MTB Coil Spring Rate Calculator

Run 25–35% of shock stroke, and start at 30% if you’re unsure. Trail bikes typically sit at 28–30%, enduro bikes at 30%, and downhill bikes at 32–35% — less sag gives support and pop on smoother, jumpier terrain, more sag gives traction and comfort when it’s rough and steep. Sag is measured on the shock: compression from fully extended to you aboard in riding gear, divided by total stroke.

These targets are conventions rather than laws. If you huck to flat or ride bike park laps all day, bias a few percent firmer; if you chase grip in slick Auckland winter roots, bias softer. The first spring is a hypothesis — confirm it with measured sag, then ride.

It sets the spring to land your target sag: rate = rear-wheel load × leverage ratio ÷ (stroke × sag), where the load is your riding weight plus the bike’s full weight, less about 4 kg of rear unsprung mass, with roughly 62% carried by the rear wheel. That’s the whole calculation — sag first, because that’s the sequence for setting up a coil: pick the spring that puts sag where you want it, then tune the feel from there.

The sag target is the tuning lever, not the weight. Less sag gives a firmer, more active bike with more support — the right direction for aggressive riding or a heavy bike that already sits deep in its travel. More sag gives a plusher, more planted ride and suits longer-travel bikes. If your frame is notably progressive, enter its progression figure in the advanced panel — it doesn’t change the number, but it tells you whether a linear coil suits the frame at all. And remember it’s a starting point: the only true way to set spring rate is to fit the spring, measure sag with the actual rider aboard in riding gear, then ride and tune.

Only if two numbers match: the spring’s rated stroke must be at least your shock’s stroke, and its inner diameter must match your shock body. A spring labelled 2–3 mm under your stroke is usually fine because manufacturers build in margin, but a free stroke genuinely shorter than the shock stroke will coil-bind before bottom-out. Inner diameters differ between brands — a Fox spring drops straight onto a Fox shock, while cross-brand combinations need the diameter checked or an adapter.

Preload is the last check: thread the collar to just-snug plus a quarter to one turn to stop rattle. Preload changes ride height only — it never changes the spring rate — and if you need more than about two turns to reach your sag target, you need a different spring, not more preload. Cane Creek’s coil spring guide covers the fitment rules in more detail.

Frames with roughly 15% or more leverage-curve progression suit a linear coil well — the linkage itself provides the end-stroke ramp that an air spring would otherwise supply. Near-linear frames are the ones that “blow through travel” on coil; they usually want a progressive-rate spring (Sprindex-style adjustable or a rising-rate coil) or are simply better left on air.

Your frame’s progression figure is published in most kinematics reviews and manufacturer charts — enter it in the advanced panel and the calculator corrects for it. Not sure what your frame runs, or whether coil is the right call at all? Book a suspension service at our Auckland workshop and we’ll set it up on the bike, or shop suspension and springs once you know your rate.