Potholes probably sell more struts and springs than anything else. When a car hits a pothole in the road, the jolt passes up through the strut and spring and hits the bearing plate at the top of the strut. The piston and hydraulic fluid inside the strut dampen the impact a bit as the suspension compresses to diminish the force of the impact somewhat, but the driver still feels a hard bang when the suspension hits the pothole.
Suspensions are pretty rugged and can take a lot of punishment without any ill effects — at least for a while. But tens of thousands of miles of constant pounding eventually wear out the ride control components and other suspension parts. And the rougher the roads, the harder the pounding these parts take, and the faster they wear out.
Winter is especially hard on shocks, struts and springs because the combination of road salt, thawing and freezing cycles breaks up the pavement and causes potholes to form. Some of these craters can turn into monsters and literally swallow up half a wheel. A slow-speed encounter with such a road hazard might cause no lasting damage, but a high-speed smackdown can cause instant damage. By the time spring arrives, months of driving over rough cratered roads can leave some vehicles in need of suspension repairs.
Symptoms that would tell you the struts or shocks may need to be replaced include:
* Badly cupped tires and/or noticeable tire shaking, wheel shimmy or vibration after hitting a bump.
* Suspension bottoming on rough roads or when backing out of a driveway.
* A bouncy ride.
* Body sway or rocking when cornering or driving in strong crosswinds.
* Nose dive when braking hard.
* Fluid leaking from a strut or shock.
YOUR MILEAGE MAY VARY
It’s difficult to estimate how many miles a set of original equipment shocks or struts should last because damper life depends on road conditions and the type of driving the vehicle owner does. Racking up a lot of miles on smooth roads produces much less wear on the suspension and ride control components than driving on rough, potholed roads, or roads with lots of tar strips and expansion joints that produce a steady thump, thump, thump as the wheels roll along.
Some aftermarket manufacturers say many original equipment shocks and struts are marginal after 50,000 miles of average driving. Replacing them can be recommended to restore like-new ride control and handling. But some shocks and struts may need replacing sooner if they have spent much of their time on rough potholed roads.
The traditional “bounce test” is still a good way to test for worn shocks and struts. Rock the suspension several times, then release it. If the suspension continues to rock more than once or twice, the dampers are not doing their job and should be replaced.
Surprisingly, many vehicles NEVER have their shocks or struts replaced. One survey found that 86 percent of the vehicles in salvage yards still have their OEM shocks or struts. The dampers were never changed because the first, second or third owner of the vehicle didn’t realize the dampers were worn out, or didn’t want to spend the money on something that didn’t seem to be absolutely necessary.
True, a worn out set of shocks or struts won’t prevent a vehicle from starting or being driven. But they can make a vehicle harder to control on a rough road, and take longer to stop on rough surfaces. So there are valid safety reasons for replacing worn shocks and struts, not just to improve ride comfort.
STRUTS OF STEEL
With struts, another reason for replacement may be mechanical damage. The tubular steel construction of a MacPherson strut makes it quite stiff and strong. Even so, struts are not invulnerable to damage. A hard hit against a curb or deep pothole can sometimes bend the strut housing or shaft, causing the wheel to go out of camber and/or caster alignment.
If the damage is bad enough to alter camber and/or caster by more than half a degree, it can cause the steering to lead or pull to one side. The driver may not notice it if the lead or pull is marginal, but over time camber misalignment can cause uneven shoulder wear on the tire. This type of damage is usually discovered when the vehicle is receiving an alignment. If the camber, caster and/or SAI (steering axis inclination) readings are out of specifications and cannot be brought back into range using the maximum amount of adjustment that’s provided, it usually indicates bent or damaged parts such as a bent strut, bent spindle or damaged control arm bushings.
THE OVERLOOKED UPPER BEARING PLATE
Struts may also need to be replaced if the upper strut bearing plate has been hammered to death. The bearing plate serves as both the upper spring support and the steering pivot. The bearings in the plate are sealed assemblies and cannot be lubricated. So if the bearing plate is rusted, loose, worn, noisy, binding or damaged, it has to be replaced.
The symptoms of a bad bearing plate include:
* Steering noise such as snapping, popping, creaking or groaning sounds when turning.
* Suspension noise such as clunking, rattling or popping on rough roads.
