Today a lot of street-legal vehicles and off-road rigs run modern coil spring suspensions. Coil springs in their various formats have a number of advantages over traditional leaf springs and torsion bars that were often popular on independent front suspensions (IFS) in the past. Unlike leaf springs that are used to support the weight of the vehicle and locate the axle under it, coil springs must be combined with some sort of linked suspension that positions the axle while the coil supports the vehicle weight.
Coil spring rate is measured in force needed to compress the spring 1 inch, expressed in pounds per inch (lb/in) of spring travel. That rate depends on three main physical dimensions. These are: coil wire diameter, coil spring body (or mean) diameter, and number of active coils. As the wire diameter increases, the spring rate increases. But, as the coil body diameter increases, the coil rate decreases. As the number of active coils goes up the spring rate drops, and fewer active coils yields a stiffer spring rate.
In its simplest form, a coil spring is used to support the vehicle over a straight axle and is held in place between the chassis and axle. Bumpstops need to be used to control the bottoming limit and shocks or limit straps typically control how far the coils can expand in droop. A spring such as this one whose coil body diameter varies over the length of the spring has a progressive spring rate, meaning the spring rate increases as the coil compresses.
There are three basic coil configurations we often find on 4WD and off-road vehicles. They vary by design construction, cost and performance. These might be described as a basic coil setup combined with an external shock, a coil strut with a spring over a shock as used on many factory vehicles today, and high-performance coilovers used to replace factory struts or used on custom suspensions. Today you’ll most often find use of the latter two on IFS as they offer packaging advantages over separate coil and shock locations.
When adding lift to these coil setups there are a few practical limits to take into consideration, and pros and cons to the different methods used to gain lift.
Lift can be attained by using a longer coil spring or by using coil spring spacers. In either case, longer shocks are usually combined to take advantage of the lift height. While lift spacers are cheaper than replacement springs and retain the factory spring rate (good or bad), they do not increase overall wheel travel. On a straight-axle vehicle, combining a spacer with the factory spring may cause it to go into coil bind (completely collapsed on itself) under full compression, ultimately limiting up-travel.
Installing aftermarket struts, such as with a leveling kit, is common and there are several ways to go about getting the added lift height in the front suspension. On the left is a factory assembled strut, and a disassembled one is shown in the center with its stock spring. On the right is a slightly longer aftermarket Bilstein strut that improves down-travel and can provide front lift. It can be used with the factory coil spring with added preload or with a longer spring as shown here. In any case, the Bilstein is designed to limit overall extended strut length to maximize droop travel without putting the IFS drive axles in a bind. It’s possible to add coil spacers on top of a factory strut to obtain lift but note that this can push the overall extended length beyond what the front axles can tolerate. It’s important to check for this potential hazard when using spacers on top of struts.
Bilstein and other replacement front struts for IFS vehicles typically offer the option to raise the front of the rig. The stock coil spring can be used at one of four height settings where the lower coil mount is positioned. Lift height from zero to several inches is possible using the stock spring, and retaining the stock spring rate. However, increased lift using the higher position settings increases preload on the factory springs so initial suspension movement will be hampered by the increased preload. Use of a longer, aftermarket spring can provide lift without having to induce so much preload on the strut, keeping the suspension more responsive to small bumps on the road or dirt surface.