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But a motorcycle suspension is usually simpler, since it does not have to contend with lateral forces such as Body Roll . The typical motorcycle has a pair of Fork Tube s for the front suspension, and a single swingarm with one or two Shock Absorber s for the rear suspension.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 277, Prentice-Hall, Inc., ISBN 0-13-160549-6 = Front suspension = See Also: Motorcycle fork The most common form of front suspension for a motorcycle is the telescopic Fork . Early front suspension designs used frames with springs. Some British manufacturers (eg Greeves) used a version of the swinging arm for front suspension on their motocross designs. A single-sided version of the idea was also used in motor scooters such as the Vespa 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 Absorber s with internal Coil Spring s. They allow the front wheel to react to imperfections in the road while isolating the rest of the motorcycle from that motion. The top of the forks are connect 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 turns. 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 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 When a motorcycle's forks are compressed or extended, the ''fork springs'' inside them are under compression or tension, respectively. Pre-load is an amount of additional compression force the springs experience when the forks are at a given position. When you adjust the pre-load, you are either increasing or decreasing the amount of that force. When you increase the pre-load, you increase the additional force at any given position of the forks under compression. This causes the forks to extend more under any given external force than they would at a lower pre-load setting. Therefore, the forks have to experience greater external force before they ''bottom out'', or reach the maximum compression travel. The pre-load can be used for suspension tuning (see below). Some motorcycles have externally accessible pre-load adjustments. Typically, this is a screw-type adjustment that moves a backing plate inside the fork against the top of the fork spring. The farther down the adjuster is screwed, the more force is applied to the spring, and the higher the preload. 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.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 291, Prentice-Hall, Inc., ISBN 0-13-160549-6 More air pressue gives more preload, and vice versa. Preload on bikes without adjusters can be changed by disassembling the fork and changing the length of the spacer between the top of the fork spring and the ''fork cap''. A longer spacer gives higher pre-load, and vice-versa. The pre-load on both forks should always be the same. Dangerous handling characteristics and possible mechanical damage can result otherwise. Damping adjustment Some stock telescopic forks have external adjustments for Damping . The adjuster is either a dial or a knob slotted for a screwdriver. Turning 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, and vice versa. This adjustment is either for compression damping alone, or for both compression and rebound damping. Fork oil Since forks act as hydraulic shocks, changing the weight of the fork oil will change the damping. Higher weight fork oil will give more damping, and vice versa. Fork oil collects impurities over time and should be changed periodically. A motorcycle's manual will give guidelines on how often the oil should be changed. Decades ago, it was often recommended to use ATF (automatic transmission fluid) as fork oil. Currently most sources recommend using an oil specifically designed to be used as fork oil. 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 ''progressive'' damping, allowing it to be stiff over small bumps, but get 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. However, changing the settings requires disassembly of the forks. ''Cartridge emulators'' are aftermarket parts that make non-cartrdige forks behave like cartridge forks. Brake Dive When a motorcyclist riding a motorcycle applies the front brake, weight is transferred to the front wheel, just as happens with many vehicles with suspensions. This is similar to the feeling of leaning forward when you brake hard in a car. eliminated and reversed brake dive]] If the motorcycle is equipped with Telescopic Forks , the added weight to the front wheel is transmitted through the forks, which compress under the additional force. Because the wheel is at the same level (the road hasn't gone down), the shortening of the forks causes the front end of the bike to move lower. This is ''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 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 no longer helping to maintain contact. Brake dive with telescopic forks can be reduced by:
However, all of these changes make make the motorcycle less pleasant to ride on rough roads, since the front end will feel stiffer. Some fork designs mitigate dive, eliminate it, or even reverse it without affecting the front suspension adversely. BMW 's older 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 nearly to 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. TELELEVER BMW introduced its Telelever Front Suspension in 1993. The Telelever uses a linkage and single hydraulic shock with external spring to replace the more common telescopic front fork suspension. The Telelever, according to BMW, "separates suspension and steering." It is most commonly praised for virtually eliminating braking dive. It is standard on BMW's R-series motorcycles and was standard on all the K-series bikes until the introduction of the K1200S. BMW introduced a copy of a front suspension system designed by Norman Hossack for the K1200S, naming it Duolever . SINGLE-SIDED The only production motorcycle to use a single-sided front ''swingarm'' suspension was Yamaha's GTS1000 , introduced in 1993. The GTS used the RADD front suspension designed by James Parker . However, 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 . = Rear suspension = ]] 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 .Wilson, H. "The Encyclopedia of the Motorcycle" p. 102 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6Wilson, H. "The Encyclopedia of the Motorcycle" p. 156 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6 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.Wilson, H. "The Encyclopedia of the Motorcycle" p. 310 Dorling-Kindersley Limited, 1995 ISBN 0 7513 0206 6 Notable manufacturers of bikes with plunger suspension include Adler , Ariel , BMW , Indian , Norton , and Zündapp SWINGARMS See Also: Swingarm The basic motorcycle swingarm is a rectangle, with one short side connected to the motorcycle's frame with Bearings so that it can pivot.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 282, Prentice-Hall, Inc., ISBN 0-13-160549-6 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. Some swing arms have only one long-side, and are know as ''single-sided'' Swingarms . Notable examples include the Honda VFR800 and the BMW R- And K-series Of Motorcycles . 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 rigidity to a conventional 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 an innovative hollow interior for reducing weight(a magnesium version is also available)and a beautifully shaped line, and the Ducati 1098, which was given a single sided swingarm purely for styling reasons. Adventure]] 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 BMWs 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. For more detail, see the article on Shock Absorber s. 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 resevoirs, 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 shocks See Also: Twinshock On motorcycles with twin-shock rear suspensions, there is a shock on each side of the swingarm, which connects to its respective side of the motorcycle's frame or subframe. There are no linkages between the shocks and the swingarm or frame. It is important on twin-shock motorcycles that both shocks be the same, and that if they are adjustable, that the adjustments on both sides be the same. Otherwise, there can be a torque to the swingarm which may cause dangerous handling and braking characteristics. Mono-shocks 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 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 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 deisgned to give a rising rate of damping for the rear.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 286, Prentice-Hall, Inc., ISBN 0-13-160549-6 Honda refers to its mono-shock designs as ''Pro-link'' suspensions. Pre-load adjustment The pre-load on a rear shock is typically adjusted via a threaded or notched collar on the shock. As the collar is rotated, the coil-over spring is compressed more or less. The more the spring is compressed, the higher the pre-load, and vice versa. Some shocks, known as ''air-assist'' shocks, allow adjustment of preload by changing the air pressure inside the shock. A valve on the shock allows air to be introduced or released from the shock. More air pressure gives more preload, and vice versa. On motorcycles with twin shocks it is very important that both shocks have the same pre-load. Dangerous handling and braking characteristics can result from dissimilar pre-load adjustments. Damping adjustment Stock rear shocks typically offer no damping adjustment, or a single adjustment for both compression and rebound damping. This adjustment is usually made by a dial at the very top or very bottom of the shock. The dial selects one of a few different orifice sizes for the damping fluid flow path. The larger the orifice, the less the damping, and vice versa. On motorcycles with two rear shocks, it is very important that the damping on both shocks be set the same. Dangerous handling and braking characteristics can result from dissimilar damping on the rear shocks. = Setting up a suspension = Many older motorcycles do not offer much in the way of suspension adjustments. Simple pre-load adjustment for the rear shock(s) and nothing more is not uncommon. However, on modern motorcycles with higher specifications both pre-load and compression adjustments are