2nd Berm Scott Community College

[NCoulter]

Hey Folks:

I wanted to hear from the court of public opinion, in case I am missing something. Mods please advise if I need to make changes to the post.

On Sunday, I went out to remove a metal fence post, which had been plaguing toes and tires on the Scott Community College trail, for years. It also had me terrified of landing on it face first, since it was mid trail on a corner and jagged metal.


Post In.jpg

Post Out.jpg
I figured, while I was out there, I would fix the 2nd berm, which had a very shallow slope to it (maybe 45 degrees). In this form, you can't really use it as a berm because if you hit it fast and leaned over, you would fly right off the top of it, since it has little slope and no lip.

I cut more slope into it and extended it about 5' because it fell away where you would still want to counter the g forces of the turn.

Well... I went out there today, and someone had packed dirt back in to remove the slope I put into it.

Can whoever did this explain why they changed it back. I assumed everyone would appreciate what I did and agree with me that it makes the corner safer. I am genuinely confused about why anyone would want the berm to be in this shape.

I recognize that it looked bumpier after I cut the slope in, but that is because I was cutting it in from packed dirt and I couldn't smooth it because the dirt was too dry and crumbly. I was going to smooth it after it had been ridden and the dirt was moist again.

For everyone's reference-

My berm:
My Berm.jpg
Berm today:
Today Berm.jpg

[rohnerj78]

I get an error 404 url not found on every photo link.

[NCoulter]

Sorry... Can you see them in the post now?

[bleedfororbea]

do you work with scott community trail crew?
if not you may want to get with them.
if you do not work with them. have not contacted them. then technically you are changing trail features without approval.
if you contact them im sure they would like the help maintaining and improving the trails! im sure you guys could come to an agreement on how certain berms should be built and where.
i think most of them work at Healthy habits bike shop .

[NCoulter]

Fyi- I just met Alex out there and we discussed it. All good and understood.

To answer your question Orbea... It's true that I did not get approval. However, I do quite a bit of small maintenance out there without approval, and I think everyone should be regularly doing things like filling braking bumps, trimming back brush hanging in the trail, rebuilding collapsed or eroded sections, etc... without involving the trail steward. Going back and forth trying to describe it in detail over email would be too cumbersome, when the solution is obvious and takes little time.

In this situation, I didn't understand why there was a problem, so that is why I made this post.

Regardless, it is all sorted now.

Additionally, I want to say that Alex and Healthy Habits have created an excellent trail for people like me to ride to straight from our front doors. I appreciate the work they put into it.

[DH001]

Opinion from a long time steward/builder/maintainer: people doing little maintenance things like clearing small branches, fixing small erosion areas, trimming back vegetation, etc. is great and saves us a bunch of time. However, rebuilding a berm is something I'd want to know about if it was happening at a trail I'm overseeing. Sending the steward a note takes seconds and they might have relevant input about that project/feature. I've seen many fixes from people trying to be helpful that end up causing more work for the trail crew. There is a science to trail-building that may not be obvious to the average trail user. Or, the trail crews may have plans to rework the issue/feature completely in which case it would be better to put effort into a different project. Communication is king.

Hard to tell from the photos, but it looks like there isn't enough "meat" to support a tire up high on that berm and it doesn't look well compacted? Rule #1 of dirtwork: don't do it when the soil can't be compacted. If we get a heavy rain that loose dirt will be washed away... The reason I don't build berms with a steep face is because they are a high maintenance item. Local soils will only hold a slope of 30-35 degrees reliably (research "angle of repose" for soils if you want to get into the weeds). Drainage towards the exit of the berm may have also been a factor, but again, hard to tell from photos. Many downhill berms require a drain at the exit and extending them too far will block water from flowing off the trail and create a mud pit.

[NCoulter]

Thanks for your reply DH001. To clarify, I didn't rebuild the berm, I cut into the slope (at most 3 inches down low) where I figured the berm had settled flat, so I didn't remove any support from the backside of the berm. The stuff on top is what I had taken away. It definitely needed more rain, to be packed in hard, but the berm face below it has years of compaction. The line you see in the photo after I cut into it is actually a root line, from vegetation. As for drainage, I realize it is difficult to tell from the pictures, but this is a downward sloping berm. The drainage point is further down the hill, and I did cut a small channel for drain exit, at the low point, just because I was there with a shovel.

However, I am interested in this statement: "The reason I don't build berms with a steep face is because they are a high maintenance item." Can you explain this further? Additionally, I am wondering where you build berms geographically and what you consider steep.

