Spring Travel and Coil Bind
Coil bind happens when all the coils of a spring are stacked on top of one another and the spring cannot compress any further. When the spring cannot compress any further, any additional force gets transmitted directly on the chassis and tires, leading to very high amounts of stress on those items (and the driver). A spring that cannot compress any further is said to be at its solid length or bound length.
Regretfully, this aspect of design is quite frequently overlooked those who create aftermarket suspension packages- both in lowering springs and in full suspension packages.
Shown below is a strut from a 2005 Subaru WRX STI:
In this picture, the distance between the upper and lower spring perches has been shown. At its shortest point, the two spring perches are 7.5 inches apart. However, we know that (from a measurement in the previous article) that this particular strut has 5.75 inches of suspension travel. Thus, when the strut reaches full compression, the spring will be compressed to a full length of just 1.75 inches at its shortest point. This is quite a small space to compress to!
As it so happens, the factory springs installed on this strut compress to just 2.25 inches. Why the discrepancy?
Because of the bump stop, the strut should never reach full compression. We also saw in the previous article that a full-length bump stop has gotten very stiff at 5 inches into the damper travel, and its terminal compression isn't much further. At 5.25 inches into the damper travel the coils of the spring will all collapse together, and this (not surprisingly) is also the terminal amount of travel the bump stop will allow. Thus, while the spring is short enough to have coil bind problems without a bump stop, it is exactly short enough for use with this particular bump stop.
Ideally a spring's solid length should be short enough to never occur in practice [i]except[/i] in the event of a bump stop failure- that is, if the bump stop were to split and become ineffective. In such a case, it may be desirable to have coil bind so that the damper itself does not get damaged, but either way this a scenario to hope never happens.
Pictured here is an aftermarket spring designed for this strut:
The spring has 1.5 extra coil winds compared to the factory spring, and each wind is thicker, too. Because of this, the spring's solid length is over 0.75 inches longer, and it easily reached its solid length in use. A shortened bump stop, as some tried, only made this problem more frequent and severe. There is no good way to correct this situation other than to replace the springs.
Not all springs are created equal. To show this, here are two springs of nearly equal spring rate and length. (Both are 9" long, but one is 300lb/in while the other is 280lb/in. That's close enough that we'll consider them both 300lb/in.) Notice how one uses thicker coils and many more of them:
Because of this, the blue spring has a significantly shorter solid length. The difference is not slight: the silver spring has about 4.2 inches of travel, while the blue spring has 5.7 inches. This is a 1.5 inch difference, or a whopping 35% more travel! Furthermore, for a car that puts 900lbs on each of these springs, we know that 3 inches of travel will be used up just by supporting the weight of the car. (This is the car's ride height.) At this point, the silver spring has 1.2 inches left for bump travel, but the blue spring has 2.7 inches remaining. This means the blue spring allows 225% more bump travel!
For factory-replacement lowering springs, you may have to take your own measurements, but do not assume the manufacturer has done that testing already. Often lowering spring manufacturers outsource the manufacture of the springs to a company that does not know the bound length requirements for the vehicle. For coilover springs, all good manufacturers list the available travel (or solid length) of their springs on their web site, so be sure to choose springs that are capable of delivering what your car needs.