"Porpoising", or bouncing up and down when going over bumps or potholes is a key sign that a car's rear suspension shocks, struts and coil springs may need to be repaired or replaced. Bouncing and shimmying during turns are another sign of rear suspension problems.
To check the rear suspension, a mechanic may do a bounce test. This is a simple examination involving pushing each corner of the vehicle down, then releasing it quickly and observing the results. A good shock absorber or strut bounces no more than twice, while a corner with a failing shock absorber or strut bounces several times.