Sag is not a suspension setting. It is a measurement that tells you whether the springs on your bike are right for your weight, and until it is correct, adjusting damping is guesswork. You need three measurements at each end: the bike fully extended on a stand, the bike on its wheels with nobody on it, and the bike on its wheels with you sitting on it in riding kit. The differences between those numbers are free sag and ride height, and they are the two figures every suspension setup sheet starts from. The whole job takes half an hour, needs a tape measure, a stand and a friend, and it is the single most useful thing most riders never do to their bike.
What preload does, and what it does not do
Preload compresses the spring before the bike is even moving. It does not make the spring stiffer — a spring's rate is fixed by its construction — and it does not change how much force is needed to compress it by a further 10 mm. What it changes is where in its travel the suspension sits when the bike is loaded, and therefore how much travel is left above and below that point.
That distinction matters because it explains the most common misdiagnosis in motorcycling. A rider who is too heavy for the standard spring winds on preload until the bike stops bottoming out, and now the suspension sits high in its stroke with almost no extension travel left. The bike stops absorbing the dips as well as the bumps, the front feels harsh, the rear skips over broken surfaces, and the rider concludes the suspension is "too stiff" and starts opening damping adjusters. The actual fault was the spring rate, and no amount of preload fixes it.
The corollary is that preload has a limited range for a reason. Öhlins publish the figure on their shock data sheets — the setup data for one of their road shocks lists a spring preload range of just 10 mm alongside a 67.5 mm shock stroke. If you need to run the adjuster near either end of that range to get sensible sag, that is the spring telling you it is the wrong one.
The six measurements
The naming below follows the Öhlins setup sheet, which is the format most suspension shops use. Do the front and the rear separately, and measure the same points every time.
| Measurement | Front | Rear | How to take it |
|---|---|---|---|
| Fully extended | F1 | R1 | Bike on a stand, both wheels off the ground, suspension hanging at full extension |
| Static, no rider | F2 | R2 | Bike on the ground, upright, nobody on it, bounced and allowed to settle |
| Laden, rider aboard | F3 | R3 | Bike on the ground, rider seated in full kit, feet on the pegs, held upright by a helper |
| Free sag | F1 − F2 | R1 − R2 | How far the bike settles under its own weight — tells you whether the spring is close |
| Ride height (rider sag) | F1 − F3 | R1 − R3 | Where the bike sits with you on it — the number setup sheets actually specify |
| Reference points | Fork slider to a fixed mark on the lower leg, or axle to a mark on the yoke | Rear axle centre to a fixed point directly above it on the subframe or bodywork | Mark them with tape so every session uses the same two points |
Doing it properly, step by step
- Set tyre pressures first. Sag measured on soft tyres is wrong at both ends, and the error is not small. Set them to the manufacturer's figures — our tyre pressure guide covers reading the placard for solo and laden conditions.
- Get the bike fully extended. Both wheels clear of the ground on a stand. If you can only lift one end, do that end and repeat for the other; see the stand selection guide for what your bike takes.
- Pick and mark your reference points. Tape on the fork lower and a mark on the slider at the front; the axle centre and a fixed point vertically above it at the rear. Measure F1 and R1.
- Lower the bike and settle the suspension. Push down on the seat and let it rise, then lift the rear and let it drop, several times. Friction in the fork bushings and linkage means the bike settles at a different height depending on which way it last moved, and the honest number is the average of a settling-down measurement and a settling-up measurement.
- Measure F2 and R2 with the bike vertical and nobody on it. A helper holding it upright is far better than a side stand, which tips the bike and corrupts the reading.
- Sit on the bike in full riding kit — jacket, boots, helmet, and any luggage you normally carry. Feet on the pegs, in your normal riding position, hands on the bars. Have your helper steady the bike and take F3 and R3.
