Post date: Jul 26, 2014 4:57:49 AM
The following document has been compiled by David Brunckhorst (rev.7/2014)*. See the PDF attached to the Sustainable Trail Building page.
Singletrack mountain bike trails usually incorporate challenging features to ride over or around. Holding just the right amount of speed around a corner, without sliding out, is a fun and valuable skill. A whole lot of factors combine to make your tyres stick, or not! Some of these include the type of rubber and air pressure in your tyres, suspension, braking forces, the trail tread composition such as soil type, texture and how wet or dry it is. Ability to carry speed around a corner also depends on how tight the turn and whether it is out-sloping (or ‘off camber’) or in-sloping (bermed).
A berm is a banked and inwardly curved turn on a trail. Its side wall provides support for the rider when turning a corner allowing them to turn smoothly while leaning the bike. In-sloping or bermed turns allow riders to maintain speed without fear of sliding out. A blue graded berm will be generally open and shallow with more difficult berms encompassing tighter turns, steeper surfaces and higher entrance speeds. The speed at which a berm is ridden increases with rider skill and experience. Berms may also be used as a trail feature in themselves, with a series of berms on a descent being a typical feature of modern bike trails.
Planning and Design
What kind of cornering do you wish to achieve? Consider the type of terrain, soil texture and composition for the type of berm – do you want high traction, a little loose, highly water resistant? A berm can have an even curve, increasing radius (opening out) or decreasing radius (getting tighter). A wall ride is a berm at the extreme end of the scale of steepness and turn.
Ensure that a berm is continued around the bend to a point where the rider is able to exit safely on the line of the trail. The trail formation – height, tread width, angle of batter – needs to be increased for berms that are to be ridden at higher speeds or are on steep gradients. Don’t forget, drainage is most important. It is best to build berms on the down slope with reverse gradients or dips for drainage before and after. Water needs to be able to flow to the inside of the berm – from the bottom (apex) of the berm face, the tread is flatter but in-sloped to flow into drainage running around the inside of the curve. Avoid building berms in flat areas or across the downslope as they will pond water.
The earth works and riding surface of a berm need the solid support of a bank of rocks or solid earth behind. Remove all vegetation and organic matter first. Generally, a 20-45° bank should be formed in a rough semi-circle with an inverted dished (concave) face and, a suitable turning angle for the grade of trail and estimated rider speed. An excavator can be used to build up the bank, sitting behind the bank and dragging material into the basic shape and foundations of the berm. Early in construction, incorporate an appropriate drainage area on the inside of the berm leading into a dip at the lowest (endpoint) of the berm to get rid of water – consider rock armouring if the trail is in a wet climate.
Now you have the general shape and drainage in place, bring on the dirt. The aim is to build up layers of dirt, tamping down each layer by hand as you go. Mechanical ‘Whacker Packers’ can sometimes be used, but are generally too large and heavy, and can flatten the shape and angle (rather than retain dished shape) of the tread surface. Do not dump a pile of dirt where you want the berm to be and simply compact the top. It is only possible to achieve the necessary compaction by tamping in layers. Layer 10-15cm of dirt on your roughly shaped berm, then compact, layer and compact, layer again and compact. Repeat until you have your desired elevation of dirt and berm shape. Scruff the top of each layer so that the layer on top of it grabs on to it – very gentle wetting can also help ‘glue’ layers and harden the surface up.
When your layered, tamped dirt pile is as high, over your rough curve of backing foundations, as you think it should be, use a shaping tool (flat edge implement like a rake hoe / McLeod tool) to carve out final shape. Imagine yourself riding it, and take some test runs on foot. If the berm feels too tight, you might need to pull the bottom side out a bit; or if the transition into the turn is too abrupt, you might need a longer run-in. Try to avoid creating ‘flat’ or straight spots. With any berm, remember to build a slight reversal in grade before and after the turn to sheet any excess water off the trail. This also allows riders coming out of the corner too ‘hot’ to scrub some speed and will help maintain the flow of the trail, as well as the integrity of the turn.
Sustainable Trail Building
Post date: Jun 2, 2014 8:06:18 AM
The following information was put together by David Brunckhorst to assist NEMTB and other trail builders to produced sustainable mountain bike trails.
The two documents are attached as PDF files below.