The Workshop · Suspension tools Beta
Coil spring calculator
Find the right coil spring for your rear shock, worked out the way I’d do it in the workshop.
You’re one of a few riders testing this before it goes public. If a number looks off or something’s confusing, tell me through the contact page.
Basic gets you close in a minute. Advanced uses measurements from your own bike for a workshop-grade number.
How it works
A coil spring has to hold up the part of you that sits over the back wheel, multiplied by your frame’s leverage, at the point in the stroke where you want it to settle. That’s the whole sum:
spring rate = load on the rear wheel × leverage ratio at sag ÷ shock travel at sag
Leverage at the sag point
Leverage ratio is how far the wheel moves for each millimetre of shock travel. Most calculators use the average, travel ÷ stroke. What actually holds you up is the ratio at the sag point, which is higher on a progressive bike. That’s what this uses, and it usually adds 2–6% to the spring.
Measured seated
Seated sag is the one you can repeat: the saddle puts you in the same spot every time. It’s also how Fox, RockShox and most frame makers quote their numbers. Sat down, about 71% of your weight goes through the back wheel. Standing in the attack position, about 65%.
Your bike, not a guess
Basic uses a typical leverage curve for each suspension design, taken from published frame data. Bikes with the same design still vary a lot, so Advanced builds a model of your own linkage from where the pivots sit and works out the curve point by point.
Checked against real bikes
The maths reproduces a published test where a 502 lb spring measured 26% sag; the calculator predicts 26–27%. The linkage model matches a hand calculation of the same linkage to within 0.002 on the leverage ratio.
Bike weight and preload
Your bike’s own weight and the spring preload roughly cancel each other out, so the quick estimate leaves both out. Advanced’s “weigh it on scales” option measures the real load on the back wheel, so it includes them.
Fit it, then check sag
Labelled spring rates aren’t exact and every shock has some friction. Treat the answer as the spring to fit first, then measure your sag. If you need more than two turns of preload to get there, go up a spring.
Sources: Pinkbike, spring rate formula · Fox, measuring sag · RockShox setup guide · ENDURO, seated vs standing sag · Race Tech, setting sag · Vorsprung, leverage rates
Got your number?
I’ll supply and fit the right spring, set your sag and fill in your settings card, so you can always get back to a setup that works.