[DH001]

Dirt berms naturally erode and the soil needs to be put back where it came from eventually (ie rebuilt/maintained). If the face angle of a berm is steeper than 30ish degrees this erosion happens much faster because the "equilibrium" angle of the soil has been exceeded. Like I mentioned, read up on angle of repose for further clarification. The critical state is when soils are very dry and easily pulverized by bike tires. Local soils are very non-cohesive when dry and break down into fine powder.

I've built dozens of berms at Illiniwek, Dorrance, as well as other local trail systems. Soil types are similar across the QC area so the same guidelines will apply; >30-35 degrees is steep for local soils and will result in accelerated erosion. When maintaining many berms across multiple trail systems with limited volunteer power it is best to create features which minimize the amount of maintenance required.

If a berm needs a face angle steeper than 30 deg, I'd start looking into using different materials (pavers, rock, wood) or increasing the turn radius/trail routing. Formulas below can be used to estimate the required berm radius/face angle. With a 30 deg face angle, a radius of 20-25ft is sufficient for up to 15mph which is a pretty quick pace for most local MTB trails. No need to go steeper unless the berm is constrained to a very small space.


Find turn radius given rider speed & berm angle:

r = v^2 / ( tan(a) * g )

Find berm face angle given rider speed & turn radius:

a = atan( v^2 / (r * g ))

Variables:
a = Berm Face Angle (degrees)
r = Turn Radius (ft or m)
v = Rider Velocity (ft/s or m/s) [1 mph = 1.46 f/s]
g = Gravitational Constant (32.2 ft/s^2 or 9.81 m/s^2 )

[NCoulter]

I'm not entirely clear regarding what these equations represent. By sufficient, do you mean capable of holding traction? Are tire type, soil condition, moisture, etc... accounted for in the equation in a manner I don't understand?

When referring to the degradation of the berm, do you have any data (anecdotal or hard) regarding the rate at which it degrades between the different slopes? I am asking so that everyone reading (including myself) has a better understanding of the trade-off between berms that don't support high speeds and berms that require increased maintenance.

When you say sufficient up to 15mph does that mean (without factoring in the variables I mentioned above) it won't hold for people riding faster than 15mph? Wouldn't that mean you were setting a pretty moderate speed limit on all of the berm features?

[Chief Illiniwek]

I'm curious about exactly how much trail building experience you have? DH has many years of trail building experience, along with being the designer and builder of two of the premier trail systems in the area. He knows very well that trying to fix or build trail features during the current dry conditions is impossible at least if you want them to last.
Clearly, you're not going to learn what you need to know by posting on a forum so my advice to you is to attend as many local trail days as possible to gain the experience needed to build and repair trail features.

[DH001]

Those equations, as I said, are a good estimate for the dimensions of a berm if you know/assume certain inputs. Tire type, soil composition, moisture, etc all deal with the frictional forces of a tire on dirt. The equations above assume zero friction forces are at play because they are balancing the centripetal force from cornering with the horizontal component of the normal force on an angled slope; one could ride a berm made of ice with tires made of Teflon at those corner speeds (given your skills are up to the task). Adding tire friction to the mix is going to result in higher cornering speeds for the same berm geometry (i.e. the equations provide a very conservative estimate and the slope of the berm could actually be a lesser angle or curvature of a tighter radius while allowing the same cornering speeds).

What do you mean by "don't support high speeds?" Typical travel speeds of MTBs on local trails are in the range of 8-13mph. Data point: the fastest racers in IL/IA at the Illiniwek Abermination, one of the fastest riding trails around, are averaging less than 13 mph for a full lap. Very rarely do riders exceed 16 mph unless on a sustained downhill and that kind of trail shouldn't include small radius berms at all. This is a non-issue for local trails IMO.

I don't have data on the rate which berms degrade. But, I've rebuild enough of them to know the steeper berms need maintenance at a higher frequency. My point is, berms don't need to be steep to do their job... Making them steeper, for no reason, creates unnecessary work.

[NCoulter]

Thanks for the reply DH. I apologize for not responding in a timely manner. I am in the midst of harvest, right now, and getting on my computer has been difficult, while responses by phone like this are likely to be replete with typos.

I am a bit confused about the mention of Abermination speed averages. Are you saying that the average speed of an xc race with fairly lengthy climbs should inform the speed estimates of berms at the same trail? I don't understand how that would be the case since berms are high speed features, by nature. Flat turns would be sufficient, if there were no intention of providing high corner speed, wouldn't they?

