Bicycle fork
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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.
[edit] Dimensions
[edit] 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.
[edit] Length
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 axel.
[edit] Steerer tube length
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.
[edit] Steerer tube diameter
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 1/4" 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, Specialized25mm thru-axle and Cannondale's Lefty system.
[edit] Suspension Fork
On most mountain bicycles, the fork contains a set of shock absorbers, in which case the blades are called legs instead, and they consist of upper and lower telescoping tubes. 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.
Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated. The amount of travel available has typically increased. When suspension forks were introduced 80-100mm of travel was deemed sufficient for a downhill mountain bike. Typically this amount of travel is now the preserve of cross country disciplines. Downhill forks can now offer in the region of 150 to 200mm of travel for handling the most extreme terrain. Some, like the Marzocchi Super Monster T, may even offer up to 300 mm (12 inches).
Other advances in design include adjustable travel allowing riders to adapt the forks travel to the specific terrain profile. eg less travel for uphill sections more travel for downhill sections. Advanced designs also often feature the ability to lockout the fork to completely eliminate or drastically reduce the fork's travel for more efficient riding over smooth sections of terrain. This lockout can sometime 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, however they are more easily designed to keep a linear spring rate 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 work by utilising the characteristic of compressed air to resist further compression. as the "spring" is provided by the compressed air rather than a coil of metal they can often be made lighter this makes their use more common in cross country designs. Another advantage of this type of fork design is that the spring rate can easily be adjusted by adjusting the pressure of the air in the spring. This allows a fork to be effectively tuned to a riders weight. One disadvantages of this design is the difficulty in achieving a linear spring rate throughout the forks action. As the fork compresses the air held inside the air spring also compresses, towards the end of the forks travel further compression of the fork requires ever increasing compression of the compressed air with the spring, this results in an increase in spring rate. 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 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, Cannondale for example, have tried other variations including a single shock built into the steering tube above the crown and a fork with just 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.
[edit] 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.
[edit] Threading
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.[1]
[edit] 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.
[edit] See also
- Bicycle
- Bicycle frame
- Bicycle and motorcycle dynamics
- Bicycle and motorcycle geometry
- Bicycle suspension
- Fork tube
- Motorcycle fork
- Stem (bike)
