Girdraulic forks featured forged alloy blades for extra strength and hydraulic damping. The design reached a peak in the "Girdraulics" used on the Vincent motorcycle. 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. BMW used Earles forks on all their motorcycles between 19. Several motorcycle manufacturers licensed the Earles patent forks for racing motorcycles in 1953, such as MV Agusta and BMW Motorcycle, while other companies (such as Douglas motorcycle) used the Earles design on their roadsters or off-road machines.
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This action can be disconcerting to riders used to telescopic forks, which have the opposite reaction to braking forces ('brake dive'). This triangulated fork causes the front end of a motorcycle to rise slightly when braking hard, as the mechanical braking forces rotate 'downward' relative to the fork's pivot point. Their construction is much stronger than teleforks, especially against lateral deflection caused by hard cornering (as when racing), or when cornering with a sidecar. The Earles fork has a very small wheelbase change under braking or under compression, unlike telescopic forks. Patented by Englishman Ernest Earles in 1953, the design is constructed of light tubing, with conventional 'shock absorbers' mounted near the front axle. The Earles fork is a variety of leading link fork where the pivot point is behind the front wheel, which is the basis of the Earles' patent. A disadvantage of this USD design is that the entire reservoir of damping oil is above the slider seal so that, if the slider seal were to leak, the oil could drain out, rendering any damping ineffective. The inverted system is referred to as an upside-down fork, or "USD" for short. This is done (i) to reduce unsprung weight by having the heavier components suspended, and (ii) to improve the strength and rigidity of the assembly by having the strong large-diameter "sliders" clamped in the yokes. On some modern sport bikes and most off-road bikes, this system is inverted, with "sliders" (complete with the spring/damper unit) at the top, clamped to the yoke, while the stanchions are at the bottom. The main advantages of the telescopic fork are that (i) it is simple in design and relatively cheap to manufacture and assemble (ii) it is lighter than older designs using external components and linkage systems and (iii) it has a clean and simple appearance that bikers find attractive.Ĭonventionally, the fork stanchions are at the top, secured by a yoke (also called a triple tree or a triple clamp), and the sliders are at the bottom, attached to the front wheel spindle. It may or may not include gaiters for protection against abrasive elements on the suspension cylinders.
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This is the most common form of fork commercially available. While the standard telescopic fork arrangement is found with few major differences among mainstream street motorcycles since the 1970s, historically there have been many variations, including trailing or leading link, springer, Earles, girder, and others, as well as non-fork steering such as hub-center steering.īMW's version of oil-damped telescopic fork, on a 1939 R12Ī telescopic fork uses fork tubes which contain the suspension components (coil springs and damper) internally. The fork and its attachment points on the frame establish the critical geometric parameters of rake and trail, which play a major role in defining how a motorcycle handles and dives during braking. Most handlebars attach to the top clamp in various ways, while clip-on handlebars clamp to the fork tubes, either just above or just below the upper triple clamp. Most forks incorporate the front suspension and front brake, and allow the front wheel to rotate about the steering axis so that the bike may be steered. Unusual "trailing bottom link" on a Honda RuneĪ motorcycle fork connects a motorcycle's front wheel and axle to its frame, typically via a yoke, also known as a triple clamp, which consists of an upper yoke joined to a lower yoke via a steering stem, a shaft that runs through the steering head, creating the steering axis.