Shock absorbers
and struts dampen the motions of the suspension to provide a smooth,
comfortable and safe ride.
Some OEM shocks 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 “rheological”
magnetic fluid that changes its viscosity when a 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 and 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 suspension.
Twin tube
shocks and struts are the more common design. The gas charge is
contained in the outer chamber (fluid reserve tube) and is typically
lower than that of a monotube.
Because the damping characteristics
of shocks deteriorate gradually over time, the decline in ride control
often passes unnoticed. Consequently, many motorists are unaware how
weak their original shocks and struts have become. They get used to the
way their vehicles ride and handle, 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 leading
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 to thinking and may reveal a
need for replacement or upgrading. Ask him how his vehicle handles when
cornering, stopping, accelerating or driving in a cross wind.
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? 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 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 or strut piston rod, broken mounting hardware, or fluid
leaking from a damper. Struts should be replaced if severely corroded.
REPLACEMENT TIPS
Shocks
and struts are generally replaced in pairs — though this isn’t always
necessary if a damaged low-mileage part is being replaced. It is
necessary when upgrading a suspension because of differences in valving
characteristics. The dampers on both sides of an axle should always
offer the same resistance.