1. ## Bermed corner radius sizing

I dont want to hijack the other thread but we will be marking out a new spur trail soon and was wondering if anyone has figured out the equation for size of radius = to speed of travel. There is a just the right size of corner for different speeds to keep the flow going with out having to break and being able to pump the corner. Not to tight and not to wide. Im really getting into this trail building aspect of mountain biking.

2. Originally Posted by GatorB
I dont want to hijack the other thread but we will be marking out a new spur trail soon and was wondering if anyone has figured out the equation for size of radius = to speed of travel. There is a just the right size of corner for different speeds to keep the flow going with out having to break and being able to pump the corner. Not to tight and not to wide. Im really getting into this trail building aspect of mountain biking.
My current approach is to start with a smooth curve that can be ridden fast, then construct the berm over what already works.

Maybe it's not the best approach, but it seems to give something that looks right. I'll post back when I my current project passes review.

Walt

3. Some recent berm work by our YCC crews needed to be modified a bit because they placed the berm just outside the existing tread. Adding more soil until the tread was slightly under the berm returned the flow. Tighter turns require more berm height, so it's more about matching the berm height to the turn radius I believe. These berms are on what I would consider pure xc trails and are basically piles of big rocks covered with dirt.

4. Thanks for the info guys, now I cant ride my local trail without rebuilding the trail in my head and making mental notes on what needs to be fixed. LOL. I used to dream of XTR derailleurs and carbon wheel sets, now I dream of mattocks and shovels .

5. Originally Posted by GatorB
. . . . was wondering if anyone has figured out the equation for size of radius = to speed of travel. There is a just the right size of corner for different speeds to keep the flow going with out having to break and being able to pump the corner. Not to tight and not to wide. . . . .
i know it inuitively, but unfortunately can't tell you the exact speeds because i've never had a speedometer on my bike.

i have used 7' and 8' radius a lot for "whippy" berms at medium speed on a trail or pump track. (i.e. if you're riding fast enough to clear 8' to 11' gap jump.) i use arms + rake outstretched in front of me for a rough 8' radius. 7', 8' and 9' radius are very common in skateboarding and bmx, not just for quarter pipes & launches, but also for bowls in cement skateparks. bowl corners are a lot like berms.

in cross country, i think you'll have a lot of turns where a 9' - 13' or 14' radius will work well. when you get larger than 16', berms will be more about soaking up speed than creating it. like that downhill berm at Keystone I posted in the other thread that had a roughly 22" radius.

i have also done the method of berming the existing turn of an xc trail. note, though, that this works best if you're already going too fast for the existing turn. if the existing turn is doable and fun at a certain speed, adding the berm allows you to go a lot faster around it. the flip side of that though, is that with the berm it will now feel sluggish--you'd have to go a hell of a lot faster around it for it to feel 'whippy'. so in reality, in that situation, you'd want to tighten up the turn to make it feely whippy.

note also that a little 30 or 45 degree elbow berm can have a tighter radius than a full 180. for whatever reason, the rider will feel more G's the more turn of a circle you have. so, for example, 8' radius will feel good for a 90 degree turn, but may feel too tight for a 180 degree turn.

you'll have to ride a lot of them to get a feel for what you want at your trail. good luck !

6. Good stuff right there cmc4130. Im currently fixing the existing xc trail to get more flow and reduce trail damage do to several different lines being made. The corners that are bermed seemed to stay nice packed and easy to maintain but the fast sweeping flat corners are getting wider, sandier and eroding. What was once a 8"-12" wide trail has turned into a 5' wide sand pit that your basically just trying to survive. We are working to reverse that but I want to make sure the new sections we cut will stay healthy and fun. The more I learn about this stuff the more I realize that its not just fun to ride but helps to protect the trail from damage.

7. Originally Posted by GatorB
Thanks for the info guys, now I cant ride my local trail without rebuilding the trail in my head and making mental notes on what needs to be fixed. LOL. I used to dream of XTR derailleurs and carbon wheel sets, now I dream of mattocks and shovels .
You are stuck now. You will never be the same again. Welcome to the club, hope you get as much satisfaction from this as I do. Dirt junky.........

