What is the dirt bike suspension

Dirt bike suspension varies in design depending on model and vintage. However, the basic idea of suspension remains constant in that there is a set of forks up front and a shock (or shocks) at the rear. The function of the suspension is to absorb bumps and impacts and to keep the wheels tracking against the ground properly. A suspension can be tuned in such a way as to facilitate good bike handling yet be incapable of absorbing big impacts. The opposite is true as well. Herein lies the art of suspension tuning, to create a suspension suited for an intended use, riding ability and combined rider and bike weight.

Concepts must exist within the suspension for it to work. When thinking about the suspension, absorbing hits is often what comes to mind first, which is essentially the whole idea. With absorbing comes other issues. To absorb a hit, a suspension must be allowed to compress while at the same time providing absorbtion in a controlled manner. Once the energy of a hit is absorbed, it is then stored until released. This absorbtion is achieved through the use of springs and pressurized gas, which also serve to return the suspension to the ride height. Some suspensions do not use springs and use only pressurized gas. Gas (usually nitrogen or air) is compressible and therefore acts as a spring, though the rate of compression is different than springs.

Simply resisting and returning the suspension is not enough, however. To properly control the absorbing and releasing of the energy, damping is used. Damping refers to the flow of oil, which is incompressible, through orifices and valves to limit (control) the speed at which a suspension moves. Damping adjusters are often provided to allow external adjustment for tuning purposes. Damping is required separately for compression (the collapsing of a shock) and extension (known as rebound). Without damping, a suspension would operate without control, would oscillate and likely do other things, none of which would be good.

Motorcycle forks and shocks look quite different from one another. However, their design is essentially the same in the function they perform. In either case, they must maintain ride height, absorb energy, control the speed of compression, control the speed of rebound, and be properly tuned for the intended use. To perform these functions, forks and shocks have similar components as well. They have pistons, valves, springs (gas and/or mechanical), orifices, accumulators and damping adjusters.

When a suspension is compressed, the spring stores energy. While movement in compression occurs, an internal piston, fixed to a rod, is displaced within an oil filled cylinder. This displacement forces oil through compression valves and/or orifices, moving oil to the opposite side of the piston, where the volume within the cylinder is increasing. At the same time, the overall suspension volume is decreasing as the length of the shock or fork is reduced. Consider a shock, for example: When compressed, the internal oil cylinder volume is reduced as the rod of the shock enters. To accommodate this reduction in volume, a gas filled accumulator is used to compensate. As oil volume is displaced by the incoming rod (or telescopic tube as in a fork) the accumulator compresses, thus providing added volume for the displaced oil.

When a suspension rebounds, there is no longer sufficient load on it to keep it in compression, so it releases the stored energy within the spring. As with compression, oil, during rebound, flows through valves and/or orifices as the piston within the oil filled cylinder is displaced. Oil moves back to the side of the piston where the cylinder volume is now increasing. As this is happening, the accumulator volume expands to accommodate the overall volumetric increase of the extending suspension.

The image below shows a simplified representation of a shock absorber. Click the image to view larger.

Different and emerging technologies in suspension design do exist, so naturally there are differences between models and years. Regardless, the basic fundamentals of physics do not change. Understanding a single design concept may be sufficient knowledge to grasp other designs as they are encountered. A list of some of the dirt bike suspension variations you may encounter is as follows. This list is by no means exhaustive and is intended to provide awareness rather than being comprehensive.

Types of Forks
  • Sealed cartridges with springs.
  • Sealed cartridge pneumatic spring forks (Ex: PSF).
  • Sealed cartridge multi-stage pneumatic spring forks (Ex: PSF).
  • Open chamber inverted forks (Ex: Many 90's vintage motocross bikes).
  • Conventional (non-inverted forks) (Ex: Many 80's vintage motocross bikes).
Variations of forks
  • Both sides dampen compression and rebound; both sides have springs (most models).
  • One side dampens compression, the other side dampens rebound; both have springs (Ex: Older KTM EXC's, possibly others).
  • One side dampens compression and rebound; the other side has a spring (Ex: RMZ SFF).
  • Varied adjusters and adjuster locations.
  • Bladder style accumulator.
  • Piston style accumulator.
  • Straight rate design.
  • Rising rate design.
  • Varied adjusters.


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