Motorcycle fork

1968BMW R60USwith conventional telescopic fork

Unusual "trailing bottom link" on a Honda Rune

Amotorcycle forkconnects amotorcycle's front wheel and axle to itsframe,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. Most forks incorporate the frontsuspensionand front brake, and allow the front wheel to rotate about the steering axis so that the bike may be steered. Mosthandlebarsattach 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.

The fork and its attachment points on the frame establish the critical geometric parameters ofrake and trail,which play a major role in defining how a motorcycle handles anddivesduring braking.^[1]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 ashub-center steering.

Variations

A variety of fork arrangements have been tried during more than one hundred years of motorcycle development, several of which remain available today.

Telescopic

Atelescopic forkusesfork tubeswhich contain the suspension components (coil springs and damper) internally. This is the most common form of fork commercially available. It may or may not includegaitersfor protection against abrasive elements on the suspension cylinders. 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.

Conventionally, the forkstanchionsare at the top, secured by ayoke(also called a triple tree or a triple clamp), and the sliders are at the bottom, attached to the front wheel spindle. 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. 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.^[2]The inverted system is referred to as anupside-down fork,or "USD" for short. 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.

Trailing link

A trailing link fork suspends the wheel on a link (or links) with a pivot point forward of the wheel axle. Most famously used byIndian Motocycle;it was also used by BMW for its early bikes.

Leading link

A leading link fork suspends the wheel on a link (or links) with a pivot point aft of the wheel axle. RussianUralmotorcycles used leading link forks onsidecarequipped motorcycles, and aftermarket leading link forks are often installed today on motorcycles when they are outfitted with sidecars. They are also very popular withtrikes,improving the handling while steering or braking. The most common example of a leading link fork is that found on theHonda Super Cub.

Springer

Thespringer forkis 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 clamp. This style of fork may be found on antique motorcycles orchoppers,and is available today onHarley-Davidson'sSoftail Springer.

While it may have an exposed spring near the triple clamp, a springer fork is distinguishable from agirder forkby 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

TheEarles forkis a variety of leading link fork where the pivot point is behind the front wheel, which is the basis of the Earles' patent.^[3]Patented byEnglishmanErnest Earles in 1953, the design is constructed of light tubing, with conventional 'shock absorbers' mounted near the front axle. The Earles fork has a very small wheelbase change under braking or under compression, unlike telescopic forks. Their construction is much stronger than teleforks, especially against lateral deflection caused by hard cornering (as when racing), or when cornering with asidecar.^[3]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. This action can be disconcerting to riders used to telescopic forks, which have the opposite reaction to braking forces ('brake dive'). Several motorcycle manufacturers licensed the Earles patent forks for racing motorcycles in 1953, such asMV AgustaandBMW Motorcycle,while other companies (such asDouglas motorcycle) used the Earles design on their roadsters or off-road machines. BMW used Earles forks on all their motorcycles between 1955 and 1968.

Girder

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 a peak in the "Girdraulics" used onVincent motorcyclesfrom 1948. Girdraulic forks featured hydraulic damping with forged alloy blades providing extra strength.^[4]

While both may have an exposed spring near the triple clamp, a girder fork is distinguishable from aspringer forkby 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.

Although girder forks are associated with early motorcycles, the BritishAriel Ace,a modernsports motorcycle,has an alloy space frame and a matching girder fork. This Ariel girder fork is just as prone to brake dive as any telescopic fork, but is claimed to be more resistant to torsional twisting.

Saxon-Motodd (Telelever)

The Saxon-Motodd fork (marketed as Telelever byBMW) has a wishbone that mounts to the frame and supports themonoshockunit. This relieves the forks of any braking and suspension forces. With a Saxon-Motodd fork, the trail andcaster angle(rake) increases during braking instead of decreasing as with traditional telescopic forks.^[5]BMW's boxer twins have been equipped with Telelever forks since 1994, but some newer boxers like theBMW R nineThave reverted to conventional telescopic forks for aesthetic and packaging reasons.

Hossack/Fior (Duolever)

The Hossack/Fior fork (marketed as Duolever byBMW) completely separates the suspension from steering forces. Developed byNorman Hossack^[6]and used by Claude Fior andJohn Brittenon racebikes, Hossack 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 BMW K models:K1200R,K1300R,K1200S,K1300S,K1200GTandK1300GT.

