"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. The coilover is a basic component of the MacPherson strut suspension system, which is distinguished from other arrangements by employing a particular design of anti-roll bar as a longitudinal constraint. This was the first widespread use of the coilover in automobile suspensions, but there are other designs. The word coilover should not be considered a synonym for the MacPherson strut arrangement. Coilover suspension systems have become a popular staple in the automotive aftermarket. Once limited to racing teams with the research and development budget to create performance parts, these suspension systems are now widely available. There are 2 different types of coilovers, full coilovers and slip on coilovers. The full coilovers are matched up with a shock from the factory, while slip on coilovers are a conversion option for factory struts with new mounting perches allowing for adjusting the springs seat up or down.