When I say grading on a curve what I really mean is curving on a grade. 🙂 Or, if you prefer, turning on a slope. Grade, expressed as a percentage, is the standard method of measuring the angle of a road, trail or slope. It is simply vertical distance divided by horizontal distance x 100 (or “rise over run” x 100). For example, a trail that rises 10 feet vertical over a distance of 100 horizontal feet has a grade of 10%. Grade plays a critical role when designing turns on a trail because the grade of the slope basically dictates the type of turn that can be used. According to IMBA’s Trail Solutions book, there are basically just three types of turns.
The first type of turn, and by far the easiest to build, is the climbing turn. However, IMBA recommends that climbing turns should only be used on slopes with a grade of 7% or less. While it’s probably fair to assume that the 7% grade could probably be stretched a little on a private trail (due to the very minimal traffic relative to public trails), stretching it beyond 10% would probably result in a trail that requires maintenance more frequently than would be desirable. On particularly steep terrain, switchbacks are the most appropriate type of turn because they can be built in tight spaces and on steep slopes. However, switchbacks have a couple of significant disadvantages. The first is that building switchbacks requires a massive effort, and the second is that switchbacks effectively disrupt the flow while riding because they generally can’t be navigated with significant speed. In between climbing turns and switchbacks are insloped (or bermed) turns. According to IMBA, insloped turns are most appropriate on a grade of 25% or less.
Although I knew all of this before selecting the tentative route for my trail, I (embarrassingly) neglected to actually measure the grade of the slope before plotting out my turns. Oops. Now, having ridden the trail one time in each direction, I know I need to rethink some of the turns. So, it was time to actually measure the grade.
First, I needed some tools. The right tool for measuring grade is a clinometer (sometime called an inclinometer) and I had previously purchased a Suunto PM-5 clinometer just for this trail building project. Unfortunately, a clinometer is generally designed to be used with a second person. On flat ground, you sight through the clinometer and note vertically where on your assistant lines up with 0% grade. Then you just sight to that spot on your assistant when you’re both standing on the slope you want to measure.
However, as this trail is a solo project and I didn’t think my wife would be up for the “Okay, stand here. Okay, now stand here. Okay, now here.” exercise, I thought I’d build some simple survey stakes to take the place of the second person. I decided to buy a few 48″ plastic step-in fence posts from Home Despot along with a few 48″ wooden dowels. I attached the dowels to the fence posts with a couple of velcro straps and tied some fluorescent flagging tape near the top of the dowel. Just put the stake at the top or bottom the slope you want to measure and slide the dowel up or down until the top of the flagging tape is at eye level. Then just sight to the top of the flagging tape and presto, super cheap and effective survey stakes!
Over the course of two days and maybe half a dozen hours, I was able to measure the grade of the slope at every turn I had plotted. It ranged from 17% to 23% for all of the turns on this first section of trail. I also measured the grade of the trail segments connecting all of those turns and found that most of them were in the single digits although a couple dipped into the low double digits. While I was at it, I figured I’d measure the diameter of each of the turns too. I was now (finally) armed with the actual information that I needed to determine the most appropriate types of turns to build. Field notes are below. 🙂