SHOCKS & STRUTS
The shock absorbers do not affect alignment directly, but they do play a major role in ride quality and handling. The shock
absorber's job is to dampen spring oscillations and the up-and-down motions of the suspension. The shock uses a
double-acting piston to displace hydraulic fluid. Valving controls the amount of resistance and the dampening characteristics of
TYPES OF SHOCK ABSORBERS
Shock absorbers come in two basic designs: double tube and monotube. The double tube design has an inner piston chamber
surrounded by an outer fluid reservoir. Fluid moves back and forth between the inner and outer chambers through valving in
the bottom of the shock housing.
The monotube design has no outer fluid reservoir. All the fluid is in the piston chamber, and the piston simply pumps the fluid
back and forth from one side of the chamber to the other. But monotube shocks also have a floating piston that separates the
fluid from a high pressure (up to 360 psi) gas charge (usually nitrogen). The high pressure gas helps prevent foaming in the
fluid when the fluid is pumped rapidly by the piston (as when driving on a rough road). Foaming is undesirable because it
reduces the resistance the shock can provide causing "shock fade" and loss of ride control. Double tube shocks are also
available with a lower pressure (typically less than 150 psi) gas charge for this same reason.
In addition to the these two basic designs, replacement shocks come in a variety of configurations:
* Standard replacement shocks have ride characteristics similar to the original equipment shocks on a vehicle.
* Heavy-duty shocks usually have a larger piston bore diameter, increased capacity and a slightly stiffer ride for applications
where additional load carrying capacity or control is desired.
* Radial tuned shocks are valved to accommodate the ride characteristics of radial tires, which tend to be somewhat harsher
than bias ply tires.
* Gas charged "premium" shocks are standard equipment on many late model cars, but can also be installed as a ride control
upgrade on older vehicles or ones that were not originally equipped with them.
* Performance shocks have a larger heavy-duty construction, are gas charged and have special valving to increase stiffness.
This type of shocks is for performance applications where ride control and handling are more important than ride quality.
* Adjustable shocks are generally for performance applications where a street driven vehicle may be occasionally raced on
weekends. Manually adjustable valving allows the driver to select standard, firm or extra firm settings by rotating the shock or
an adjustment screw.
* Overload shocks are heavy-duty shocks with an outer coil helper spring that can boost a vehicle's load carrying capacity. A
better alternative here might be variable rate replacement springs or air springs because the shock mounts are not designed to
* Air shocks contain an inflatable air bladder that can be pressurized to maintain ride height when hauling additional weight or
towing a trailer. Some vehicles have air shocks as standard equipment as part of an automatic load leveling or electronic ride
* Truck and sport/utility shocks are also available in various configurations to suit different applications. Off-road shocks
typically provide increased travel as well as added capacity and stiffness. Specially tuned shocks that provide a smoother,
quieter ride for trucks and sport/utility vehicles are also available.
* Electronic shocks are used as original equipment on some vehicles, and offer variable valving (usually computer controlled).
A stepper motor or solenoid on the shock rotates a rod to change the valving. These shocks are very expensive and may be
available only through a new car dealer. On some vehicles, less expensive conventional shocks or struts may be used to
replace electronic shocks if the original units are bad.
PROBLEMS CAUSED BY WORN SHOCKS
Worn shocks of any type can allow any or all of the following conditions:
* A rough, bouncy ride
* Suspension bottoming
* Nose dive when braking hard
* Rear squat when accelerating hard
* Uncontrolled suspension and body motions that result in poor handling and traction when driving on rough roads
* Excessive body sway when cornering or driving in cross winds
Worn shocks also contribute to increases stress and wear on other steering and suspension components, and may cause tire
Shocks replacement should be recommended if any of these conditions exist, and required if the shock is leaking fluid (seal
failure), dry (no fluid inside) or damaged.
