How to Set Sag on a Mountain Bike: Fork and Shock Setup for Trail Riding, Step by Step

To set sag, slide the O-ring against the seal, sit on the bike in full riding gear in your normal position, step off without bouncing, and measure how far the O-ring moved. Divide by total travel and multiply by 100. For trail riding, L'Atelier's target is 25-30% for both the fork and the shock. Too much sag: add air, 5 psi at a time. Too little: let air out. Rebound, compression and tokens all come after.

This guide is for trail and all-mountain riders setting up suspension for the first time, on a new bike or after buying a used one. It follows L'Atelier's suspension guides (la-telier.com). Pressure figures are starting points: your fork or shock maker's chart always comes first.

What sag is, and why it comes first

Sag is how much your suspension compresses under your weight when you are sitting still on the bike. It matters because suspension has to work in both directions: compress over obstacles and extend into dips. With too little sag (too firm), the wheel lifts off in holes and you lose traction. With too much sag (too soft), you burn through travel and bottom out on big hits. L'Atelier is blunt about it: if sag is wrong, no rebound, compression or token adjustment can make up for it.

Step by step: the O-ring method

1. Set a starting pressure. Fully deflate the fork, then pump it back up. L'Atelier's baseline is your weight in kilograms as the pressure in psi. A 176 lb rider weighs about 80 kg (176 ÷ 2.2046 ≈ 79.8), so start around 80 psi.
2. Open the dampers. Turn rebound and compression fully counter-clockwise.
3. Reset the O-ring against the dust wiper at the top of the stanchion (or against the shock body seal).
4. Gear up. Helmet, pack, water: everything you ride with. Have a friend hold the bike or lean against a wall.
5. Take your natural riding position. Not standing on the pedals, not sitting upright like on a road bike: your normal trail stance, feet on the pedals.
6. Step off gently without compressing the suspension again.
7. Measure the gap between the O-ring and the wiper in millimeters. That is your sag.

The formula: sag (%) = static compression ÷ total travel × 100. L'Atelier's worked example: a fork with 150 mm of travel and an O-ring that moved 40 mm gives 40 ÷ 150 = 26.7%, right inside the trail window. No O-ring on your fork? A zip tie around the stanchion does the same job; hold the riding position about 5 seconds before stepping off.

Sag targets by discipline

L'Atelier's other suspension articles give slightly different windows (for example 20-25% for a trail fork in its RockShox note, or 25-30% for enduro in its settings overview), which tells you these are ranges, not exact numbers. The real goal is to use all your travel on a typical ride without harsh bottom-outs: if you bottom out often, add pressure; if you only ever use about 80% of travel, take some out.

Adjusting air pressure (and reading the pump)

• Too much sag (too soft): add air, 5 psi at a time, and re-measure.
• Not enough sag (too firm): release air, 5 psi at a time.
• Use a high-pressure shock pump, not a tire pump.
• Expect to lose about 5-10 psi when you unscrew the pump head, and account for it.
• Air shocks can also lose 5-10 psi per month, so recheck pressure monthly.
• Coil shock? Sag is set with the spring rate (lb/in) or preload, and preload should never exceed 5 mm. Need more? Change the spring.

Tokens and volume spacers are not a sag adjustment

This is the most common beginner confusion. Tokens (RockShox Bottomless Tokens) and volume spacers (Fox) reduce the volume of the positive air chamber. The result is a more progressive spring: it gets firmer the deeper it compresses. That does not change sag; it changes how the suspension behaves at the end of its travel and helps prevent bottoming out.

• More tokens: more progressive, firmer at the end of the stroke. For aggressive riders or long travel.
• Fewer tokens: more linear and comfortable. For lighter riders or XC.
• Typical trail setup: 1-2 spacers in the fork and 1-2 in the shock (XC 0-1, enduro 2-3).
• When to add one: you still bottom out regularly even with high-speed compression closed. Remove one if you never reach full travel and the end-stroke feels harsh.

Rear shock specifics: air vs coil

The shock is measured the same way as the fork: O-ring against the seal, sit on the bike in your riding position, step off gently, measure, and divide by the travel figure given in the shock's documentation. L'Atelier's rear shock guide uses the same trail target of 25-30%. What changes is how you correct it. An air shock (Fox Float X, RockShox Deluxe Air, Cane Creek DB Air) is light and adjustable for any rider weight and progression via tokens, which is why it is the majority choice for XC, trail and enduro. A coil shock (Fox DHX2, RockShox Super Deluxe Coil, EXT Storia) is more sensitive on small bumps and has a linear curve; L'Atelier notes that many trail and enduro riders are going back to coil and accepting 200-300 g of extra weight. On coil, you do not pump: you choose a spring rate and set preload, within the 5 mm preload limit.

Common first-setup mistakes

• Measuring without your gear. A hydration pack and full bottle change the reading; measure in what you ride in.
• Bouncing on or off the bike. It pushes the O-ring further than static sag and makes the suspension look too soft.
• Leaving the dampers closed. Open rebound and compression before measuring.
• Forgetting the pump loss. The 5-10 psi you lose when unscrewing the pump can move you out of the target window.
• Adding tokens to fix sag. Tokens change progression, not sag. Fix sag with pressure first.
• Using a tire pump. Suspension needs a dedicated high-pressure shock pump.

After sag: rebound and compression

With sag right, move on in this order. Rebound (usually the red dial): start fully open (fast), push down hard on the bike and release, then slow it click by click until it returns quickly without bouncing past its sag point. Too fast and the bike pogos; too slow and the suspension packs down over successive hits. Low-speed compression controls pedal bob and weight shifts; L'Atelier gives 3-5 clicks from open as a typical trail fork setting and 2-4 clicks for a trail shock. High-speed compression, when your fork or shock has it, handles roots, rocks and landings.

For model-specific numbers, the L'Atelier bike mechanics base answers questions like "What's the recommended SAG for a trail mountain bike fork and shock?" and points to the guide it used.