8. Not to get too nerdy.... but a berm doesn't have to have a constant radius either.

Say for example you're doing a 90 degree turn from "-a" to "b" above (West to North)... elliptical radius--tight to mellow--works well when you're at a slower speed but as you're turning you're dropping down elevation. Because of the decline, the rider is speeding up through the berm--therefore the radius gets beigger. The reverse is good when you're going fast into a tight uphill left or right turn. In this case the rider would be losing speed turning uphill, therefore the berm tightens up.

Another thing I've learned is that people don't necessarily ride a berm the way they're "supposed" to. If you do what I call a pure 90 bowl corner berm, a lot of people (especially newbs) will not do the appropriate bike tilt/shoulder drop to actually ride the center of the transition all the way around. Instead they will skip the first part of the berm then ride 'up' the face of the berm at the apex or right after it. The sketchier riders will then have their front wheel go over the berm. There are different things you can do about that. True, you can just make the berm taller.... But you can also do what I call "ribbon" lead-in and lead-out. For like 8 or 10 feet before the berm you start banking the trail like a twisting ribbon so that it sort of forces the person to start tilting, then you have the normal bowl corner design for the actual radius-arc, then you have another 8 or 10 feet of straight lead-out where the ribbon unwinds back to flat.

Another version is where you make it where you are 'supposed' to carve up the face of the berm and exit high, then you can sort of land on a roller/exit.
Like this one, from a pump track I worked on.

BEFORE:

AFTER:

you can't quite see it from this angle, but the exit on the right of this berm we were working on also had sort of a backside/roller out:

9. To follow up what I was saying a minute ago about elliptical radius turns when you are turning into a decline verus turning into an incline.... check this out:

Originally, roller-coaster designers made circle-shaped loops. In this design, the angle of the turn is constant all the way around. In order to build an acceleration force strong enough to push the train into the track at the top of the loop, they had to send the train into the loop at a fairly high rate of speed (so it would still be going pretty fast at the top of the loop). Greater speed meant a much greater force on the rider as he entered the loop, which could be fairly uncomfortable.

The teardrop design makes it much easier to balance these forces. The turn is much sharper at the very top of the loop than it is along the sides. This way, you can send the train through the loop fast enough that it has an adequate acceleration force at the top of the loop, while the teardrop shape creates a reduced acceleration force along the sides. This gives you the force you need to keep everything running, without applying too much force where it might be dangerous.

How Roller Coasters Workhttp://science.howstuffworks.com/eng...r-coaster7.htm

10. Great information that.

However, you blew your "not to get too nerdy" disclaimer.

11. Originally Posted by slocaus
Great information that.

However, you blew your "not to get too nerdy" disclaimer.
heh. i blame it on these guys:

check this out. how to draw an "cycloid." i've been wondering about having a 180 berm (or skatepark bowl) with this shape....

elliptical transitions for mini ramp?
http://forums.oldmanarmy.com/ellipti...mp-t16560.html

12. I dunno about cycloids, but what I do like about berms is how when you stand facing them with a mattock, your golf-tennis swing arc describes the curve of the berm as does your reverse-croquet swing as does your square-nose shovel stroke from above the lip down into the base of the radius. (If you don't use machines)

cmc, that was really so simply insightful that you must be an antidisestablishmentarian.

13. Thanks for the insights cmc4130, you have increased my understanding of berms at least a hundred percent!

I dunno about cycloids, but what I do like about berms is how when you stand facing them with a mattock, your golf-tennis swing arc describes the curve of the berm as does your reverse-croquet swing as does your square-nose shovel stroke from above the lip down into the base of the radius. (If you don't use machines)

cmc, that was really so simply insightful that you must be an antidisestablishmentarian.

and no, i like the First Amendment.

