Road Bike Forks

Offset
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.

Length
The length of the fork is usually measured parallel to the steer tube from the bottom of the lower bearing race to the center of the front wheel axle.

Steer Tube Length
The steer tube is sized either to just accommodate the headset bearings, in the case of a threaded headset, or to contribute to the desired handlebar height, in the case of a threadless headset.

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. The blades, 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.

Aero Forks
Aero forks are made to be very stiff and to minimize wind resistance. Because time trials and triathlon events focus on speed over a relatively short distance, the overall ride-ability of the fork doesn't matter quite so much as wind resistance and efficiency.

Road Forks
This category accounts for about 80% of all the forks on the market. They are meant to be lightweight and provide a comfortable ride over a long distance (as opposed to an aero fork).

Cyclocross Forks
Due to the rigors of crossing mixed terrain, cyclocross forks are usually stronger and heavier than their normal counterparts. Furthermore, a cyclocross fork is made to incorporate a bigger tire than a normal road brake fork, and they often have the necessary break bosses to incorporate a MTB style rim brake instead of the standard road caliper brake.