Rear Suspension Preload and Setting Sag

Preload does not make the suspension stiffer. Knowing what it actually does fixes most complaints.

The most common belief about preload is that it makes the suspension harder, and it is wrong. Preload does not change the spring rate. The spring is as stiff as it was when you bought it. What preload changes is where in its travel the bike SITS with you on it — the ride height, and therefore how much travel is left for bumps. Understanding that one distinction fixes most "my suspension is terrible" complaints without buying anything.

Tools

• Tape measure
• A helper (this cannot be done properly alone)
• Spanner or tool for your preload adjuster
• Shock pump if yours is air-adjustable
• Service manual

What preload actually does

A spring compresses a certain distance per pound of load, and that relationship is fixed. Preload compresses the spring BEFORE you sit on it, so the bike starts higher in its travel and there is more of it left before you run out.

Too little preload and you sit too deep in the travel. The suspension then runs out on medium bumps and crashes into its bump stop — which feels exactly like "too hard", so riders add stiffness they do not need or, worse, remove the preload that would fix it.

Too much preload and the bike rides high, the suspension barely moves over small inputs, and the ride feels harsh and skittish because the wheel is not following the road.

If the bike is genuinely too soft or too harsh WITH sag set correctly, that is a spring rate problem — the wrong spring for your weight — and no amount of adjustment fixes it.

Measuring sag, which is the whole job

Sag is how far the suspension compresses under load. You need two measurements and a helper; doing it alone gives numbers you cannot trust.

FREE SAG: with the rear wheel fully off the ground and the suspension fully extended, measure from the rear axle to a fixed point vertically above it. Write it down. Use the same two points every time.

RIDER SAG: with the bike upright, you on it in your normal riding gear, feet on the pegs and a helper steadying it, measure again between the same two points.

The difference is your sag. The target figure is model-specific — take it from your service manual. Measure with the gear and luggage you actually ride with; setting sag in jeans and then loading panniers and a passenger makes the number meaningless.

Bounce the bike and let it settle before each reading. Suspension has friction ("stiction") and a reading taken without settling can be out by a useful amount.

Adjusting

Add preload if sag is too much; reduce it if sag is too little. Change one thing, re-measure, repeat. Adjusting both ends and the damping at once tells you nothing.

On air-adjustable shocks, use a proper shock pump and the pressure range in your manual. A garage compressor will overshoot instantly and can damage the shock seals. Air shocks also lose pressure slowly, so check them periodically rather than assuming last year's setting is still there.

Set BOTH shocks identically on a twin-shock bike. A difference side to side makes the bike feel like it is steering itself.

Re-check drive belt tension after any significant ride-height change — the geometry between the gearbox and the rear wheel has moved.

Parts for this job

Check fitment for your own model and year before ordering — add your bike and we will flag what fits.

Other Harley how-to guides

This guide covers procedure and judgement. Torque figures, fluid capacities, gaps and tension settings are specific to your model and year and must come from the factory service manual for your motorcycle — we deliberately do not print numbers we cannot stand behind for your bike. Independent aftermarket retailer, not affiliated with Harley-Davidson, Inc.