Coaxial steering front suspension

Developed byMotoCzyszfor their C1 and awarded United States Patent 7111700 on September 26, 2006. Defined as a motorcycle or bicycle front end having 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. ^[7]

This particular fork, as implemented on the MotoCzysz C1, also has adjustable trail, from 89 mm to 101 mm.^[8]

Non-forks

There have been several attempts to implement front steering and suspension without using anything that could be described as a "fork". Examples includehub-center steering,used as early as 1920 on theNer-a-Car,^[9]and implemented in the 1990s on theBimotaTesi^[10]and theYamaha GTS1000^[11]

A single-sided girder "fork" was used by the German firmImme^[12]^[13]between 1949 and 1951.^[13]

Mead & Tomkinson Racingcompeted in endurance racing in the 1970s with "Nessie," aLaverda-based racing motorcycle with hub-centre steering.^[14]

References

^Foale, Tony."TO DIVE OR..."Archived fromthe originalon 2013-02-04.Retrieved2006-12-08.
^Wilson, Hugo (1993).The Ultimate Motorcycle Book.Dorling-Kindersley. p. 181.ISBN 0-7513-0043-8.
^^a ^bGB patent 693646,Earles, Ernest, "Motor cycle front wheel forks", published 1953-07-01
^Robert Smith (May–June 2011)."A Brief History of Girder Forks".Motorcycle Classics.Retrieved2011-05-20.
^Cossalter, V."BMW Telelever and telescopic forks: which is better?".Archivedfrom the original on 2010-05-16.Retrieved2012-04-07.
^Hossack DesignRetrieved 2014-03-16
^"Coaxial steering and suspension for motorcycle".
^"MotoCzysz".2006. Archived fromthe originalon 1 December 2006.Retrieved2006-12-14.
^Willoughby, Vic (1977) [1975]. "Ner-a-Car".Classic Motorcycles(Third impression ed.). Hamlyn. pp. 30–32.ISBN 0-600-31870-2.
^Foale, Tony."STEER FOR THE FUTURE".Retrieved2006-12-09.
^McClellan, Don."The Bike, Yamaha USA".Archived fromthe originalon 2007-05-28.Retrieved2006-12-09.
^Wilson, Hugo (1995). "The A-Z of Motorcycles".The Encyclopedia of the Motorcycle.London: Dorling Kindersley. p.100.ISBN 0-7513-0206-6.
^^a ^bWilson, Hugo (1995). "The Directory of Motorcycles".The Encyclopedia of the Motorcycle.London: Dorling Kindersley. p.241.ISBN 0-7513-0206-6.
^"Lot 417: The ex-Mead & Tomkinson, 1976 Laverda 1,000cc 'Nessie' Endurance Racing Prototype - Bonhams".Bonhams. 20 October 2007.Retrieved3 February2014.

[Foale-1] Foale, Tony."TO DIVE OR..."Archived fromthe originalon 2013-02-04.Retrieved2006-12-08.

[UltiMoto181-2] Wilson, Hugo (1993).The Ultimate Motorcycle Book.Dorling-Kindersley. p. 181.ISBN 0-7513-0043-8.

[Earles,_patent_693646-3] GB patent 693646,Earles, Ernest, "Motor cycle front wheel forks", published 1953-07-01

[4] Robert Smith (May–June 2011)."A Brief History of Girder Forks".Motorcycle Classics.Retrieved2011-05-20.

[Cossalter-5] Cossalter, V."BMW Telelever and telescopic forks: which is better?".Archivedfrom the original on 2010-05-16.Retrieved2012-04-07.

[6] Hossack DesignRetrieved 2014-03-16

[Google-7] "Coaxial steering and suspension for motorcycle".

[MotoCzysz-8] "MotoCzysz".2006. Archived fromthe originalon 1 December 2006.Retrieved2006-12-14.

[ClassicMotorcycles-9] Willoughby, Vic (1977) [1975]. "Ner-a-Car".Classic Motorcycles(Third impression ed.). Hamlyn. pp. 30–32.ISBN 0-600-31870-2.

[Foale2-10] Foale, Tony."STEER FOR THE FUTURE".Retrieved2006-12-09.

[RADD-11] McClellan, Don."The Bike, Yamaha USA".Archived fromthe originalon 2007-05-28.Retrieved2006-12-09.

[EncycloMoto100-12] Wilson, Hugo (1995). "The A-Z of Motorcycles".The Encyclopedia of the Motorcycle.London: Dorling Kindersley. p.100.ISBN 0-7513-0206-6.

[EncycloMoto241-13] Wilson, Hugo (1995). "The Directory of Motorcycles".The Encyclopedia of the Motorcycle.London: Dorling Kindersley. p.241.ISBN 0-7513-0206-6.

[14] "Lot 417: The ex-Mead & Tomkinson, 1976 Laverda 1,000cc 'Nessie' Endurance Racing Prototype - Bonhams".Bonhams. 20 October 2007.Retrieved3 February2014.

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