Bicycle and motorcycle geometry

Bicycle and motorcycle geometry is the collection of key measurements (lengths and angles) that define a particular bike configuration.

[1] 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.

For example, Lemond[2] 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).

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

For example, Moto Guzzi[6] offers: The fork offset is the perpendicular distance from the steering axis to the center of the front wheel.

Wilson and Papodopoulos [citation needed] argue that mechanical trail may be a more important and informative variable,[15] although both expressions describe very nearly the same thing.

As telescopic forks compress due to load transfer during braking, the trail and the wheelbase both decrease.

[19] Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground.

Wheel flop is caused by the lowering of the front end of a bicycle or motorcycle as the handlebars are rotated away from the "straight ahead" position.

This lowering phenomenon occurs according to the following equation: where: Because wheel flop involves the lowering of the front end of a bicycle or motorcycle, the force due to gravity will tend to cause handlebar rotation to continue with increasing rotational velocity and without additional rider input on the handlebars.

Bicycle Quarterly magazine states, "A bike with too little wheel flop will be sluggish in its reactions to handlebar inputs.

A bike with too much wheel flop will tend to veer off its line at low and moderate speeds.

Increasing the length of the fork, for example by switching from rigid to suspension, raises the front of a bicycle and thus decreases its head angle.

[26] The state of North Dakota (USA) has minimum and maximum requirements on rake and trail for "manufacture, sale, and safe operation of a motorcycle upon public highways.

For motorcycles the other main geometric parameters are seat height and relative foot peg and handlebar placement.