MTB Fork and Shock Sag Setup for Trail Riding: a Beginner's Step-by-Step Guide

Sag is the share of travel your fork or shock uses under your weight at rest. Measure it with the O-ring, work out sag (%) = compression ÷ travel × 100, and aim for 25-30% front and rear on a trail bike, according to L'Atelier's SAG guide. Adjust with air pressure in 5 psi steps. Tokens, rebound and compression come later, because none of them can fix the wrong sag.

This walk-through is written for trail and all-mountain riders doing their first proper setup. It is based on L'Atelier's suspension guides (la-telier.com). L'Atelier's guides give pressures in psi and weights in kilograms, so this article does the same. Always cross-check against the pressure chart for your exact fork or shock.

Before you start

• A shock pump (high-pressure, suspension-specific; a track pump won't do).
• A ruler or calliper in millimetres, and your fork and shock travel figures.
• Your full riding kit: helmet, pack, bottle. Sag is measured with the weight you actually ride with.
• A friend to hold the bike, or a wall to lean on.
• Starting pressure: L'Atelier's rule of thumb is rider weight in kg = fork pressure in psi. An 80 kg rider starts at about 80 psi.

Measuring sag with the O-ring

1. Fully deflate the fork, then re-inflate to the starting pressure.
2. Open rebound and compression fully (dials anticlockwise).
3. Push the O-ring down against the dust wiper (or the shock seal).
4. Climb on in kit and settle into your natural riding position, feet on the pedals, without bouncing.
5. Step off gently, without compressing the suspension again.
6. Measure the distance the O-ring has moved, in millimetres.

Then do the maths. L'Atelier's example: 150 mm of travel, 40 mm of O-ring movement, so 40 ÷ 150 × 100 = 26.7%. On a 140 mm fork, the same 26.7% would be about 37 mm (140 × 0.267 ≈ 37.4), which shows why you should always calculate from your own travel rather than copy someone else's millimetres. If your fork has no O-ring, a zip tie around the stanchion works: hold your riding position for about 5 seconds, step off, measure.

Repeat exactly the same routine on the rear shock, with the O-ring pushed against the shock body seal, and check the travel figure you divide by in the shock's documentation. Write down the pressures and the sag you measured for fork and shock: when you later change tyres, add a heavier pack or swap a part, you can come back to a known baseline in a couple of minutes instead of starting again from the rule of thumb.

Target sag by discipline

Treat these as starting points. Other L'Atelier articles quote slightly different windows (its RockShox note gives 20-25% for a trail fork; its settings overview puts enduro at 25-30%), so the ride itself is the final judge. Bottoming out often? Add pressure. Never using more than about 80% of the travel? Remove some.

Correcting sag: pressure, springs and the pump trap

• Too much sag means too soft: add air 5 psi at a time.
• Too little sag means too firm: let out 5 psi at a time.
• The pump trap: you lose roughly 5-10 psi when unscrewing the pump, so overshoot slightly or re-measure after removing it.
• Coil shocks: adjust with the spring rate (lb/in) or preload. Never more than 5 mm of preload; beyond that, fit a different spring.
• Monthly check: a rear shock can lose 5-10 psi a month.

Tokens and volume spacers: progression, not sag

Volume spacers (Fox) and Bottomless Tokens (RockShox) take up space in the positive air chamber, so the spring ramps up harder towards the end of the stroke. They do not change sag. They change progression: more spacers resist bottom-outs, fewer make the suspension more linear and supple. L'Atelier's typical counts are 0-1 for XC, 1-2 for trail, 2-3 for enduro, in both fork and shock. Fox spacers come in 0.2, 0.4, 0.6 and 0.8 cubic-inch sizes.

Next steps: rebound and compression

Once sag is set, L'Atelier's setup order is pressure, rebound, low-speed compression, high-speed compression, then spacers if needed, fine-tuning one or two clicks at a time. Change one setting at a time and ride 15-20 minutes before the next change.

1. Rebound: set it fully open (fastest), push hard on the bars or saddle and let go, then slow it a click at a time until the suspension returns quickly without bouncing past its sag point. Too fast and the bike is unstable over successive hits; too slow and it packs down, sinking deeper into the travel by the end of a descent.
2. Low-speed compression (LSC): ride for about 10 minutes and adjust for pedal bob. More LSC rides higher and pedals better; less is more supple. L'Atelier's typical trail fork setting is 3-5 clicks from fully open, and 2-4 clicks for a trail shock.
3. High-speed compression (HSC): set it on a technical descent. Bottoming out often? Close it. Harsh on big hits? Open it. Many trail forks do not have this adjustment.
4. Spacers last: only add one if you still bottom out regularly with HSC closed.

Mistakes beginners make

• Measuring in shorts and a T-shirt instead of full kit with pack.
• Bouncing when getting on or off, which overstates sag.
• Measuring with compression closed.
• Ignoring the 5-10 psi lost when the pump comes off.
• Adding tokens to cure a sag that is simply too high.
• Using a track pump instead of a shock pump.
• Changing three dials before the next ride, then not knowing which one helped.

When the problem is the shock, not the setup

If you cannot get a sensible sag reading, or the suspension behaves oddly whatever you do with the dials, the issue may be mechanical. L'Atelier's rear shock guide lists the signs that mean a service rather than another round of adjustments:

• A metallic clunk at bottom-out: the internal floating piston (IFP) needs checking.
• Oil seeping from the seals: worn shaft or air can seals.
• Rebound no longer responds to the dial: worn damper piston.
• Lockout no longer works: compression valve failure.
• More than 200 hours since the last service: degraded oil and reduced performance.
• Play at the shock eyelets: replace the DU bushings before they damage the frame.

Air or coil also changes how you work. An air shock is light and adapts to any rider weight, with progression tuned by spacers; that is why it dominates XC, trail and enduro. A coil shock is more sensitive on small bumps with a linear curve, and L'Atelier notes that many trail and enduro riders are returning to coil and accepting 200-300 g of extra weight. With coil, sag is set by spring rate and preload rather than the pump.

Want the reference pressure for your fork? The L'Atelier bike mechanics base answers with the guide behind it, for instance to "What's the recommended SAG for a trail mountain bike fork and shock?".