Shocks may also be replaced to upgrade the suspension (replacing standard shock with gas-shocks or performance shocks,
Replacing shocks in pairs (same axle) is recommended.
Struts are more than overgrown shock absorbers because on most applications they are also an integral part of the
suspension and play a direct role in wheel alignment and steering. A worn strut won't alter wheel alignment, but it can allow
excessive suspension motions that do create undesirable changes in alignment as the vehicle is being driven. A bent strut, on
the other hand, can cause undesirable changes in camber and caster alignment.
Like shock absorbers, struts come in double tube and monotube designs, with or without gas charge, and in standard,
heavy-duty, specially tuned, adjustable and electronic configurations. Some struts are sealed but others are rebuildable and
can be opened up to change the damper cartridge inside.
SHOCK & STRUT CHECKS
Shocks and strut wear is difficult to gauge on mileage and age along alone because a variety of factors affect how quickly the
dampers wear. These include climate, vehicle weight, load, usage, driving speeds and overall vehicle condition. Shocks and
struts wear gradually, so many motorists don't notice the progressive deterioration in ride quality and handling performance
that occurs over time. They become used to it without even realizing it.
A simple bounce test will tell you if the shocks or struts are really weak, but not much else. A bounce test id performed by
rocking the suspension up and down, then releasing it -- and watching to see if it stops bouncing. One or two gyrations after
releasing the suspension is usually okay, but more means the dampers are weak and are doing a poor job of controlling
A better way to test shocks and struts is to test drive the vehicle so you can get a firsthand feel for how well the dampers
perform under various real world driving conditions. Be sure to include sharp turns in both directions, a stretch of rough road
and a quick stop.
A visual inspection should also be made anytime a problem is suspected or the vehicle is up on a lift for other service or
repairs. Look for the following:
* Fluid leakage around the shaft seal. A little fluid on top of a shock or strut may be normal. But fluid trickling down the side is
not normal and tells you the seal is leaking. Leaky shocks and struts should be replaced because if they haven't already failed,
they soon will.
* Broken, loose or deteriorated shock mounts. If the shock isn't solidly connected at both ends, it cannot function properly.
Cracked or collapsed bushings can cause noise and prevent the shock from controlling small movements of the suspension.
* Broken or damaged piston rod. If bent or broken, replacement is required.
* Severe rust or cracks on struts that structurally weaken the housing. Failure could lead to suspension collapse.
* Worn upper bearing assemblies on struts. If worn or loose, these can cause steering noises such as snapping, popping,
creaking or groaning sounds when turning, or suspension noises such as clunking, rattling or popping on rough roads. Worn
bearing assemblies can also increase steering harshness. If binding or frozen, the result may be poor steering return, memory
steer, increased steering effort or possibly even steering snap back after turning due to spring wind up.
The reasons for replacement are the same as those for shock absorbers: a deterioration or loss of ride control, or a need to
"upgrade" to a better or special type of unit.
Struts can be replaced individually, but high mileage struts should usually be replaced in pairs.
Replacement may also be necessary if a strut is found to be bent. This is often the result of collision damage or a severe road
impact. Bending struts as a method of correcting alignment problems is not recommended because it can weaken the housing
increasing the risk of failure. Alignment must always be checked after strut replacement.
The condition of the upper strut mounts is also very important because the bearing assemblies support the weight of the
vehicle and serve as the upper steering pivots. The upper mounts should always be inspected when rebuilding or replacing a
strut, and should also be replaced if worn or binding.
When replacing a strut, use the correct type of spring compressor for the application (and don't overcompress the spring).
Mark the position of the camber bolts and upper strut mounts before loosening anything to make realigning the wheels easier
once the new struts have been installed.
Always follow the installation instructions regarding the use of spacers or washers under the body nut on rebuildable struts.
Differences in height among replacement cartridges make the use of such spacers necessary.
Also note that many replacement struts are marked "left" or "right" depending on which side they fit. Don't interchange them.