Metal springs for coil rear shocks – as opposed to air-sprung suspension units – are replaceable in order to upgrade (e.g. replace a steel spring with a lighter titanium spring) or to fine-tune your rear suspension performance. Changing your rear shock spring enables your suspension to tuned for factors including your own weight, your chosen riding discipline and the suspension design of your bike.
Read on for more information on how to choose the right spring for your coil shock.
Choosing the correct spring for your coil shock is crucial towards getting the best out of your suspension.
There are a number of things to take into account when picking a new spring, the most important being material, spring length and spring rate.
• Material: Coil springs are made from one of two materials – steel or titanium. Performance-wise there’s not much between them. Steel springs are relatively inexpensive, but heavy. Titanium springs can represent a significant weight saving, but with a premium price tag. The choice is yours – and will largely depend on your budget.
• Spring Length: Springs for coil shocks are rated using two numbers (you will see these stamped on the spring, e.g .450 X 2.25). The first number is the weight (in pounds) needed to compress the spring by an inch (aka spring rate – see below) and the second is the travel length of the spring (in inches). However his is not the same as the travel length of the bike’s suspension, which is determined by the suspension design used (see ‘Rear Shock Springs: In-Depth’ for detailed information on how to choose the correct spring for your shock/frame).
• Spring rate:As above, the spring rate (the latter of the two numbers stampe on the spring) represents the weight (in pounds) needed to compress the spring by an inch. Lighter riders generally need lower-rated springs, while heavier riders needing higher-rated).
If you are replacing or upgrading your rear spring and want to buy like-for-like, the choice is simple – just look at the numbers stamped on your existing spring and buy one with the same length and spring rate. If you wish to experiment with your spring rate – e.g. try a heavier or lighter spring for more or less active suspension – simply choose one with the same length but a spring rate above or below your existing spring and experiment until you find the sweet spot for your weight and riding.
However if you have no information on your existing spring – e.g. you are building a frame with a springless rear shock, you will need to do a bit of groundwork in order to choose the correct size.
In order to choose the correct size spring for your shock you will need to do a bit of groundwork. There are three things you will need to determine about your setup – the stroke of your shock, the maximum spring free length that your shock is designed to take, and your ideal spring rate.
• Shock Stroke: This is the actual travel of the shock itself, or the maximum movement of the shock from unloaded to fully compressed (please note this is not the same as travel length, which is the maximum amount the bike’s rear axle will move when the suspension is fully compressed). To find your shock stroke this, measure the visible part of the shock shaft (the part that disappears when the shock is fully compressed) in inches (1″ = 25.4mm).
• Max spring free length: This is the maximum length of (uncompressed) coil spring that your shock will take. To figure this out, wind the preload off completely and measure from the inside of the spring collar to the inside of the preload nut.
• Spring Rate: As above, the spring rate is measured in lbs and refers to the stiffness of the spring. Different spring rates will be appropriate depending on your weight, riding style/discipline, the leverage ratio of the bike, and the suspension design of the frame. As a general rule you are trying to achieve the correct amount of sag (sag is the amount your suspension travels when you’re sitting on your seat, feet on your pedals and hands on the bars). This should be around 1/3 of the total travel on a DH bike and 1/4 of the total travel on an XC or Enduro/All-Mountain bike (again, this can vary due to suspension design and personal preference).
Once you have established all of the above you need to choose your spring using the following rules:
• Spring stroke should be equal to or greater than stroke of the shock;
• Spring length should be less than the maximum free length of the shock (this is so the spring physically fits);
• Spring rate should be correct to suit your body weight, bike and riding style
For example, if your shock stroke is 2″, your max spring free length is 130mm and your spring rate is 450lbs, you need a spring that is 2.25″ x 450Lbs x 125mm (if you don’t see a spring that has the exact stroke of your shock you can use a spring with a longer stroke, as long as the free length will fit).