Suspension Sag Calculator
Rider sag and static sag from three measurements — and the static figure tells you whether the spring itself is right for your weight.
Measure axle to a fixed point on the bodywork, the same two points every time. The absolute number does not matter — only the differences between the three do.
Is the spring right for you?
The bit everyone gets backwards
Rider sag is set with preload, so you can always hit the target. What it costs you to hit it is what static sag reveals — and the direction is the opposite of what almost everyone guesses.
A spring that is too soft needs a lot of preload to bring rider sag down to target. All that preload is exactly what stops the bike's own weight compressing the spring, so static sag comes out small. A spring that is too stiff needs hardly any preload, so the bike's weight moves it relatively more and static sag comes out large.
So: too little static sag means the spring is too soft, and too much means it is too stiff. No amount of preload fixes either — preload moves where the spring sits, not how stiff it is. That is a spring change, and it is the reason a heavier or lighter rider than the factory assumed cannot dial the bike in with a spanner.
Targets by discipline
| Riding | Rider sag | Static sag | Notes |
|---|---|---|---|
| Road / sport | 30–35 mm | 5–15 mm | About a third of the rear travel, which on a sportbike is 30 to 35 mm. Touring two-up wants the top of that range or a little past it. |
| Track | 25–30 mm | 5–12 mm | Slightly less than road, to keep the geometry sharper and the rear from squatting under power out of a corner. |
| Adventure / touring | 40–55 mm | 15–30 mm | More travel and more weight, so more sag in absolute terms while staying near the same fraction of the stroke. |
| Motocross / enduro | 100–108 mm | 30–40 mm | The classic 100 mm figure, roughly a third of a 300 mm stroke. The static window of 30 to 40 mm is what tells you whether the spring rate is right. |
Measuring it properly
- Measure from a fixed point on the axle to a fixed point on the bodywork, and use the same two points every time. The absolute number does not matter; the difference between the three measurements is the whole measurement.
- Free length means the wheel fully off the ground with the suspension completely extended. On a bike with a side stand you need a paddock stand or a friend, and doing it half-heartedly poisons both other numbers.
- Rider sag is measured in full gear, sitting normally, feet on the pegs, with someone steadying the bike. Gear is several kilograms and it is on the bike every time you ride.
- Bounce the suspension and let it settle from both directions before each reading. Stiction can hide five millimetres, which on a road bike is most of the adjustment range.
- Sag is the first setting, not the last. Damping adjusters cannot compensate for the wrong spring, and time spent on clickers before the sag is right is time wasted.
How to use
- Pick what you ride so the tool knows which targets apply.
- Measure fully extended, on its wheels, and with you aboard in full gear.
- Set rider sag with preload first, then read the spring verdict from the static figure.
Frequently asked questions
What is the difference between rider sag and static sag?
Rider sag is how far the suspension settles with you on the bike; static sag is how far it settles under the bike’s own weight. Rider sag is what you set with preload. Static sag is what tells you whether the spring rate is right.
What should my sag be?
Roughly a third of the travel. On a road or sport bike that is 30 to 35 mm of rider sag; on a motocross bike the classic figure is 100 mm. Adventure bikes sit between, with more travel and more weight.
Too little static sag means the spring is too what?
Too soft — which is the opposite of what most people guess. A soft spring needs a lot of preload to bring rider sag to target, and all that preload is what stops the bike’s own weight compressing it, so the static figure comes out small.
Can I fix a wrong spring with preload?
No. Preload changes where the spring sits in its travel, not how stiff it is. If static and rider sag cannot both be in range at once, that is a spring change — which is why a rider much heavier or lighter than the factory assumed cannot dial the bike in with a spanner.
Do I measure in full gear?
Yes, sitting normally with feet on the pegs and someone steadying the bike. Gear is several kilograms and it is on the bike every time you ride, so leaving it off measures a machine you never actually ride.
Why bounce the suspension before each reading?
Stiction. Let it settle from both directions and take the average, because friction in the seals can hide five millimetres — which on a road bike is most of the usable adjustment range.
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