15. Originally Posted by bsieb
Thanks for the insights cmc4130, you have increased my understanding of berms at least a hundred percent!
How true. Probably 500% for me. Give the guy some + rep!

16. Originally Posted by slocaus
How true. Probably 500% for me. Give the guy some + rep!
heh, thanks.

here's another stoke:

The Physics of a Curved Wallride
By Ryan Fudger
Wed, Dec 17 2008

http://bmx.transworld.net/1000074334...rved-wallride/

In the January 2007 issue of Ride, we ran a sequence of Ryan Sher doing the mother of all curved wallrides while on the Shadow Euro Tour (which will surely be in Shadow’s just-released video, Into The Void—appropriately titled for this trick). The wallride sent Ryan down a spiraled stair case and straight into an adjacent wall—although I always imagine that the wall just goes on forever and he’s stuck in an eternal-hell of a never-ending wallride, but whatever—it really caught people’s eye simply because how wild the setup was and shear wildness of ducking into a dark hallway. It sparked even more interest once we threw the animated sequence up on RideBMX.com…although it was forever lost and I had to re-make it for the purpose of showing you this: The Physics of Ryan Sher’s Curved Wallride, as written by Tom Schug.

You learn a lot of useless things in school, but applied science isn’t one of them. Read on, learn something about the “why” and “how” of what you do on a daily basis works, and geek out a bit. It’s okay sometimes. Be sure to leave any comments or questions, maybe Tom will chime in and elaborate.

“What’s In This Picture?”
Ryan Sher’s Curved Wallride
By Tom Schug

The picture presented is of pro BMX rider and owner of Subrosa, Ryan Sher. This is a photo of him doing a curved wallride down a set of stairs in Lyon, France. The picture was taken by Keith Mulligan and printed in the January 2007 issue of RideBMX magazine (86-87). What makes this a great picture is that Ryan rides along a curved vertical wall without falling over as if he was on flat ground. There are many actions taking place here that can be explained by applying fundamental physics ideas. The question addressed here is how fast would Ryan have to be traveling in order to not fall over.
The first step in solving this physics problem is to draw a free body diagram, identify the forces and analyze the situation. (See Fig.1) Focusing on just the contact forces, the three forces acting in this situation are gravity, normal force, and a force of static friction. Because Ryan is moving with centripetal motion, the normal force is considered to be the force of the wall pushing against the tire towards the center of the curve. Also, because Ryan is not sliding up or down the wall, he is in vertical equilibrium. This means that the force of gravity pulling down on his tire is equal to the force of static friction between the wall and his tire keeping him from slipping downward. With these concepts and forces understood, some information can be derived in order to estimate his needed speed.
By the networking capabilities of Myspace, Ryan was contacted directly for the information pertaining to his bike and body weight. Ryan stated, “Well I weigh around 155 lbs., and my bike at the time was probably around 26 lbs.” Another known piece of information is the coefficient of static friction between rubber and concrete. This was found in the textbook Physics For Scientists and Engineers A Strategic Approach by Randall D. Knight. Due to lack of definite certainty, the radius of the curve must be assumed.

•Mass of Ryan & bike = 181 lb. = 82.10 kg.
•Radius R = 7.5 ft. = 2.29 m.
•Force of gravity = 9.8 m/s
•Rubber on Concrete ?s = 1.00

In physics, the force of gravity is equal to the product of the mass and force of gravity acting on it . The force of gravity is always positive or negative 9.8 meters per second depending on the coordinate system used. In this case gravity is positive because it is pulling Ryan down at a positive rate towards the earth. The textbook describes the force of static friction as, “The value of depends on the force or forces that static friction has to balance to keep the object from moving. (133)” In this case then, is equal to the product of and the normal force. The normal force in this situation is a special kind called centripetal. Centripetal force occurs when an object moves at a constant speed in a circular motion. For simplicity of this problem, the fact that Ryan is not moving at a constant speed and that he is following the slope of the stairs downward on the wall will be discarded. It will be assumed he is in uniform circular motion. Newton’s second law defines centripetal force as the product of mass and velocity squared all divided by the radius. The force of static friction is equal to the force of gravity as stated above because Ryan is in vertical equilibrium. Now, to solve the problem of how fast Ryan has to be going is simply just an equation with one unknown variable.
The calculations show that Ryan needs to be going approximately 10.5 miles per hour to make it around the wallride without falling over, which seems to be a logical speed. It must be realized though that this example only takes into account the exact point where Ryan’s tire touches the wall. In reality Ryan’s mass is not just all at his tire but in fact offset and at an angle to the wall. To account for this and adjust Ryan’s speed, a different method can be used.

