Suspension (motorcycle)
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[edit] 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 absorbers.
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.
[edit] Twin shocks
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.
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.
[edit] 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 designed to give a rising rate of damping for the rear.[7]
Honda refers to its mono-shock designs as Pro-link suspensions.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.[8]
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. Even when the sag is set correctly sometimes the springs have to be replaced. This is dependent on the weight of the rider. If the rider is too light for the design of the springs the ride will be harsh, even when sag is correct. If the rider is too heavy the ride may be mushy, brake dive may be excessive, etc.
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 but the shock itself may be incompatible with a different rate spring if it lacks sufficient damping adjustment.
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.[9] For most modern sports bikes, progressive rate springs are not recommended unless factory fitted as the suspension linkage will usually have a progressive action. Fitting a progressive spring to such a linkage would cause the suspension to be too soft at one end of the linkage's movement and too hard at the other.
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.
[edit] 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 terms “low-speed†and “high-speed†are somewhat confusing as they do not refer to the speed of the motorcycle. Rather, the terms refer to the speed at which the shock absorber shaft is moving. Low-speed damping adjustments are used to ensure the bike is stable over gentle undulations in the road surface. High-speed damping deals with sharper variations in road surfaces and when the bike transitions from heavy acceleration to heavy braking or vice versa. The nuances of properly adjusting all these settings is beyond the scope of this article.
[edit] See also
1976 KZ900
2003 ZX12R
2007 FZ1000
2004 ninja 250R for wife
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