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Branton & Andette
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Step-by-step guides

The basics

Set your suspension sag

Time 20 min Medium

Sag is the share of travel your suspension uses when you're simply sitting on the bike. Set right, it gives a bike that grips the ground without bottoming out. It's the first adjustment to make, before any other.

What you need

  • A high-pressure shock pump (with a gauge)
  • A tape measure or a ruler
  • A wall, or someone to hold you steady

Why

Air suspension is set with air: the more there is, the firmer it gets. Too much and it barely moves, the bike bounces off rocks; too little and it sinks too far, bottoms out (hits the end of its travel) and the bike dives on descents. Sag measures that balance: how far the suspension sinks under your weight alone.

Values to aim for

Riding Fork Rear shock
Cross-country, smooth trails 15 to 20% 20 to 25%
Trail riding 15 to 20% 25 to 30%
Enduro, downhill 20 to 25% 30 to 35%

These are starting points. Fork makers (RockShox, Fox, SR Suntour…) most often give 15 to 20%; at the rear, RockShox recommends around 30% for most full-suspension bikes. Fox starts from 25% front and rear, and advises the same percentage at both ends of a full-suspension bike, a few points more for aggressive riding, a few less for cross-country. Your fork or bike manual gives its own range and a pressure chart by rider weight: that's the one to follow.

What you measure

1 2 3 4 5
1 stanchion · 2 O-ring · 3 dust seal · 4 travel · 5 measured sag

On the stanchion (1), the smooth part of the fork, a small O-ring (2) slides freely. Pushed down against the dust seal (3) before you get on, it moves up as the fork compresses and stays put when you get off: the distance (5) between the O-ring and the dust seal is the sag. You compare it with the travel (4) stated by the manufacturer (for example 120 mm).

Steps

1. Kit up as if you were riding

Helmet, shoes, a full backpack, a bottle: everything you carry counts towards your weight. Set the compression adjusters (blue dial, lockout lever) to open.

2. Inflate to the manufacturer's chart

Unscrew the valve cap, screw on the pump and inflate to the pressure recommended for your weight. The chart is often printed on the fork leg or under the cap; if not, look up your model's manual on the manufacturer's website. Measuring the sag then corrects that starting point.

When you unscrew the pump, a little air escapes: that's the air in the hose, not in the suspension. Don't pump up to make up for it.

3. Push the O-ring back down

Push the O-ring against the dust seal, on the fork and on the shock body.

4. Get on the bike, without bouncing

Lean against a wall or have someone hold you. Get on gently, take your riding position (seated for a cross-country bike, standing on the pedals for an enduro bike), let the suspension settle, then get off gently, without pushing down.

5. Measure and work it out

Measure the distance between the dust seal and the O-ring, then divide by the travel: for example, 25 mm on a 120 mm fork gives 21%. For the shock, compare it with its stroke (often printed on the body, for example 50 mm): many shocks even have sag markings in percent.

6. Adjust and repeat

  • Too much sag: add air, in steps of 5 psi (fork) or 10 psi (shock).
  • Too little sag: let a little air out with the pump's button.

Measure again each time, until you're within the target range. Note down the pressure.

7. Set the rebound

The red dial (rebound) sets how fast the suspension comes back up. Start in the middle of its range. Drop the front wheel off a kerb: it should come back up quickly without bouncing a second time. Too slow, and the suspension "packs down" over a series of bumps; too fast, and the bike kicks you back like a spring.

Check it's done right

  • Riding your usual terrain, you use almost all the travel on the biggest hits, without bottoming out hard.
  • The bike stays stable when pedalling and follows the ground through rough sections.
  • After a ride, the O-ring has stopped a few millimetres from the end of the stanchion.

Common mistakes

  • Measuring without your kit: a 5 kg backpack changes the sag.
  • Bouncing as you get on: it throws the measurement off.
  • Comparing with the wrong number: the visible smooth part is longer than the actual travel.

What next

After every muddy ride, wipe the stanchions with a clean rag: mud wears out the seals. A trace of oil or dirt stuck above the seals means a service is due. The logbook reminds you to check the suspension every month. Internal services are counted in riding hours: RockShox plans a lower leg service (oil and seals) every 50 hours, Fox a full service every 125 hours or once a year. They are often done at a bike shop.

Sources

This guide was checked against the following documents:

Values vary between models: your part's manual always takes precedence.