Three measurements, two answers
Every sag figure comes from the same pair of reference marks — one on or near the axle, one directly above it on the bodywork or fender — read in three different states.
| Measurement | State of the machine | What it gives |
|---|---|---|
| Extended | Wheel off the ground, suspension topped out | The zero everything else is measured from |
| Static | On its wheels, nobody aboard | Free sag = extended − static |
| Laden | On its wheels, rider aboard in gear | Rider sag = extended − laden |
Free sag is what the machine sinks under its own weight. Rider sag — also called race sag — is what it sinks under machine plus rider plus whatever is strapped to it. Both are measured from the extended figure, not from each other, which is why an error in the extended measurement poisons both at once.
Getting the extended measurement right is the fiddly part. The wheel has to be genuinely unloaded and the suspension genuinely at the top of its travel. A machine on a stand that is still taking part of the load through the swingarm gives an extended figure that is too small, which makes both sags read low, which sends you off adjusting preload to fix a measurement error.
What the two numbers tell you together
Rider sag on its own does not say much, because you can hit any rider sag figure you like by winding preload in or out. What the two sags say together is how you got there.
Think of it as the spring having a job to do. The machine plus rider compresses it by some amount that depends on the spring rate; the preload then decides where in the stroke that compression starts. Set preload high and the machine sits up, with very little free sag because it takes almost the whole machine weight just to move the spring off its preload. Set preload low and free sag grows.
So if you reach a given rider sag with almost no free sag, the spring is being asked to carry the load with a lot of preload, which is what happens when the rate is high for the weight on it. Reach the same rider sag with a large free sag and the opposite is true. This relationship is the reason both figures get measured, and it is also where this page stops. Reading it into a spring rate recommendation depends on the machine, the linkage, the rider and the intended use, and it is a suspension specialist decision rather than an arithmetic one.
Preload turns, and the linkage in between
A preload adjuster moves the spring seat. Move the seat by one millimetre and, to a first approximation, the spring sits one millimetre further into its travel and the sag at the spring drops by one millimetre. On forks, where the spring moves with the wheel, that is also one millimetre at the wheel.
At the rear of a linkage machine it is not. The wheel moves further than the shock does, by the motion ratio — commonly somewhere between two and three, and not constant through the stroke on a rising-rate linkage. A millimetre of preload at the shock is therefore two or three millimetres of sag change at the axle. The calculator above asks for that ratio rather than assuming it, and uses 1 for forks and direct-mount shocks.
The turns figure then needs the movement per turn of the adjuster. For a threaded collar that is the thread pitch, often between 1 and 2 mm. For a stepped ramp adjuster there is no such thing as a turn and the figure does not apply. If you do not know it, wind the collar a measured number of turns and measure how far the seat moved.
Repeatability beats precision
The measurements are worth less than the technique. Stiction — friction in the seals and bushings — means a suspension that has been pushed down and released settles higher than one that has been lifted and released, and the difference on a set of forks can exceed the adjustment you are chasing. Bouncing the machine and letting it settle, repeating each reading two or three times, and having the same person hold the machine each time all matter more than reading to a tenth of a millimetre.
Rider sag also moves with the rider. Gear, a tank bag, a pillion, a full tank of fuel: all of them are load, and load is what sag measures. A figure taken in shorts on an empty tank is not the figure that applies on a loaded machine, which is why the measurement is taken in riding kit.
The line this page will not cross
There is no correct sag figure here, and there is no spring rate recommendation. Published targets vary widely between road machines, motocross bikes, trail machines and ATVs, between manufacturers, and between what a manual says and what a tuner sets for a specific rider. This page converts your tape measure into millimetres, percentages and adjuster turns, and compares them against the target that you supply.
Everything past that — spring rates, damping, ride height, geometry — changes how a machine behaves under braking, over bumps and at lean, sometimes in ways that only reveal themselves at the worst moment. That is suspension work, and it belongs with someone who does it for a living.
Questions people ask
What is the difference between free sag and rider sag?
Free sag, sometimes called static sag, is how far the suspension settles under the weight of the machine alone with nobody on it. Rider sag, or race sag, is how far it settles with the rider aboard in full gear and in the normal riding position. Both are measured downward from the same fully extended reference, not from each other. They answer different questions: rider sag is about where in the travel the machine sits when it is being ridden, and free sag is mostly used as a cross-check on whether the rider sag was reached with a sensible amount of preload. Measuring only one of them tells you where the machine is sitting but not how it got there, which is why the procedure asks for three readings rather than two.
How do I measure sag on my own?
The extended and static readings are straightforward alone. The laden reading is not, because you cannot sit on the machine in the riding position and read a tape at the axle at the same time, and any attempt to hold yourself up with a foot or a hand changes the load you are trying to measure. Two helpers is the usual answer: one steadying the machine upright, one reading. With one helper, the rider steadies against a wall with a fingertip. Doing it alone with a zip tie on the fork leg gives you maximum travel used rather than sag, which is a different and also useful measurement, but it is not sag.
Does adding preload make the suspension stiffer?
Not in the sense of changing the spring rate, which is a property of the spring and is fixed. What preload changes is where in the spring travel the machine sits at rest, and therefore how much travel remains before the spring is fully compressed and how much force is present at the start of the stroke. Riders often describe the result as feeling stiffer, because with more preload the suspension sits higher and needs more force before it starts moving from static, and because it now has more travel available before bottoming. But over a given bump, the change in force per millimetre of movement is exactly the same as it was. If the machine needs a different force per millimetre, that is a spring rate question, and it is not one preload can answer.
Should I measure sag with a full tank of fuel?
Measure it in the condition the machine is actually ridden in, and be consistent about it, because fuel is load and load is what sag measures. A large tank going from empty to full can be twenty pounds or more of it, and on a machine where much of that weight sits ahead of the rider it moves the front and rear sag differently. The same applies to luggage, a pillion, tools, water and armour. What you cannot do is measure with an empty tank, set the sag, then ride loaded and expect the setting to hold. Riders who tour and riders who race often keep two sets of figures for exactly this reason.
Why do my two sag measurements not agree between attempts?
Almost always stiction plus reference points that moved. Friction in the fork bushings, the seals and the linkage bearings means the suspension does not settle to a single position — pushed down and released it sits low, lifted and released it sits high, and the band between the two can be several millimetres on forks. Bouncing the machine and letting it settle gently, then repeating the reading two or three times, closes most of the gap, and some tuners deliberately take a reading from each direction and use the midpoint. The other half of it is the reference marks: pointing a tape at approximately the same place on the fender each time is not the same as measuring between two marks made with a pen, and the difference shows up as exactly this kind of scatter.