Coil Overs are a pre-engineered spring/dampener package that precisely matches the spring rate with the dampener (shock). The spring and the shock are arranged so the spring ?coils over? the shock and act/move essentially as one piece.

The other advantage of coil overs is that the spring sits on an adjustable ?perch?. This allows you to set the ride height?higher or lower?with in a specified limit.

Some manufacturers also give you the ability to control/set the rate of compression and rebound (up to 16+ different settings for both compression and rebound).

By setting the rate of compression and rebound, and adjusting your ride height, you can fine tune your suspension to a specific track or your driving preference.

In other words, a good set of coil overs give you precise control of ride height and dampening rates. That allows you to go from a track set up to a daily driver setup with the same suspension.