A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. 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 mounted in the head tube.
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 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.
The length of the fork is usually measured parallel to the steerer tube from the bottom of the lower bearing race to the center of the front wheel axle.
The width of the fork, also called spacing, is measured colinear with the front wheel axle between the inside edges of the two dropouts. Most modern adult sized forks have 100 mm spacing. Downhill mountain bike forks designed for through axles have 110 mm spacing.
The steerer 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.
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" fork in a frame designed for a 1 1/8" steer tube or a 1 1/8" fork in a 1 1/2" 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 25mm thru-axle and Cannondale's Lefty system.
On most mountain bicycles, the fork contains a set of shock absorbers, in which case the blades typically consist of upper and lower telescoping tubes and are called "legs." 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.
Popular makers of suspension forks include Cannondale, Fox, Manitou, Marzocchi, and RockShox.
Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated and diverse in design. The amount of suspension travel available has increased over time. When suspension forks were introduced, 80-100 mm of travel was deemed sufficient for a downhill mountain bike. Typically this amount of travel is now used in XC disciplines, with downhill forks now offering 150 to 200 mm of travel for handling extreme terrain. This is not the limit of suspension fork travel; for example, the Marzocchi Super Monster T offers 300 mm (12 inches) of travel.
Other advances in design include adjustable travel allowing riders to adapt the fork's travel to the specific terrain profile. Typically, less suspension travel is needed for uphill or flat terrain than for downhill terrain. Advanced designs also often feature the ability to lock out the fork to eliminate or drastically reduce the fork's travel for more efficient riding over smooth sections of terrain. This lockout can sometimes be activated remotely by a cable and lever on the handlebars.
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 choice of spring material has a fundamental effect on the characteristics of the fork as a whole. Coil spring forks are often heavier than designs which use compressed air springs, but they are more easily designed to exhibit a linear, Hookean response throughout their travel. Substituting steel coils for titanium coils in a design can decrease the weight of the design but leads to an increase in expense. Air springs utilize the thermodynamic property of gases that their pressures increase as they are compressed adiabatically. As the "spring" is provided by air rather than a coil of metal, forks with air springs are often lighter. This makes their use more common in XC designs. Another advantage of this type of fork design is that the spring constant can be adjusted by adjusting the air pressure. This allows a fork to be tuned to a rider's weight. One disadvantage of this design is the difficulty in achieving a linear response, as pressure varies approximately inversely (not linearly) with volume in a gas. As the fork compresses, the effective spring constant of the fork increases. Increasing the volume of the air inside the spring can reduce this effect, but the volume of the spring is ultimately limited, as it needs to be contained within the dimension of the fork blade.
The damper usually forces oil to pass through one or more small openings or shim stacks in the suspension fork. 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, Cannondale for example, have tried other variations on the suspension fork design, including a single shock built into the steering tube above the crown and a fork with a single leg 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.
Forks have been made from steel, aluminum, carbon fiber, titanium, magnesium, and various 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.
Fork steerer 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 steerer tube may be threaded with an appropriate die if necessary. The thread pitch is usually 24 threads per inch except for some old Raleighs which use 26.
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.