Motorcycle Suspension Sag Calculator

Suspension sag is how much the bike compresses under its own weight (free sag) and under the rider’s weight (race sag, also called rider sag). Getting sag right keeps the suspension working in the middle of its stroke, with room to extend into a dip and room to compress over a bump.

Two measurements:

• Free sag (static sag): Bike on stand with no rider. Measures spring preload.
• Race sag (rider sag): Rider aboard in riding position. This is the key setup number.

Target race sag values:

• Sport/superbike: 25-30 mm rear, 30-35 mm front
• Naked / roadster: 30-35 mm rear, 35-40 mm front
• Adventure / dual-sport: 85-100 mm rear (longer travel), 55-65 mm front
• Cruiser: 30-40 mm rear, 30-40 mm front

How to measure:

1. Measure full extension (bike on paddock stand, wheel hanging free) = L1
2. Measure under rider weight = L2
3. Race sag = L1 - L2

Interpreting results:

• Race sag too HIGH (>target): Add preload (compress spring more)
• Race sag too LOW (<target): Reduce preload
• Free sag should be 5-10 mm at the rear. If free sag is 0 with correct race sag, the spring rate is too stiff for rider weight.

Why the targets differ so much between classes. Sag in millimetres only means something next to the travel it is eating. A sportbike has around 120 mm of rear wheel travel, so 28 mm of sag uses a bit under a quarter of it. An adventure bike has 200 mm or more, so its 90 mm target is a much larger slice, deliberately, because that suspension needs room to extend down into ruts and holes rather than only room to compress. A cruiser sits at the other extreme: short travel, so a normal-looking 35 mm of sag is already most of what it has, which is exactly why cruisers bottom out over speed bumps.

Two practical consequences. Quoting a sag target without saying what travel it assumes is close to meaningless, and if your bike’s travel is unusual for its class, enter it below so the percentages you get back are yours rather than a class average.

Front sag is measured differently. At the rear you measure from the axle to a fixed point on the tail. At the front you measure the exposed fork tube, from the dust seal up to the bottom triple clamp, which is why the numbers are smaller and why you cannot reuse a rear target on the front. Front travel is usually 110 to 130 mm on a road bike and 190 to 220 mm on an adventure bike.

Spring rate guide: A rough starting point is 1 N/mm of spring rate per 10 kg of total weight (rider plus gear plus luggage).