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Motorcycle fork
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A motorcycle fork is the portion of a motorcycle to which the front wheel and the handlebars are connected, usually incorporates the front suspension and front brake, and allows the rider to steer and balance the motorcycle. In the case of a telescopic fork, it consists of two fork tubes (sometimes also referred to as forks) which hold the front wheel axle and a triple tree which connects the fork tubes and the handlebars, perhaps through a riser, to the frame with a pivot that allows for steering.
The fork, along with its attachment points on the frame establish the critical motorcycle geometry parameters of rake and trail, which in turn contribute to wheelbase.
The front brakes are connected to the fork, and act against a rotor or drum attached to the front wheel.
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Encyclopedia
A motorcycle fork is the portion of a motorcycle to which the front wheel and the handlebars are connected, usually incorporates the front suspension and front brake, and allows the rider to steer and balance the motorcycle. In the case of a telescopic fork, it consists of two fork tubes (sometimes also referred to as forks) which hold the front wheel axle and a triple tree which connects the fork tubes and the handlebars, perhaps through a riser, to the frame with a pivot that allows for steering.
The fork, along with its attachment points on the frame establish the critical motorcycle geometry parameters of rake and trail, which in turn contribute to wheelbase.
The front brakes are connected to the fork, and act against a rotor or drum attached to the front wheel. The front fender is also usually attached to the front fork.
The fork, how it is implemented and adjusted, plays a major role in defining how a motorcycle handles, especially how much it dives during braking.
Variations
Over the more than one hundred years of motorcycle development, several variations of front forks have been tried. Several are commercially available today.
Telescopic fork
A telescopic fork uses fork tubes which contain the suspension components (coil springs and damper) internally. This is the most common form of fork commercially available.
The main advantage of the telescopic fork is that it is relatively simple in design and therefore relatively easy and inexpensive to manufacture and assemble. It is also relatively light compared to older designs based on external components and linkage systems.
Conventionally, the fork stanchions are at the top of the fork assembly, clamped to a triple tree, also called a yoke, pivoting around the headstock, and sliding in and out of the spring/damper unit at the bottom of the assembly. On many sport bikes, this system is inverted, with the spring/damper unit clamped to the yoke while the stanchions are at the bottom of the assembly. This is done for two reasons: to reduce unsprung weight by having the heavier components be suspended, and to improve the strength and rigidity of the assembly by having the bulkier and stronger component being directly supported by the pivot. Such a system is referred to by many as upside-down forks or USD for short.
The disadvantage of the inverted design is that the entire reservoir of damping oil is stored directly over the slider seal such that, if the slider seal were to leak, the oil would drain out, reducing the effectiveness of the fork.
The first production motorcycle with hydraulically damped telescopic forks was the Danish Nimbus in 1934. However, undamped telescopic forks were used on bikes made by The Scott Motorcycle Company from the beginning of production in 1908.
Fork tube
The fork tubes link a motorcycle's front wheel to the front of the frame and are generally used in pairs, called motorcycle forks. Fork tubes house the front suspension assembly and compress and extend to adjust for inconsistencies in the road, allowing the front tire to maintain contact with the road for better handling and braking without the rider feeling the up-and-down movement of the wheel.
Some of the motorcycle's weight is riding on the forks, so inside the tubes there are springs and fork oil to make the fork tubes act as a shock absorber. There is also pressurized air inside the forks.
Some forks allow the air pressure in the fork to be adjusted through a valve in the top of the fork.
A more common form of adjustment is a screw at the top of the fork which presses down on the fork spring. Turn the screw increases or reduces the pre-load on the spring, letting the rider adjust the amount of negative fork travel. This has no effect, however, on the spring rate except if the springs have a progressive rate, in which case the 'softer' part is used up first when increasing pre-load, letting them appear 'stiffer'.
Some forks also allow damping adjustment. This is typically done through a screw-type adjuster in the top of the fork. The adjuster is connected to a rod that, when rotated, selects differently-sized orifices that control the flow of fork oil. The larger the orifice, the more freely the oil flows, and the less damped is the fork.
Cartridge forks provide better performance by having regressive damping. Cartridges in the forks contain damping orificies that are covered by springs. With small forces on the fork, the springs resist the flow of oil and the damping is higher. With higher forces on the fork the springs are forced back allowing the flow of more oil and less damping. Thus the fork is stiffer when responding to small bumps in the road, but will soften as larger bumps are encountered.
The fork tubes are held together by a part called a triple clamp. There is an upper and a lower triple clamp to stabilize the forks; together these constitute the triple tree. At the bottom of the forks is has a special slot to hold the axle. The front wheel fits between the two forks, the axle harnessing the lower ends of the forks together.
Triple tree (also triple clamp or yoke)
A triple tree or "triple clamp" is a two-part motorcycle component that attaches the fork tubes to the frame to comprise the fork and make steering possible.
