Shock therapy

The first real sign that something is wrong comes at night.

After you pull out of the drive, your headlights are illuminating parts of the trees that you’ve never noticed before. Then you realize that the car is rocking fore and aft for a few seconds every time you stop for a light.

But the real eye-opener comes when you hit a speed bump that you’ve encountered a thousand times before — and I do mean “hit.” For years you’ve traversed this nuisance at a cautious 12 mph without spilling your cup of coffee, but this time you hit the same bump at the same speed, and your next stop is the dry cleaner after your suspension tops out and bottoms out with a thump.

Diagnosis: Your shock absorbers are worn out.

To begin with, shock absorbers don’t absorb shock. What we in the United States call shock absorbers are more accurately referred to in the rest of the civilized world as “dampers.”

Your suspension, if it is working correctly, has little friction in its travel. Consequently there’s a tendency for the wheel to overshoot during normal up-and-down movement, which can make the vehicle bounce up and down for several cycles after a disturbance. When the car passes over a large bump, a poorly controlled axle may slam into either the top or bottom limits of its travel. Not only is this uncomfortable, it can make the vehicle difficult to control.

What’s happening inside the shock absorber as your suspension scales that speed bump? A piston forces oil through carefully tuned orifices and valves as the wheel moves up into the wheel well. When you hit the top of the bump, a different set of valves allows the oil to return, resisting the spring’s return force. At higher speeds some additional orifices remain open to provide less resistance and a smoother ride.

As the miles pile on, the piston and its sealing rings wear, allowing more fluid to pass through. The valves and their springs or spring washers wear out, permitting even more fluid flow. Eventually the calibrated damping rates degrade, leaving you with too little damping for the spring rates and, therefore, a wallowy ride.

In more extreme cases the seal around the piston road starts to leak, allowing oil to escape. Since there are only a few ounces of oil inside, almost any evidence of leaking oil on the outside of a shock makes it a candidate for immediate replacement.

Always replace both shocks on a given end of the car at the same time, or you’ll wind up going over that speed bump in a corkscrewing motion as one side of the vehicle responds differently than the other.

Traditional shocks use two concentric tubes, one to act as a reservoir for hydraulic fluid. Recently, however, single-tube shocks, also called gas shocks, have become more popular.

This type uses an inert gas under high pressure — 200-plus pounds per square inch — to prevent aeration of the fluid under rough conditions. This slightly more expensive design has a superior ride, albeit a stiffer one, and one odd feature: Single-tube shocks tend to extend themselves to their full travel when not installed. They’re shipped with a strap of some sort to keep their damper rods retracted, to prevent damage and to make the packaging smaller.

For the most part, replacing shock absorbers is a straightforward matter of remove-and-replace. It’s accomplished with simple hand tools and some elbow grease.

Unless, that is, your vehicle uses coil-over struts. These require the use of a spring compressor, and may prove a more difficult job than the average Saturday-morning mechanic wants to bite off. You’ll need to rent or borrow the compressor, which will allow you to take all the tension off the coil spring while you change the strut or its innards.

Warning! Compressed springs have enough stored force to lift a car — or to exit the wheel well with considerable velocity if the spring compressor slips, which the tool has a tendency to do. If something important, such as your face, is in the way, damage can occur.

There are a couple of alternatives. Most vehicles allow you to remove the entire strut with the spring still attached. You can then take the strut to a shop and have a mechanic swap the shock out. A better solution might be a Monroe Quick Strut, a complete preassembled unit, including top bearing, all ready to swap for the old unit. It’s a good idea, especially if your car has high mileage or one of the springs has started to sack out and let that corner droop.

As always, check a good service manual for your vehicle to see if there are any serious hurdles before taking something apart.

•Assuming that you’ve assured yourself that the job is within your capability, start by blocking the wheels to the front and rear. Loosen all the lug nuts on whichever end of the car you’re doing first.

•Now jack up the car and secure it on stands. You’ll be twisting big bolts with a great deal of force while working partly under the car — be certain it won’t fall off! Concrete blocks are not an acceptable substitute for proper jackstands. They’re way too brittle and prone to suddenly reverting to their gravely ancestry when provoked.

•No floor jack? You can use the car’s scissor jack to lift it far enough, one corner at a time.

•Remove the wheel. If you’re changing a rear shock, it’s usually necessary to remove only the hardware at the top and bottom. You may need to dive into the trunk or hatchback to remove the upper bolt, which might even mean removing or displacing some interior parts to gain access to it.

•Loosen the top and bottom bolts. Before removing them, however, hold up the suspension with a jack to keep it from drooping under the pressure from the spring. Check the bushings and the bump stop. Most shocks will come with fresh hardware, locknuts and rubber bushings, which I recommend using even if the old ones look fine.

•The fronts generally represent a stiffer challenge. You’ll probably need to remove a clip to free the brake line. Late-model cars may also have antilock-sensor wiring that needs to be dealt with. Work carefully and avoid damaging the brake line or wiring.

•Now you can jack up the suspension. If your vehicle makes its camber adjustment at the strut/shock bottom attachment bolts with integral eccentric washers, mark the index position of the bolts for later.

•Remove the upper bolt from the shock’s central damper rod, then remove the shock from the vehicle. Important: Now you can check the upper bearing, if you have one, for wear. That may have been the source of the thumping noise you thought was a bad ball joint. On many front suspensions the steering pivots in this bearing and, if it’s worn, bad things will happen whenever you steer.

•Now thoroughly inspect each bump stop, which may be a chunk of black rubber between the control arm and the frame, shaped like either a pillow or a cone, or perhaps a doughnut surrounding the damper rod hidden under a weather boot. If it’s deteriorated, you must replace it to prevent damage to the new shock caused by overtravel.

•Oil leaking from the shock body or from the engine is usually tough on these rubber bumpers. If they are oily, out they go. While you’re under there, check the ball joints, tie-rod ends and CV joints as well.

•Now you can button up again. You’ll probably need to use the jack holding up the suspension to let you push the upper damper rod back into its bearing or bushing. And don’t forget to torque the wheel lug nuts with a proper torque wrench.

If your camber adjustments have been disturbed, you’ll need to have the front end realigned.