Caution Lets Suspension Strut Its Stuff

Question: I have heard conflicting advice on replacing the struts on my front-wheel-drive car. Just what are struts and when do they need to be replaced?--I.I.

Answer: Struts are a costly item to replace, and even though all cars eventually need struts at some time, they are often replaced prematurely. It is not uncommon for front and rear struts to cost more than $200 to replace. So it pays for a motorist to question closely a recommendation that the struts are worn.

Struts are very similar to shock absorbers and perform the same function, that of damping body motion. But they are also more complicated, cost more to replace and require more labor to install. They are commonplace on today’s front-wheel-drive cars, which use a McPherson-type front suspension system.

Suspended on Springs

Your car’s body is suspended on springs, but it must not be allowed to bounce up and down as you drive over holes or other irregularities in the road. The strut is designed to keep the car steady and reduce vibrations that develop in the steering system. Thus, it is a key safety item, and it affects the durability of many other things, such as front-end parts and tires.

Struts are simple to understand, operating much like a screen-door damper that prevents the door from slamming. They consist of a piston that moves fluid or gas through a cylinder. They typically stop doing their job once the fluid or gas leaks out.

In diagnosing struts, you can’t be really certain when they need replacement. The state of California is currently investigating whether some shops and manufacturers have defrauded motorists by prematurely recommending the replacement of struts.

Warning Signs

There are five signs manufacturers have developed that can warn a motorist of worn struts, but in many cases they are vague. The signs are:

--Oil leakage from the struts.

--Cupped wear patterns on the tires.

--Excessive bouncing or sway in turns.

--More than three or four rebounds if you push down on the bumper when the car is parked.

--More than 25,000 miles on the car or the struts.

The problem with these warning signs is that struts wear out gradually, and they may display some signs and still be within acceptable limits. It is not impossible for struts to last more than 100,000 miles, though some mechanics will want to replace them at 25,000 miles, according to an engineer at one major strut manufacturer.

Struts don’t all wear out at the same rate. Rough conditions, such as driving on dusty roads that have lots of potholes, can wear out struts fast. The dirt tends to contaminate the strut rod, wear out the seal and allow the oil to leak out.

The only way to scientifically test a strut is to pull it off the car and put it on a device called a dynamometer that tests its damping ability. That would cost more than replacing the struts, however.

The good news on struts is that they are getting less expensive to replace. Early McPherson suspension systems had heavy structural struts that had to be entirely replaced. Today’s struts are lighter in weight and easier to get at.

The smart motorist will not want to wait so long to replace his or her struts that the tires begin to exhibit a cupped wear pattern. At that point, you not only have bum struts but a ruined set of tires. The weak struts allow the tire to bounce excessively, which sets up the cupped wear. Once the cupped wear is established on a tire, it will continue to wear that way even if you fix the struts.

The only solution is to watch your tires closely, have your mechanic monitor the struts for leakage and pay careful attention to the way the car handles on the highway. Push down on the bumper occasionally and see if the car stops bouncing after one or two rebounds.

Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.