Suspension (motorcycle)


Suspension (motorcycle)

A motorcycle's suspension serves a dual purpose: contributing to the vehicle's handling and braking, and providing safety and comfort by keeping the vehicle's passengers comfortably isolated from road noise, bumps and vibrations.

The typical motorcycle has a pair of fork tubes for the front suspension, and a swingarm with one or two shock absorbers for the rear suspension.[1]

Contents

Front suspension

The most common form of front suspension for a motorcycle is the telescopic fork. Early front suspension designs used frames with springs.

Vincent Black Lightning with Girdraulic front suspension

Some British manufacturers (e.g. Greeves) used a version of the swinging arm for front suspension on their motocross designs. A single-sided version of the idea is also used in motor scooters such as the Vespa.

The Hub-center steering as developed by Ascanio Rodorigo, on a concept associated to Massimo Tamburini is a complex front swingarm alternative system that entails suspension and steering, as seen in projects such as Bimota Tesi and Vyrus motorcycles.

Telescopic forks

In 1934 Nimbus was the first manufacturer to produce a motorcycle with hydraulically damped telescopic forks. Most motorcycles today use telescopic forks for the front suspension. The forks can be most easily understood as simply large hydraulic shock absorbers with internal coil springs. They allow the front wheel to react to imperfections in the road while isolating the rest of the motorcycle from that motion.

Telescopic forks on a 1969 BMW

The top of the forks are connected to the motorcycle's frame in a triple tree clamp (otherwise known to British riders as the top yoke and bottom yoke), which allows the forks to be turned in order to steer the motorcycle.

The bottom of the forks are connected to the front axle around which the front wheel spins.

On typical forks, the upper portion, known as the fork tubes, slide inside the fork bodies, which are the lower part of the forks. As the tubes slide in and out of the body they are telescoping, thus the term telescopic forks. The fork tubes must be smooth to seal the fork oil inside the fork, and typically have a mirrored finish, though some fork tubes, especially those on off-road motorcycles, are enclosed in plastic protective sleeves, known as gaiters.

"Upside-down" (USD) forks, also known as inverted forks, are installed inverted compared to typical forks, with the tubes at the bottom and the bodies at the top. This decreases the unsprung weight of the motorcycle and improves its handling. USD forks are usually found on sportbikes, though Honda's large power-cruiser, the Valkyrie, sported USD forks.

Pre-load adjustment

Motorcycle suspensions are designed so that the springs are always under compression, even when fully extended. Pre-load is used to adjust the initial position of the suspension with the weight of the motorcycle and rider acting on it.

The difference between the fully extended length of the suspension and the length compressed by the weight of the motorcycle and rider is called total sag. Total sag is set to optimize the initial position of the suspension to avoid bottoming out or topping out under normal riding conditions. Bottoming out" occurs when the suspension is compressed to the point where it mechanically cannot compress any more. Topping out occurs when the suspension extends fully and cannot mechanically extend any more. Increasing pre-load increases the initial force on the spring thereby reducing total sag. Decreasing pre-load decreases the initial force in the spring thereby increasing total sag.

A few motorcycles allow adjustment of pre-load by changing the air pressure inside the forks. Valves at the top of the forks allow air to be added or released from the fork.[2] More air pressure gives more preload, and vice versa.

Damping adjustment

Some stock telescopic forks have external adjustments for damping. The adjuster turns a rod inside the fork which brings different sized orifices into alignment with the damping fluid flow path inside the fork. Smaller orifices restrict the flow of the fork oil more and give greater damping.

Fork oil

Since forks act as hydraulic shocks, changing the weight of the fork oil will change the damping.

Cartridge forks

Cartridge forks use internal cartridges with various leaf springs covering orifices to control the damping of the fork.

Some of the leaf springs lift with little force allow fluid to flow through the orifice. Other springs require greater force to lift and allow flow. This gives the fork digressive damping, allowing it to be stiff over small bumps, but get relatively softer over larger bumps.

