The engineering design of your Plymouth shocks is based on the principles of fluid displacement and thermodynamics. Shocks act through the movement of a piston within a cylinder, which is damped by the passing of oil through one or more orifices. Modern engineering has improved Plymouth shocks so they react to road conditions by offering more resistance when they encounter bumpy roads. That's because shock absorbers are velocity sensitive, which means the faster the piston moves (because of bumpy roads) the more resistance or damping will result. Consult one of the laws of fluid dynamics, which states that a fluid's resistance to flow through any given orifice will increase directly as the square function of flow velocity. So in other words, the harder the damper works, the harder the shocks work. But no matter how well designed and engineered your Plymouth shocks are, they will need to be replaced in time. Check your Plymouth shocks every 12,000 miles or so to see if they need to be replaced. Do not stint on the task: Failure to do so can have a catastrophic effect on your car, by leading to premature damage to your tires and reduced braking performance. Purchase a new set of Plymouth shocks here and do the job yourself - it should take about an afternoon to complete.