![]() The relationship between head angle, rake and trail in a bicycle Most bikes have positive trail, though a few, such as the Python Lowracer have negative trail. The measurement is considered positive if the front wheel ground contact point is behind (towards the rear of the bike) the steering axis intersection with the ground. Trail, or caster, is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground. The length of a fork is measured parallel to the steer tube from the lower fork crown bearing to the axle center. Other, less-common motorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms. In motorcycles with telescopic fork tubes, fork offset can be implemented by either an offset in the triple tree, adding a rake angle (usually measured in degrees from 0) to the fork tubes as they mount into the triple tree, or a combination of the two. As most roads became paved, bicycles forks were made steeper, which also gave lighter steering. Before most roads were paved, fork rake had a lower angle so the fork would be loaded axially on rougher surfaces. ![]() Most fatigue failures of forks result in a fork blade breaking at the rear edge of the fork crown from repeated vertical road shocks. Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock. Today, some fork blades are straight, having their offset introduced by an angled fork crown. The term "rakish angle" means steep, rather than that the fork has a curl at its end. With the bare fork in hand, rake is undefined, although one might believe the term refers to similarity with a leaf rake whose tines curl in a similar manner. The terms "rake" and "offset" became confused when bicyclists misunderstood the reason for the curl at the fork end, shown in the adjoining diagram, believing its purpose was shock absorption. Virtually all road racing bicycle forks have an offset of 43-45mm due to the almost-standard frame geometry and 700c wheels, so racing forks are widely interchangeable. In bicycles, fork offset is also called fork rake. The fork offset is the perpendicular distance from the steering axis to the center of the front wheel. a 2007 Nevada Classic 750 with a rake of 27.5° (27.5 degrees). ![]() In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. ![]() a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size.In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. The steering axis angle usually matches the angle of the head tube. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention. Telescopic forks on a BMW motorcycle reveal the head angle or rake File:Chopper with long rake.JPGĮxample of a chopper with an unusually large rake Short bikes are much more likely to perform wheelies and stoppies. Wheelbase has a major influence on the longitudinal stability of a bike, along with the height of the center of mass of the combined bike and rider. It is similar to the term wheelbase used for automobiles and trains. Wheelbase is a function of rear frame length, steering axis angle, and fork offset. Wheelbase is the horizontal distance between the centers (or the ground contact points) of the front and rear wheels.
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