To compare speeds determined by the next method, two more variables and a new free body diagram will be needed. The first variable is an estimated angle of Ryan riding on the wall and the second is an estimate where his center of gravity is.

• Angle Ryan is riding = 40°
• Distance up from tire Ryan’s center of gravity is = 4 ft. = 1.22 m.

This method uses the concepts of torque incorporated with centripetal motion. The diagram shows the torque, gravitational and centrifugal forces. (See Fig. 2) Centrifugal force is not actually a real force, but instead it is an imaginary force that is not shown on a free body diagrams. The centrifugal force in this picture is what Ryan feels pulling him towards the wall but in reality there is no force pulling him, hence imaginary force. For this picture though, the centrifugal force is defined by the equation for centripetal force. Torque is dependent on three factors; the magnitude of force, distance from the pivot point to the object, and the angle. All of these can be computed by using simple trigonometric functions. Since we will assume Ryan’s angle is not changing, the torque pulling Ryan counterclockwise is equal to the torque pulling him clockwise (rotational equilibrium). If one torque were greater than the other, his angle would continue to either increase or decrease. Since one torque is dependent on velocity and both are equal to each other, a simple equation can be solved for velocity using algebra.

This method of finding Ryan’s speed shows that he must be going 9.04 miles per hour or he will fall over. This number is slightly less than the previous calculation when only the points of contact were considered. This also seems to be a reasonable outcome. It may be even more reasonable than the first outcome because it places Ryan’s weight at a more realistic position. The angle in the second method has a big factor in the speed needed to travel around the wall. Although many other combinations of riding angles or centers of gravity could be used to find different outcomes, it is reasonable to say that between the two methods used here, Ryan was riding around the wall at a rate of around 10 miles per hour.

17. Good read, Im ready to go apply some of this on a horrid section to trail this weekend.

18. What have you gotten yourself into now? Nice!

19. Haha, whats up Mitch. We are renovating Spruce Creek and going to make it flow. The club out there vaporized last summer so we have organized and started to do renovations and fixing problems. And yea im in it now, knee deep.

20. This thread is awesome. Truthfully, I have to go back and read the last few posts afterwards, but any thoughts on this are appreciated....

It involves a reducing radius, cmc4130 berm with an uphill exit and a serious falline entry. This will be primarily a downhill trail but I think the hard-core XC types will climb it regularly, hopefully by bike and not by razzafrazzin horse as per the destroyed trail this will bypass and close.

At the bottom of this turn the grade of slope is getting close to 18%. Large rainfall events are common. Although it is next to a creek and there are almost optimum drainage opportunities above the berm (the turn exits into the creek and crosses where it is dry unless raining), there will be lots of water trying to pool in the low point of the berm.

There are heaps of rocks from small to 50-60cm available on the line and nearby. The creek crossing will be rock-armoured (it's already naturally most of the way there) for slower riders, but the berm should throw riders up onto a naturally banked bit of creekbed, into a floaty right hander and then launch over the other side of the gravelly knoll and .....sorry

As it crosses the falline, I plan to cut the inner line of the berm into the slope about 50-80cm, save all the soil and rocks and create a platform to really kick the rider out despite going uphill.

To avoid pooling water, after cutting the inner border of the line, I plan to level from the inside edge of tread to outside edge, more or less in line with the falline grade (ie sloping down the hill).

