BICYCLE FORK

A 'bicycle fork' is the portion of a bicycle that holds the front wheel and allows one to steer. A fork consists of two dropouts which hold the front wheel axle, two ''blades'' which join at a fork ''crown'', and a ''steerer'' or ''steering tube'' to which the handlebars attach (via a stem) allowing the user to steer the bicycle. The steerer of the fork interfaces with the frame via a set of bearings known as a headset.

Contents

Offset

Suspension

Materials

Sizes

Threading

Attachment points

See also

External links

Offset

Bicycle forks usually have an offset, or rake (''not to be confused with a different use of the word rake in the motorcycle world'' [1]), 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', 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, 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.
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.

Suspension

Main articles: bicycle suspension

On most mountain bicycles, the fork contains a set of shock absorbers. The suspension travel and handling characteristics vary depending on the type of mountain biking the fork is designed for. For instance, manufacturers produce different forks for cross-country (XC), downhill, and freeride riding. Forks designed for XC racing are typically lighter, less robust and have less suspension travel than those designed for rougher terrain and more extreme conditions.
The shock absorber usually consists of two parts: a spring and a damper or dashpot. The spring may be implemented with a steel or titanium coil, an elastomer, or even compressed air. The damper is usually implemented by forcing oil to pass through one or more small openings or shim stacks. On some models, the spring, the damper, or both may be adjusted for rider weight, riding style, terrain, or any combination of these or other factors. The two components may be separated with the spring mechanism in one blade and the damper in the other.
Some manufacturers, especially Cannondale, have tried other variations including a single shock built into the steering tube above the crown and a fork with just a single blade that has a shock built into it. Others have marketed suspension forks that employ linkages to provide the mechanical action instead of relying upon telescoping elements.

Materials

Forks have been made from steel, aluminum, carbon fiber, titanium, magnesium, and their combinations. For example, a fork may have carbon fiber blades with an aluminum crown, steer tube, or dropouts.
In rigid forks the material, shape, weight, and design of the forks can noticeably affect the feel and handling of the bicycle. Carbon fiber forks are popular in road bicycles because they are light, and also because they can be designed to lessen and absorb vibrations from the road surface.

Sizes

When sizing a fork to a frame, the diameter of the fork steerer or steer tube (1" or 1 1/8" or 1 1/2") 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 1/8" fork in a frame designed for a 1 1/2" steer tube. 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, Specialized25mm thru-axle and Cannondale's Lefty system.

Threading

Fork steer tubes may be threaded or unthreaded, depending on the headset used to attach the fork to the rest of the bicycle frame. An unthreaded steel steer tube may be threaded with an appropriate die if necessary.

Attachment points

Forks may have attachment points for brakes, racks, and fenders. These may be located in the crown, along the blades, and near the dropouts. These are often holes, threaded or not, and may be located on tabs that protrude.

See also

★ Bicycle

★ Bicycle frame

★ Bicycle and motorcycle dynamics

★ Bicycle and motorcycle geometry

★ Bicycle suspension

★ Fork tube

★ Motorcycle fork

★ Stem (bike)

External links

★ Adapter for bicycle forks, article

★ Bicycle Fork Lengths