bike - Bicycle and motorcycle geometry
Photograph by Velo Steveon Flickr.
For example, Moto Guzzi offers: The fork offset is the bike Single-speed bicycle perpendicular distance from the steering axis to the center of the front wheel. In bicycles, fork offset is also called fork rake. Their relationship can be described by this formula: where Rw wheel radius, Ah is the head angle measured clock-wise from the horizontal and Of is the fork offset or rake.
The state of North Dakota (USA) actually has minimum and maximum requirements on rake and trail for manufacture, sale, and safe operation of a motorcycle upon public highways. 4. A 90° head angle would be vertical.
A 0° rake would be vertical. These parameters have a major influence on how a bike handles. Wheelbase is the horizontal distance between the centers (or the ground contact points) of the front and rear wheels.
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. The terms rake and offset became confused when bicyclists misunderstood 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. Bicycle and motorcycle geometry is the collection of key measurements (lengths and angles) that define a particular bike configuration.
For motorcycles the other main geometric parameters are seat height and relative foot peg and handlebar placement. . Wheelbase is a function of rear frame length, steering axis angle, and fork offset.
The term rakish angle means steep, rather than that the fork has a curl at its end. Note that, on a bicycle, as rake angle increases, head angle decreases. Trail can vary as the bike leans or steers.
All motorcycles, except three-wheel motorcycles, must meet the following specifications in relationship to front wheel geometry: Manufacturer s specifications must include the specific rake and trail for each motorcycle or class of motorcycles and the terms rake and trail must be defined by the director by rules adopted pursuant to chapter 28-32. For other aspects of geometry, such as ergonomics or intended use, see the Bicycle frame article. 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.
For example, Lemond offers: 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. In the case of traditional geometry, trail decreases (and wheelbase increases if measuring distance between ground contact points and not hubs) as the bike leans and steers in the direction of the lean. Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground.
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. Most fatigue failures of forks result in a fork blade breaking at the rear edge of the fork crown from repeated vertical road shocks.
It may also be referred to as normal trail. Although the scientific understanding of bicycle steering remains incomplete, Forks may be modified or replaced, thereby altering the geometry of the bike. Increasing the length of the fork, for example by switch from rigid to suspension, raises the front of the bike and decreases the head angle. Most bikes have positive trail, though a few, such as the Python Lowracer have negative trail. Trail is often cited as an important determinant of bicycle handling characteristics , and is sometimes listed in bicycle manufacturers geometry data, although Wilson and Papodopoulos argue that mechanical trail may be a more important and informative variable. Trail is a function of head angle, fork offset or rake, and wheel size.
The steering axis angle usually matches the angle of the head tube. 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. Other, less-common motorcycle forks, such as trailing link or leading link forks, can implement offset by the length of link arms. The length of a fork is measured parallel to the steer tube from the lower fork crown bearing to the axle center. Trail, or caster, is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground.
Primary among these are wheelbase, steering axis angle, fork offset, and trail. Short bikes are much more likely to perform wheelies and stoppies. The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention.
The larger the rake angle the larger the trail. It is similar to the term wheelbase used for automobiles and trains. 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.
The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. Trail can be increased by increasing the wheel size, decreasing or slackening the head angle, or decreasing the fork rake or offset.
Today, some fork blades are straight, having their offset introduced by an angled fork crown. Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock. Before most roads were paved, fork rake had a lower angle so the fork would be loaded axially on rougher surfaces.
