Bicycle fork

A bicycle fork is the part of a bicycle that holds the front wheel.

A fork typically consists of two blades which are joined at the top by a fork crown. Above the crown, a steerer tube attaches the fork to the bicycle and the handlebars (via a stem) allowing the user to steer the bicycle. The steerer tube of the fork interfaces with the frame via bearings called a headset mounted in the head tube.

At the bottom of the fork, dropouts hold the wheel. Usually, either the axle is bolted to the fork, or a skewer passes through a hollow axle, clamping the axle to the fork.

The term fork is sometimes also used to describe the part of a bicycle that holds the rear wheel, which on 19th Century ordinary or penny-farthing bicycles was also a bladed fork. On most modern bicycle designs the rear wheel is now attached to a rear triangle made up of multiple triangulated tubes, rather than an actual fork, but the rear fork usage persists.

Forks have several key dimensions which include: offset, length, width, steerer tube length, and steerer tube diameter.

Bicycle forks usually have an offset, or rake (not to be confused with a different use of the word rake in the motorcycle world), that places the fork ends forward of the steering axis. This is achieved by curving the blades forward, angling straight blades forward, or by placing the fork ends forward of the centerline of the blades. The latter is used in suspension forks that must have straight blades in order for the suspension mechanism to work. Curved fork blades can also provide some shock absorption.

The purpose of this offset is to reduce 'trail', the distance that the front wheel ground contact point trails behind the point where the steering axis intersects the ground. Too much trail makes a bicycle feel difficult to turn.

Road racing bicycle forks have an offset of 40-55mm. For touring bicycles and other designs, the frame's head angle and wheel size must be taken into account when determining offset, and there is a narrow range of acceptable offsets to give good handling characteristics. The general rule is that a slacker head angle requires a fork with more offset, and small wheels require less offset than large wheels.

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Wikipedia