Wound up Tight – Coil Springs Part 2

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Last issue, we began our look at coil springs – both conventional jobs and coil over examples. There’s a lot more to cover and we’ll complete the series with this segment. As we mentioned earlier, the rate of the spring is determined by the diameter of the spring, the number of active coils in it and the diameter of the wire used. In the case of coil over springs, most are built to the same overall diameter. This means the spring manufacturer must decide upon the diameter of the wire along with the number of active coils needed to produce the desired rate. If the designer chooses a smaller than normal diameter of wire (this has a tendency to "soften" the rate), they will have to compensate by using fewer active coils (which has the effect of stiffening the rate) to achieve the desired spring rate.

AFCO points out two possible reasons for a spring designer to use a smaller than normal wire diameter for a specific rate spring:

(1) The ideal diameter wire may not be made and using the next larger wire (which requires more active coils) would produce a spring with insufficient spacing between the coils. This could cause the spring to bind during normal operations.

(2) Cost could be the prime consideration, and by using a smaller diameter wire and fewer coils (shortening the length of the wire used) material cost is reduced. Unfortunately, this "cost is an object" manufacturing perspective is more common than you might think, and the end result can be a number of different problems which can plague the chassis tuner.

According to AFCO, many hot rodders and even seasoned pro racers mistakenly believe extra spacing between the spring coils indicates a better spring. While a spring must have sufficient stroke capacity, it must also have sufficient material to absorb the load placed upon it. If the spring material is not sufficient for the load placed upon it, the material will become over-stressed and the spring will take a set. This usually results in a loss of height which, in turn, can have a profound effect upon the way your car works. Worse yet, this is an extremely difficult problem to detect unless, of course, you pay very close attention to the springs in your car (which is seldom the case for most enthusiasts).

Given the long stroke requirements for many types of current springs, material strength must sometimes be sacrificed to achieve significant stroke. Couple this with the fact that the ideal wire diameter is not always made and you can see why some springs have a real potential to take a set. AFCO claims they have seen some springs lose as much as 15/16" of free height during normal operation. The AFCO solution is to answer the stress problem by design. They've found that, if done correctly, the springs can be pre-set during manufacture. Done correctly, this pre-setting manufacturing process will usually eliminate any potential for added spring set, even when springs are designed with smaller-than-ideal wire. AFCO points out that shot peening will further enhance the durability of a spring.

What if a spring does in fact take a "set?" When a spring "sets," it will normally stabilize at the new height. AFCO reminds us that the rate effectively remains the same, since no appreciable changes have been made to any of the three factors that determine the rate of the spring. Other than creating a need to readjust the chassis (to restore the original setup and ride heights), the spring would provide satisfactory performance. AFCO claims it is not uncommon for springs (even those which are well designed and properly manufactured) to settle up to 1% of their free height. There is a catch, though: If the spring is a poorly designed or manufactured example, and it is subject to extreme over-stressing, the height many never stabilize. The spring could continue to change height (both shortening and lengthening) as long as the spring is worked. Because of this, the setup on your car physically changes each time the spring height changes. Obviously, this can lead to some pretty severe chassis tuning headaches. The only easy solution is to carefully monitor ride height in your car. If it changes regularly, you definitely have a spring "set" problem.

Coil bind is another factor that can plague modified cars. Coil bind occurs whenever a spring is compressed and one or more of the active coils in the springs contact another coil. The rate of the spring increases whenever a coil binds since the bound coil (or coils) is no longer active (if you think back to the parameters of spring design, this changes one of the three rate-determining factors). In simple terms, if the spring is compressed to solid height (all coils touching) during suspension movement, the suspension will cease to work. AFCO offers a simple inspection process: "If any coils have bound, the finish between them will show contact marks that appear as though they were drawn with a lead pencil. Normally, any spring that is binding should be replaced with a taller spring. Be aware there are racing springs on the market that are built with wire that is heavier than required. These springs will coil bind before others constructed with the proper wire size."

Under very extreme conditions, coil binding can cause a spring to unwind slightly. This can cause the mean diameter of the spring to increase and reduce the rate of the spring. AFCO points out that the potential for coil bind is increased whenever short springs are used. Bottom line? Always match the spring to the job.

The final thing you have to consider in regards to coil springs is "bowing." Springs that have lengths greater than 4 times their diameter have a natural tendency to "bow" when loaded. Because of this, tall springs tend to bow more than short springs, and small diameter springs tend to bow more than larger diameter springs. Generally speaking, the more a spring is compressed, the more it will "bow." AFCO reminds us of something that should be obvious: "The rate of a spring will increase if an active coil rubs another part of the car." AFCO offers the following tips to minimize spring bowing:

• Use correctly fitting coil over hardware or install weight jack assemblies so that the spring mounting surfaces are kept as parallel as possible during suspension travel.

• Use springs that do not lean excessively (when positioned on a flat surface). This indicates that the ends are ground parallel to each other. This reduces the tendency for a spring to bow. You should check both ends.

• If a coil over spring is rubbing the shock, try reversing the spring so the bowed part of the spring is around the shaft, where there's more clearance.

• Use coil over springs that have straight sides rather than an hourglass shape. This maximizes the clearance between the shock and the spring.

• Use springs that are wound straight. You can roll the spring on a flat surface to check for straightness.

As you can see, there's much more to coil springs than first meets the eye. Select your springs carefully, and inspect them occasionally. You won't regret it.

Extra spacing between coils does not indicate a superior spring. AFCO advises that a spring must have sufficient stroke capacity. At the same time, it must have sufficient material to absorb the loads placed upon it. If it doesn't, then there's a good chance the spring will take a set. More on this topic in the text.

Moroso and others offer a wide range of replacement springs for many common passenger cars.

While primarily designed for racing purposes, these springs offer a wide range of rates for different front-end weights.

Here’s a close up of the two different Moroso springs shown in the previous photos. These springs are both for the same application, but as you can see, they are very different. What’s with that? Simple: They’re designed for cars with different front end or “corner” weights.