Have you noticed a little too much bounce when you drive your car over bumps? Get out and push down on each corner of the car. If it bounces more than once or twice, it's time to replace the shocks or struts on that corner.
Most monocoque cars today use MacPherson strut suspensions, not because they're better than other designs, but because they're cheap to assemble and don't take up a lot of space. That's how they were able to fit that inline four-cylinder engine and automatic transmmission next to each other, sideways in your front wheel drive Honda or whatever.
Here's how to change struts when they no longer do the job they're supposed to: damping bumps and road vibrations.
Disclaimer: This is a general guide to strut replacement, not a verbatim how-to instruction manual for any particular vehicle. Working with suspension parts is dangerous, so if you're accident prone, perpetually drunk, or have never heard of a MacPherson strut, don't try to do this job yourself.
When MacPherson struts wear out, they can be a pain in the ass to replace, but it's doable. It just takes some patience and a willingness to think through simple problems. You'll know when it's time to get your hands dirty. If the front struts (or shocks) are worn, the car won't want to steer over bumps, because the wheels aren't always in contact with the ground. When the back ones crap out, the car feels like it's being steered from the rear when you hit a bump on the freeway. When both sides go, driving down city streets feels like a buckboard ride, and freeway curves are terrifying as you struggle to maintain control through the turn.
Either way, you can save a bunch of money by doing the work yourself, and if you have the right kind of car, could possibly spend the money you would have forked over to a mechanic on some fancy high performance struts. Why the hell not? But you should be careful, because working with springs can be dangerous.
As you can see from this illustration, struts are fairly straightforward. The spring and shock absorber are packaged into one piece to save space. The top is mounted to a reinforced hollow in the vehicle's body, and the bottom is usually connected to the steering knuckle (in the front) or wheel hub (on the rear), either directly or via a balljoint.
How this is all connected affects the way the strut assembly comes out of the vehicle — some are more complicated than others. Sometimes, if the car has a sway bar, it's attached to the struts. Other times, like on my '86 Subaru GL, the strut is stuffed down into a receiver in the knuckle, making it necessary to disconnect the balljoints in order to have enough room to pull the knuckle down and the strut out from its tower.
But basically, all you have to do is unfasten the top mounts, remove the bolts that hold the strut in place at the bottom, and pull the whole thing out in one piece.
First thing first. While you still have the car on the ground and are thinking about it, Loosen the wheel lug nuts, as well as the three or four nuts that secure the top of the strut to the frame and the big nut that holds the spring in place on the strut.
IMPORTANT: Loosen these nuts, but don't remove them. The whole thing is still under load and you could seriously injure yourself by taking things off now.
Now that you have all the nuts that are difficult to loosen when the car is off the ground loosened, jack the side of the vehicle you're doing off the ground and put the car securely on jackstands. Make sure you have the wheels on the other side of the vehicle chocked to prevent it from rolling while you're working. Also, use jacking points, frame crossmembers and other sturdy parts to place your jack and jackstands.
IMPORTANT: Don't rely upon a jack — especially a hydraulic one — to hold up the car while you're working on it. That's what jackstands are for. They're metal, they don't move, and that's the safe way to keep the vehicle off the ground. Also, don't jack the car up using suspension components. The jack/jackstands can slide on them (because they move), and without the suspension arms free to move once you remove the strut mounts, it will be impossible to get the struts out of the car.
When you've raised the vehicle and supported it securely on jackstands, walk around the car and give each corner a jiggle to make sure your setup is stable.
Again, loosen the top nut (the strut's Jesus nut, so to speak) before you take the strut out of the car. You'll be bummed if you take the strut out and spend a bunch of time compressing the springs with those miserable screw compressors only to find out that you can't take the assembly apart because the top nut is on too tight.
IMPORTANT: DO NOT take the top nut all the way off. It's dangerous to do so.
Disconnect everything from the bottom of the strut:
To get the bottom part of the strut loose, you usually have to remove the brake caliper and rotor. In most cases, you'll have to unscrew the brake hose from the caliper to get it off of a little bracket that connects it to the strut. That means you'll have to bleed the brakes when you're finished with the whole job. After you've disconnected the line, reconnect it while you're working on the other parts so that a) brake fluid doesn't get all over the place, and b) you don't get dirt in the caliper's brake fluid gallery.
Put PB Blaster or some other sort of penetrating oil on the bolts you have to remove. Suspension parts are always at the mercy of road dirt and salt, and can get sticky. You'll have to remove whatever huge bolts/nuts connect the strut to the knuckle/hub, and you may have to disconnect a sway bar link from the strut as well.
IMPORTANT: Make sure you note how alignment parts that affect suspension geometry were oriented. You'll have to get an alignment done anyway, but it's better not to have to drive the car to the alignment shop with the tops of the wheels sticking out.
Unfasten the top of the strut:
When you take those three or four nuts that hold the top of the strut to the body, make sure you support the strut with your free hand so it doesn't fall out of the bottom of the car onto your foot/helper or something. Once it's free, you may have to use a pry bar to move suspension parts out of the way so that it slides out of its tower.
Don't force anything. If you have to, remove tie rod ends and things until you have enough space to get the strut assembly out. This is where those problem solving instincts come in handy.
Now that you have the strut assembly out, you'll need a set of special tools to compress the spring. Basically, it's a pair of hooks that hold the spring and clamp it down with huge screws. These things can be dangerous if you don't use them correctly, or if they're worn out.
Place the hooks as wide apart on the spring as possible, so that it will compress enough to relieve pressure on the strut mounting plates. The two hook/screw tools should be on opposite sides of the spring. Putting them too close together won't work. Tighten them a little at a time, first on one side, then the other; back and forth.
Once you've compressed the spring enough (you'll start to see a little space between the spring and one or both mounting plates), you can unscrew the Jesus nut and take the top plate off. Note how things look when you take it apart. The spring should be oriented the same way when you mount it on the new strut tube. Also, don't forget the rubber dampers that go between the spring and the mounting plates. Leave these out and your car's suspension will clank like Thor's hammer when you get it back on the road.
With the assembly back together, Snug down the Jesus nut and loosen the compressor screws. Once the compressors are off, you're ready to put the strut assembly back into the vehicle. The top goes in first, with the mounting nuts secured loosely so that you can wiggle the bottom around and get it mounted right. Then fasten the bottom parts.
When everything is in and where it's supposed to be, tighten everything to the proper torque (which you can usually find online for any particular vehicle). Then, go back and re-check all of your connections to make sure they're on tight. This is your suspension we're talking about, so it's important not to screw it up.
With the struts back in the car, recheck everything and then bleed the brakes (if you've had to disconnect brake lines to get the struts in and out). Then, replace your wheels and lower the car. Once the car is on the ground, tighten the lug nuts to the proper torque and also tighten the strut top mount nuts and Jesus nut.
Got everything? Did you check and re-check that things were all connected the way they're supposed to be? Good. Take the car for a low speed test drive in the neighborhood. If everything works, congrats! You did it!
You could always get a repair shop to do this stuff for you, but if you're reasonably intelligent, you can do it yourself. Besides, you didn't have anything better to do after work, did you?
Photo credit: Benjamin Preston