Starting with the smallest stones at the base and getting larger with the height of the berm, the plan is to build a rock berm that fills in all the cut-out area and allows water to drain through it down the falline.

After filling over the rocks and compacting the soil (apart from just lightly smoothing the low point - the drainage and climbing line), plus stabilizing the back of the berm, hey presto, a solid berm that drains through itself without any imported materials.

Anyone tried this?????

21. a lil' observation of different berm styles, plus jumps can act as turns too (in bmx and skateboarding an off-set landing is called a "hip"), and curved wallrides are basically berms too...

First note that in a berm, the riding line is not always the same. You can have berms where you enter low, then go high in the apex, then exit low. Or you can have berms which are the reverse--where you enter high, go low in the apex, then exit high again. One of the reasons that pump track builders say to put a roller at the entrance and exit of a berm is not just for speed--it's also to basically push the rider down into the low part of the berm then exit upwards. For example, in this video, where the rider is going through the "M" section, notice how he drops down into the berm, gets sideways, whips around, then comes back up. I would call that a high-low-high 180. Pump Track "M" bermwww.youtube.com/watch?v=JhM022wo0PA

Here's a low --> high --> low. (Pay attention to the packed riding line!)

different style of low --> high --> low. (Note the shape of the packed riding line--the groove as it's called in bmx trails).

cool ribbon/wave style....

Note that when you build a berm that is looks like a skatepark bowl, the rider gets to pick their line. They can go high-low-high or low-high-low, whichever.

So, like this skater... is at the high position (at 6 o'clock in this view of the bowl). He could have skated it differently coming down from a high carve at 3 o'clock, then low, then exiting high again at 9 o'clock.) The same can be true of dirt berms on a lesser scale.

Jumping into a berm can be done different ways. The landing can blend directly into the berm, or than can be space before you get to the berm...
landing down "into" berm . . . riding line is low... thus the berm does not need to be tall.

landing, then some space, then berm. you can go high in the apex.

good example of entering high, going low around the apex, then exiting high

chiller version of this:

rad combo of the landing blending into a wave-ish berm with blended waterfall out....

g-force!!

super-tight 180 (leelikesbikes.com)

classic curve wallride berm.

tilted launch to tilted landing

no separate landing at all. jumping directly to berm wall.

short & steep

steep wall 180

massive speed berm:

bermed launch to straight landing

another launch-berm to straight landing (at cattywoods)

the opposite: straight launch to berm landing

launch up to 180 wallride (at Ray's indoor mtb park)

tilted (not vertical) wallride

another way to do 180 degrees. dirt berm with slanted curve wall - (Winter Park, CO)

if you need to turn around 180, a quarter pipe to vert wall (or just a way over-vert quarter) will generate a lot more speed than a 180 berm will. because you air up... you can leave a vert wall going faster than when you came into it. (Highland MTB Park)

a lot of these photos are from the What Is Trails? thread on ridemonkey:

help with jump line into berm" http://www.ridemonkey.com/forums/sho...d.php?t=241208

22. in this example, we had a huge-turnout work day and built a berm on the existing trail radius... I went for a curve at the right, then a flat extension to drop-down/waterfall roller...

some more berm work at Walnut. we did the short steep-face berm mainly because we were doing this by hand with only 3-4 guys in one day.
As long as the steepness is there in the face of the berm, a good rider can use it with speed....

Winter Park Colorado. A crew of us Austin guys will be heading up there again in July. Great stuff up there.

23. Originally Posted by cmc4130
in this example, we had a huge-turnout work day and built a berm on the existing trail radius... I went for a curve at the right, then a flat extension to drop-down/waterfall roller...

some more berm work at Walnut. we did the short steep-face berm mainly because we were doing this by hand with only 3-4 guys in one day.
As long as the steepness is there in the face of the berm, a good rider can use it with speed....

Winter Park Colorado. A crew of us Austin guys will be heading up there again in July. Great stuff up there.
I'll take a guess your berms will not get braking ruts like the one in the last pic.

