In this article, we use the words "twists" and "turns" to describe gentle curves or bends in the trail, not full-fledged changes in direction such as switchbacks.
Flow and Rhythm are key considerations on trails popular with cyclists. The experience of riding a trail where one turn leads into the next, and every descent leads into the next rise, creates a rhythm that mountain bikers love. A trail with even flow tends to be more durable, because riders are not forced to jam on their brakes or miss turns, which can cause tread damage and trail widening.
Three Types of Trail Flow:
- Open and Flowing. These trails are relatively gentle, featuring sweeping turns, smooth surfaces, wider tread, long sight lines, and fewer technical challenges. They appeal to less-skilled riders and those who enjoy the sensation of speed.
- Tight and Technical. These routes are more difficult, with sharper turns, rougher surfaces, and a narrower tread. They provide mountain bikers with challenge while keeping speeds low, which can decrease user conflict.
- Hybrid. This type of trail blends "open and flowing" with "tight and technical." These trails may be wider, yet incorporate a technical tread. Trees, brush and obstacles are generally below eye level, allowing for long sight lines. Slightly wider trails allow users to easily pass each other while the addition of technical challenges keep speeds down.
Gradual transitions are essential between changes in trail flow. Abrupt transitions are likely to produce skidding, braking bumps and may even force users off trail.
Flat Terrain. We recommend locating trails on sideslopes whenever possible because flat areas accumulate water. However, if flat terrain is all that's available, twists and turns are great ways to add challenge. Note: serpentine twisty trails can seem artificial or contrived and people may shortcut sharp turns if no obstacle blocks their way. Use trees, bushes, rocks and other natural features to prevent shortcuts. Extremely turny trails won't be successful in open grasslands or forests - the temptation for shortcutting is too great.
Sideslopes. There are two types of turns along a variable sideslope (a hillside with protrusions and indentations). Inside turns are concave - the hillside helps keep the rider on the trail by providing a berm. Outside turns are convex and more difficult to ride because they feel like they're off-camber and centrifugal force pulls the rider outside and away from the trail. Compensate for these forces by designing smaller radius (sharper) turns on inside corners and larger radius (gradual) turns on outside corners.
Ups and Downs. A turny trail traversing an undulating hillside should include ups and downs. This will improve drainage and help manage rider speed. Design slight downhills leading into inside turns - it is essential to cross low-lying drainage features with a grade reversal. The trail should gradually descend into drainages and then gradually climb up out of them. Ascend to the apex of outside turns - a rise in the trail to an outside turn will slow the rider and allow them to more easily negotiate the turn.
Surf the Contour. When designing a trail across a featureless hillside, include twists and turns to avoid long straightaways. Subtle curves will create gentle ups and downs that add interest and control rider speed.
Testing Your Design. Mark the proposed route with pin flags placed every three feet for tight and twisty trails to help clearly define the route. For open and flowing trails, place flags every six feet. Run the trail in both directions to sample the flow. The route must flow well at a variety of speeds.
IMBA's books offer our most comprehensive advice on trail building and other topics. Consider picking up copies of Trail Solutions: IMBA's Guide to Building Sweet Singletrack and Managing Mountain Biking: IMBA's Guide to Providing Sweet Riding at IMBA's online store.