1. Definition & Purpose
- A berm is made to help riders change direction. The fact that it is raised produces support to lean the bike on and helps cornering with smaller radius and higher speed.
- Well-designed berms allow riders to maintain speed and momentum between different sections of trail.
- In addition to changing direction, smaller radius corners can also be built to add fun or control speed (see speed management features)
- Every berm must be built according to 1) the user level 2) the section’ speed
- The higher the intended level and entering speed is, the more support the berm should offer. Support depends on ground tilt and texture.
- Adapting the berm to the situation is essential, There are 3 parameters that can be influenced: the radius, the height and the length of the berm.
- Change in direction often involves reducing speed. Which means breaking, ground erosion and breaking bumps. Every turn must be carefully planned with a design that allows one to naturally loose speed before reaching the entrance. (See anatomy of a berm).
- Water management can be tricky when planning and building turns. Always consider trail gradient and the 7 rules for a sustainable trail (see sustainable trail basics chapter).
- For building details, refer to the building tips below
2. Anatomy of a berm
Set up feature
The set-up feature is:
- uphill or concave to reduce the speed and offer best braking control
- uphill or concave to drain the water out before the turn
- inslopped to send the rider on the outside of the turn for an ideal trajectory
The entrance is a gentle outslope part that prepares the rider to lean for the apex. It is also where the water coming from the land above will travers to join the inside of the turn. If the setup feature isn’t big enough, this is where braking bumps will occur.
The apex is the part where the track changes direction and this is the place where the G-forces are the biggest. The curve must be steep and high to properly retain the rider. Because of the steepness and the forces applied at this point, it is also the most fragile place. If damages are too consequent, consider reducing the entry speed or increasing the turn radius.
Exit / Tail
The tail of a berm is what will guide the rider on a good line for the following of the trail. It must be inslopped and high enough long enough to provide support as long as riders aren’t fully aligned with the next section. This is a critical part to build as it often needs a retaining wall to support the building material against the terrain slope.
3. Key Considerations
3.1 Typical Characteristics
180° Turns can be easily damaged by water if built incorrectly. In steep terrain, when reaching the fall line, the grade can get steeper and water erosion can start. Big size berms can as well cover large surfaces and accumulate a lot of rain. That’s why it is important to exit the water right before and after every turn. In hard packed terrain where water could run down the hill side, a crossing drain (in orange) can be dug right above the entrance and apex to protect from sheet flow.
Find additional images for teaching practices in this media folder
- Felton, V. (2004). Trail Solutions: IMBA’s Guide to Building Sweet Singletrack (International Mountain Bicycling Association)
7. Further reading
- IMBA Trail Solutions p 245; Guidelines for a quality trail experience p. 25, 28