standard, on both front and rear suspension. Some models as of 2006 offer both high and low speed compression adjustment. Even those motorcycles that do offer adjustments sometimes cannot be adjusted far enough for some riders. Properly tuning motorcycle suspension often requires assistance from an expert due to the range of adjustment possible. It is very difficult for the average rider to determine what adjustments need to be made, as a poorly set up rear end may cause front-end problems, and vice versa. TRAVEL The total ''travel'' of a suspension is the distance the suspension travels between total compression and total extension. The travel distance on off-road and dual purpose bikes tends to be very high; the rear suspension travel on cruisers tends to be relatively little. This value is usually listed in the motorcycle's manual, or available on-line. Adjusting the suspension based on the travel is the easist place to start tuning a suspension for a given rider. Ideally, the suspension should ''sag'' under the weight of the rider by 30 percent of the total travel. Start by measuring a distance between two points along the suspension's travel with the bike upright but without any of the rider's weight on the bike. For example, measure the distance from the front axle to where the fork enters the bottom of the Triple Tree . On the rear, measure from the rear axle to a point on the frame directly above it. Next, the rider should put as much of his or her weight on the bike while holding it upright. Measure between the same two points, and find the difference in the two measurements. This should be approximately 30 percent of the total travel of the suspension. Increase the preload to reduce the sag; decrease the pre-load to increase the sag. SPRING RATE A spring's ''rate'' is a measure of how much force is require to compress the spring a given distance. The higher the rate, the more force it takes to compress it a given distance, and the less it compresses under a given force.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 279, Prentice-Hall, Inc., ISBN 0-13-160549-6 If the sag of a motorcycle's suspension for a given rider cannot be properly set using pre-load adjustments, typically the spring must be replaced with one of a different rate. If the sag is too great, a higher rate spring must be used, and vice-versa. In most telescopic forks, the springs can be replaced in a straight-forward manner. The coil-over springs on the rear shocks can be another matter. Not only can they be of a unique design, the shock itself may be incompatible with a different rate spring. Typically, rear shocks are changed along with the springs. Progressive-rate springs are springs whose rate changes as the spring is compressed. As the spring is compressed the rate increases. Springs can be progressive either by having the coils at one end of the spring wound differently than the other, or by actually being two separate springs with different rates held together by a spacer.Motorcycle Mechanics Institute,''The Complete Guide to Motorcycle Mechanics'', 1984, p. 279-280, Prentice-Hall, Inc., ISBN 0-13-160549-6 Progressive rate springs are intended to give the best of both worlds: a smooth ride, yet response handling over rough surfaces. For maximum suspension performance, however, straight-rate springs are usually recommended. DAMPING Damping prevents the springs in the motorcycle's suspension from Oscillating , and the motorcycle from bouncing down the road like a Bobblehead Doll . When, for example, the front wheel encounters a Convex bump in the road, the suspension should compress when going up the bump, and extend when going down the bump, maintaining contact with the road at all times. Then the suspension should stop moving. Without proper damping, the suspension may continue compressing after the top of the bump has been reached, causing the tire to jump off the road. Or, it may continue to extend and compress after the bump, causing the front end of the motorcycle to bounce. Compression damping affects the compression of the suspension, and '''rebound''' damping affects the extension of the suspension. In the convex bump example, compression damping affects the suspension's behavior traveling up the bump, and rebound damping affects its behavior down the bump. Most stock suspensions do not offer separate rebound and compression damping. The handling of a motorcycle with too little damping will feel soft and bouncy. The handling of a motorcycle with too much damping will feel stiff and jittery. The proper damping will vary with the rider, the rider's style of riding and the road conditions. To tune the damping for street riding, it is usually easiest to start with the damping too soft, then adjust the damping progressively stiffer until the motorcycle's handling no longer feels soft and bouncy. High-end racing shocks offer low-speed compression damping, high-speed compression damping, low-speed rebound damping and high-speed rebound damping. The nuances of properly adjusting all these settings is beyond the scope of this article. = See also =
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