Sag is how much your suspension compresses under your static weight in riding position. It is the first thing to set on a new bike, the first thing to check after a long off-season, and the cheapest fix for a bike that feels wrong. Get sag right and most other dials become a lot more honest.
This guide walks through how to set sag on both your fork and rear shock, what gear you need, how the percentages map to your style of riding, and the small mistakes that cost you time on the trail.
What sag is, and why it matters
Sag is the percentage of total suspension travel that compresses when you sit on the bike in your normal riding stance. A 160 mm fork at 20% sag has settled 32 mm into its travel before you have ridden a single foot.
That settled position is where your suspension starts working. With too little sag the wheel cannot drop into holes, so small bumps feel sharp and the bike loses traction in corners. With too much sag the wheel is already partway through travel before a bump arrives, so you blow through the rest of the stroke on every hit and the bike wallows in turns.
Most riders are running too little sag. Air pressure recommendations on the fork leg are starting points, not final settings, and they assume an average rider in average kit on average trails. None of that describes you.
What you need
You can set sag with about ten minutes and three things:
- A shock pump. Standard MTB shock pumps go up to 300 psi or so. Cheap ones lose a few psi when you remove them. The good ones do not. If you do not own one, borrow it. Floor pumps will not work.
- A wall, a doorframe, or a friend. You need something to balance against without putting weight on your hands or feet.
- Your full riding kit. Helmet, pack, water, shoes, knee pads, the works. Five extra pounds shifts your sag noticeably.
That is it. No ramps, no scales, no apps required.
Setting fork sag
Start with the fork because it is easier to read and harder to bias.
- Push the rubber o-ring on the fork stanchion all the way down against the wiper seal. If your fork does not have an o-ring, wrap a thin zip tie around the stanchion and slide it down.
- Stand the bike upright. Mount up like you would for a real ride: hands on the bars, feet on the pedals, weight evenly distributed. Sit back slightly into your normal trail-riding stance, not perched forward like you are climbing.
- Brace yourself against a wall with one hand or have a friend steady you. Do not bounce. Do not push down. Let your weight settle for two or three seconds.
- Step off carefully. The o-ring stayed where the fork settled.
- Measure from the wiper seal to the o-ring. That number, divided by your fork's total travel, is your sag percentage.
So a 160 mm fork with the o-ring 32 mm from the wiper is at 20% sag. A 140 mm fork with the o-ring at 35 mm is at 25%.
If you are above your target, add air. If you are below, let air out. Both go in 5 psi steps. Push the o-ring back down between each check, settle on the bike again, step off, measure. Three or four iterations gets most riders close.
Setting shock sag
The shock is the same idea with one extra wrinkle: how you stand on the bike matters even more.
- Push the o-ring on the shock shaft against the air can.
- Sit on the saddle with your feet level on the pedals, weight forward over the bottom bracket like a real attack position. Sitting straight up biases sag toward the rear and you will set the shock too soft.
- Brace, settle, step off without bouncing.
- Measure from the air can to the o-ring. Divide by the shock stroke, not the rear wheel travel. The stroke is printed on the shock itself, usually something like 65 mm or 55 mm.
- Adjust pressure in 10 psi steps until you land in the 25–30% window.
If your bike has a flip chip or geometry adjustment, set sag in the position you actually ride. Switching from low to high after setting sag changes leverage ratios and shifts your numbers.
What percentage should you actually run?
The 20–25% fork and 25–30% shock starting point covers most trail riding. From there, ride style nudges the numbers:
- Cross-country and short-travel trail bikes: lower end of both ranges. 18–20% fork, 22–25% shock. You want a firmer ride that pedals well and does not bob.
- Trail and all-mountain: the middle of the range. 22% fork, 28% shock is a solid all-around target.
- Enduro and freeride: upper end. 25–28% fork, 30% shock. You want the wheel to track aggressive terrain and you are giving up some pedaling efficiency for traction and confidence.
- Park and downhill: 28–30% fork, 30%+ shock. Every hit is a big hit. You need the bike sitting deep into its travel.
These are starting points. The right number for you depends on terrain, riding style, and how the bike feels after a few rides. The 30% sag rule has its own caveats that are worth understanding once you are past the basics.
Common sag mistakes
A few things trip up riders on every bike forum:
- Setting sag in pajamas. A full kit can weigh five to fifteen pounds depending on your pack and pads. That can move shock sag by 5%. Always set sag dressed to ride.
- Forgetting to recheck after a service. Every fork and shock service changes the volume slightly, even on a perfect rebuild. Check sag the first ride after any service. The fork service intervals guide explains why.
- Reading off the rear wheel travel instead of shock stroke. Sag is measured on the shock, not on rear axle movement. Use the stroke length printed on the shock body.
- Tuning damping before sag is dialed. Compression and rebound feel completely different on a bike with bad sag. Get the spring right first, then start working on the dampers. The compression damping guide is the next stop.
- Setting it once and never checking again. Air springs lose pressure slowly. Coil springs settle and seat. New riders gain or lose weight. Sag drifts. Check it once a month if you ride often.
When to recheck
Add this to your tune-up rhythm:
- After every fork or shock service.
- Whenever your weight changes by more than five pounds.
- After any major kit change like a heavier pack, new body armor, or going from flats to clipless with different shoes.
- The first ride of the season if your bike has been sitting all winter.
- Any time the bike starts feeling off and you cannot pin down why.
It takes ten minutes. There is no setup task with a higher ratio of impact to effort.
Where to go from here
Once sag is set, the rest of suspension setup actually starts to make sense. Your dampers were always doing their job, but they were doing it from the wrong starting position. Move on to rebound damping, then compression. If your bike still feels wrong after that, the reading suspension symptoms guide can help you translate what the bike is telling you into the right dial to turn.