A Good MTB Coil Calculator

February 23rd, 2025

This is a post about the mtb coil calculator I built. Why I built it, and what it does differently to existing calculators. I originally built it some years back, and then a few months ago I added some more features to it. It’s still not showing up on any search engines or anything but if it does one day here is an explanation of the tool for those interested.

When it comes to rear suspension on a mountain bike you have 2 options.

An air sprung shock
A coil shock

The first is dead simple… sure, there are some options to pick from, but you choose something that fits your budget (and your bike) and maybe your favorite color. When it arrives you can install it and set it up. Cake.

Where things get a little more complicated is the second option: A coil shock. Unlike an air shock you actually gotta know a thing or 2 before you hit buy. Yes, you need to know the specs of your bike, but you also need to know the correct spring rate to choose. And you also need to know your bikes suspension platform. And you also need to know who is going to be riding the bike.

If you don’t have a buddy or a friendly bike shop with an inventory of demos your gonna be ordering new coils until you find the right one. There is no setup step that can make the wrong coil right. Air shocks are infinitely adjustable, coil shocks cannot be adjusted at all.

Why can't you use the preload adjuster?

Well, yeah, preload is an adjustment you can use. However, you need to understand what preload actually does. When you add preload to your coil shock you're increasing the initial force required to compress the suspension.

A coil shock with no preload is supple and responsive to miniscule amounts of force, whereas a preloaded coil will sit you higher in the suspension with less sensitivity. The rate, however, does not change. A coil that's heavily preloaded out of necessity will have poor small bump compliance and will be prone to bottom outs frequently.

A simplified example with math, ignoring leverage ratios, coil length, weight distribution, etc.

Lets say you you weigh 200lbs, and you've installed a 100lb/in coil (unrealistic for many reasons, but easy math). With no preload you sag the suspension 2 inches. If you've only got 2 inches of suspension (lets assume you do) then thats 100% sag. You bottom out the shock just by sitting on the damn thing. An obvious sign you have the wrong coil.

But, you think to yourself "I'll just use that preload adjuster". So you crank the preload until you acheive a 25% sag. "Perfect" you think. You head out to the driveway and do a couple bounce tests, only to realize that you still bottom out the shock by bouncing up and down on the bike.

This is because the coil still compresses 1 inch for every 100lbs of force added, and as a 200lb person, it is trivial to apply an additional 100lbs of force. Meanwhile, a 50lb person, who this coil is actually intended for, would struggle to apply 100lbs of force without hitting a big drop or doing something extreme. You've bought a childrens coil, and there is nothing you can do about it.

Preload should be looked at as a way to tune the ride characteristics of the bike & shock, not to make a sag measurement acceptable. A preload adjuster will not save you from the wrong coil.

Typically what you’ll do is a quick web search for “mtb coil calculator” to try figuring out what coil you need. You’ll either land on some page with 1 or 2 inputs that makes you question how (or if) it actually works, or you’ll find the fox calculator with its 30 different form inputs, assume its black magic industry secrets and blindy trust the output as the only acceptable answer.

What’s missing is a calculator that lets you test a few spring rates for your exact bike while giving you an intuition for what goes into picking a coil in the first place. With intuition comes perspective and the confidence you can only get from knowing what you’re doing.

This is what the mtb coil calculator I built is supposed to do. Like the others, it’s still a theoretical approximation and is never going to be a perfect predictor of reality. However, I think it’s much better than the alternative calculators available for a few reasons.

The calcuation produces a sag value, not a theoretical perfect coil. By outputting sag instead of coil rate the user can play around with a few different coils and see how it does (or doesn’t really) affect the sag. Most calculators output a coil value like its the only one that’ll work, and users don’t realize how little affect on sag different coil rates may have.
Actual leverage ratio curves. Some calculators ask for suspension platform (“single pivot” “4 bar” etc) as if they can assume a leverage curve from that. They cannot. Others ask for a static number, which is equally ridiculous. With the fully customizable leverage curve in this calculator you can truthfully match your bikes suspension kinematics.
It updates the sag in realtime. This is particularly helpful in building an intuition around how leverage curves affect sag. Just drag the curve around and the sag will change with it. Most people say things like “progressive suspension platform” but don’t actually understand what it means or what it affects.
A shock to test out (on desktop). Admittedly this one is kinda a gimick. I mostly just wanted to draw a shock. But, it is a physically accurate simulation and helps ground what you’re doing into a recognizable reality.

Ultimately, it’s still just a calculator, but its a bit different than the others. I have some ideas to make it better too (a huck to flag metric for conveying bottom out force, a database of bike leverage curves, visualization of sag point on leverage curve, a rider weight vs coil rate chart, etc) but am not sure when or if I’ll get to those. Give it try if you’re interested, your milage may vary.

If you like math or code go tear it apart on GitHub. Factoring a fully customizable leverage curve in to the calculation was particularly challenging for me.