* Increased steering effort (most noticeable with manual steering) brought on by binding in the bearing plate.
* Steering snap-back after turning caused by a frozen upper strut bearing assembly and spring wind up.
* “Memory” steer or poor steering return where the car doesn’t want to go straight after turning due to binding in the upper mount.
One of the most common mistakes do-it-yourselfers and some professional technicians make when replacing struts is to reuse the old original bearing plates. It’s like rebuilding an engine with a new crankshaft and reusing the old bearings.
REPLACING SHOCKS AND STRUTS
As a rule, shocks and struts should always be replaced in pairs (both fronts, both rears, or all four) to maintain consistent ride control side-to-side.
Shocks are a popular DIY item because on most vehicles they are fairly simple to replace. The hardest part of the job is getting at the upper shocks mounts, which on some vehicles can only be accessed from inside the trunk or passenger compartment (behind the back seat).
Gas shocks do not need to be primed before installation, but most non-gas shocks do in order to purge air from the piston chamber. This is done by holding the shock upright and pulling it all the way out, then turning it upside down and pushing it all the way in. Repeat several times until the shock pumps smoothly in both directions. Never reuse old shock mounting hardware. And don’t overtighten or crush the rubber bushings. Torque all fasteners to the vehicle manufacturer’s recommendations.
Struts are much more challenging to replace. On vehicles where the spring is on the strut, replacing the strut requires the use of a special spring compressor to disassemble the strut. This can be a dangerous job if not done carefully because of the considerable tension the spring exerts on the upper bearing plate assembly. A strut spring compressor is not something many DIYers have or know how to use properly. Care must be taken to clamp the spring properly, and to not damage plastic coated springs during the job.
The fastest and easiest way to change a strut is to change the entire strut assembly. Aftermarket strut suppliers now offer preassembled replacement struts for many applications. The strut assembly includes a new spring, new upper bearing plate, new jounce bumper (if used) and dust shields. The struts are ready-to-install, so there is no need for a spring compressor. Like complete halfshaft assemblies and steering racks, these preassembled struts save both time and trouble. However, wheel alignment still needs to be checked and reset to specifications after the struts have been installed.
CHANGING SHOCK TECHNOLOGY
In recent years, more monotube gas shocks and struts have been used as original equipment dampers on performance and luxury vehicles. In a twin tube direct-acting shock, there is an inner tube for the piston cylinder, and an outer tube that serves as a fluid reservoir. Valves in the piston and in the bottom of the shock control fluid flow in both directions, and determine the dampening characteristics of the shock. In a monotube shock, there is no outer fluid reservoir. There is only a single tube, and a floating piston separates the fluid inside from a high pressure gas charge inside the shock or strut.
The gas charge in both twin tube and monotube shocks and struts is there to reduce fluid foaming when the damper is pumping hard. Without the gas charge, the fluid cavitates and forms tiny air bubbles as the piston pumps up and down. The fluid turns to foam and the shock starts to fade (weaken). Adding the gas charge stops this from happening, and keeps the shock firm even on the roughest roads.
Over time, however, the gas charge may slowly seep out of the shock or strut, causing it to lose much of its original ride control ability. That’s why aging shocks and struts often have to be replaced, even if they are not yet leaking fluid.
Monotube shocks are more expensive to manufacture than twin tube shocks because they require a highly polished inner surface. Even so, they do have some advantages over twin tube shocks and struts:
* The monotube design provides better heat dissipation and cooling than a twin-tube shock. There is no outer tube or fluid reservoir to inhibit heat flow, so a monotube shock runs cooler and delivers more consistent ride control.
* A monotube shock is lighter than a twin-tube shock that has the same external diameter. This reduces unsprung weight and allows the wheels and tires to follow the road more closely.
* A monotube shock can be mounted in any position (right side up, upside down or even sideways). A twin-tube shock uses gravity to drain the fluid down through the valving in the bottom, and to maintain the gas charge in the outer reservoir.
* A monotube shock has a larger diameter piston than a twin-tube shock with the same external dimensions, which gives the shock greater sensitivity for small piston motions.
* Monotube shocks are used for many aftermarket coil-over applications because of their compact design. Many also have adjustable valving so the ride characteristics can be fine tuned or changed depending on how the vehicle is being driven.