MTB Coil Spring Rate Calculator

Multiply the sprung weight on your rear wheel by the bike’s leverage ratio, then divide by the shock stroke times your target sag — the result is your spring rate. Leverage ratio is rear wheel travel divided by shock stroke: a 160 mm bike with a 62.5 mm stroke runs 2.56:1, so every newton at the rear axle becomes 2.56 newtons at the shock. Your rear wheel doesn’t carry all of your weight, either — standing neutral on the pedals, most mountain bikes put 60–66% of the rider on the rear wheel (roughly 1 − chainstay ÷ wheelbase).

The calculator above runs exactly that maths, and on frames with a progressive linkage it uses the leverage ratio at the sag point rather than the average — the correction that simple calculators skip and the main reason their recommendations run soft on progressive bikes. If you want to go deeper on how linkage design changes what your shock feels, Vorsprung’s guide to leverage curves is the best explanation published.

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.

Because it counts your bike’s sprung mass, which measured sag physically includes but rider-weight-only calculators ignore. When you measure sag, the shock is already carrying the frame, motor, battery and everything else above the swingarm — around 11–20 kg of sprung bike on a modern enduro bike or eMTB. Ignore that, as most brand calculators do, and the spring you’re told to buy will sag noticeably more than promised once it’s on the bike.

Expect this tool to read roughly 5–15% firmer than the simple calculators — about one spring increment. That matches the common real-world experience of buying the calculator’s recommendation and ending up one spring firmer after testing. If you’d rather compare like-for-like with other calculators, untick “include bike sprung mass” in the advanced panel.

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.