Aftermarket shock suppliers lament the days when shocks were sold by the truckload. The aftermarket for replacement shocks and struts is currently estimated to be upwards of $800 million annually - that equates to about 22 million units a year. That's still a lot of product. Even so, the shock and strut replacement market has shown little, if any, growth in recent years. With the exception of performance shocks and struts, sales have been flat - which is bad news for shock suppliers, distributors, parts stores and installers.
Some say the sales slump is because struts are now used on most late-model cars instead of shocks. Struts are larger and more robust than shocks, and typically last longer than shocks. But they are also more complicated than shocks and are much more difficult and expensive to replace.
A MacPherson strut is actually part of the suspension rather than an "add-on" device like a shock. The strut housing has a shock-dampening piston and valving inside just like an ordinary shock absorber, but the piston rod is much larger. Around the outside of the strut is a coil spring supported by a seat welded to the housing. The spring is compressed and held in place by an upper bearing plate that is attached to the top of the piston rod. The whole strut assembly is held together by a single large nut on the end of the piston rod.
A MacPherson strut replaces the upper control arm, upper ball joint and coil spring in an older SLA (short/long arm) style suspension. The bottom end of the strut usually bolts to the steering knuckle, and the top of the strut is mounted inside a tower in the inner fender (usually two or three bolts). The strut not only serves as the steering pivot but also supports the weight of the body. Consequently, it is both a steering and suspension component.
Replacing a strut requires raising the vehicle, removing the wheel, unbolting the bottom of the strut from the steering knuckle and unbolting the top of the strut from the strut tower. The job may take upwards of an hour or more - on each side.
If a brake line is attached to the strut, that must also be disconnected - which sometimes means opening the brake line if the line or hose cannot be slipped out of its mounting bracket. This, in turn, requires bleeding the brakes later to remove air from the brake line, otherwise the brakes will feel soft and spongy.
Once the strut is out of the vehicle, it must then be disassembled with a spring compressor to replace the housing with a new one. This changes wheel alignment and requires the wheels to be realigned after the new strut has been installed.
Few DIYers own the type of spring compressor needed to safely disassemble a strut and have no means of realigning the wheels other than to take their vehicle to a shop. There is also a risk of bodily injury and/or damaging the spring if the strut is disassembled incorrectly. The spring may be under several hundred pounds of tension and must be held securely by the spring compressor when the large nut is removed from the top of the strut rod. If the person disassembling the strut isn't aware of this hazard, they may get more than they bargained for when that top nut comes off!
On some older vehicles, the strut housing is rebuildable and has a large hex nut that can be removed so a new shock cartridge can be dropped into the housing. On some of these applications, only the top of the strut has to be disconnected from the strut tower. The top of the strut can then be pulled out from under the fender and disassembled to replace the cartridge (provided there is enough room for the suspension to drop down so the top of the strut will clear the fender). Most struts, though, are not rebuildable and must be replaced if the shock-absorbing elements are worn or the piston seal is leaking.
On a vehicle with a "modified" strut suspension, there is no spring around the strut, but the strut still serves as the upper steering pivot. No spring compressor is needed to replace this type of strut. On vehicles with "wishbone" strut suspensions, the strut is more like a "coil-over" shock and supports the weight of the vehicle but is not attached to the steering knuckle. The strut is attached to the lower control arm and chassis or strut tower. This type does need a spring compressor to replace unless the strut comes preassembled with a spring.
One very important item that is often overlooked when replacing a strut is the upper bearing plate. This plate supports the weight of the vehicle and affects steering effort and return. If the bearing plate is worn or corroded, it can cause steering noise, increased steering effort and/or poor steering return. Few DIYers are aware of what the bearing plate does or how to inspect it, and fewer still realize it should probably be replaced when the strut is replaced.
In an effort to make this whole process easier for DIYers (as well as professional installers), some strut suppliers now offer pre-assembled struts with the spring and a new upper bearing plate already installed. This allows the strut to be replaced as an assembly and eliminates the need to use a spring compressor.
With shocks, installation is much easier so motorists are more apt to replace a set of shocks themselves. The wheels usually do not have to be removed and there are only a couple of mounting bolts (usually one on the bottom, and one or two on the top). On some vehicles, access to the upper mounting bolts may be limited, and may require removing a rear seat or getting inside the trunk to get at the fasteners. But changing shocks does not affect wheel alignment, require a spring compressor or require opening any brake lines. Most shocks can be changed in 20 to 30 minutes.