24. Funny; I didn't really think too much about this thread when I replied to cmc's last post. Then I looked back through the unbelievable beauty in pics of berms crafted with artisan skills and found one of my posts from nearly 2 years ago. It asked for advice about a berm we intended to build. One that crossed the falline in its entry and before the exit. Here is that berm and how it changed over time.

We decided to build the impossible and drain water through the berm. It was bound to fail eventually, but we had an exit plan. Somewhere in the following pics you will see radius sizing - otherwise this is just bermvelution.

In its infancy and during the end of a dry spell

The low point is below the pile of soil

The low point was dug really deep and wide and filled with large to massive stones leading to an exit drain of > 1m x 1m.

1800mm+ of rain fell on the berm in 2012 with several 200mm deluges, most of it in the first 8 months. It was also popular as the entire trail only opened after this berm was sound and it is a fun trail.

Over time the drain failed as expected. At first it drained a 100mm in 1 hour rainfall like a bathtub. Then slower. Then we had to be there to unplug the surface paving stones (we had a number of flat 50cm plates with gaps over the ballast) if it rained really heavily, or it would flow over the berm. In the end we could not lift enough stones to drain the pond and it was time for phase 2.

The plan was to lower the exit by removing all the ballast and digging the last part of the berm and the first part of the exit deeper. This moved the drainage point a little farther along to meet the exit point of the berm, but the radius of the turn and the line were almost unchanged (we believe your eyes change your chosen line as much as the trail does, so the last bit is only our opinion!!!). A better draining and deeper exit was the aim.

We made the changes immediately before another 200mm (in 24 hours) rain event hit. It had been pretty dry for months and we knew it could disable the trail for months if the berm blew out. Here's how it looked after we finished work and after maybe 50mm of lead-up "drizzle"

As you can see from some tracks in the last pics, some were anxious to bugger the turn before it was ready for re-opening, but it survived the deluge with loss of only topsoil

and in the end the balance of pleasure was restored

25. where i live in florida it's pretty sandy and we get lots of monsoon rain events. if i was to get a truckload of dirt to build some berms should it get straight red clay or sand/clay or something else? what do you use to compact the soil? i've got one of those 8"x8" steel tampers but i could rent a power compactor. i don't want to build something and then have it be all rutted and sloppy.

26. Some pics from my recent trip to Steamboat CO.

Apparently they've been spending a lot of time and money on DH flow trails. One trail I checked out had really consistent 180 descending roller-coaster style turns. They were clearly purpose-built and not jsut adding a berm to an existing v-shaped switchback trail.

view from top down...

view from bottom back up the trail....

Looked like they used pavers on some of the higher-altitude stuff.. Probably due to the dirt being more exposed (and maybe the dirt quality itself as well).

Cool wall-ride berm:

27. Originally Posted by cmc4130
Some pics from my recent trip to Steamboat CO.

Apparently they've been spending a lot of time and money on DH flow trails. One trail I checked out had really consistent 180 descending roller-coaster style turns. They were clearly purpose-built and not jsut adding a berm to an existing v-shaped switchback trail.

view from top down...

view from bottom back up the trail....

Looked like they used pavers on some of the higher-altitude stuff.. Probably due to the dirt being more exposed (and maybe the dirt quality itself as well).

Cool wall-ride berm:
Where does the water go when it rains

Where does the water go when it rains
quite a few of the berms have pipe drains under them. you can see in the second pic above the little pile of white rocks. pretty sure that was covering one of the drains. i took a different pic of one of the drains but i didn't upload it yet...

29. That looks like a blast to ride.

30. Originally Posted by GatorB
That looks like a blast to ride.
Agreed and thanks cmc for the comment re drainage

Agreed and thanks cmc for the comment re drainage
This is a pic of one of the drains:

32. Also, here was one of the rad S-berm sections at Granby Ranch CO (formerly known as Sol Vista). i was there for opening day (May 30, 2013):

33. Snake style

34. Originally Posted by cmc4130
Snake style

Another snake - not yet complete

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