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Wide Singletrack With Flow Elements: Either hand built or machine built, this style blends traditional trail construction with frequent bike-optimized elements. These trails are usually 3-4 feet wide. Dirt can be harvested from borrow basins or imported to the site to facilitate raising and tilting the tread. This style of trail is well suited for somewhat steep to lower-angled hillsides, usually in the range of 20 to 35 percent grades. Often, but not always, signed for one-way and/or bike-preferred use.
Full-Featured Gravity/Flow Trails: Created with machine-built construction in most cases, the corridors for these trails are typically 4-7 feet wide. Side slopes are typically 10 to nearly 30 percent, resulting in average trail grades in the range of 5 to 8 percent—that’s plenty steep for big-time air and high speeds with minimal braking. There are several modern-day classics to emulate, such as Whistler’s A-line; Coldwater, Alabama’s Sparkleberry; or the Vapor trail at Snowmass Mountain in Colorado. These trails are usually most successful when signed for one-way and/or bike-preferred use.
Out-slope to In-slope: The shift from out-sloped to in-sloped trail tread is a good place to start such a discussion. Traditional rolling contour trails, the kind that IMBA’s first two books focus on, are built with a constantly out-sloped tread to allow water to sheet off the trail surface—this remains a very legitimate trail building technique.
However, heavy trail use tends to degrade that out-sloped tread over time, as both users and natural forces steadily displace the soil. Water will eventually channel down the tread, causing gullies and cupping. Instead of continuous out-sloping, we have learned that a combination of out-sloping and in-sloping—along with pronounced grade dips—will greatly extend the life of the trail.
From a rider’s perspective, replacing constantly out-sloped features with ones that are in-sloped (usually called “bermed” by riders) changes the riding experience. Even a few degrees of in-sloping, especially when applied to wide-radius turns, rewards mountain bikers with a swooping sensation and the ability to dramatically lean their bikes toward the center of the turn.
Similarly, grade reversals can be built so the dips are out-sloped and the crests are in-sloped, allowing the rider to