Front fork

The front forks (short telescopic fork ) or dipping fork is a front suspension for two-wheelers. It is the most common type of motorcycle fork and the fork of bicycles. 1935 BMW introduced the telescopic fork in a bike. The superiority over other constructions, is reflected in the Motorcycle World Championship ( except the sidecar class), which is obtained since the 1960s, only motorcycles with telescopic fork.

• 3.1 stability
• 3.2 steering head
• 3.3 Suspension and damping
• 3.4 vote
• 3.5 air chamber
• 3.6 bellows
• 3.7 Advantages and disadvantages

History

Already in 1905 there have been various attempts a suspension in a front fork to integrate. The British motorcycle manufacturer since 1909 Scott dealt with this problem. The " Scott - dipping fork " could not have the perfect encapsulation as well as the hydraulic damping of the fork. [Note 1] A similar dipping fork is patented in 1911 by Joseph Merkel " Truss fork ", which was applied in the Flying Merkel.

1935 BMW introduced to his newest types BMW R 12 and R 17 BMW the dipping fork before. The suspension springs lay in metal sleeves, outside of the guide tubes, the travel was 100 mm. BMW called the dipping fork telescopic fork, as two stand pipes telescopically slid into each other. The first dive forks were fitted according to the " upside-down ", that is, the standpipe (also inner tube ) was down to the wheel axle, the inner tube (also outer tube ) mounted on top of the steering head and the crown. The sliding tube stand pipe up, down on the wheel axle - In the 1960s, with the appearance of Japanese manufacturers, the fork was inverted, so to speak, " Downside Up". BMW also used this design since 1969 - according to the interim takeover of the Earles fork ( 1955 to 1969 ) - in their production models.

The Dutch manufacturer White Power suspension introduced in the 1980s, the original version ( upside-down ) restores. 1984 was first installed an upside -down fork of White Power at KTM motocross model KTM 495 MX. Today, the upside-down design is important not only for sports bikes than the prior art.

Types

Upside-down fork

The fork legs upside-down forks ( also USD forks, in the Anglo-Saxon also Inverted fork ) are arranged so that the inner tubes are down and carry the axle of the front wheel. The Yokes on which work great leverage as lead, the verwindungssteiferen, outer tubes with the larger diameter. Although the inner fork tubes lose torsional stiffness, because they lack the rigid fixation missing by the reverse incorporation by the triple clamps can be used as sliding tubes but are further apart bearings are out, so that overall a greater torsional stiffness of the wishbone is achieved.

Be further reduced by this construction the unsprung masses, which basically results in an improvement of the wheel guidance. However, the total weight of an upside -down fork by design larger and the steering angle because of the thicker tubes is above low. In addition, the fork seals are theoretically subject to greater wear due to dirt, but this can be minimized by specially shaped fenders.

Classic telescopic fork

With the classic design, which until the 2000s was the standard design from the 1960s, the crown takes the steering head to the standpipes. These slide into the lower lying dip tubes or sliders carrying the wheel axle and brake. Almost all manufacturers are equipping today motorbikes in low price segment of it, as the classic front fork is inexpensive to produce. The key disadvantages of the classic front fork against the upside- down fork are the higher unsprung masses and the lower torsional stiffness.

A telescopic fork is usually made of two fork legs, which is connected at the top by a yoke. Between these a third, short tube is attached to the steerer. It stores the triple clamp mounted in the steering head bearings. The leg slides in the dip tube and is supported with an inner coil spring. For dampening the movement of a piston is usually also placed in the dip tube of the fork oil presses with movement of the fork leg by means of small bores and therefore acts as a shock absorber, as shown below.

Technology

A telescopic fork consists of two fork bridges [note 2] and two fork legs. The two fork legs each consisting of standpipe and dip tube, also called inner tube. Inside the sliding tube are suspension springs, dampers and damper piston tube, which take over the suspension / damping of the front wheel. At the front forks are at the top of the triple clamp handlebar mounted at the bottom of the front wheel and mudguard ( fender). On certain models, the headlights and the horn (horn) is also installed on the front forks.

Stability

The stability of the telescopic fork is mainly from

• Stanchion diameter [note 3]
• Stability of the triple clamp and the
• Radachsensteifheit

Affected. A fork brace between the fork tubes attached - In the classic front fork was from the 1970s - in order to increase the bending stiffness.

