Bicycle and motorcycle geometry

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 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.

The steering axis angle, also called caster angle or head 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 offers:

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 62° (depending on frame size).

At least one manufacturer, Cane Creek, offers an after-market threadless headset that enables changing the steering axis angle. 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. A 0° rake would be vertical. For example, Moto Guzzi offers:

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