Telescopic fork

from Wikipedia, the free encyclopedia
Telescopic fork
Upside-down (Moto Guzzi Falcone, 1950–67)
Upside-down (2003)
Classic telescopic fork (1974)

The telescopic fork (short telescopic fork , also diving fork ) is a front wheel suspension for two-wheelers . It is the most common form of motorcycle fork and suspension fork on bicycles. In 1935 BMW introduced the hydraulic telescopic fork in motorcycles. The superiority of the concept over other front suspensions can be seen in the motorcycle world championship (except for the sidecar class), which has only been won by motorcycles with telescopic forks since the 1960s.

history

Beginnings

As early as 1905 there were various attempts to integrate a suspension into a front fork. The English motorcycle manufacturer Scott had been dealing with this problem since 1909. However, the “Scott diving fork” could not have the complete encapsulation and hydraulic damping of the telescopic fork. A similar diving fork is the “truss fork” patented by Joseph Merkel in 1911, which was used in the Flying Merkel . In 1929 Monroe developed the first 2-way hydraulic shock absorber. In 1934 the Danish motorcycle manufacturer brought Nimbus model Type C , called " Humlebi" (Hummel ) with telescopic fork without hydraulic damping out.

Hydraulic diving fork

In 1935, BMW introduced the hydraulic immersion fork on its latest models, the BMW R 12 and BMW R 17. The suspension springs were in sheet metal sleeves outside the guide tubes, the spring travel was 100 mm. BMW called the diving fork telescopic fork, as two standpipes slid telescopically into one another. The first diving forks were attached according to the "upside-down" principle, that is, the standpipe (also the inner tube ) was attached to the bottom of the wheel axle, the sliding tube (also the outer tube ) was attached to the top of the steering head and the fork bridge. In the 1960s, with the appearance of Japanese manufacturers, the fork was turned around, so to speak "downside-up" - the stanchion on top, the sliding tube on the bottom of the wheel axle. BMW has also been using this design since 1969 - after it had since taken over the Earles fork (1955–1969) - on its production models.

The Dutch manufacturer White Power Suspension restored the original version (upside-down) in the 1980s. 1984 at KTM in Motocross 495 MX for the first time installed an upside-down forks from White Power model KTM. Today, the upside-down construction is considered the state of the art, and not just for sports motorcycles.

Types

Classic telescopic fork

In the classic design, which was the standard design since the 1960s to the 2000s, takes crown at the steering head the standpipes on. These slide in the immersion tubes or sliding tubes below , which carry the wheel axle and brake. Almost all manufacturers to this day equip motorcycles in the low price segment with it, because the classic telescopic fork can be produced inexpensively. The decisive disadvantages of the classic telescopic fork compared to the upside-down fork are the higher unsprung masses and the lower bending stiffness.

A telescopic fork usually consists of two tubes that are connected at the top by a fork bridge . Between these is a third, short tube, the steerer tube. It rotates the fork bridge in the steering head bearing . The fork leg slides in the dip tube and is supported by an internal helical spring . To dampen the movement, a piston is usually also attached in the dip tube, which presses the fork oil through small holes when the fork leg moves and thus acts as a shock absorber , see below .

Triple clamp

Upside-down fork

The fork legs of upside-down forks (also USD fork , in Anglo-Saxon also inverted fork ) are arranged in such a way that the inner tubes are at the bottom and carry the thru axle of the front wheel. The fork bridges, on which large leverage forces act, guide the more rigid outer tubes with the larger diameter. The inner fork leg tubes are less stiff because they are not fixed by the fork bridges due to the reverse installation, but can be guided as sliding tubes in sliding bearings that are further apart, so that the overall rigidity of the fork leg is achieved.

Furthermore, this design has lower unsprung masses, which fundamentally improves wheel control. However, the overall weight of an upside-down fork is greater due to the design and the steering angle is lower due to the thicker tubes at the top. In addition, the fork sealing rings are theoretically subject to greater wear due to contamination, but this can be reduced by using specially shaped protective plates.

technology

The upper fork bridge (also called cover plate), on which i. d. R. the handlebar is attached. The handlebar stubs of this model are attached directly to the stanchions (common in racing).

A telescopic fork consists of two fork bridges and two fork legs. The two fork legs each consist of a standpipe and an immersion pipe (also a sliding pipe), which are sealed by two plugs. Inside the sliding tube there are suspension springs, damper tube and damper piston, which take over the suspension / damping of the front wheel. The handlebars are attached to the telescopic fork at the upper end of the fork bridge, and the front wheel and mudguard (mudguard) are attached to the lower end. On certain models, the headlight and the horn (horn) are also attached to the telescopic fork.

stability

The stability of the telescopic fork is largely determined by the

  • Standpipe diameter, the
  • Stability of the fork bridge and the
  • Wheel axle stiffness influenced.

The stanchion diameter of the sportiest models was 36 mm in the early 1970s and is now 46 mm. In traditional telescopic forks one was in the 1970s between the fork legs additionally fork stabilizer appropriate to increase the bending stiffness.