A common (and debated) modification to the triple tree and/or to the forks is known as 'raking'. The term rake is used to indicate the angle of the steering head measured in degrees from a line 90 degrees to the ground. Essentially a 'raked' front-end is one that has been adjusted to a more pronounced forward angle.
In the United Kingdom they are more commonly known as the top and bottom yokes.
Girder Fork
One of the earliest types of motorcycle front suspension, the girder fork consists of a pair of uprights attached to the triple clamp by linkages with a spring usually between the top and bottom triple clamps. The design reached its peak in the "Girdraulics" used on "The Vincent" motorcycle.
While it may have an exposed spring near the triple clamp, a girder fork is distinguishable from a springer fork by the wheel being fixed firmly to the (usually a long diamond shape) upright. The pivot points are short links mounted to the top and bottom triple clamps. The spring is (usually) mounted to the girder and compressed against the upper triple clamp.
Trailing Link Fork
A trailing link fork suspends the wheel on a link (or links) with a pivot point forward of the wheel axle. Most famously used by Indian Motorcycles, it was also used by BMW for its early bikes.
Leading Link Fork
A leading link fork suspends the wheel on a link (or links) with a pivot point aft of the wheel axle. Russian Ural motorcycles still use leading link forks on sidecar equipped motorcycles, and aftermarket leading link forks are often installed today on motorcycles when they are outfitted with sidecars. The most common example of a leading link fork is that found on the early Honda 50.
Springer fork
The springer fork is an early type of leading link fork. A springer fork does not have the suspension built into the fork tubes, but instead has it mounted externally, where it may be integrated into the triple tree. This style of fork may be found on antique motorcycles or choppers, and is available today on Harley-Davidson's Softail Springer.
While it may have an exposed spring near the triple clamp, a springer fork is distinguishable from a girder fork by its two parallel sets of legs. The rear is firmly fixed to the bottom triple clamp (usually brazed or welded). A short leading link holds the wheel and the forward leg which actuates the springs (usually mounted on the triple clamp).
Earles fork
The Earles fork was a variety of leading link fork where the pivot point was aft of the rear of the front wheel - this was the basis of the Earle's patent. Designed by Englishman Ernest Earles, this triangulated fork actually caused the front end of a motorcycle to rise when braking hard — the reverse of the action of a telescopic fork. It was designed to accommodate sidecars, and from 1955 to 1969, BMW used the fork even though most of its motorcycles were sold as solo bikes.
Saxon-Motodd (Telelever) fork
The Saxon-Motodd (marketed as Telelever by BMW) has an additional swingarm that mounts to the frame and supports the spring. This causes the trail and castor angle (rake) to increase during braking instead of decreasing as with traditional telescopic forks. In the 21st century, BMW's boxer twins are equipped with Telelever forks.
Hossack/Fior (Duolever) fork
The Hossack/Fior (marketed as Duolever by BMW) separates completely the suspension from steering forces. It was developed by though used by Claude Fior and John Britten on racebikes. Hossack himself described the system as a 'steered upright'. In 2004 BMW announced the K1200S with a new front suspension that appears to be based upon the design. As of 2006, the Duolever is on the K1200S, K1200R, and K1200GT.
Coaxial steering front suspension
Developed by MotoCzysz for their C1 and awarded United States Patent 7111700 on September 26, 2006. It is a fork with "coaxial steering and suspension components, and having telescopic forks. Swing weight of the forks is dramatically reduced by removing their suspension components to the central location, coaxially within the steering tube. Ride height can be adjusted without loosening the forks in the triple clamps. A shock tube disposed substantially coaxially within the steering tube wherein the shock tube includes a passage therethrough substantially coaxial with the steering axis; an upper triple clamp and a lower triple clamp coupled to the shock tube; a pair of sliding-tube forks each having an upper fork tube coupled to the upper triple clamp and to the lower triple clamp, and a lower fork tube; a coil-over shock disposed within the shock tube; a front wheel rotatably coupled to the lower fork tubes; a pair of bearings rotatably coupling the shock tube to the steering tube; and a top bolt coupling the shock tube to the upper triple clamp and having a passage therethrough substantially coaxial with the steering axis; wherein the coil-over shock includes a setting adjustment mechanism which is accessible via the passages through the top bolt and the shock tube."
This particular fork, as implemented on the MotoCzysz C1, also has adjustable trail, from 89mm to 101mm.
Non-forks
There have been several attempts to implement front steering and suspension without using anything that could be described as a "fork". An example is Hub-center steering as implemented on the Bimota Tesi and used as early as 1920 on the Ner-a-Car, or the RADD by James Parker implemented on the Yamaha GTS-1000. A single-sided girder "fork" was used by the German firm Imme between 1949 and 1951.
See also
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