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Bicycle and motorcycle geometry is the collection of key measurements (lengths and angles) that define a particular bike configuration. Primary among these are wheelbase, steering axis angle, fork offset, and trail. These parameters have a major influence on how a Bike Handles . WHEELBASE Wheelbase is the ''horizontal'' distance between the centers (or the ground contact points) of the front and rear wheels. Wheelbase is a function of rear frame length, steering axis angle, and fork offset. 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 gravity. Short bikes are much more likely to perform Wheelie s and Stoppie s. STEERING AXIS ANGLE 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 '''steering axis''' is the Axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots.
FORK OFFSET 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''. Virtually all road racing Bicycle Fork s have an offset of 43-45mm due to the almost-standard frame geometry and 700c wheels, so racing forks are widely interchangeable. TRAIL Trail is the ''horizontal'' distance from where the Steering Axis intersects the ground to where the front wheel touches the ground. The measurement is considered ''positive'' if the front wheel ground contact point is behind (towards the rear of the bike) of the steering axis intersection with the ground. Trail is often cited as an important determinant of bicycle handling characteristics and is sometimes listed in bicycle manufacturers' geometry data, although [http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10063 Wilson and Papodopoulos argue that Mechanical Trail may be a more important and informative variable. : where wheel radius, is the head angle measured clock-wise from the horizontal and 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 Or Offset . Trail decreases as head angle increases (becomes steeper), as fork offset increases, or as wheel diameter decreases. Motorcyclists tend to speak of trail in relation to Rake Angle . The larger the rake angle the larger the trail. Note that, on a bicycle, as rake angle increases, head angle decreases. MECHANICAL TRAIL Mechanical trail is the ''perpendicular'' distance between the Steering Axis and the point of contact between the front wheel and the ground. It may also be referred to as ''normal trail''. LEGAL REQUIREMENTS 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 "4. All motorcycles, except three-wheel motorcycles, must meet the following specifications in relationship to front wheel geometry: :MAXIMUM: Rake: 45 degrees - Trail: 14 inches centimeters positive :MINIMUM: Rake: 20 degrees - Trail: 2 inches centimeters positive 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." OTHER ASPECTS For other aspects of geometry, such as ergonomics or intended use, see the Bicycle Frame article. For Motorcycle s the other main geometric parameters are seat height and relative foot peg and handlebar placement. REFERENCES SEE ALSO EXTERNAL LINKS |
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