A coilover is an automobile suspension device. "Coilover" is short for "coil spring over strut". It consists of a shock absorber with a coil spring encircling it. The shock absorber and spring are assembled as a unit prior to installation, and are replaced as a unit when the shock absorber has leaked. This provides for optimal damping without torsional loads. Some coilovers allow adjustment of ride height and stiffness, using a simple threaded spring perch similar to a nut. More advanced adjustable coilover systems will use a threaded shock body, coupled with an adjustable lower mount for ride height adjustment, while an adjustment knob is used to adjust damping, commonly referred to as "stiffness." Spring rate also plays a role in stiffness.