# Building corners and switchbacks



## butryon (Aug 12, 2005)

Does anybody know a good formula for laying out a switchback...like radius of turn in regards to speed coming into it...etc? I tried to search it but didn't get anything.
Thank you


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## butryon (Aug 12, 2005)

So I search and I don't get ****! but after posting the thread, similar threads appear at the bottom...!
Anyway, some good info. found, if you have some links and stuff, I appreciate them.
Ryon


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## DaveVt (Jun 13, 2005)

Find flat spots and natural benches for turns through the fall line. Make the radius as big as possible, bigger then you think you need. You can compensate for over tight turns with higher and steeper inslope but if you are on a steep side hill you will be moving a lot of dirt/materials. Grade reversal before the turn for a few meters can help bring speeds down, or give a better area to brake. A grade reversal after the turn will dump the water you trapped with an insloped turn. (omega shape)


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## pinkrobe (Jan 30, 2004)

+1 on the bigger radius. I've read that 8' is the minimum, but I find that 12' is barely doable for most people unless they spend a lot of time riding berms. The last couple I built were ~20' radius, and worked well climbing or descending. Further +1 on the grade reversals [a.k.a. jumps] before and after the berm.


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## Trail Ninja (Sep 25, 2008)

Quick and dirty. Your arm and your Mcleod handle is 8 feet. For a wider turn (11 feet) stick a pole in the ground and hold on to it with one hand while holding the Mcleod out with the other.


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## zrm (Oct 11, 2006)

pinkrobe said:


> Further +1 on the grade reversals [a.k.a. jumps] before and after the berm.


I wasn't aware that grade reversals = jumps, especially where slowing people down without skidding into switchbacks is the goal.


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## aero901 (Apr 11, 2012)

The slope of the hill where the turn is to be located is the controlling factor on what kind of turn can be built. Anything steeper than 20% usually requires a switchback with a retaining wall and significant excavation.

For slopes below 10%, insloped turns are usually possible with proper drain work. A radius of 20' with an inslope of 45 deg. will work well for speeds up to 15 mph. Adjust the angle of the berm face or the turn radius according to these formulas:

*Find the turn radius given rider speed & berm angle*

_r = v^2 / ( tan(a) * g )_

*Find the berm 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)
g = Gravitational Constant (32.2 ft/s^2 or 9.81 m/s^2 )

*Example:*
r = ???
v = 15mph = 22 ft/s
a = 45 deg

_r = (22 ft/s)^2 / ( tan(45) * 32.2 ft/s^2 )
r = 15 ft
_
Another option is to shape the face of the berm with a cup to handle riders of many different speeds (as in the image below). The dimensions in the image show the varying angle of contact along the face of the berm. At a face angle of 90 degrees a rider can, theoretically, carry as much speed as they want through the turn independent of the radius. Not all dirt will allow this steep of an angle and if it is possible it won't last very long.

View attachment 933229


Switchbacks are slow speed corners by nature because the turn radius is minimized to reduce the amount of materials required to build them. Anything smaller than a 7' radius is difficult to navigate for most riders. A 10' radius turning platform is a good starting point. Use a shorter radius on steeper slopes. We built a 7' radius switchback on 45% slopes and it is manageable but still pretty tight.

For a level, equal cut/fill turning platform, the height of the retaining wall can be approximated by:

Max Retaining Wall Height = Turn Radius x Slope Gradient (%)

Example (10' radius, 50% slope):
Retaining Wall Height = 10' x 0.50
Retaining Wall Height = 5'


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## Mark E (Feb 7, 2006)

This was a popular IMBA blog post on a turn style dubbed the "Switchberm."

https://www.imba.com/blog/mark-eller/advanced-trail-building-introducing-switchberm

Joey Klein is a long-time member of IMBA's trail building team. In this entry he gives an overview of an advanced construction technique - meet the "Switchberm."

If you've read IMBA's trail building advice in Trail Solutions or Managing Mountain Biking you're already familiar with the rolling crown switchback. "Rolling crown switchbacks drain water like a dream, but tend to interrupt the flow of a trail and thus can be awkward to negotiate," reads a description from Trail Solutions. "While this isn't an issue for hikers and horseback riders who are traveling relatively slowly, a person on a bike would prefer to maintain their speed through the turn."

That was true in 2004, and it's still true today. In the past decade, IMBA's trail building staff has continued experimenting with insloped turns, berm-building techniques and other tactics that reward riders with flow without sacrificing durability.

The insloped platform turn, AKA the "switchberm," is our latest alternative to the rolling crown switchback. Switchberms work great on really steep sideslopes, especially for shared-use and dual-direction trails where hikers, trail runners, equestrians and mountain bikers are all searching for their prefered style of flow.

The pics in this post come from a project that Chris Kehmeier, Chris Leman and myself teamed up on in Utah this fall. Many of these were placed on 40%-70% sideslopes. Equestrian traffic is high, and foot travelers often bypassed the intended trail by cutting the original switchbacks.

We made extensive use of drainage dips above and below the turn platform and a bit of insloping for the water to run through the inside of the turn. These turns have proven to be bombproof, holding up to equestrians and descending MTB traffic. Runners, hikers and even our dog-walker friends are no longer shortcutting.

