If you have a coil shock, finding the right spring rate can be tricky. In an ideal world, you'd experiment with a few different options until you find the right one. But with some springs running into three figures, that can be very expensive, not to mention time-consuming, so you want to have a very good idea of what spring rate you need before you order one.
For clarity, the spring rate, or stiffness, is the amount of force required to compress the spring by a certain distance. This is usually measured in pounds per inch (lb/in) or occasionally Newtons per millimeter (N/mm). So, for a 400 lb/in spring, it would take 400 lb of force to compress it one inch, or 800 lb to compress it two inches, and so on.
What spring rate you need will depend on your weight and on the suspension design of your bike. In particular, its leverage ratio, which is how far the axle moves for every millimeter the shock compresses. You can easily work out the average leverage ratio by dividing the wheel travel by the shock stroke. A higher leverage ratio means you'll need a stiffer spring to provide the same sag.
When it comes to estimating what spring rate you need, there are lots of online calculators to help you do this. Here are some links to spring rate calculators from TFTuned
and The Suspension Lab
These calculators are great - honestly, most people will be happy enough using any one of them. But they don't always agree with one another because they work in slightly different ways. The calculators from TF and The Suspension Lab rely on subjective inputs like riding style and trail speed, which maybe makes more sense to some people but it's hard to be precise with such terms. Meanwhile, the ones from Fox and MRP allow you more control over the objective input parameters like sag and, in particular, how much your weight is biased towards the rear wheel.
Experienced riders usually have a rough idea of how much sag they need based on their riding style. If you want something super comfortable, you might run as much as 35% sag; for a more responsive ride, you'll be nearer 25%. But for most bikes, 28-30% is the sweet spot. But how do you know what your weight bias is? And how do these calculators work anyway?
To calculate the spring rate you need to achieve a given sag, you can use the below equation. Unless you want to play around with different values of the leverage ratio (more on that below), you don't actually need to use this equation
because the online calculators from Fox and MRP do it for you. It's just there to show you how they work and what all the variables mean.Spring rate in lb/in = W*B*L^2/(T*S)
= Your riding weight in pounds. Make sure to add the weight of your riding kit.B
= The weight bias, or the percentage of your weight on the rear wheel when standing on the pedals without any pressure on the grips. This is given by the ratio of your bike's rear center divided by the wheelbase, but a good approximation for most bikes is 65%.L
= The bike's leverage ratio, which is usually just the travel divided by the shock stroke. Alternatively, you could find a leverage ratio graph
for your bike (from the manufacturer, a review, a blog
or linkage software
) and find the leverage ratio at the sag point. This will give you a more accurate estimate of the sag you'll get. But remember, with a more progressive bike you'll usually want to run more sag (and visa-versa); using the average ratio takes this into account to some extent.T
= Your bike's travel in inchesS
= The percentage sag you want (e.g. 0.3 for 30% sag)A note on preload:
MRP and Fox's calculators allow you to see the effect preload will have on sag. But remember that preload does not affect spring rate - it's just a way to adjust the ride height (sag) independently of the spring rate. I'd recommend using zero preload, both in the real world for better sensitivity, and when calculating spring rate because sag is an indicator of the spring rate you need, not a goal in itself. Yes, you'll need a little preload to stop the spring from rattling and there is some breakaway force from the damper to consider too, but roughly speaking, these tend to be cancelled out by the weight of the bike, which is why bikes don't tend to sag much (if at all) under their own weight. That means, to a reasonable approximation, if you ignore the turn or so of preload needed to stop rattle you can ignore the weight of the bike too. Here's an example
I recently reviewed the Starling Spur
, which has 170 mm (6.69") of travel and a 65 mm stroke shock, giving it an average leverage ratio of 2.62. Because the leverage ratio
barely changes through the travel, we don't need to consider using the ratio at sag instead of the average ratio. In full riding kit, I weigh about 200 lbs, and by dividing the rear center by the wheelbase we get a weight bias of 64%.
Since it's a pretty linear bike with a very active suspension design, I wanted less than the standard 30% sag. If I put 28% (0.28 ) into the equation, along with all the numbers above, it suggests a 467 lb/in spring. If we plug in 26% sag for a firmer setup, the equation suggests a 503 lb/in spring. In the real world, I used a 502 lb/in spring, which gave me bang-on 26% sag.