Correct rear suspension setup can improve traction, steering response, braking stability and rider control on a motocross motorcycle.
Use the calculator below to measure static sag and rider sag. The result gives you a useful starting point for checking spring preload and whether the fitted spring rate is in the right range.
Raise the rear wheel clear of the ground so the suspension is fully extended.
Measure from the rear axle to your chosen fixed reference point and record this as measurement A.
Place the motorcycle on level ground without the rider.
Bounce the rear suspension lightly, allow it to settle, then record the measurement as measurement B.
Sit or stand on the motorcycle in your normal riding position while wearing your normal riding gear.
Have another person record measurement C.
Static sag = measurement A minus measurement B.
Rider sag = measurement A minus measurement C.
| Measurement | General Starting Point |
|---|---|
| Rider Sag | Approximately 30% to 35% of total rear wheel travel. |
| Static Sag | Approximately 5% to 12% of total rear wheel travel. |
These figures are general starting points only. Modern motocross manufacturers may specify a particular sag value in millimetres rather than a percentage.
Always check the motorcycle owner's manual, workshop manual or suspension setup information for the specific model where available.
Spring preload changes the motorcycle's ride height and sag.
Increasing preload reduces rider sag. Reducing preload increases rider sag.
Preload does not change the actual spring rate.
Spring rate determines how much force is required to compress the spring.
If correct rider sag requires excessive preload or almost no preload, the spring rate may not be suitable for the rider.
Compression damping controls resistance as the shock compresses under bumps, acceleration and landings.
Too little compression damping may make the rear feel soft or wallowy. Too much can make the motorcycle feel harsh and reduce traction.
Rebound damping controls how quickly the shock extends after being compressed.
Too little rebound damping can allow the rear to kick or bounce. Too much can prevent the suspension recovering quickly enough between repeated bumps.
Some modern motocross shocks provide separate high-speed and low-speed compression adjustment.
These terms refer primarily to shock-shaft movement speed rather than motorcycle road speed.
Low-speed compression generally affects chassis movement such as acceleration squat, braking and rolling terrain, while high-speed compression has greater influence on faster shock movements caused by sharp bumps and hard landings.
| Symptom | Possible Cause |
|---|---|
| Rear sits too low | Too much rider sag, insufficient preload or spring rate too soft. |
| Rear sits too high | Too little rider sag, excessive preload or spring rate too stiff. |
| Rear kicks upward over bumps | Rebound damping may be too light or the rear may be riding too high. |
| Rear packs down through repeated bumps | Rebound damping may be too heavy. |
| Harsh over square-edged bumps | Compression damping may be excessive, spring rate may be unsuitable or the shock may require servicing. |
| Bike wallows or feels unstable | Incorrect sag, insufficient damping, worn linkage components or shock condition. |
| Shock leaks oil | Damaged seal, worn shaft or shock requiring service. |
| Little or no damping | Internal shock problem, loss of oil or gas pressure, or service required. |
Rear suspension setup depends on more than the shock itself.
Inspect the swingarm, linkage and shock bearings for free movement, lubrication, corrosion and excessive play.
Worn or seized linkage bearings can affect ride height, suspension movement and the accuracy of sag measurements.
Start with the motorcycle manufacturer's standard settings where available.
Count adjuster clicks from the specified reference position and never force an adjuster beyond its mechanical stop.
If the shock has recently been serviced or replaced, confirm that the spring rate, preload and baseline settings suit the motorcycle and rider before making large damping changes.
Confirm the motorcycle make, model, year, shock type and exact component required before ordering springs, bearings, seals or replacement suspension parts.
Aftermarket suspension components may use different specifications from the original equipment.
Note: Suspension settings are rider- and model-dependent. Use the calculator as a setup aid, not as a substitute for the motorcycle manufacturer's specifications.