Additionally, I would argue that there are very good reasons to go steeper or provide a steeper portion at the top of these berms:

1) A steeper berm provides a lot more tire feedback, which helps you stay locked into the most supportive part of the berm. With shorter berms, the margin for error is pretty small. If you have a constant 30 degree slope, your hands and feet don't tell you if you are riding high or low on the berm. That means you need to get the initial turn exactly right or you risk coming off of it, while your eyes are on the exit and not looking down to see exactly where the front tire is tracking.

2) Safety- If someone gets the entry angle, speed, or lean angle incorrect and needs to grab some brake, make weight shift adjustments, etc... the loss of traction doesn't just keep happening and send them off the back of the berm. Eventually they hit a pitch that keeps them in the berm.

3) It gives riders the option to ride the berm how they want to ride it. If someone wants to really rail the berm or pump really hard into it, then more slope is necessary, from what I have experienced. The berms at the bottom of the North loop downhill section at Illiniwek are a pretty good example that we are both familiar with. When I get to the berms in this section I am definitely going faster than 15 mph, and I need to grab a lot of brake because I don't think they can hold the speed. If I am alone in this and other riders feel comfortable ripping these as hard as they would more vertical berms, then so be it. However, I would wager that isn't the case, and I have ridden many similar radiused berms with increasing slopes where I didn't feel the need to decelerate so dramatically.

[NCoulter]

I apologize for replying with a novel. Feel free to pick and choose specific things you want to respond to.

Also, in regard to your equation... It is assuming a constant centripedal force, isn't it? It is essentially saying this slope will provide adequate support to a rider going this speed to keep them on the berm for a full 360⁰ rotation. The problem I see with using this as a guide is that constant force isn't how we ride. We weight and unweight the tires, vary the angle of the bike, and rarely enter a berm already riding on that radius, so the tires are getting a lot of different looks at the berm face.

[DH001]

Firstly, remember the discussion around said equations should be framed by the emphasized word in this quote:

Those equations, as I said, are a good estimate for the dimensions of a berm if you know/assume certain inputs.

Yeah, I'm saying the average speed from an XC race, where top riders can ride up the sustained hills at over 10mph and where the climbs are less than 10% of the total lap distance/time, is a good indicator of an upper limit for typical speeds (not maximum speeds) on this specific trail. Check out the Strava segment speeds for this trail system if you want more minutia.

1) I'm not saying don't build steep berms. Most berms on local trails don't need to be steep. Typical corner radii and speeds don't require it. Use materials other than dirt if they do. You seem to be conflating my replies will all berms? Perhaps I wasn't 100% clear on this. I was specifically talking about dirt berms built from local soils since that is what you started this thread for/about.

2) MTB is an inherently risky activity. Riders are ultimately responsible for being in control at all times. Washing out on a corner is one of the least serious types of crash that can be had IMO. Arguing for safety and increased riding speeds is somewhat contradictory. I like riding fast and at the limit, but don't expect it to be inherently safe.

3) The design intent of the berm you referenced at the bottom of the North Descent, and turns before/after it, are to gradually slow riders. Some of the preceding turns are getting straightened/cut and are in need of rework to improve this. Note how the next 180 deg. corner doesn't have a defined berm; also to slow riders. In fact, the final "twisty" section of that trail is/was a series of speed checks before crossing the old overlook stairway hiking trail. Especially the leaning tree. Not a good idea to have riders crossing a busy hiking trail with limited visibility at high speeds and then shortly after rejoining a busy trail with bi-directional travel.

The issues you mention about riding line precision, mid berm corrections, etc are better solved by making the face (riding surface) of the berm wider instead of steeper IMO. I understand what you mean about a steeper top part of the face catching a slipping tire. I've built a few berms with a face angle ranging from 0-90 deg, from wood, and have experienced it a few times when coming in too fast and too low. My experience with these "curved face" or variable slope berms is that many people don't know how to ride them properly. They don't lean the bike enough when getting into areas of higher incline. This can cause the front tire to climb up and over the face of the berm causing a "high-side" or OTB type crash. I'd argue this mode of crashing is worse than sliding out on a constant slope berm where the most typical type of crash is "laying the bike down."

The equations don't account for a riders mass. It is irrelevant. Weighting/un-weighting the bike has zero effect on how tight/steep the berm needs to be to allow a specific cornering speed. A 200lb rider and a 100lb rider will have the same max cornering speed on the same berm even though they exert different amounts of force into the berm. If you are talking about "pumping" berms, that is an entirely different topic and requires a berm with varying turn radius or other specific geometry.

[NCoulter]

Well, I appreciate your candor regarding the merits of low slope berms. I am perfectly happy to let my previous arguments in favor of steeper berms stand.

To answer your question: Throughout the conversation, I only understood you to be talking about earth berms.

Ride On