Repair guides › Suspension
Set fork sag
A fork with a spring (coil or air) and damper so the front wheel can move up over bumps. Stanchions (upper tubes) slide into lowers on bushings. A spring holds you up; a damper controls how fast it rebounds. Wiper seals keep dirt out. Air forks are tuned with a shock pump; coil forks with a preload knob.
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Tools
- Tape measure + 6" steel ruler: Stanley 16 ft tape measure ($)
- Shock pump: RockShox High Pressure Shock Pump ($$)
- Sag O-rings (suspension setup): ZDBB rubber O-ring assortment kit ($)
Compare Good, Better and Best for each tool
Steps
- Look up the fork travel (printed on the lowers or in the manual).
- Air fork: set pressure to the chart on the fork leg for your weight. Coil: start with preload knob in the middle.
- Push the O-ring on the stanchion down to the seal.
- Set compression fully open. Air forks: take the pump off, then slowly compress the fork 25–30% of its travel 5 times to equalize the air chambers, and re-check the pressure (never compress it with the pump on).
- Get on in riding gear in your normal riding position (seated for XC and trail; standing attack position only for DH/park), bounce lightly, let it settle, then step off gently.
- Measure how far the O-ring moved. Aim for 15–20% of travel for XC and trail (Fox and RockShox), up to 20–25% for enduro/DH.
- Too much sag: add air (5 psi at a time) or preload. Too little: let air out. Coil fork that can't reach sag within the preload range: fit a different spring rate.
- Set rebound so the fork comes back fast but does not kick you on a curb drop.
Diagram
Side view, chain side toward youFork
Signs it needs doing
- Fork bottoms out with a clunk
- Oil ring on the stanchions
- Sticky, notchy movement
- Fork tops out with a clunk (rebound too fast)