 12" with a peak-to-peak distance of no less then 12 feet. Most of them are gap-able y the more experienced riders and can be pumped or manual-ed by anyone.
Originally Posted by aero901
You could probably make them 18" high, but you'll want to make sure you make them somewhat flat, not peaky. Remember to keep at least a bike length between the bases on the rollers, i.e. in the trough, but not much more than that. Good luck!
I have not ever clocked speed, so I really cannot speak to the 20mph parameter. BUT, I have built and ridden a lot of rollers.
They can vary immensely. Dburatti is right: never peaky. He says not less than 12', and I definitely agree with that, although I would push them out even more and say not less than 15'-17'.
I paced these rollers at Whistler at 20'-22'-ish feet peak to peak:
The faster the speed, the more stretched the roller itself needs to be (if you intend the rider's wheels to stay on it). Think about a wheelbase up the front, a wheelbase over the top, and a wheelbase down the backside, at a minimum for a higher speed roller. You can even do wheelbase up, 2 wheelbases across the top, then wheelbase down (which I'm about to get to with "roller-tables" and "monster rollers."
DJ , Pump Track plans
I would also advise using what I call the "roller-table."
A roller-table allows a rider to deliberately skim/scrub the top of a jump by staying low. You round off the lips on both ends to allow the un-jump to happen....
D. L. at WCPT
DJ , Pump Track plans
You can also have "monster rollers" which are almost like a tabletop with a little dome action on the top.
See in action: wcpt random edit - spring 2010 on Vimeo
Serious BMX trails "monster rollers" (note the flatness, as they get stretched):
You never want a high speed monster roller to look like this (although it could be rad in slower speed situations)....
This is actually a slow-speed monster roller.
A high speed monster roller looks like this (scrub by the veteran Brian Foster):
Or at my local spot:
. the "take off point" to "landing point" is the same distance as a jump in the line, but you're un-jumping it.... meaning that the dome is a full 11 feet long in this case. most people don't think of rollers like this, but it's a legit way to do it in higher speed sections, when you want a "full-bike" pump on the transition.
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