Suspension design across models (and years) of dirt bikes varies significantly, though the functional concept remains unchanged in the basic sense. With regard to springs in the forks, for example, there may be a spring on one side but not the other, springs on both sides or none at all (as with pneumatic spring forks). In all cases, spring action must be present to hold and return the suspension to the ride height. This means the springs must absorb energy when loaded and release energy when unloaded. For the springs to do so properly, the spring rate must be set according to the combined rider and bike weight.
What then is the spring rate? The spring rate is the amount of displacement that occurs based on the amount of force applied to the spring. For a conventional steel wound spring, the spring rate is linear and is represented mathematically as k=F/x where k is the rate, F is the applied force and x is the amount of displacement. Typically, within the realm of dirt bike suspension, steel wound springs are specified as having rates with units of kgf/mm (kilograms of force per millimeter of displacement). If, for example, you have a fork spring with a rate of 0.49 kgf/mm, this means that for every 0.49 kgf applied to the end of the spring, 1 mm of displacement will occur. If 0.98 kgf is applied, 2 mm of displacement would occur and so on. For a bike with a fork spring on each side, the total combined spring rate is twice that of a single spring. If two identical springs were stacked together end to end, the effective spring rate would be halved.
Most shocks and forks on motorcycles have springs, but they also have nitrogen gas or air in them. Of course, some forks do not have springs and rely on compressed gas as the spring. Regardless, the gas has its own characteristic spring rate but it is not linear like steel wound springs. The gas adds a progressive component to the spring rate. As the forks (or shock) compresses, the gas compresses as well. For every incremental increase in gas compression, the effective spring rate exhibited by the compressed gas also increases. This concept eludes to my skepticism about the use of pneumatic spring forks. With no steel springs, using only compressed gas as the spring, adjustability is reduced because the linear component is absent. Sprung, sealed cartridge forks with springs are more tunable because you can adjust the linear spring rate by changing the springs and you can adjust the progressive component by adjusting the oil level (which serves to change gas volume, thus changing the progressive action). With pneumatic spring forks, there is no linear spring rate adjustment.
In a suspension, spring rate is critical for optimum performance. Equally important is spring preload. By preload we mean the amount a spring is compressed when a suspension is static (not moving) with a rider on the bike in the neutral riding position. In general, the suspension stroke should sag approximately 1/3 (33%) of its full travel. Of course, the percentage of sag may vary depending on suspension design, rider ability, riding style and rider preference. Sag is important because the suspension must not only be able to compress when hit, it must also be able to extend into holes and dips. It is important to realize that sag can be correctly set with an incorrect spring rate. If a shock spring is too soft, for example, more spring preload would need to be added to set sag to the proper distance. However, this results in too little free sag (sag when the rider is not on the bike). Free sag is important because the suspension needs free sag to prevent a shock from fully extending prior to releasing all of the stored energy. With insufficient free sag, a rear shock would have the tendency to kick the rear of the bike up on rebound.
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