You're half right. The only time you would ever replace just one strut is when they are fairly new and one gets damaged in a crash. Other than that, you could buy just one new shoe, but would you? You could buy just one new mitten to replace the one that got a hole in it, but would you?
Struts are always replaced in pairs to maintain even ride quality and stability. Struts have nothing to do with the weight being carried in the car, as in number of passengers. All shock absorbers do is reduce the bouncing that takes place. Struts do the same thing, AND they hold that wheel up straight. When you're discussing the weight the car is carrying, that has to do with the coil springs. Each one on front is holding up about 1,200 pounds of vehicle weight. Adding a passenger will barely be noticed.
You should understand too what is involved with replacing struts. The upper mounts you're wondering about can't be fully-inspected without totally disassembling the strut. One thing you can look for first is to have the vehicle standing still with the engine running, then slowly turn the steering wheel left and right. If you hear a clunking or popping noise, inspect the upper mounts by reaching over a tire and wrapping your fingertips around part of the coil spring, then have a helper slowly turn the steering wheel back and forth. You should feel the spring rotate smoothly with the tire. If the upper mount is binding, you'll feel the spring wind up and build tension, then suddenly pop free and turn.
If the upper mounts do let the springs rotate smoothly, they also have to be inspected after they are removed. The center hole can rust out and let the strut's shaft wobble back and forth, and the bearing can fall apart. Those things can't be found any other way than by taking the strut assembly apart. As a result, a lot of people come in for a pair of struts, then the mechanic has to tell them more parts are needed after they have everything apart. We hate having to do that, so a lot of shops automatically include new upper mounts in their estimates.
Replacing the struts requires removing the assemblies from the vehicle, then the coil springs have to be squeezed a little to remove their tension. That takes time and involves a procedure that can be somewhat dangerous. A new alternative is to install a pair of "Quick Struts" that are the entire strut, coil spring, and upper mounts already assembled. With the right tools, I can replace just the struts in an hour, but anyone can install a pair of Quick Struts in about 15 minutes. I'm going this route right now with a friend's car because it's the less-expensive route. The additional cost of parts is less than the labor to do it the "cheap" way.
You also must be aware that any vehicle needs to be aligned after front struts are replaced. While the numbers on the alignment computer may appear to be perfect, you will still have accelerated tire wear if ride height is not correct, and that's where your old coil springs become an issue. When the front suspension sags with age, (not mileage or pot holes you hit), the entire geometry of the suspension and steering systems changes. The wheels will go through the wrong motions as the suspension travels up and down. This adversely affects handling, steering response, and balanced front-to-rear braking.
There's no rhyme or reason as to why one strut fails first or how it fails. There's no way to say it was because you hit the same pot hole every day. That's like saying one light on your Christmas tree burned out sooner because it was sitting in the sun light and got hotter. Why does one head light bulb burn out before the other one? Always replace struts in matched pairs. If you do only one, you need to have the vehicle aligned. If you do the second strut a year later, you have to pay for a second alignment, so how are you dollars ahead?
Monday, August 17th, 2015 AT 6:21 PM