Motorcycle Suspension Forks and Shocks are the most ignored fundamental components of every motorcycle. Motorcycle Suspension Forks and Shocks should be serviced on a consistent basis. To ensure proper performance and ride safety contact Moto Sage. We can resume proper motorcycle suspension service.
Motorcycle Suspension Forks and Shocks History
Motorcycle Suspension Forks(Telescopic forks)
Scott produced a motorcycle suspension with telescopic forks in 1908, and would continue to use them until 1931. In 1935 BMW became the first manufacturer to produce a motorcycle suspension with hydraulically damped telescopic forks. Most motorcycles suspension today use telescopic forks for the front motorcycle 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 suspension from that motion.
The top of the forks are connected to the motorcycle’s frame in a triple tree clamp (known as a “yoke” in the UK), which allows the forks to be turned in order to steer the motorcycle. The bottom of the forks is connected to the front wheel’s axle.
On conventional telescopic forks, the lower portion or fork bodies (“fork sliders” in the UK), slide up and down the fork tubes (“fork stanchions” in the UK). The fork tubes must be mirror-smooth to seal the fork oil inside the fork. Some fork tubes, especially on early roadsters and off-road motorcycles, are enclosed in plastic protective “gaiters.
“Upside-down” (USD) forks
Also known as inverted forks, are installed inverted compared to conventional telescopic forks. The slider bodies are at the top, fixed in the triple clamps, and the stanchion tubes are at the bottom, fixed to the axle. This USD arrangement has two advantages: (i) it decreases the unsprung weight of the motorcycle; and (ii) it increases torsional stiffness, which can improve handling. Two disadvantages of USD forks are: (i) they are more expensive than conventional telescopic forks; and (ii) they are liable to lose all their damping oil should an oil seal fail. USD forks are typically found on sportbikes, though the Honda Valkyrie featured USD forks.
Motorcycle suspension 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 motorcycle suspension with the weight of the motorcycle and rider acting on it.
The difference between the fully extended length of the motorcycle suspension and the length compressed by the weight of the motorcycle and rider is called “total sag” or “race sag”. Total sag is set to optimize the initial position of the motorcycle suspension to avoid bottoming out or topping out under normal riding conditions. “Bottoming out” occurs when the motorcycle suspension is compressed to the point where it mechanically cannot compress any more. Topping out occurs when the motorcycle 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.
Some 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. More air pressure gives more pre-load, and vice versa.
Basic fork designs use a simple damper-rod system, in which damping is controlled by the passage of fork oil through an orifice. Though cheap to manufacture, it is hard to tune such forks, as they tend to give too little damping at low slider speeds, yet too much damping at higher slider speeds. Any adjustment setting will always be a compromise, giving both over-mushy and over-stiff damping. Since forks act as hydraulic dampers, changing the weight of the fork oil will alter the damping rate. Some telescopic forks have external adjustments for damping.
A more sophisticated approach is the cartridge fork, which use internal cartridges with a valving system. Damping at low slider speeds is controlled by a much smaller orifice, but damping at higher slider speeds is controlled by a system of flexible shims, which act as a bypass valve for the fork oil. This valve has a number of such shims of varying thicknesses that cover the orifices in the valve to control the damping of the fork on high and medium speed bumps.
Some of the shims (or “leaf springs”) lift with little force allowing 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, yet relatively softer over larger bumps. Also, the springs (or shims) 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 damper-rod forks behave virtually as cartridge forks. The damping orifice in the damper-rod is made so large that it has virtually no effect on damping, and instead an “emulator” takes over the damping function. The emulator has a very small orifice for low fork-speed damping, and an adjustable shim-stack for high fork-speed damping.
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. If the motorcycle suspension 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. telescopic forks are particularly prone to this, unlike leading link designs.
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 suspension. 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.
While excessive brake dive is disconcerting, and bottoming out can cause loss of traction, a certain amount of brake dive reduces the rake and trail of the motorcycle, allowing it to turn more easily. This is especially important to racers trail braking on entrance to corners.
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.
Saxon-Motodd (Telelever) fork
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 motorcycle 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 motorcycle suspension that is based upon this design.
A single-sided front swingarm motorcycle suspension was used on the Yamaha GTS1000, introduced in 1993. The GTS used the RADD, Inc. front motorcycle suspension designed by James Parker. A single sided girder fork was use on the German Imme R100 motorcycle between 1949 and 1951, and the Vespa scooter has a single-sided trailing-link fork. More recently, between 1998 and 2003, the ItalJet “Dragster” scooter also used a single-sided swingarm suspension, though unlike the GTS1000 there was no upper control arm; the upper part of the motorcycle suspension on the Dragster served only to transmit steering input.
Hub-center steering is characterized by a swingarm that extends from the bottom of the engine/frame to the centre of the front wheel instead of a fork. The advantages of using a hub-center steering system instead of a more conventional motorcycle fork are that hub-center steering separates the steering, braking, and motorcycle suspension functions.
With a fork the braking forces are put through the motorcycle suspension, a situation that leads to the motorcycle suspension being compressed, using up a large amount of motorcycle suspension travel which makes dealing with bumps and other road irregularities extremely difficult. As the forks dive the steering geometry of the bike also changes making the bike more nervous, and inversely on acceleration becomes more lazy. Also, having the steering working through the forks causes problems with stiction, decreasing the effectiveness of the motorcycle suspension. The length of the typical motorcycle suspension fork means that they act as large levers about the headstock requiring the forks, the headstock, and the frame to be very robust adding to the bike’s weight.
Motorcycle Suspension Shock Absorbers
The hydraulic shock absorbers used on the rear motorcycle suspension are essentially the same as those used in other vehicle applications.
Motorcycle suspension shocks do differ slightly in that they nearly always use a coil-over spring. In other words, the spring for the rear motorcycle suspension is a coil spring that is installed over, or around, the shock.
In terms of adjustment, rear shocks span the range from no adjustments whatsoever to pre-load adjustments only to racing shocks with adjustments for length, 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 into account the characteristics of the motorcycle, the weight of the rider, and the rider’s preferred riding style/aggressiveness.
Twin shock absorbers
Twin shock 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, rear motorcycle suspension design and performance underwent tremendous advances. The primary goal and result of these advances were increased rear wheel travel, as measured in how far the rear wheel could move up and down. Before this period of intense focus on rear motorcycle 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 motorcycle 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. The performance of single shock absorber motorcycle suspension was vastly superior to twin shock motorcycle suspension. Accordingly, this design distinction is readily used to categorize motorcycles.
Single shock absorber
On a motorcycle with a single shock absorber rear motorcycle suspension, a single shock absorber 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 swing arm. Such linkages are frequently designed to give a rising rate of damping for the rear. In 1972, Yamaha introduced the Mono-Shock single shock absorber rear suspension system on their motorcycles competing in the Motocross World Championships. The suspension which was designed by Lucien Tilkens, became so successful that other motorcycle manufacturers developed their own single shock absorber designs. Honda refers to its single shock absorber design as Pro-link suspensions, Kawasaki as Uni-Trak and Suzuki as Full-Floater. Honda’s Unit Pro-Link, used first on the Honda RC211V MotoGP racer, and then on the 2003 Honda CBR600RR sport bike, is intended to isolate the frame and the steering head from undesirable forces transmitted by the rear suspension by having the dampers upper mount contained within the rear swingarm subframe, rather than connecting it to the frame itself.
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