VMX Owners Australia

Rear Suspension Setup & Sag Calculator

Correct rear suspension setup can improve traction, steering, braking stability and rider comfort on vintage motocross, enduro and trail motorcycles.

Use the calculator below to measure static sag and rider sag. The results provide a general starting point for adjusting shock spring preload.

Measuring tip: Take all three measurements from the rear axle to the same fixed point directly above it on the rear guard or frame.

Rear Suspension Sag Calculator

How to Measure Rear Suspension Sag

1. Fully Extended

Raise the rear wheel clear of the ground so the suspension is fully extended.

Measure from the rear axle to a fixed point above it and record this as measurement A.

2. Static Sag

Place the motorcycle on level ground without the rider. Bounce the rear suspension lightly and allow it to settle.

Record this as measurement B.

3. Rider Sag

Sit or stand on the motorcycle in your normal riding position while wearing your usual riding gear.

Have another person record measurement C.

4. Calculate Sag

Static sag is measurement A minus measurement B.

Rider sag is measurement A minus measurement C.

General Rear Sag Starting Points

Measurement General Starting Point
Rider sag Approximately 30% to 33% of total rear wheel travel.
Static sag Approximately 5% to 10% of total rear wheel travel.
Vintage twin-shock motorcycles May require model-specific settings due to shorter travel and different suspension geometry.
Trail and enduro motorcycles May use slightly more rider sag to improve comfort and traction.

These figures are general starting points only. Check the motorcycle workshop manual or suspension specialist recommendations where available.

Rear Suspension Adjustments

Spring Preload

Spring preload adjusts ride height and sag. It does not change the actual stiffness of the spring.

Increase preload to reduce rider sag. Reduce preload to increase rider sag.

Spring Rate

Spring rate determines how much force is required to compress the spring.

A soft spring may require excessive preload. A hard spring may produce too much static sag when rider sag is correct.

Compression Damping

Compression damping controls how quickly the shock compresses when hitting bumps, landing jumps or accelerating.

Too little may feel soft or wallowy. Too much may feel harsh and reduce traction.

Rebound Damping

Rebound damping controls how quickly the shock extends after being compressed.

Too little can cause bouncing. Too much can prevent the suspension from recovering between bumps.

Common Rear Suspension Problems

Symptom Possible Cause
Rear sits too low Too much rider sag, insufficient preload or spring too soft.
Rear sits too high Too little rider sag, excessive preload or spring too stiff.
Rear kicks upward over bumps Rebound damping may be too light.
Rear packs down through repeated bumps Rebound damping may be too heavy.
Harsh over square-edged bumps Compression damping may be too heavy, the spring may be too stiff or the shock may require servicing.
Bike wallows or feels unstable Insufficient damping, worn shock components or incorrect sag.
Shock is leaking oil Damaged seal, worn shock shaft or shock requires rebuilding.
Little or no damping Low oil, failed seals, contaminated oil or internal shock damage.
Safety notice: Rear shock springs are stored under significant pressure. Do not remove the spring or open a gas-pressurised shock without the correct equipment and experience. Use a qualified motorcycle suspension specialist where required.