Another factor that makes shocks and struts challenging to sell is the fact many motorists don't appreciate the importance of ride control and how it affects driving, handling and braking safety.
According to one consumer survey, 70 percent think the primary function of shocks and struts is to provide a comfortable ride. That's probably why they place such a low priority on replacing worn shocks and struts. Only 21 percent recognized the fact that new shocks and struts can improve handling and driving safety.
Not replacing a worn set of shocks or struts may seriously compromise the ability of the vehicle to handle bumps, dips, crosswinds or extra weight. Worn shocks and struts will also increase tire wear and wear on other steering and suspension components, too.
On rough roads, worn dampers that fail to keep the wheels in firm contact with the road may increase stopping distances. Tests have shown that stopping distances increase significantly when the tires bounce and lose grip on the road. This also hurts traction when accelerating and cornering.
Shocks and struts should be replaced when they no longer provide adequate ride control or fail to meet the driver's expectations. Replacement is also required if a shock or strut is leaking, broken or damaged. A bent strut, for example, should be replaced to restore proper wheel alignment and eliminate any risk of future breakage.
The best way to evaluate the condition of a vehicle's dampers is a test drive on a variety of road surfaces (smooth and rough) and under various driving conditions (stopping quickly, cornering and changing lanes). Excessive body roll, sway or rocking would tell you the dampers are not up to the task and need to be replaced. For parts stores, of course, this method is not really practical. However, the old "bounce test," can be easily done right in the store's parking lot. To do the bounce test, rock the suspension up and down several times, then release it to see how many times it rebounds. As a rule, good dampers should stop the chassis from rocking almost immediately. Worn ones may allow it to rebound several times.
A visual inspection can also identify shocks or struts that are failing or have failed. Oil on the outside of the housing means the piston rod seal is leaking. The shock may still be doing its job, but for how much longer is anybody's guess.
RIDE CONTROL UPGRADES
A more compelling reason to replace shocks and struts these days is to upgrade handling and ride control performance. This approach seems to work best with motorists who want to improve the way their vehicles rides or handles.
If a vehicle is used to pull a trailer, a ride-control upgrade may provide a more sure-footed track without wagging or whipping, better stability in cross winds and better handling on curvy roads.
Popular upgrades include high-pressure gas shocks and struts with firmer valving or adjustable valving, shorter, stiffer springs to lower the center of gravity, stiffer sway bars to keep the body flat, and firmer suspension bushings to reduce suspension compliance.
Though most late-model vehicles come factory equipped with gas-pressurized shocks or struts, many OEM dampers are valved more for ride comfort than ride control. Soft valving provides a nice boulevard ride, but the trade-off is reduced body control and more roll - exactly what you don't want on a vehicle with a high center of gravity like your typical SUV.
Valving is what determines the performance characteristics of a shock. Some use "deflective disc" valving while others use spring-loaded check valves. Some shocks and struts incorporate "road-sensing" technology that allows the shock to act progressively stiffer as suspension travel increases. Small slots or grooves in the piston chamber wall allow fluid to bypass the piston in the piston's normal operating range. This effectively softens valving and decreases resistance for a smoother ride. As the piston travels beyond this range, resistance increases for improved ride control.
Other shocks and struts use an "impact-sensing" valve that absorbs sudden impacts by momentarily bypassing fluid, then closes to dampen the rebound. All of this occurs within a few milliseconds, and it produces a significant reduction in both body roll and pitch.
According to one shock manufacturer, the valving on their aftermarket replacement shocks is about 15 percent firmer on compression and rebound than the original shocks to compensate for suspension wear. On their performance shocks, the valving can be much stiffer.
Gas shocks come in one of two basic varieties: single tube (monotube) and double (twin tube). The single tube variety has all its major components contained within a single large tube (thus the name) and typically uses a very high-pressure charge (280 to 360 psi). The gas charge is separated from the hydraulic fluid by means of a floating piston in the top or bottom of the tube. This type of shock must be manufactured with a heavier gauge cylinder and a highly polished internal surface (some are Teflon-lined).
A less-expensive alternative for upgrading ride control performance is the double- or twin-tube gas shock. Available from many suppliers of single-tube shocks, the double-tube design is essentially a gas-pressurized conventional shock with lower pressure. Some are in the 70-130 psi range while others are 112 to 130 psi or higher.