The fork offset is the perpendicular distance from the steering axis to the center of the front wheel.
In bicycles, fork offset is also called fork rake. Road racing bicycle forks have an offset of 40-45mm.
Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock. Most fatigue failures of forks result in a fork blade breaking at the rear edge of the fork crown from repeated vertical road shocks. Before most roads were paved, fork rake had a lower angle so the fork would be loaded axially on rougher surfaces. As most roads became paved, bicycles forks were made steeper, which also gave lighter steering.
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. Other, less-common motorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms.
Read more about this topic: Bicycle And Motorcycle Geometry
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