While Shocks and Struts are both designed to limit body movement and give you a smooth ride, they are two completely different parts. A Strut is a part of the vehicle Suspension while a Shock is not. Alignment settings are adjusted on the Strut as well.
A Shock Absorber is designed to keep your tires in constant contact with the road; by absorbing impacts and controlling the spring oscillations. This is done by internal hydraulics. Any loss of fluid affects the Shock’s ability to dampen the road effects. If you see any fluid wetness on a Shock it should be replaced. If a Shock loses enough fluid the Tires will not maintain contact with the road and can cause loss of Steering control. If you are driving on a very rough road, the Tires will bounce wildly and without the solid contact with the road, your ability to Steer and Brake the vehicle is greatly diminished.
Shock Absorbers also affect Tire Wear as Cupping will develop with weak Shock Absorbers.
Have you ever seen a vehicle driving down the road and one tire is bouncing up and down like a yo-yo? Most people see that and think the tire is out of balance when in fact the Shock Absorber is worn out and allowing the Tire to bounce. That bounce is what causes the “Cupping” of the tire. Every time the tire leaves the road and makes contact again, it is scraping the road instead of rolling, much like an airplane landing (to a much lesser degree) and you see the smoke from the wheels as they touch the ground. A little piece of rubber is worn away each time.
Did you know Shock Absorbers also help control the sway or lean of a vehicle when cornering? Bad shock absorbers also contribute to the wear of ball joints, steering linkages, the springs and CV joints.
A Strut has all the internal workings of a shock absorber portion built into it but as an integral part of the Suspension system it also has a Coil Spring and replaces the Upper Control Arm. Vehicles with Strut Suspension will only have a lower Control Arm and Ball Joint. A Strut will have all the hydraulic components of a shock with the addition of a coil spring and an upper strut bearing. To replace the Shock Absorber portion of a Strut, you need a spring compressor to compress the Coil Spring and remove the upper bearing. Once the Strut is replaced, reassembled and installed, the vehicle must be Aligned.
It is almost impossible to accurately measure how worn your Struts or Shocks are. Look for fluid leaks, broken springs, damage and tire cupping. Also perform a bounce test. Bouncing the bumper up and down and releasing on the down position, the bumper should stop on the way up at the normal resting position of the vehicle. This will show a badly worn shock or strut. A test drive of the vehicle to compare to “like new” capabilities with a quick stop, steering and swerve maneuvering test as well as an acceleration check will see how much control or excessive body movement is present.
Most industry experts agree that Shocks and Strut have enough wear to suggest replacement by 50,000 miles. If you are concerned about the handling of your vehicle have your Shocks and Struts inspected.