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!