QUESTION: How
do I know when it's time to replace my shock absorbers?
I saw an ad on TV recently that said my car could be
dangerous to drive if I didn't replace them every 50,000
miles. Is this actually true? Also I confess to not
really understanding the difference between a shock and
a strut — how can I tell which kind my car has?
—Sue Talman
ANSWER: It's flat wrong to use scare
tactics to sell services. Shocks and struts play an
important role in keeping your tires firmly planted on
the ground and providing vehicle stability, but aren't
nearly as important — in my opinion — as properly
maintained tires and prudent driving practices.
Your car's suspension system uses
springs at each of its corners to provide a smooth ride.
Your tires, wheels, brake parts, and suspension members
combine as un-sprung weight. Upon hitting a bump they
enter what's called jounce. The spring is compressed,
storing energy, which is soon released in rebound, as
one passes the bump. Unfortunately, the combined mass of
parts overshoots its original position and additional
oscillations occur, disrupting tire/ground traction and
vehicle stability.
Heavier wheels, tires, brakes or
suspension parts make matters worse, and are a
performance trade-off. Modern shocks and struts use a
piston that plunges back and forth in a tube filled with
hydraulic fluid, controlled by some smart valves, to
dampen suspension movement. The suspension jounces and
rebounds in a more controlled fashion, and subsequent
oscillations are greatly reduced.
Shock absorbers (usually one per
corner) are typically found on trucks and older/larger
cars and are a fairly inexpensive bolt-on part. Front
wheel drive cars need greater width in the engine bay to
fit a sideways engine and transmission, so a more
compact, integrated suspension system, known as a
MacPherson strut suspension, is used. The shock
absorber, upper control arm and coil spring are
integrated into a single vertical suspension member
called a strut. Struts may also be found in the rear of
many vehicles to reduce weight and/or improve
performance.
When it's time to renew the shock
absorber, either the entire strut (minus spring) is
renewed, or a shock cartridge is inserted into the
original strut. Replacement typically costs more than a
shock renewal due to the greater labor required in
disassembly and reassembly. Also, realigning the
suspension is recommended, as the new parts may differ
very slightly in dimension and reinstallation position.
If you peek above or behind each tire,
a strut (and its coil spring) are usually visible, about
an inch or two behind the tire. If shocks are used,
they're usually further inboard/lower, do not have a
spring encircling them, and are less visible.
Worn shocks and struts sneak up on you
gradually. One day while slowing on a curving, bumpy
road, you'll get a queasy feeling the vehicle just isn't
as nailed down as it should be. Or, expansion joints on
a rough section of road make you feel seasick, or your
tires develop choppy tread wear. Also, if a shock or
strut shows signs of fluid leakage, it's time to renew
it.
Every vehicle is different, as well as
roads travelled, driving habits, and operator
expectations. Replacement at 50,000 milkes is a good
recommendation for a bulky vehicle driven on windy,
bumpy roads, or if stellar handling is desired. Being a
skinflint, I go half again further than this for my
around-town, grocery-getter car. After replacement I
usually say, "Wow, there is a difference. I should
have done this sooner!"
Here's a neat link for a historical
perspective on automotive suspension: http://www.motorera.com/history/hist08.htm/