Also, the springs only allow flow in one direction, so one set of springs controls compression damping, and another rebound damping. This allows the dampings to be set separately.

Cartridge emulators are aftermarket parts that make non-cartrdige forks behave like cartridge forks.

Gas-charged cartridge forks

In 2007 the gas-charged bolt-in cartridge set for modern sportbike forks became available. This kit is legal for supersport styled classes of racing, which regulations do not allow a complete fork replacement, and force competitors to use the stock fork casings.

Brake dive

Applying the brakes of a moving motorcycle increases the load borne by the front wheel and decrease the load borne by the rear wheel due to a phenomenon called load transfer. For a detailed explanation and a sample calculation, see the braking section of the Bicycle and motorcycle dynamics article.

BMW's 1955-1969 Earles fork eliminated and reversed brake dive

If the motorcycle is equipped with telescopic forks, the added load on the front wheel is transmitted through the forks, which compress. This shortening of the forks causes the front end of the bike to move lower, and this is called brake dive.

Brake dive can be disconcerting to the rider, who may feel like he or she is about to be thrown over the front of the motorcycle. If the bike dives so far as to bottom out the front forks, it can also cause handling and braking problems. One of the purposes of a suspension is to help maintain contact between the tire and road. If the suspension has bottomed out, it is no longer moving as it should, and is no longer helping to maintain contact.

Brake dive with telescopic forks can be reduced by either increasing the spring rate of the fork springs, or increasing the compression damping of the forks. However, all of these changes make the motorcycle less pleasant to ride on rough roads, since the front end will feel stiffer, in the 1980s various manufacturers attempted to get round this by methods of anti-dive such as:

  • ACT: Developed by Marzocchi and fitted to Buell motorcycles such as the Buell RR 1200 (1988).
  • ANDF (Anti Nose Dive Forks): This was fitted to a number of Suzuki GSX models and the RG250.
  • AVDS (Automatic Variable Damping System): This was fitted to a number of Kawasaki motorcycles.
  • NEAS (New Electrically Activated Suspension): As fitted to the Suzuki GSX-R 1100 and GSX-R 750 Limited Edition.
  • PDF (Posi Damp Fork): This was fitted to the Suzuki RG500 and GSX-R 750 and worked by brake fluid pressure closing a valve in the mechanism when the brakes are applied, restricting the flow of damping oil and slowing fork compression. The valves are spring-loaded so if the wheel hits a bump when the brakes are on, they bounce off their seats and restore the flow of oil for a moment to allow the suspension to absorb the shock.
  • TCS (Travel Control System): Anti-dive system with variable damping. TCS was introduced on the FZ 400 R (1984, only for the Japanese market).
  • TRAC (Torque Reactive Anti-dive Control): This was fitted to a number of Honda motorcycles such as the CB1100F, CB1000C, and VFR750F and worked by utilizing a pivoting caliper that activated a valve in the fork leg.

Another method to reduce or eliminate brake dive in telescopic forks is to use a reactive link or torque arm to connect the braking components to the motorcycle frame via the triple clamp.

Some fork designs mitigate dive, eliminate it, or even reverse it without affecting the front suspension adversely. The Earles fork is among the latter; when braking the front brake hard, the front end of the motorcycle actually rises. BMW's Telelever fork is designed to nearly eliminate dive, but could have been designed to eliminate it completely if the manufacturer chose to do so. Leading link front forks, such as used on some Ural motorcycles, can also be designed either to reduce or eliminate dive.

Saxon-Motodd (Telelever) fork

BMW's Telelever

The Saxon-Motodd (marketed as Telelever by BMW) has an additional swingarm that mounts to the frame and supports the spring. This causes the rake and trail to increase during braking instead of decreasing as with traditional telescopic forks.

