Suspension Setup Basics: Sag & Air Pressure

What sag actually measures

Sag is how much your suspension compresses under your own body weight alone, just standing or sitting on the bike at rest, before you hit a single bump. It's usually expressed as a percentage of total travel, and it's the starting point nearly every suspension setup procedure begins from — get sag roughly right first, then dial in rebound and compression damping against that baseline. Sag is set by air pressure on an air-sprung fork or shock, and by physical spring selection (plus a small preload adjustment) on a coil-sprung one.

Two major makers, two different starting numbers

It's worth knowing upfront that suspension makers don't converge on one universal sag target. RockShox's own guidance splits by air-spring family: it recommends roughly 25% sag for its Solo Air rear shocks and roughly 30% for its DebonAir rear shocks, with about ±5% left as rider preference. Fox, by contrast, publishes one simpler starting point — roughly 25% sag for both fork and rear shock — and then has riders adjust up or down from there by riding style (more sag, lower pressure, for descending-focused riding; less sag, higher pressure, for pedaling-focused riding). These aren't a disagreement to resolve — they're genuinely two different manufacturers' own published starting points for their own products, and the honest takeaway is to follow the guidance published for the specific suspension you own rather than treat one number as universally correct.

The setup procedure, in outline

A typical air-spring setup follows a similar shape regardless of brand: open any external damping adjusters fully, pressurize the air spring from empty up toward a starting point based roughly on your body weight (a common rule of thumb is to start near your weight in pounds as a psi figure, then adjust from there), and cycle the suspension through several strokes before taking a final reading. That cycling step matters — a fresh air spring has separate positive and negative air chambers that need to equalize against each other before the pressure gauge and the sag measurement mean anything reliable; skipping it can give you a sag reading that doesn't match how the suspension will actually behave once ridden in.

Sag vs. "travel used" — two different things people conflate

Sag, measured at a dead stop, is a completely different measurement from how much travel you actually use while riding. Suspension makers commonly recommend tracking a sliding o-ring (or a zip tie) against the stanchion or shock body to see how deep into the travel a real ride actually pushes the suspension — and the general guidance is that using most of your travel on a typical ride, with the occasional full bottom-out on a bigger hit, is normal and even desirable. Don't confuse a static sag percentage with how much of the travel gets used dynamically; they're measuring two different moments.

How BuildMyMTB checks this — and where it doesn't

Sag and air pressure are entirely outside what the compatibility checker looks at. The builder checks whether a fork or shock physically fits your frame — travel, eye-to-eye, stroke, mount type, and so on — but it has no concept of your body weight, your riding style, or what pressure you've actually pumped into the part once it's installed. Setting sag correctly is something you do with a shock pump against the maker's own published guidance for the specific fork or shock you bought, after the build is assembled — it isn't a fit question the tool is equipped to answer, and it shouldn't be read as something a green build verdict has any opinion on.

Practical takeaways

• Look up the sag target published for your specific fork or shock — RockShox and Fox genuinely differ, and mixing up their guidance will land you at the wrong starting pressure.
• Cycle a fresh air spring through several strokes before trusting the pressure reading or sag measurement — skipping this step gives you numbers that won't match how the suspension rides.
• Don't treat sag (a static, at-rest measurement) and travel used (a dynamic, while-riding measurement) as the same thing — they answer different questions.
• A compatible build says nothing about whether your suspension is set up correctly — that's a separate step you handle with a shock pump after the parts are assembled.