Steering head

The fork is rotatably mounted on the steering head via the fork bridge connecting element . The bearing that enables the rotation is called the steering head bearing. It is designed as an axial deep groove ball bearing or as a tapered roller bearing.

Suspension and damping

Today only hydraulically damped constructions with helical compression springs are used on motorized two-wheelers with telescopic forks . In sports activities, linearly coiled springs of different hardness are often used depending on the nature of the route; typical values ​​of the spring rate are around 10 N / mm. There are progressively wound springs that enable greater driving comfort, as they respond more gently in the most frequently used area and provide more reserves due to the higher spring rate when the load is high.

The sliding tube and standpipe are sealed against each other by a shaft sealing ring (Simmerring) . The fork legs contain an oil filling and a damper rod with bores that moves in it and acts as a hydraulic shock absorber . The viscosity of the fork oil used influences this damping and is in the range of viscosity class SAE 5 to SAE 20 (comparative value).

In the case of a cartridge fork, the constant oil hole on the damper is replaced by a series of spring washers (shim package), which release a different cross-section depending on the compression speed. The goal is a soft cushioning on small bumps, which among other things ensures better ground contact of the tire. The hard damping, on the other hand, is aimed at with strong compression. The effect can usually be influenced by adjusting screws, separately for compression stage (compression, adjuster below) and rebound (rebound, adjuster above).

poll

High-quality telescopic forks offer the possibility of chassis tuning . With a "fully adjustable" suspension, the following can be set:

  • Spring preload (spring base): This allows the fork to be adapted to different loads, for example when riding with a passenger. The aim is usually to keep the ratio of negative to positive suspension travel the same under different loads, for example at 30:70. Öhlins describes the adjustment of the spring base as follows: In the first step, the front wheel is fully relieved (wheel floating freely) and the length from the upper triple clamp to the axle mount is measured. In the loaded condition (without driver) the difference between the completely unloaded front wheel should be 15 to 30 mm, with the driver between 35 and 50 mm.
  • Rebound ( rebound damping ) and compression ( compression damping) are set separately by adjusting screws. These screws influence the cross-section of an oil channel, so that the oil flow resulting from the pumping movement is dampened differently. The compression stage takes effect during compression and the rebound stage during rebound. The latest development here is the separate setting of the pressure level in high and low speed. This refers to the compression speed (fast and slow), not the vehicle speed. In general, when setting the tension and compression level, the basic setting in the operating instructions or the manufacturer's information should always be used as a basis. Öhlins recommends that you always proceed in small steps from the tension to the compression stage and only ever make one setting.

Air chamber

The main part of the suspension is provided by the coil spring in the fork leg; the air volume above the oil filling also acts as a gas spring , the effect of which increases continuously with the deflection and thus reaches its maximum before the stop. The amount of oil filled in changes the volume of this gas spring and therefore influences, albeit within narrow limits, the spring characteristic at full deflection. A telescopic fork can thus be prevented from “walking on the block”.

The volume of the air chamber is adjusted by a screw on the fork closure plug. Reducing the height of the air chamber changes the effect of the enclosed air volume as a gas spring. In some cases, telescopic forks (such as the Öhlins FG 670) are built, which build up or increase the spring effect wholly or in combination with compressed air; However, this design has so far only prevailed in racing.

Classic telescopic fork with bellows

Bellows

The sliding path of the standpipes leading into the immersion tubes was for a long time protected against dust by bellows . For about 30 years, however, their use in road vehicles has been decreasing and the submerged area of ​​the standpipes remains visible. Instead, the protective effect should be provided by additional scraper rings , sometimes also dust caps or fork protectors , which protect the standpipes and the seals from dirt and insects.

Advantages and disadvantages

The advantages of the telescopic fork are its compact design, the low moment of inertia around the steering axis (control head) and the relatively good rigidity. The disadvantages of the telescopic fork are the pronounced brake diving. The telescopic fork compresses relatively deeply in the event of high braking decelerations. The strong compression not only reduces the positive suspension travel, but also shortens the caster and makes the steering head angle steeper; this reduces driving stability. As the standpipe diameter increases, in order to prevent the high bending forces during braking, the friction forces and thus the breakaway force due to the larger fork seals increase. This influences the response behavior, especially on small bumps in the road.

When mounting a disc brake on one side, the fork's twisting influences the driving stability during braking.

Mechanical, hydraulic and electromechanical devices to reduce brake diving ( anti-dive ) came onto the market in the 1980s, but mostly did not have a satisfactory effect and thus could not prevail in the long term.