Steering head

About the connecting element fork bridge the dip tubes are rotatably mounted on the steering head. The camp, which allows the rotational motion is called steering head bearings. It runs as a thrust ball bearings or tapered roller bearings.

Suspension and dampers

At motorized two-wheelers with telescopic forks today only hydraulically damped structures are used with coil springs. In sports inserts of varying hardness linear wound springs are often used depending on the road surface; typical values ​​of the spring rate will be around 10 N / mm. Also progressively wound springs are used in the original and partly accessories that allow higher driving comfort, as in the most used area is a lower spring rate available, but are due to the higher spring rate more reserves to the limit. Sliding tube and support tube are sealed by a shaft seal ( shaft seal ) against each other. The fork legs contain an oil fill and a damper rod is moving with holes, which acts as a hydraulic shock absorber. The viscosity of the filled fork oil affects this loss and is normally in the range of 5 to SAE SAE 20 ( comparison value).

Vote

High quality telescopic forks offer the possibility to tune the suspension at a " fully adjustable " suspension can be set:

• Preload ( spring base): For optimum fork can be adapted to different loads, eg when carrying a passenger. This is generally desired to maintain the ratio of negative to positive of travel at different loads equal, such as 30:70. Öhlins describes the adjustment of the spring base as follows: In the first step, the front wheel is fully credited ( wheel free-floating ) is measured and the length of the upper triple clamp to the axle. In the loaded state ( without driver) is the difference between the completely empty front wheel 15 to 30 mm, be with drivers between 35 and 50 mm.
• Rebound ( rebound damping ) and compression ( compression damping ) can be set separately by set screws. These screws affect the cross-section of an oil passage, so that the pumping action created by the oil flow is attenuated differently. When compression affects the compression stage, during rebound rebound. The latest development here is the separate compression adjustment in high-and low -speed. Among the compression rate is to be understood (fast and slow), not the vehicle speed. In general, should always be considered when setting the train and rebound from the basic setting of the manual or the manufacturer of the information. Öhlins recommended by the train - on the pressure level, only small steps approach and always make only one setting.

Air chamber

The essential part of the suspension makes the coil spring in the fork leg, but also the volume of air present above the oil fill in the fork leg is a gas spring which is particularly effective on the last road before the attack. The quantity of oil changes the volume of the gas spring and affects therefore, though. Within narrow limits, the spring characteristic at full deflection The " On - block - walking" a telescopic fork can be prevented.

The volume is set as " air chamber ", where the oil level in the spar without the spring is measured at full deflection. Partly also telescopic forks are constructed (for example, Ohlins FG 670 ) that build or enhance all or combined with compressed air, the spring action; this construction has, however, been implemented so far in racing.

Bellow

Leading into the dip tubes sliding path of the standpipes was protected for a long time by means of bellows against dust. However, since about 30 years decreases their use in road vehicles, and the submerged area of the standpipes remains visible. The protective effect should instead take on additional wiper, partly dust caps or fork protectors, which protect the inner tubes and the seals before anfliegendem dirt and insects.

Pros and Cons

Advantages of the telescopic fork, their maintenance, their safe wheel location and large suspension travel. The disadvantages of the front forks are in the higher starting torque (compared to the arm ) and the strong nose dive. The front fork is exposed to strong braking high bending forces, and to support a relatively deep. Through the compression not only the positive travel is reduced, but also shortens the caster and the steering head angle steeper; stability must be reduced. [note 4] When installing a disc brake vehicle stability when braking through the entanglement of the fork affected. [Note 5]

Mechanical, hydraulic and electro-mechanical devices to reduce the jounce (Anti Dive ) came in the 1980s on the market, but most showed no satisfactory effect and it's not over the long term prevail.

Alternatives

The alternative to telescopic fork was the Earles fork in the 1950s. The rare double-pivot steering, and more recently the steering wheel hub as front suspension remained due to their complexity and higher cost niche products; getting used to the look, which certainly contributes well to this. 1993 BMW developed the Telelever, which is used in a variety of BMW motorcycles; a mix between dipping fork and high mounted wing. 2004 arose again from the BMW Duolever, an evolution of the Hossack fork with bonds at the Keystone fork. The unsprung wheel is connected by means of two longitudinal control arm to the frame; the transfer of the steering movement takes a scissor -like joint.