- Do the arithmetic and compare against the target for your bike. Free sag is F1 − F2 and R1 − R2; ride height is F1 − F3 and R1 − R3. The target figures come from your owner's manual, or from the setup sheet supplied with an aftermarket shock or fork kit — they differ substantially between a sportsbike with 120 mm of travel and an adventure bike with 210 mm, which is why quoting a universal number is meaningless.
- Adjust preload and re-measure. Change one end at a time, in known increments, and take a full fresh set of measurements after each change. Record everything, including where the adjusters started, so you can go back.
The diagnostic that comes out of this is the reason to do it at all. If ride height is correct but free sag is very small — the bike barely settles under its own weight — the spring is too stiff for you and preload is masking it. If free sag is large and you have wound the adjuster near its limit to get ride height correct, the spring is too soft. Either way the answer is a different spring rate, not more turns of the adjuster.
What changes when you load the bike
Sag set solo is wrong the moment you add a pillion and luggage. Combined weight can rise by 40 percent or more, and the suspension sits deep in its travel with little left for a mid-corner bump. This is what the rear preload adjuster is for on a road bike, and it is why touring machines put a hydraulic remote adjuster on the shock — so you can add preload for two-up riding and take it off again without tools.
Set your solo baseline first and record it. Then set a two-up figure with your usual pillion and luggage aboard, record that, and treat the two as named settings rather than something you guess at in a hotel car park. Tyre pressures change for the same reason and in the same direction: manufacturers publish a higher laden figure, and running solo pressures fully loaded overheats the carcass. A Bluetooth TPMS sensor is genuinely useful here, because it shows you what the tyres are actually at when the bike is loaded rather than what you set them to last week.
When to stop and hand it over
Preload adjustment is within reach of any owner with a C-spanner and patience. Some things that sit next to it are not. Changing a spring means removing the shock and compressing the spring off the bike, which needs a proper spring compressor and is genuinely dangerous improvised — a compressed motorcycle shock spring stores a great deal of energy. Anything inside a cartridge fork or a shock body is a service-centre job. And if your measurements show that the suspension will not extend fully, or that one fork leg sits differently from the other, that is damage or seized components rather than adjustment.
Two condition checks are worth doing while the bike is on the stand. Look for oil on the fork sliders and around the shock shaft, and check for play at the swingarm and head bearings. Fork oil leaks and worn suspension components appear in roadworthiness standards for a reason — the UK's DVSA inspection manual for wheels, tyres and suspension sets out how those defects are graded, and it is a fair benchmark wherever you ride. If a fork is weeping, sag numbers are the least of it — see our fork oil and seal guide.
Once sag is right, damping is worth touching. Not before — the damping adjustment guide explains why compression and rebound clicks only make sense on a correctly sprung bike. And if the bike develops a new wobble or weave afterwards, work through the vibration checks before assuming the suspension is at fault. All figures in this article are indicative: use the values in your own owner's manual and factory service manual.
FAQ
What is the correct sag for a motorcycle?
It depends entirely on the bike, because sag targets are expressed relative to that model's suspension travel. A sportsbike with 120 mm of front travel and an adventure bike with 210 mm have very different figures. Use the number in your owner's manual or on the setup sheet that came with your shock or fork kit.
Does preload make the suspension stiffer?
No. It compresses the spring before you get on, which changes where the bike sits in its travel, but the spring rate is unchanged. If you are winding on preload to stop the bike bottoming, you probably need a stiffer spring rather than more preload.
Can I measure sag on my own?
Not the rider measurement. F3 and R3 need someone else holding the bike upright and reading the tape while you sit in your normal riding position — leaning over to read it yourself moves your weight and changes the number.
Should I set sag with my riding gear on?
Yes, and with anything you normally carry. Kit, boots, helmet and a full tank all count, and a set of loaded panniers changes the rear figure substantially. Set a solo baseline and a separate two-up setting, and write both down.
Do I need to redo sag after fitting new tyres or a new shock?
After a new shock or fork kit, always — start from the supplier's setup sheet. After new tyres, only if the profile or size changed, since that alters ride height slightly. Re-check anyway once a season, because springs settle and your own weight changes.
