Coil spring compressors are tools that are used to service certain types of automotive suspensions. Some vehicles use components known as struts, which combine a shock absorber and spring into an integral component of the suspension system. The springs are very strong, and under significant tension, so simply taking a strut apart can be quite dangerous. To safely accomplish this task, and to facilitate the reassembly of a strut, a coil spring compressor is typically required.
Struts have three primary components, which are a housing, cartridge, and coil spring. The housing typically bolts up to a steering knuckle and a strut tower, and the cartridge fits inside. A strut plate typically sits on top of the cartridge, and a coil spring is installed between the housing and the plate. In most cases, some type of bushing will also be installed between the plate and the tower. A coil spring compressor is usually required to permit the replacement of items such as cartridges and bushings.
The purpose of a coil spring compressor is to remove the tension from a strut assembly so that it can be safely disassembled for service and then reassembled afterwards. This is accomplished by inserting the feet or hooks of the compressor into the spring, and then tightening it with a pneumatic ratchet or hand tool. When the coil spring compressor is tightened, its feet or hooks cause the spring coils to draw closer together. That effectively removes the tension from the strut assembly so that the plate can be unbolted, and the cartridge may safely be replaced.
It is sometimes possible to take a strut assembly apart without this type of tool, if the technician is careful and works very slowly, though it is typically dangerous to attempt this. Since the coils are very strong, and under extreme pressure, they may move suddenly when the pressure is released. If a technician is in the way when that happens, severe injury, or even death, may occur. Moreover, at that point it would be difficult or impossible to put the assembly back together with a new shock absorber cartridge. Consequently, it is generally much safer and more practical to use a coil spring compressor when servicing strut assemblies.