Hossack/Fior (Duolever) fork

The Hossack/Fior (marketed as Duolever by BMW) separates completely the suspension from steering forces. It was developed by Norman Hossack though used by Claude Fior and John Britten on racebikes. Hossack himself described the system as a 'steered upright'. In 2004 BMW announced the K1200S with a new front suspension that is based upon this design.

Single-sided

A single-sided front swingarm suspension was used on the Yamaha GTS1000, introduced in 1993. The GTS used the RADD front suspension designed by James Parker. A single sided girder fork was use by the German firm Imme between 1949 and 1951, and the Vespa scooter has a single-sided trailing-link fork. More recently[when?], the ItalJet "Dragster" scooter also uses a single-sided swingarm suspension, though unlike the GTS1000 there is no upper control arm; the upper part of the suspension on the Dragster serves only to transmit steering input.

Rear suspension

Plunger rear suspension on a BMW R51/3

Early rear suspensions

While front suspensions were almost universally adopted before World War I, several manufacturers did not use rear suspension on their bikes until after World War II. However, motorcycles with rear suspension were offered to the public before World War I. Notable among these are the 1913 Indian Single with a swingarm suspended from a leaf spring and the 1913 Pope with wheels supported on a pair of plungers which were each suspended by a coil spring.[3][4]

Plunger suspension

Several motorcycles before and immediately after World War II used plunger suspension in which the vertical movement of the rear axle was controlled by plungers suspended by springs.[5]

Notable manufacturers of bikes with plunger suspension include Adler, Ariel, BMW, BSA, Indian, MZ, Norton, and Zündapp

Swingarms

The basic motorcycle swingarm is a rectangle, with one short side connected to the motorcycle's frame with bearings so that it can pivot.[6] The other short side is the rear axle around which the rear wheel turns. The long sides are connected to the motorcycle's frame or rear sub-frame with one or two shocks with coil-over springs.

In production motorcycles, swingarms are not exactly rectangular, but their function can be more easily understood by thinking of them as such.

Moto Guzzi's CA.R.C.

When a swingarm is present on only one side of the motorcycle, this is known as a single-sided swingarm. Notable examples include the Honda VFR800 and the BMW R- and K-series. Single-sided swingarms make rear-wheel removal easier, though they generally increase the unsprung weight of the rear suspension. This is due to the additional material required to give identical torsional rigidity to a conventional (two-sided) swingarm setup. For this reason sports bikes are rarely seen using the setup. Notable exclusions are the Ducati 916 which was intended to be taken endurance racing, the MV Agusta f4 which has a hollow interior for reducing weight (a magnesium version is also available), and the Ducati 1098, which was given a single sided swingarm purely for styling reasons.

BMW's Paralever rear suspension on a R1200GS

On many shaft-drive motorcycles the drive shaft is contained in one of the long sides of the swingarm. Notable examples include all post-1955 BMW models prior to BMW's use of the single-sided swingarms, Urals, many Moto Guzzi twins, the Honda Goldwing, the Yamaha XS Eleven, and the Yamaha FJR1300.

The BMW R- and K-series combine a shaft-drive contained in the swing arm with a single-sided swingarm, and the combination is marketed as the Paralever. Newer Moto Guzzi motorcycles use a similar arrangement marketed as the CA.R.C. ("CArdano Reattivo Compatto" - Compact Reactive Shaft Drive).

For motorcycles with chain drives, the rear axle can be adjusted forward and back in relation to the swingarm, to adjust chain tension.

Shock absorbers

The hydraulic shock absorbers used on the rear suspensions of motorcycles are essentially the same as those used in other vehicle applications.

Motorcycle shocks do differ slightly in that they nearly always use a coil-over spring. In other words, the spring for the rear suspension is a coil spring that is installed over, or around, the shock.

In terms of adjustment, rear shocks span the range from pre-load adjustments only to racing shocks with adjustments for pre-load, and four different kinds of damping. Most shocks have internal oil reservoirs, but some have external ones, and some offer air-assisted damping.

