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# Bicycle and motorcycle geometry

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 Title: Bicycle and motorcycle geometry Author: World Heritage Encyclopedia Language: English Subject: Collection: Motorcycle Dynamics Publisher: World Heritage Encyclopedia Publication Date:

### Bicycle and motorcycle geometry

Bike wheelbase, head angle, fork offset (AKA rake), and trail

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.

## Contents

• Wheelbase 1
• Steering axis angle 2
• Fork offset 3
• Fork length 4
• Trail 5
• Mechanical trail 6
• Wheel flop 7
• Modifications 8
• Changing fork length 8.1
• Changing fork offset 8.2
• Legal requirements 9
• Other aspects 10
• References 12

## 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 mass of the combined bike and rider. Short bikes are much more suitable for performing wheelies and stoppies.

## Steering axis angle

Telescopic forks on a BMW motorcycle reveal the steering axis angle, also called the rake angle
Example of a chopper with an unusually large rake 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. 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 from the horizontal. A 90° head angle would be vertical. For example, Lemond[1] offers:

• a 2007 Filmore, designed for the track, with a head angle that varies from 72.5° to 74° depending on frame size
• a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25° to 74°, depending on frame size.

Due to front fork suspension, modern Mountain Bikes as opposed to Road Bikes, tend to have slacker head tube angles, generally around 70° although they can be as low as 68° (depending on frame size).[2]

At least one manufacturer is offering after-market headsets that enable changing the steering axis angle.[3] When all else remains the same, this alters the trail of the bicycle.

In motorcycles, the steering axis angle is called the rake angle or just rake and is measured from the vertical.[4] A 0° rake would be vertical. For example, Moto Guzzi[5] offers:

• a 2007 Breva V 1100 with a rake of 25°30’ (25.5 degrees)
• a 2007 Nevada Classic 750 with a rake of 27.5° (27.5 degrees)

## 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. Road racing bicycle forks have an offset of 40–45 mm (1.6–1.8 in).[6]

The offset may be implemented by curving the forks, adding a perpendicular tab at their lower ends, offseting the fork blade sockets of the fork crown ahead of the steerer, or by mounting the forks into the crown at an angle to the steer tube. The development of forks with curves is attributed to

• Motorcycle Steering Geometry
• Bicycle Fork Lengths
• Illustration of a dimension that is sometimes referred to as caster angle on bicycles.
• BIKE GEOMETRY PROJECT
• Motorcycle rake, trail and offset explained