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.

The 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, along with the position of the center of mass of the combined bike and rider, on the longitudinal stability of a bike: a short wheelbase and high center of mass increase the likelihood of pitching over the front the rear wheel when braking, or lifting the front wheel off the ground when accelerating. Short bikes are much more suitable for performing wheelies and stoppies.

The steering axis angle is called caster angle when measured from vertical axis or head angle when measured from horizontal axis. 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 measured from the horizontal and called the head angle; 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 geometry setting).

At least one manufacturer, Cane Creek, offers an after-market threadless headset that enables changing the head angle.

In motorcycles, the steering axis angle is measured from the vertical and called the caster angle, rake angle, or just rake; a 0° rake is therefore vertical. For example, Moto Guzzi offers: