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Telescopic fork

A telescopic fork is a form of motorcycle front suspension whose use is so common that it is virtually universal. The telescopic fork uses fork tubes and sliders which contain the springs and dampers.

Conventional (on left) and USD forks

The main advantages of the telescopic fork are that it is simple in design and relatively cheap to manufacture and assemble, and that it is lighter than older designs using external components and linkage systems. Telescopic forks sometimes have gaiters to protect the fork tubes from abrasion and corrosion.

A more modern (and more expensive) version of the conventional telescopic fork is the inverted or "USD" (upside-down) fork.

BMW's patented telelever front suspension appears at first glance to be conventional telescopic fork, but the fork tubes contain neither springs nor damping. Instead, a wishbone and an inboard monoshock perform suspension duties, and the forks serve to locate the front wheel and control steering.[1]

History edit

 
Early oil-damped telescopic fork on a 1939 BMW R12

The first production motorcycles with hydraulically damped telescopic forks were the German BMW R12 and R17 in 1935.[2] However, undamped telescopic forks were used on bikes made by The Scott Motorcycle Company from the beginning of production in 1908,[3] and the Danish Nimbus used them from 1934 on.[4]

Design edit

 
1968 BMW R60US with conventional telescopic fork
 
Telescopic fork in upside down design, with stanchions at the bottom.

Conventional telescopic forks invariably have a pair of fork tubes, or "stanchions", at the top, clamped to a triple tree (also called a triple clamp or a yoke), and the sliders are at the bottom, attached to the front wheel spindle.

Inside most tubes are springs, fork oil, and air, creating a shock absorber. Some forks allow pressurized air to be added through a valve in the top of the fork to stiffen the suspension. Quality forks have a flexible air tube between both fork tubes in order to balance the air pressure in both forks. Modern fork tubes have a method which employs a screw to compress fork spring to increase or decrease spring pre-load.

Damping edit

Cartridge forks provide regressive damping. Self-contained cartridges within the forks contain spring-covered orifices regulating fork oil flow. The springs resist low forces and thus provide high damping rates. Higher forces compress the springs, allowing more oil flow and less damping. Thus the fork is stiffer when responding to small bumps but will soften as larger ones are encountered.

Most modern forks also are suspended on springs and control fork movement by damping through adjustable valving to control movement by controlling the flow of fork oil. The larger the orifice, the more free the flow and the less damped the fork. A selector atop the fork engages the desired hole size or valve and corresponding damping rate in compression, while usually the lower tube has a mechanism to control rebound (extension) damping.

Single action damping is currently being manufactured, such that compression occurs in one fork tube while its partner tube controls the rebound damping. This is because the oil controlling the damping in each tube, then, has only to control one direction of damping and is therefore less aerated by it working in both directions, and it develops less heat while working, which causes a variation to the thickness of the oil, thereby adversely affecting the damping rate.

Upside down edit

On most modern motorbikes this system is inverted, with the "sliders" and the spring/damper unit at the top, clamped to the yoke, while the stanchions are at the bottom. This is done (1) to reduce unsprung weight by having the heavier components suspended, and (2) to improve the strength and rigidity of the assembly by having the strong large-diameter "sliders" clamped in the yokes.[5] The inverted system is referred to as an upside-down fork, or "USD" for short. One disadvantage of the USD design is that the entire reservoir of damping oil is above the slider seal so in case of leak the oil drains out, rendering any damping ineffective.

Triple tree edit

 
The upper triple clamp joins the top of the fork tubes and the handlebar to the frame of a Honda 919

A triple tree ("triple clamp" (US) or "yoke" (UK)) attaches the fork tubes to the frame via steering head bearings, which allow the fork assembly to pivot from side to side, and therefore steer the motorcycle. Most bikes have upper and lower triple trees, providing two solid clamping points that keep the fork tubes parallel while also connecting the forks to the frame via the steering head bearings.

With the tendency to employ fork tubes with single action damping, fork triple trees need to be reinforced more than when the forks shared both damping roles, because the rigidity of the triple trees is relied upon to distribute the forces within the forks without flex.

The BMW Telever fork has only a single triple tree, which allows its fork sliders to be longer than those on telescopic forks. This in turn allows for greater overlap of the sliders over the stanchion tubes, which improves stiffness and helps to reduce flex.

References edit

  1. ^ Cossalter, V. "BMW Telelever and telescopic forks: which is better ?". from the original on 2010-05-16. Retrieved 2012-04-07.
  2. ^ Setright, L. J. K. (1977). Bahnstormer: The story of BMW Motor cycles. Transport Bookman Publications. p. 47. ISBN 0-85184-021-3.
  3. ^ Wilson, Hugo (1995). "The A-Z of Motorcycles". The Encyclopedia of the Motorcycle. London: Dorling Kindersley. p. 166. ISBN 0-7513-0206-6. The advanced design included a two-speed gear, all-chain drive, a triangulated frame, telescopic forks, and a kickstarter.
  4. ^ Duchene, Paul (May 2008). Duchene, Paul; Lombard, Stefan (eds.). "Bike Buys: Nimbus–The Prince of Denmark". Sports Car Market. 20 (5). Portland, OR USA: Sports Car Market: 136. ISSN 1527-859X. Retrieved 2012-12-30. Nimbus developed telescopic forks in 1933 and introduced hydraulic damping in 1939.
  5. ^ Wilson, Hugo (1993). The Ultimate Motorcycle Book. Dorling-Kindersley. p. 181. ISBN 0-7513-0043-8.

External links edit

  Media related to Telescopic forks at Wikimedia Commons

telescopic, fork, main, article, motorcycle, fork, telescopic, fork, form, motorcycle, front, suspension, whose, common, that, virtually, universal, telescopic, fork, uses, fork, tubes, sliders, which, contain, springs, dampers, conventional, left, forks, main. Main article Motorcycle fork A telescopic fork is a form of motorcycle front suspension whose use is so common that it is virtually universal The telescopic fork uses fork tubes and sliders which contain the springs and dampers Conventional on left and USD forks The main advantages of the telescopic fork are that it is simple in design and relatively cheap to manufacture and assemble and that it is lighter than older designs using external components and linkage systems Telescopic forks sometimes have gaiters to protect the fork tubes from abrasion and corrosion A more modern and more expensive version of the conventional telescopic fork is the inverted or USD upside down fork BMW s patented telelever front suspension appears at first glance to be conventional telescopic fork but the fork tubes contain neither springs nor damping Instead a wishbone and an inboard monoshock perform suspension duties and the forks serve to locate the front wheel and control steering 1 Contents 1 History 2 Design 2 1 Damping 2 2 Upside down 2 3 Triple tree 3 References 4 External linksHistory edit nbsp Early oil damped telescopic fork on a 1939 BMW R12The first production motorcycles with hydraulically damped telescopic forks were the German BMW R12 and R17 in 1935 2 However undamped telescopic forks were used on bikes made by The Scott Motorcycle Company from the beginning of production in 1908 3 and the Danish Nimbus used them from 1934 on 4 Design editThis section does not cite any sources Please help improve this section by adding citations to reliable sources Unsourced material may be challenged and removed February 2015 Learn how and when to remove this message nbsp 1968 BMW R60US with conventional telescopic fork nbsp Telescopic fork in upside down design with stanchions at the bottom Conventional telescopic forks invariably have a pair of fork tubes or stanchions at the top clamped to a triple tree also called a triple clamp or a yoke and the sliders are at the bottom attached to the front wheel spindle Inside most tubes are springs fork oil and air creating a shock absorber Some forks allow pressurized air to be added through a valve in the top of the fork to stiffen the suspension Quality forks have a flexible air tube between both fork tubes in order to balance the air pressure in both forks Modern fork tubes have a method which employs a screw to compress fork spring to increase or decrease spring pre load Damping edit Cartridge forks provide regressive damping Self contained cartridges within the forks contain spring covered orifices regulating fork oil flow The springs resist low forces and thus provide high damping rates Higher forces compress the springs allowing more oil flow and less damping Thus the fork is stiffer when responding to small bumps but will soften as larger ones are encountered Most modern forks also are suspended on springs and control fork movement by damping through adjustable valving to control movement by controlling the flow of fork oil The larger the orifice the more free the flow and the less damped the fork A selector atop the fork engages the desired hole size or valve and corresponding damping rate in compression while usually the lower tube has a mechanism to control rebound extension damping Single action damping is currently being manufactured such that compression occurs in one fork tube while its partner tube controls the rebound damping This is because the oil controlling the damping in each tube then has only to control one direction of damping and is therefore less aerated by it working in both directions and it develops less heat while working which causes a variation to the thickness of the oil thereby adversely affecting the damping rate Upside down edit On most modern motorbikes this system is inverted with the sliders and the spring damper unit at the top clamped to the yoke while the stanchions are at the bottom This is done 1 to reduce unsprung weight by having the heavier components suspended and 2 to improve the strength and rigidity of the assembly by having the strong large diameter sliders clamped in the yokes 5 The inverted system is referred to as an upside down fork or USD for short One disadvantage of the USD design is that the entire reservoir of damping oil is above the slider seal so in case of leak the oil drains out rendering any damping ineffective Triple tree edit nbsp The upper triple clamp joins the top of the fork tubes and the handlebar to the frame of a Honda 919 A triple tree triple clamp US or yoke UK attaches the fork tubes to the frame via steering head bearings which allow the fork assembly to pivot from side to side and therefore steer the motorcycle Most bikes have upper and lower triple trees providing two solid clamping points that keep the fork tubes parallel while also connecting the forks to the frame via the steering head bearings With the tendency to employ fork tubes with single action damping fork triple trees need to be reinforced more than when the forks shared both damping roles because the rigidity of the triple trees is relied upon to distribute the forces within the forks without flex The BMW Telever fork has only a single triple tree which allows its fork sliders to be longer than those on telescopic forks This in turn allows for greater overlap of the sliders over the stanchion tubes which improves stiffness and helps to reduce flex References edit Cossalter V BMW Telelever and telescopic forks which is better Archived from the original on 2010 05 16 Retrieved 2012 04 07 Setright L J K 1977 Bahnstormer The story of BMW Motor cycles Transport Bookman Publications p 47 ISBN 0 85184 021 3 Wilson Hugo 1995 The A Z of Motorcycles The Encyclopedia of the Motorcycle London Dorling Kindersley p 166 ISBN 0 7513 0206 6 The advanced design included a two speed gear all chain drive a triangulated frame telescopic forks and a kickstarter Duchene Paul May 2008 Duchene Paul Lombard Stefan eds Bike Buys Nimbus The Prince of Denmark Sports Car Market 20 5 Portland OR USA Sports Car Market 136 ISSN 1527 859X Retrieved 2012 12 30 Nimbus developed telescopic forks in 1933 and introduced hydraulic damping in 1939 Wilson Hugo 1993 The Ultimate Motorcycle Book Dorling Kindersley p 181 ISBN 0 7513 0043 8 External links edit nbsp Media related to Telescopic forks at Wikimedia Commons Retrieved from https en wikipedia org w index php title Telescopic fork amp oldid 1200689696, wikipedia, wiki, book, books, library,

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