Race sag is the change in distance from full suspension extension to ride height with riding on bike.
1. Put bike on stand so that wheels are off of the ground.
2. Measure from the center of the rear axle straight up to a point on the fender. Record this distance.
3. Have the rider sit on the bike in a normal riding position with the bike off of the stand. The rider should be wearing all of his or her gear.
4. Measure the distance form the rear axle up to the same point on the rear fender.
5. Calculate the difference between the two measurements. The difference should be approx. 1/3 of the suspension travel. The desired setting will differ from model to model but most 125cc-650cc bikes will require 100mm-110mm. Most 85cc bikes will require 95mm-100mm of race sag. 65cc bikes have a starting point of 80-85mm sag and 50cc bikes require 65mm of race sag.
6. If the difference is not within the desired range, adjust the preload on the shock spring to obtain the correct measurement.
Race sag should be properly set prior to checking static sag. Use the same method to measure static sag that is used to measure race sag. To calculate static sag you need the measurement with the bike on the stand and the measurement with the bike off of the stand without a rider on it. This will reveal the amount that the rear sags under its own weight. The appropriate range for a 125cc-650cc bike is 20mm-45mm. If the bike sags more than the desired range, the rider will need a softer shock spring. If the bike sags less than the desired range then the rider will need a stiffer spring.