Sag

Key facts

Origin

English “to sag” — to sink under a load

Suspension should not sit at the top of its travel: some of it has to be used just holding you up, so the wheel can drop into holes as well as rise over bumps. That pre-used part is the sag.

Typical values

• Fork: about 20 % of travel; 15 % on cross-country bikes.
• Rear shock: 25–30 % of stroke; up to 35 % on downhill bikes.

Measuring it

Push the rubber O-ring down against the seal, stand on the bike in the attack position without bouncing, step off gently and measure how far the ring moved. Divide by the total travel or stroke. The full routine is in suspension setup.

Related articles

• Full-suspension bike — A mountain bike with suspension at both ends. The rear shock keeps the back wheel on the ground over roots and rocks, which means more grip, more control and less fatigue.
• Suspension fork — The telescopic front fork that lets the front wheel move up over bumps instead of throwing them into your hands. It is what steers the bike through rough ground.
• Rear shock — The spring and damper that control the rear wheel of a full-suspension bike. Its tune and setup decide whether the back of the bike grips, pedals and lands well.
• Suspension setup — Half an hour with a shock pump makes a bigger difference than most upgrades. Set the air pressure by sag, then rebound, then compression — one change at a time.
• Suspension travel — How far a wheel can move on its suspension, in millimetres — the first number in any mountain bike's description. More travel absorbs bigger hits; less travel pedals and weighs less.