Better yet, MTBers of all abilities really enjoy these turns. When descending, they can focus on the turn first, then deal with the most severe changes in trail grade. When climbing, they crank onto the platform, then get a nice rest through the turn and beyond. The inslope gives them a bit of a psychological "guardrail."

What makes switchberms different than rolling crown switchbacks?

- the leg just above the turn is a gentle grade, keeping speeds in check for descending riders (5%-8%).
- the turn is on a nearly level platform and slightly bermed (insloped 6%-9%).
- the climbing leg below the turn is brief but quite steep (15%-20%, or just a bit more if you need to push it).
- the turning diameter is between 14 feet and 18 feet depending on width of trail, sideslope and intended users.
- do all you can do not to exceed grades above 20% for the lower leg steepness -- this is where overall trail analysis is needed before you start digging.

Obviously, for bike parks and fully bike-optimized turns these would be used only as a last resort. Bike park turns, and turns designed for one-way bike traffic with higher speeds, generally should be built on fairly gentle sideslopes, allowing for a much bigger turning radius with a diameter of 22 feet or more.

Hope the advice and photos are useful!

- Joey


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## twd953 (Aug 21, 2008)

zrm said:


> I wasn't aware that grade reversals = jumps, especially where slowing people down without skidding into switchbacks is the goal.


That probably depends on the type of rider the trail is designed for among numerous other factors. On a beginner (green) trail, it would be better to use a much gentler grade reversal (i.e. just enough to shed the water). On an advanced (black) trail using a grade reversal as a jump into the berm is probably OK depending on soil type, armoring etc. We have a few of these on our trails where you can jump right into the berm without ever touching the brakes. But, the overall flow of the trail needs to be accounted for. You cant have a 30 mph straight leading to a grade reversal jump into a tight 10 mph berm. If that is the case, either the corner/berm is too tight, or the trail above needs something to either keep speeds in check or slow riders down more gradually.

Most of the major switchback skidding problems I have seen are on old school hiker/equestrian built trails that have those super fast straights leading to a tight 2 mph off camber switchback.


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## DaveVt (Jun 13, 2005)

aero901 said:


> The slope of the hill where the turn is to be located is the controlling factor on what kind of turn can be built. Anything steeper than 20% usually requires a switchback with a retaining wall and significant excavation.
> 
> For slopes below 10%, insloped turns are usually possible with proper drain work. A radius of 20' with an inslope of 45 deg. will work well for speeds up to 15 mph. Adjust the angle of the berm face or the turn radius according to these formulas:
> 
> ...


Thanks for that stuff. We always just ride it until it feels right but have learned the basic math that applies by feel. That will save us some time.


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## pinkrobe (Jan 30, 2004)

zrm said:


> I wasn't aware that grade reversals = jumps, especially where slowing people down without skidding into switchbacks is the goal.


I'm not talking about a monster booter, but for the riders that have the skill, having a small jump/roller immediately before the berm means that they can air into it. The majority of riders will simply roll over the bump and into the berm. The basic function of a grade reversal is to divert water off the trail. If you can extract additional utility from it, why wouldn't you?

edit: I'll also mention that we cup the face of all of our berms. We build in soils with a very high clay content, and once set up, the berms are very resistant to rider traffic.


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## bpressnall (Aug 25, 2006)

To get a round turn. you can use a string as a compass for "drawing" the circle. The engineer types will write equations that can also be used for determining how to get a rocket to circle Jupiter. It's interesting stuff, but you can probably do just as good by going out and riding corners and seeing what works on various trails. Some folks, like myself, like those tight switchbacks that require a rear wheel hop to negotiate, but that won't work on those "flow" trails. I think it best to try to avoid switchbacks where possible and make as long of runs between them as possible. You have to be careful about building those all to frequent steep, fall line turns that are an invitation to rutting and erosion. That "switchberm" shown above is on a steep slope, but looks like a lot of soil was moved to create the flat platform. Still, I would call it a last resort if you can't find natural flat spots. On gentle terrain you can play with the radius more, but as it gets steeper, your options decrease, but you can add a berm if it seems too tight to carry speed.


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## Markhpnc (May 5, 2011)

Those are some pretty rad formulas factoring in speed and all Aero! Not sure if I'm smart enough to utilize them, but awesome that it has been worked out.

Something already mentioned that I often think about is making the grade reversal leading up to the turn longer and steeper. If you are able to get the speed and flow calculated well enough, you could slow riders down just enough without using brakes and at the same time gain some elevation to use on the exit to get back up to speed.

Nothing is more annoying to me than having to scrub speed on an epic downhill to make a turn, and this seems to be what I encounter more frequently than not.

Great posts guys! :thumbsup:

Edit: I meant to specify that the idea above is in the context of a one way downhill trail or downhill section of a one way loop.



aero901 said:


> The slope of the hill where the turn is to be located is the controlling factor on what kind of turn can be built. Anything steeper than 20% usually requires a switchback with a retaining wall and significant excavation.
> 
> For slopes below 10%, insloped turns are usually possible with proper drain work. A radius of 20' with an inslope of 45 deg. will work well for speeds up to 15 mph. Adjust the angle of the berm face or the turn radius according to these formulas:
> 
> ...


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