Bicycle forks usually have an offset, or rake, that places the dropouts
forward of the steering axis. This is achieved by curving the blades
forward, angling straight blades
forward, or by placing the dropouts
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', which is 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.
Virtually all road racing bicycle forks have an offset of 43-45mm due to the almost standard frame
geometry and 700c wheels,
so racing forks are widely interchangeable. For touring bicycles and other designs, the frame's head angle and wheel
size must be taken into account when determining offset (keep in mind that 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.
Fork offset influences geometric trail, which affects a bicycle's handling characteristics. Increasing offset results in decreased trail, while decreasing offset results in increased trail.
Steer Tube Diameter
When sizing a fork to a frame,
the diameter of the fork steerer or steer tube (1" or 1 1/8") must not be larger than that of the frame,
and the length of the steerer tube should be greater than but approximately equal to the head tube
length plus the stack height of the headset.
Adapter kits are available to enable use of a 1" fork in a frame
designed for a 1 1/8" steer tube or a 1 1/8" fork in a 1 1/4" frame.
of course, must be the proper length to both accommodate the desired wheel
and provide the approximate steering geometry intended by the frame
designer. The functional length of the fork is typically expressed in terms of Axle-to-Crown race length (A-C). Also, the axle
on the wheel
must fit in the fork dropouts
(usually either a 9mm solid or hollow axle,
or a 20mm thru-axle). Some manufacturers have introduced forks and matching hubs
with proprietary standards, such as Maverick's 24mm axle,
Specialized's 25mm thru-axle and Cannondale's Lefty system.