Underhood: ABS Diagnostics
Most shocks and struts today are “gas-pressurized” with nitrogen to minimize fluid foaming when the piston is pumping back and forth.
Shock absorbers and struts dampen the motions of the suspension to provide a smooth, comfortable and safe ride. Some OEM shocks even have electronic valving that allows the driver or a body control module to adjust the dampening characteristics of the shocks or struts to changing driving conditions. Electronic dampers may use a solenoid or an electric stepper motor for this purpose. The latest technology is to use a special “magneto-rheological” fluid that changes its viscosity when a magnetic current is passed through it.
Most shocks and struts today are “gas-pressurized” with nitrogen to minimize fluid foaming when the piston is pumping back and forth. Foaming creates bubbles in the fluid, which offer less resistance to the piston. The result is “shock fade” as the damper loses its ability to provide adequate ride control.
Gas shocks and struts come in one of two basic varieties: monotube or twin tube. Monotube dampers have all the major components contained within a single large tube and typically use a very high-pressure charge. The gas charge is separated from the hydraulic fluid by means of a floating piston in the top or bottom of the tube. Monotube shocks are used primarily on performance vehicles with stiffer-handling suspensions.
Twin tube shocks and struts are the more common design. The gas charge is contained in the outer chamber (fluid reserve tube) that is typically lower than that of a monotube design.
Because the damping characteristics of shocks deteriorate gradually over time, the decline in ride control often occurs unnoticed. Consequently, many motorists are unaware of how weak their original shocks and struts have become. They get used to the way their vehicle rides and handles, and may not realize they need new shocks or struts.
Although you won’t find a recommended replacement interval for shocks or struts in a vehicle owner’s manual, one aftermarket shock supplier says shocks and struts should be replaced every 50,000 miles — and has solid research to back up the recommendation.
Ask For the Sale
Asking your customer how his vehicle has been riding lately may get him thinking, and may reveal a need for replacement or upgrading. Ask him how his vehicle handles when cornering, stopping, accelerating or driving in a crosswind. Excessive body sway or rocking is a sure sign of inadequate ride control. How does the vehicle ride over tar strips or on rough roads? Let them know that 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?
A “bounce test” is still a valid means of checking the dampening ability of the shocks and struts. If the suspension continues to bounce more than once after bouncing and releasing the bumper or body, it indicates weak shocks and/or struts that should be replaced.
If the original dampers are worn out or not up to the task, recommend a new set of shocks and/or struts as a way to rejuvenate or upgrade ride control performance. Replacement would certainly be necessary if a vehicle has a bent or damaged shock, strut or piston rod, broken mounting hardware or fluid leaking from a damper. Struts should be replaced if they are severely corroded.
Shocks and struts are generally replaced in pairs — though this isn’t always necessary if a damaged, low-mileage part is being replaced. Replacing in pairs is often necessary when upgrading a suspension because of the differences in valving characteristics. The dampers on both sides of an axle should always offer the same resistance.
Upper-strut bearing assemblies on front struts are often over-looked, but may also need to be replaced. The bearings are sealed assemblies and cannot be lubricated. If they are rusted, loose, worn, noisy, binding or damaged, they must be replaced.
• Steering noises such as snapping, popping, creaking or groaning sounds when turning;
• Suspension noise such as clunking, rattling or popping on rough roads;
• Increased steering effort brought on by binding in the bearing plate;
• Steering snap-back after turning caused by a frozen bearing assembly and spring windup; and
• “Memory steer” in which the car doesn’t want to go straight after turning due to binding in the upper mount.
Strut replacement changes the alignment of the front wheels, so a wheel alignment is usually necessary after new front struts have been installed. Also, if brake lines have to be opened to replace a strut, the brakes must be bled after the job is finished to remove trapped air.
Additional chassis parts that may have to be replaced include springs, tie rod ends, lower ball joints, lower control arm bushings, steering rack mounts and wheel bearings.
Article courtesy Brake & Front End.