This story originally appeared in the summer 2015 issue of IMBA Trail News.
IMBA published Trail Solutions: IMBA’s Guide to Building Sweet Singletrack in 2004, and followed with Managing Mountain Biking: IMBA’s Guide to Providing Great Riding in 2007. Those books had a profound influence on trail design and construction, helping thousands of trail builders, bike advocates and land managers learn how to create sustainable trails.
Just as mountain bike technology has continued to evolve, so, too, have trail building techniques. In 2014, IMBA published Bike Parks: IMBA’s Guide to New School Trails, which is already helping create the next generation of bike-specific facilities. The emphasis in Bike Parks, however, is on organizing, funding and overseeing bike park efforts. Unlike IMBA’s first two books, it does not go into great detail about the specific design and construction methods that produce bike-optimized trails.
This article is meant to help fill that gap. Of course, a few pages in Trail News are not a sufficient resource to convey all the knowledge required to create a masterpiece in dirt. But it is quite possible for builders who have gained skill and confidence with the trail building advice in IMBA’s first two books to adapt and expand their trail building repertoires.
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, 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 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 shift her weight up and down, and from inside to outside, to enhance momentum—a sensation similar to riding a pumptrack.
As these features become more pronounced, the trail tends to feel increasingly bike-specific, both in terms of aesthetics and ride qualities. A trail with high-bermed turns and frequent grade reversals that are big enough to launch fast-moving riders into the sky is recognizable to riders and non-riders alike as a bike-specific trail design. The most common term for this building style in mountain bike circles is “flow trail.”
Lifted and Tilted
If the desired outcome is a strongly bike-flavored trail, the emphasis will shift toward bigger features and more pronounced in-sloping. These “lifted and tilted” builds require lots of dirt to create the oversized features, and may require additional rockwork to contain the soil and ensure it stays in place.
It is entirely possible to create this type of trail with hand tools and human power. IMBA’s Trail Solutions professional trail building team has recently built trails in China and Mexico where power machinery was unsuitable, but skilled and strong human labor got the job done. However, there is no doubt that mechanized trail building equipment offers a timesaving advantage for flow trail construction.
The skill of machine operators, not the size of the equipment, has the most influence on the end result. While it is true that smaller machines may produce a narrower trail bed with more of a natural singletrack feel, it’s also true that bigger machines are often required for large-scale projects.
Finding the right location for this type of trail is an important consideration. As the angle of the side slope increases, the degree of “lifting” required—in other words, the amount of earthworks required to create big berms and jump-friendly grade reversals—increases substantially. The recently completed Paradise Royale trail in Northern California features segments where the side slopes range from 50- to 70-percent grades. Due to the steepness of the hillside, an enormous effort and massive amounts of soil were needed.
“When I analyze a site and start planning a trail system, I try to use the steep side slopes for climbing trails or shared-use segments,” said Joey Klein, veteran trail designer and builder with IMBA’s Trail Solutions. “More gently angled side slopes, from five to, at most, 40 percent, are the best match for either entry-level singletrack or gravity, flow and downhill trails.”
“It’s counterintuitive that gently angled hillsides are the best places to build both beginner trails and gravity trails,” continues Klein. “But when we are trying to boost riders into the air we do not need super-steep grades. To create big turns, fun jumps and happy landings we want a consistent, moderate trail grade matched with enough side slope for proper water drainage.”
Maintainable Or Sustainable?
There’s no doubt that bike-optimized trails featuring in-sloped tread, abundant rollers and high berms are a hoot to ride. When a new gravity or flow trail opens to the public, the response is usually rampant enthusiasm and a big increase in bike traffic—with all ability levels taking to the trail to experience something new and different. This often leads to erosion, particularly because inexperienced riders need to refine their braking, cornering and jumping techniques.
“There’s nothing inherently more or less sustainable about lifted and tilted trails compared to traditional singletrack,” says Klein. “But if a trail gets super popular because it’s so fun to ride, and at the same time riders are sliding through turns and constantly hitting their brakes, there’s definitely going to be some significant soil loss. I tell local groups to expect this, especially in the first few seasons, and to be prepared to perform a relatively high volume of maintenance, including patching berms and smoothing out the trail tread.”
In this sense, bike-optimized trails can be thought of as maintainable, rather than self-sustaining. (Of course, all trails require some degree of regular maintenance.) But the trade-offs are often well worth the increased load on volunteer maintenance crews. In New Mexico, IMBA's chapter the Santa Fe Fat Tire Society recently built a new flow trail called Hustle and Flow—with assistance from Klein—that is earning positive feedback in the community.
“The trail required a huge effort and pushed our volunteer trail crews to their limits,” said Tim Fowler, the chapter president. “We probably underestimated the time required to build all the berms and jumps. Still, there are no regrets. The end result is fantastic—it was especially gratifying to see high school-aged riders showing up to help build the features, and getting excited about a new style of trail.”
Teddy Jaramillo and Lewis Starvrowsky, both 16 years old, are two of the young riders who joined the work crews. “I’m super stoked to see more people getting into flow trails and having them available in our community,” said Jaramillo, who is a four-time state champion in BMX and the reigning national champion for the 15-18 age group in category 3 downhill racing. “I will definitely join our local IMBA chapter in Santa Fe and help with future trail projects.”
New Trail Types
Singletrack With Out-Sloped and In-Sloped Tread: Typically hand built and from 1.5 to 3 feet wide, these trails closely resemble traditional trail construction, but add bike-optimized flavor by out-sloping the dips and in-sloping the crests of grade reversals. Turns can be in-sloped on the flats but may be out-sloped for better drainage when diving into gullies. Side slopes of 40 percent or less are best—steeper slopes require lots of earthworks to build turning platforms. This building style works well for both shared-use and single-use trails, generally with two-way traffic.
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-prefered 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 bigtime 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-prefered use.