Quad Shock, 1985-2004 solid-axle V8 Mustang axle damper (BUY 2 FOR EACH CAR)
A pair of rear horizontally mounted shocks was part of what Ford called the "Quadra-Shock rear suspension system". Most commonly called "quad shocks", these are also known as "axle dampers", "kicker shocks", and "kickers".
Quad shocks were installed to reduce the axle wind-up that causes wheel hop. They did this by damping out the axle's tendency to rotate during acceleration.
Torque reaction during acceleration makes the axle housing rotate in the direction of raising the forward (pinion side) of the axle housing. The considerable amount of deflection allowed by the eight stock rubber rear control arm bushings of the Mustang four-link let the axle wind up, and then rotate back, in a cyclic manner. This causes wheel hop, which literally does mean that the tire loses traction as it leaves the ground. Wheel hop can become violent enough to cause parts breakage.
Installation of the MM Torque-arm eliminates the axle wind-up that causes wheel hop. Installation of most aftermarket rear lower control arms will usually eliminate enough of the deflection to allow removal of the quad shocks. Removal does depend upon the total rear suspension package; it is an experiment. Remove the quad shocks; if wheel hop persists, then reinstall.
When you do need quad shocks, these gas-pressurized Bilstein units are the best available.