Rake and trail

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Rake and trail are two of the primary measurements that make up a motorcycle's frame geometry.^[1]

1 Rake
2 Trail
3 Offset
4 Modifications
5 References
6 See also

[edit] Rake

Rake, or castor angle, is the angle of the steering head in relation to a perpendicular line going from the ground through it. (It dictates the angle of the forks to the ground as well, since they are usually parallel to the steering head.) A steeper angle yields a shorter wheelbase, a larger angle yields the chopper-style longer wheelbase.

[edit] Trail

The trail dimension is determined by drawing an imaginary line through the steering axis to the ground, dropping a perpendicular line from the front axle to the ground (which usually falls some distance behind the first line), then measuring the distance between these two points (the name arises because the drop point "trails" the steering axis point).^[2]

Trail ultimately defines the handling characteristics of a motorcycle, how stable it will be at higher speeds, how easy it will flick through S-turns, and how easy it will be to control at sub-idle parking lot maneuvers. Bikes with shorter trail will be very easy to handle at slow speeds and quite responsive when navigating S-turns, but at higher speeds these bikes will be more vulnerable to the road conditions and feel more unstable. On the other hand, bikes with longer trail dimensions may handle like a wheel barrow in the parking lot, require a bit more encouragement to tackle an S curve, but track straight and true like an arrow at freeway speeds, offering little response to the road until the handlebars are activated by the rider. The trick for the motorcycle designer is to find some happy medium for the intended design.^[3]

[edit] Offset

Typically bikes with greater rakes have longer trail dimensions. However, the design of the triple trees is what eventually defines the final trail dimension. Offset is measured by drawing an imaginary line from the center of one fork tube to the other, then dropping a perpendicular line from the center of that line to the center of the steering stem. The distance of this perpendicular line is the triple tree offset. What this does is position the front axle to a point where the trail can be made to a suitable position. This means that the closer the forks are to the steering stem (shorter offset) the longer the trail can be modified, as the axle is being positioned further back from the steering axis to ground contact point. Conversely, the farther apart the forks are from the steering stem (longer offset) the shorter the trail, as the axle is positioned closer to the steering axis line.).^[4]

[edit] Modifications

A certified motorcycle mechanic should be involved in any major design changes while altering a factory motorcycle such as a Harley-Davidson, or while designing a custom motorcycle such as Jesse James' West Coast Choppers. Altering the rake and trail of a factory motorcycle may be needed when creating a chopper.^[5]

To change the rake on a motorcycle, the designer may also change the trees to raked trees. Raked trees are designed so the lower tree sticks out further than the upper tree, thus increasing the rake of the forks in relation to the steering head rake. What this does, for those still following along all the imaginary lines, is position the axle closer to the frame rake measurement line, or shortening the trail. Thus, when adding raked trees to a raked frame (which sports a longer trail), the trail is shortened to a more manageable level. However, adding raked trees to a frame with short rake and trail can be hazardous, as shortening an already short trail measurement can lead to an unstable situation as speed increases. Misuse of raked trees can be quite dangerous, so a bit of research is in store before turning the first wrench on any chopper project.