Part 2: Getting the Right Coils for Your Combo
Last month we covered the basics about rebuilding your coilover shocks, but we just flat ran out of room before we got to anything about the coils on those shocks. The beauty of the coilover is in thecoil-carrying capacity of the shock. It's adjustable, easily replaceable, and easily tunable. Since the coilover shock only has the duties of supporting the vehicle's weight and absorbing the inconsistencies in the road, you can finely tune your coils and shock to give you the utmost performance out of your suspension. The coils are readily available in different spring rates and are universal to a 2-, 2.5-, or 3-inch-diameter shock body, on almost any manufacturer's coilover shock. This way you can just step into your local off-road shop and buy the coils you want right over the counter. No custom coil orders, just easy-to-drop-in coils at about a hundred bucks a pop.
Hopefully you'll never have to do any coil maintenance, and if you do, it will be picking them up and throwing them in the trash (or making a nice ashtray out of 'em for your mom). Coils either work or they don't. They can wear out over time, but you'll notice them getting a little soft, and you'll hopefully realize that a new coil is in order. You can prematurely kill the spring rate of the coils by collapsing them to the point when the coil winds touch. Once this happens, the coils have now been stressed to the point when their labeled spring rate will no longer apply. Most coilover shocks are dual-rate coilovers, meaning they use two differently rated coils to come up with an initial spring rate. There are some triple-rate coilovers we've seen, some custom single coils, and some that use keeper coils (almost zero-rate coils that basically hold the other coils in place during full shock extension). But for all intents and purposes, we're covering dual rates, and most other configurations are similar enough that you can make the connection in your head.
Dual-rate coilovers give the advantage of allowing you a net (initial) rate to ride around on, but upon clapout of the upper secondary coil, progress into a stiffer second spring rate (the sole rate of the lower primary coil). The primary coil sits on the lower coil plate of the shock, with a spring slider that divides the main coil and upper secondary coil. As the shock compresses, so will the coils, until a point where the slider/divider will be stopped by the slider stop ring. You'll need to make sure you set the slider stop ring to a point where it stops the upper coil's compression before it is fully collapsed. Once the slider has stopped, the upper coil is no longer part of the spring rate and the rest of the spring compression is placed on the lower primary (stiffer) coil, therefore boosting the spring rate to whatever the lower coil's rate is.
Confused? Two differently rated coils make up one single rate when stacked. And it's not as simple as adding them together. Even two 250-pound rate coils stacked on top of each other do not equal a 250-pound spring rate.
Here is a simple equation to help you figure out your rate:
|Initial rate||=||Upper coil x lower coil|
|Upper coil + lower coil|
This initial spring rate will be your spring rate until your coilover compresses enough for the coil slider to stop and the lower coil to become the only actively moving coil.