A number of companies offer custom-built rear shocks for motorcycles. These shocks are assembled for a specific motorcycle and rider combination, taking in to account the characteristics of the motorcycle, the weight of the rider, and the rider's preferred riding style/aggressiveness.

Twin shock absorbers

Twinshock refers to motorcycles that have two shock absorbers. Generally, this term is used to denote a particular era of motorcycles, and is most frequently used when describing off-road motorcycles.

During the late 1970s and 1980s, motorcycle rear suspension design and performance underwent tremendous advances. The primary goal and result of these advances were increased rear wheel travel, as measured in the how far the rear wheel could move up and down. Before this period of intense focus on rear suspension performance, most off-road motorcycles had rear wheel travel of about 3.5–4 inch (9–10 cm). At the end of this period, most of these motorcycles had rear wheel travel of approximately 12 inch (30 cm). At the beginning of this period, various rear suspension designs were used to reach this degree of performance. However, by the end of this period, a design consisting of using only one shock absorber (instead of two) was universally accepted and used. Motorcycles with only one shock absorber are called monoshock motorcycles. The performance of monoshock motorcycles was vastly superior to twin shock motorcycles. Accordingly, this design distinction is readily used to categorize motorcycles. Since monoshock motorcycles have been the norm since the 1980s, the term "twinshock" is now used to categorize vintage motorcycles. This distinction is important in that it provides classes used for vintage motorcycle competition. For example, vintage motocross races are held for older motocross motorcycles. To prevent the better-performing monoshock motorcycles from dominating the competition, there are separate competition classes for monoshock and twinshock motorcycles, which prevents them from competing directly against each other.

Mono-shock

On a motorcycle with a mono-shock rear suspension, there is only one shock that connects the rear swingarm to the motorcycle's frame. Typically this lone shock absorber is in front of the rear wheel, and uses a linkage to connect to the swingarm.

Mono-shocks eliminate torque to the swingarm and provide more consistent handling and braking. They are also easier to adjust, since there's only one shock absorber to adjust, and there is no worry about matching two shocks. Also, the linkages used to connect the shock to the swing-arm are frequently designed to give a rising rate of damping for the rear.[7]

Honda refers to its mono-shock designs as Pro-link suspensions, Kawasaki as Uni-Track, Suzuki as Full-Floater and Yamaha as Monocross.

See also

References

  1. ^ Motorcycle Mechanics Institute,The Complete Guide to Motorcycle Mechanics, 1984, p. 277, Prentice-Hall, Inc., ISBN 0-13-160549-6
  2. ^ Motorcycle Mechanics Institute,The Complete Guide to Motorcycle Mechanics, 1984, p. 291, Prentice-Hall, Inc., ISBN 0-13-160549-6
  3. ^ Wilson, H. "The Encyclopedia of the Motorcycle" p. 102 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6
  4. ^ Wilson, H. "The Encyclopedia of the Motorcycle" p. 156 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6
  5. ^ Wilson, H. "The Encyclopedia of the Motorcycle" p. 310 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6
  6. ^ Motorcycle Mechanics Institute,The Complete Guide to Motorcycle Mechanics, 1984, p. 282, Prentice-Hall, Inc., ISBN 0-13-160549-6
  7. ^ Motorcycle Mechanics Institute,The Complete Guide to Motorcycle Mechanics, 1984, p. 286, Prentice-Hall, Inc., ISBN 0-13-160549-6

Further reading

  • Cameron, Kevin (1998). Sportbike Performance Handbook. St. Paul MN: Motorbooks. ISBN 978-0-7603-0229-3. 
  • Foale, Tony (2002). Motorcycle Handling and Chassis Design. Spain: Tony Foale Designs. ISBN 84-933286-3-4. 
  • Robinson, John (1990). Motorcycle Tuning: Chassis. Oxford: Butterworth Heineman. ISBN 0-7506-1840-x. 

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