How to measure sag
- L1, fully extended: lift the bike on a stand so the wheel hangs. At the rear, measure straight up from the rear axle centre to a fixed mark on the rear fender or subframe. At the front, measure the exposed fork from the dust seal to the axle clamp, or use the same axle to fixed point method.
- L2, bike on its own weight: put the bike on its wheels, upright, and measure again between the same two points.
- L3, rider on: sit on the bike in riding gear, in your normal riding position, feet on the pegs with a helper holding the bike upright. Measure again.
- Travel: the wheel travel of that end, from your owner's manual or service manual.
Push the bike down and let it rise, then lift it and let it settle, and average the two readings. Seal friction can hold the suspension a few millimetres off either way.
The formula
static sag = L1 − L2 rider sag = L1 − L3 % of travel = sag ÷ travel × 100Worked example
Rear: L1 600 mm, L2 565 mm, L3 495 mm, 300 mm of travel. Static sag is 600 − 565 = 35 mm (11.7% of travel). Rider sag, also called race sag, is 600 − 495 = 105 mm, which is 35% of travel. With a target of 100 mm, that is 5 mm too much sag.
What target to use
Use the rider sag figure in your owner's manual or the one your suspension tuner gives you. Targets differ between dirt bikes, road bikes and linkage designs. On dirt bikes a common starting point is around one third of the rear travel; road bikes are usually set with less. If the rider sag is right but the static sag looks very small or very large, the spring rate may not suit your weight: that is a question for the manual or a tuner.
Adjusting
Spring preload moves the sag. More preload means less sag; less preload means more. On an air shock or air fork, air pressure does the same job. Change it in small steps and measure again.
Questions
How do you calculate sag on a motorcycle?
Measure the distance from the axle to a fixed point with the wheel hanging (L1), with the bike on its own weight (L2) and with you on it (L3). Static sag is L1 − L2 and rider sag is L1 − L3. With 600, 565 and 495 mm that is 35 mm and 105 mm.
What is the difference between static sag and rider sag?
Static sag is how far the bike settles under its own weight. Rider sag, also called race sag, is how far it settles with you on it in riding gear.
How much sag should a dirt bike have?
Use the figure in your owner's manual. A common starting point is around one third of the rear travel, which is 100 mm on a bike with 300 mm of travel.
How do I work out sag as a percentage?
Divide the sag by the suspension travel and multiply by 100. 105 mm of rider sag on 300 mm of travel is 35%.