Measure suspension sag

Home mechanic (basic tools) · Time: 20 to 30 min (editorial estimate) (estimate)

Sag is how far the suspension sinks under the bike's own weight (static sag) and with the rider on board (rider sag). It shows whether the spring and preload suit the load. On a commuter bike it is also a simple way to see if a spring has become tired: a tired spring lets the bike sit lower than before. This page gives the method. No target number is given here, because it depends on the bike and the load; use your manual or the shock maker's figure if one is published, and otherwise compare left with right and today with your earlier records.

Safety

• One person sits on the bike and one measures. Keep the bike upright and steady; do not let it fall on the measurer.

You need

• A tape measure, a helper and a notebook.

Steps

1. Choose two fixed points at the rear: the centre of the rear axle and a point straight above it on the frame, grab rail or bodywork. Mark the upper point with tape.
2. Fully extended: lift the rear wheel off the ground (centre stand) so the suspension hangs. Measure from axle to the mark. Call it A.
3. Static sag: take the bike off the stand, hold it upright with no one on it, push down and let it settle. Measure again. Call it B.
4. Rider sag: the rider sits in their normal position with feet on the pegs, wearing their usual gear and carrying their usual load, while the helper holds the bike upright. Measure again. Call it C.
5. Work out: static sag = A minus B. Rider sag = A minus C.
6. Front: fit a cable tie round one fork stanchion and push it down against the dust seal. Lift the front wheel off the ground, then put it down, sit the rider on, and let them get off carefully. Measure how far the cable tie moved up the tube. Also look at the tie after a ride with a heavy load: if it is at the very top of the tube, the fork used all its travel.
7. Twin rear shocks: check both are on the same preload step before you measure.
8. Write all readings and the preload setting in your service record.

Check your work

• You have A, B and C written down with the preload setting.
• If your manual gives a sag value, compare it. If rider sag is too large, add preload with Adjust rear spring preload; if it is still too large at maximum preload, the spring is too soft or tired.
• If static sag is much bigger than when the bike was new or than an identical bike, the spring is tired.

If it did not fix the problem

• Rear too low: Rear sits too low.
• Fork uses all its travel: Front forks bottom out.