A. The primary function of shocks and struts is to dampen the motions of the suspension as the suspension reacts to bumps and dips in the road. They do this by pumping hydraulic fluid through orifices and spring-loaded valves that offer a calibrated amount of resistance. As the piston pumps up and down in the shock, fluid is forced back and forth through the restrictions. This creates friction and slows the motions of the suspension.
Rapid pumping can churn the hydraulic fluid into foam. When this happens, the shock loses much of its dampening ability and begins to “fade.” This occurs because the bubbles in the foam offer less resistance to the piston.
In “gas” shocks, a charge of pressurized gas (usually nitrogen) is added to the shock when it is manufactured. The pressure exerted by the gas on the fluid reduces the formation of bubbles, which greatly improves the shock’s dampening ability under hard use. Most “twin tube” shocks contain about 100 to 200 psi of gas, while “monotube” performance shocks carry 280 to 360 psi.
The ride characteristics and stiffness of a shock or strut depends on the size of the pistons (bigger generally offer more resistance), the valving inside (soft, firm or adjustable), the type of shock or strut (monotube or twin tube) and the amount of gas pressure inside.
Q. When should shocks and struts be replaced?
A. Normally, most motorists don’t even think about their dampers until the ride control is so bad it’s obvious something needs to be done. Some shock manufacturers say replacing the dampers at 50,000 miles can maintain the like-new ride control provided by the dampers. But most shocks and struts are not replaced until the odometer flips over 100,000 miles. Even then, they may be ignored.
Because the damping characteristics of shocks and struts deteriorate gradually over time, the decline in ride control often passes unnoticed. Ask your customer how their vehicle has been riding lately. This may prompt your customer to consider replacing or upgrading their dampers.
How does the vehicle handle when cornering, stopping, accelerating or driving in a cross wind? Excessive body sway or rocking is a sure sign of worn shocks/struts that need to be replaced.
How does the vehicle ride over tar strips or on rough roads? A rough or bouncy ride could be improved with new shocks or struts.
Does the suspension bottom out when the vehicle is heavily loaded, or does the steering wheel shudder at every railroad crossing? The sure cure for such conditions is new set of shocks and/or struts.
A “bounce test” is still a valid means of checking the dampening ability of shocks and struts. If the suspension continues to gyrate more than one or two times after rocking and releasing the bumper or body, it’s time for new dampers.
Shocks and struts are generally replaced in pairs, unless an individual strut needs to be replaced because of collision damage or because it’s bent. Shocks are a simple bolt-on installation in most applications, but struts require the use of a spring compressor – unless you sell your customer a set of preassembled ready-to-install struts. These are very popular with both professional and DIY customers because of the labor they save.
Wheel alignment is not necessary after replacing shocks, but it is required after changing a pair of struts. Skipping this step may result in uneven tire wear and/or a steering pull to one side.
Finally, keep in mind that shocks and struts are not the only parts in the suspension. Springs can sag with age or break. Control arm bushings and ball joints can wear out, too. All of these parts must be in good condition for safe driving and responsive handling.