Alternatives

As an alternative to the telescopic fork, the pushed long-arm swing arm was particularly popular in the 1950s and 1960s . It prevents diving in during braking and still allows generous spring travel and a particularly fine response to uneven road surfaces. However, due to its high weight and the associated high steered mass, it was not able to establish itself permanently. Short swing arms were also widespread on simple motorcycles , but these are still common today on motor scooters with small running wheels. The rare stub axle steering and, more recently, wheel hub steering as front suspension have remained niche products because of their complexity and higher costs; the unusual appearance certainly also contributes to this. In 1993 BMW developed the Telelever , which is used in various BMW motorcycles; a mixture between a diving fork and a high swing arm. In 2004 BMW again developed the Duolever , a further development of the Hossack fork with borrowings from the trapezoid fork . The unsprung wheel fork is connected to the frame via two longitudinal wishbones; a scissors-like joint transmits the steering movement.

literature

  • Helmut Werner Bönsch: Introduction to motorcycle technology. 3. Edition. Motorbuch Verlag, Stuttgart 1981, ISBN 3-87943-571-5 .
  • Helmut Werner Bönsch: Advanced motorcycle technology. 1st edition. Motorbuch Verlag, Stuttgart 1985, ISBN 3-613-01054-2 .
  • Michael Gressmann, Franz Beck, Rüdiger Bellersheim: Expertise in motorcycle technology. 2nd Edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2013, ISBN 978-3-8085-2232-5 .
  • Jürgen Stoffregen: Motorcycle technology: basics and concepts of engine, drive and chassis. 7th edition. Vieweg Verlag, Braunschweig 2010, ISBN 978-3-8348-0698-7 .

Web links

Remarks

  1. ↑ is usual singular fork bridge although it consists of upper and lower fork bridge, steerer and steering bearings.
  2. ↑ In an example with a braking deceleration of 5 m / s², the caster was shortened from 97 to 78 mm, the steering head angle became steeper from 63 degrees to 70 degrees. See Helmut Werner Bönsch: Progressive motorcycle technology. 1985, p. 233.
  3. ↑ With braking at 8 m / s² and a distance of the brake disc of 60 mm to the wheel center plane, the deceleration introduces a torque of over 300 Nm into the fork and rotates it by 2 to 3 degrees. See Helmut Werner Bönsch: Progressive motorcycle technology. 1985, p. 229.

Individual evidence

  1. ^ Oskar Koch: The current state of motorcycles. In: Polytechnisches Journal . 321, 1906, pp. 294-298.
  2. ^ Helmut Werner Bönsch: Introduction to motorcycle technology. 1981, p. 51.
  3. ↑ Monroe - inventor of the shock absorber ( Memento from February 19, 2014 in the Internet Archive ); Tenneco, accessed November 15, 2013.
  4. ↑ Helmut Krackowizer : Milestones in motorcycle history, 1st edition. Motorbuch Verlag, Stuttgart 1987, ISBN 3-613-01141-7 , p. 88.
  5. ↑ Roger Hicks: The international encyclopedia motorcycles. 1st edition. Motorbuch Verlag, Stuttgart 2006, ISBN 3-613-02660-0 , p. 378.
  6. ↑ lannungsmuseumsfond.dk ( Memento of the original dated December 10, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Humlebien from 1934 (accessed September 19, 2015) @1@ 2Template: Webachiv / IABot / lannungsmuseumsfond.dk
  7. ↑ Leonard John Kensell Setright: The Guinness Book of Motorcycling Facts and Feats. Guinness Superlatives, Enfield 1982, ISBN 0-85112-255-8 , p. 30.
  8. ^ Norbert Adolph: chassis - link to the road. In: Christian Bartsch (Ed.): A century of motorcycle technology . VDI Verlag, 1987, ISBN 3-18-400757-X , pp. 190-191.
  9. ↑ Motorcycle Catalog 1984, ISSN  0949-0892 , p. 187.
  10. ↑ KTM 495 MX (1984) ( Memento of the original from December 27, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.vintagedirtbikeparts.net
  11. ↑ Jürgen Stoffregen: Motorcycle technology. 2010, p. 295.
  12. ↑ Jürgen Stoffregen: Motorcycle technology. 2010, p. 294.
  13. ↑ Screwdriver tips for steering head bearings. (No longer available online.) POLO, archived from the original on January 15, 2014 ; accessed on January 14, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.polo-motorrad.de
  14. ↑ Comparison table for fork oils. (PDF) Goede Motorsport, accessed on January 16, 2014 .
  15. ↑ motorradonline.de: Front wheel damping - The dream setup , accessed November 25, 2017.
  16. ↑ ohlins.eu Road-Track Shock Absorbers. P. 5 (accessed December 27, 2013)
  17. ↑ ohlins.eu voting part 3 (accessed on December 27, 2013)
  18. ^ Wolfgang Matschinsky: Wheel guides of road vehicles. 3. Edition. Springer Verlag, Berlin / Heidelberg 2007, ISBN 978-3-540-71196-4 , p. 371.
  19. ↑ Jürgen Stoffregen: Motorcycle technology. 2010, p. 294.
  20. ^ Helmut Werner Bönsch: Progressive motorcycle technology. 1985, p. 233.
  21. ↑ Jürgen Stoffregen: Motorcycle technology. 2010, p. 303.