—Sue Talman

ANSWER: It's flat wrong to use scare tactics to sell services. Shocks and struts play an important role in keeping your tires firmly planted on the ground and providing vehicle stability, but aren't nearly as important — in my opinion — as properly maintained tires and prudent driving practices.

Your car's suspension system uses springs at each of its corners to provide a smooth ride. Your tires, wheels, brake parts, and suspension members combine as un-sprung weight. Upon hitting a bump they enter what's called jounce. The spring is compressed, storing energy, which is soon released in rebound, as one passes the bump. Unfortunately, the combined mass of parts overshoots its original position and additional oscillations occur, disrupting tire/ground traction and vehicle stability.

Heavier wheels, tires, brakes or suspension parts make matters worse, and are a performance trade-off. Modern shocks and struts use a piston that plunges back and forth in a tube filled with hydraulic fluid, controlled by some smart valves, to dampen suspension movement. The suspension jounces and rebounds in a more controlled fashion, and subsequent oscillations are greatly reduced.

Shock absorbers (usually one per corner) are typically found on trucks and older/larger cars and are a fairly inexpensive bolt-on part. Front wheel drive cars need greater width in the engine bay to fit a sideways engine and transmission, so a more compact, integrated suspension system, known as a MacPherson strut suspension, is used. The shock absorber, upper control arm and coil spring are integrated into a single vertical suspension member called a strut. Struts may also be found in the rear of many vehicles to reduce weight and/or improve performance.

When it's time to renew the shock absorber, either the entire strut (minus spring) is renewed, or a shock cartridge is inserted into the original strut. Replacement typically costs more than a shock renewal due to the greater labor required in disassembly and reassembly. Also, realigning the suspension is recommended, as the new parts may differ very slightly in dimension and reinstallation position.

If you peek above or behind each tire, a strut (and its coil spring) are usually visible, about an inch or two behind the tire. If shocks are used, they're usually further inboard/lower, do not have a spring encircling them, and are less visible.

Worn shocks and struts sneak up on you gradually. One day while slowing on a curving, bumpy road, you'll get a queasy feeling the vehicle just isn't as nailed down as it should be. Or, expansion joints on a rough section of road make you feel seasick, or your tires develop choppy tread wear. Also, if a shock or strut shows signs of fluid leakage, it's time to renew it.

Every vehicle is different, as well as roads travelled, driving habits, and operator expectations. Replacement at 50,000 milkes is a good recommendation for a bulky vehicle driven on windy, bumpy roads, or if stellar handling is desired. Being a skinflint, I go half again further than this for my around-town, grocery-getter car. After replacement I usually say, "Wow, there is a difference. I should have done this sooner!"