~from the ever-glorious Wikipedia
“A factor that influences how easy or difficult a bike will be to ride is trail, the distance that the front wheel ground contact point trails behind the steering axis ground contact point. The steering axis is the axis about which the entire steering mechanism (fork, handlebars, front wheel, etc.) pivots. In traditional bike designs, with a steering axis tilted back from the vertical, trail causes the front wheel to steer into the direction of a lean, independent of forward speed.[5] This can be seen by pushing a stationary bike to one side. The front wheel will usually also steer to that side. In a lean, gravity provides this force.
The more trail a bike has, the more stable it feels. Bikes with negative trail (where the contact patch is actually in front of where the steering axis intersects the ground), while still ridable, feel very unstable. Bikes with too much trail feel difficult to steer. Normally, road racing bicycles have more trail than mountain bikes or touring bikes. In the case of mountain bikes, less trail allows more accurate path selection off-road, and also allows the rider to recover from obstacles on the trail which might knock the front wheel off course. Touring bikes are built with small trail to allow the rider to control a bike weighed down with baggage. As a consequence, an unloaded touring bike can feel unstable. In bicycles, fork rake, often a curve in the fork blades forward of the steering axis, is used to diminish trail.[10] In motorcycles, rake refers to the head angle instead, and offset created by the triple tree is used to diminish trail.[11]
Trail is a function of head angle, fork offset or rake, and wheel size. Their relationship can be described by this formula:[12]
where Rw is wheel radius, Ah is the head angle measured clock-wise from the horizontal and Of is the fork offset or rake. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the fork rake.
A small survey by Whitt and Wilson[5] found:
* touring bicycles with head angles between 72° and 73° and trail between 43.0 mm and 60.0 mm
* racing bicycles with head angles between 73° and 74° and trail between 28.0 mm and 45.0 mm
* track bicycles with head angles of 75° and trail between 23.5 mm and 37.0 mm.”
