HOW
TO CHECK CAR STEERING AND SUSPENSION
If you suspect that you have a problem with your cars steering or
suspension,or you just want a periodic check, here is a basic examination
you can make.
BASIC SUSPENSION CHECK
Push down on top of the wing each side (individually) just once
and let go, The vehicle should go down and come back up once and
stay put. If the vehicle bounces up and down and appears soft this
indicates a faulty shock absorber strut.
Further Examination
With the
car jacked up at the front, place on axle stands for safety , put steering
wheel in the unlocked position. Move the road wheel from side to side
as if you were turning the wheels to steer. Feel for any excessive free
play or movement in the steering joints ie top/bottom swivel joints,
track rod ends ,steering rack and suspension bushes. Then hold the road
wheel at the top and bottom grabbing hold of the tyre pushin in and
out and feel for any free movement there. Any excess free play should
be picked up and the offending part noticed and replaced.
Examine rubber gaiters or (boots) as they are sometimes called on the
steering rack and rubber insulator bushes on the anti-roll bar.
Examine front suspension struts for leakage and split dust covers,this
can be observed by looking inside the coil spring, any signs of oil
around this area indicates a worn shock absorber strut and must be replaced.
Check also for broken coil springs.
A Primer on Suspension Testing
Introduction
A safe vehicle must be able to stop and maneuver over a wide range of
road conditions. To be able to stop and maneuver quickly there must
always be good contact between the tires and the road. The suspension
is responsible for keeping the tire firmly planted on the road. The
suspension also provides a comfort buffer between the rough road and
the passenger cabin.
There are many different types of suspensions. Each is a compromise
between comfort, cost, and road holding. To meet these compromises for
each vehicle, many types of suspensions have been developed. Each design
shares two essential componentssprings and shock absorbers.
Some vehicles have struts instead of shocks. In reality, a strut is
just a shock absorber built into a suspension link. The strut is generally
replaceable as one unit. In this discussion, any reference to shock
absorbers will also apply to struts.
There are also some variations in the type of springs used on vehicles.
There are coil springs, leaf springs, air springs, and torsion bars.
Despite the differences in design, these variations all react with the
suspension in the same manner. Any further reference made to springs
will be general and includes any of these types.
The Spring
Springs isolate the driver from road imperfections by allowing the tire
to move over a bump without drastically disturbing the chassis. If the
chassis remains fairly steady then the tires are better able to follow
road contours.
While springs do an excellent job of smoothing over bumps, they will
keep bouncing once started. In other words, the chassis continues swaying
and the tires keep hopping long after the vehicle strikes a bump. Left
uncontrolled, springs give an uncomfortable ride with very poor tire
to road contact. To control this undesirable behavior, a shock absorber
keeps the spring from overreacting to every bump or dip and prevent
excess movement of the tire and chassis.
Springs are durable items and are easily inspected. If the ride height
of a vehicle has decreased excessively or a coil/leaf has broken it
is advisable to replace the springs in axle sets. Consumers also often
change springs to alter their vehicle’s ride and handling characteristics.
Spring problems are generally easy to identify.
The Shock Absorber
The shock controls spring motion by damping (absorbing) energy from
the spring. A shock absorbs energy by forcing oil through valves whenever
it is moved. It takes a lot of energy to push oil through the valves
so when the spring is done pumping, it doesn’t have much energy left
to keep bouncing. Imagine running 50 yards on bare ground – you could
do it and have plenty of energy to run back again. Now imagine running
the same 50 yards through 6 inches of mud. How much energy would you
have for a return pass?
Shocks also control the reaction of the body to road undulations. A
stiffer shock tends to transmit more road irregularities to the driver
but will also not pitch and roll as much as a vehicle with softer shocks.
Thus shocks, like springs, can be changed to obtain a personalized ride.
Shock absorber condition is difficult to evaluate. Shocks are wear items,
like air filters or tires, and do need to be replaced occasionally.
The Car Care Council recommends inspection at 25,000 miles and every
6,000 miles thereafter. There are some exterior signs of a damaged shock
that indicate a need for replacement, but frequently a shock absorber
will have stopped working without any visible indicators.
What if the shock absorber is just worn out? It is not possible, simply
by examining the shock, to tell if it is functioning. One visual indication
of tired shocks is worn out and cupped tires – a very expensive diagnostic
technique. The only way, up until recently, to check shock performance
was a test drive or bumper jounce test – both of which are very subjective.
In any case it was difficult to identify the worn part because all the
shocks are working together.
Shock Absorber Design
All modern shock absorbers use oil to absorb energy. As the shock compresses
and extends, oil must be forced through valves in the shock piston (see
diagram). The valves provide resistance to the flow of fluid, which
absorbs energy from the suspension. The valves let a different amount
of fluid through at different shock piston speeds. That means that the
shocks absorb different amounts of energy depending on how fast the
suspension is moving. Thus, valves allow the suspension to provide a
comfortable ride by controlling chassis movement (low piston speed)
and, at the same time, provide firm contact with the road surface by
controlling suspension movement (high piston speed).
There are two basic types of shock absorbers on the market today. All
shocks are designed to provide comfort and safety but, like all products,
some are better than others. Some shock absorber designs are better
suited to handle rough road conditions without loosing effectiveness.
Other differences in shock design center around the quality of components
and how that affects the life expectancy of the shock.
The oldest design still in use today is the two tube shock absorber.
In this type, an inner tube holds oil which the piston moves up and
down through. An outer tube holds reserve oil and air. When the piston
moves up then down it draws then forces oil to the outer tube. There
are usually valves on the piston, as well as, between the inner and
outer tube in this design. Under very bumpy conditions, motion can mix
the air into the oil in little bubbles, creating a foam. Foaming oil
does not absorb energy very well. This type of shock absorber stops
working, as well, when you really need it most – on very bumpy roads.
Recently, this design has been updated by the use of low pressure nitrogen
in the place of air. These are called “GAS” shocks. The nitrogen under
pressure is much less prone to mixing with the oil and cause foaming.
This is a significant improvement over the standard twin tube design.
The second basic type of shock absorber is called a monotube design.
It is also a “GAS” shock because high pressure gas is used to account
for volume changes inside the shock instead of a reservoir tube. The
gas is pressurized to as high as 360 psi. When this type of shock is
not installed, the piston rod will extend completely due to gas pressure.
The gas is separated from the oil tube by a second, free floating piston.
By not having air mixing with the oil, this type of shock eliminates
the possibility of foaming. All the valves for monotube shocks are
located in the piston.
An interesting variation on the monotube design is a “GAS” shock that
intentionally mixes the pressurized gas with oil. Because the gas is
under very high pressure the oil does not foam. The gas just travels
around with the oil in little bubbles. The valving on this type of shock
is located on the piston and is specially designed to handle the airoil
mix. A rule of thumb for all types of shocks and struts is that “GAS”
shocks are better for almost all circumstances. In addition, the larger
the tubes used to construct the shock, usually the higher quality it
is. Larger tubes hold more oil which then doesn’t get as hot. Cool oil
retains its molecular properties longer than hot oil.
There are additions to these basic designs on the market. Some vehicles
have electronic controls that bypass some damping under different vehicle
conditions or driver choice. Aftermarket shocks that have softer damping
rates at normal travel but get firmer at extreme positions are becoming
popular. These designs give a comfortable ride under most conditions
but become firm when aggressive vehicle maneuvers are required. There
are also very high performance shocks available that have multiple to
infinite damping adjustments, allowing the vehicle owner to precisely
tune suspension performance.
The Need for Suspension Testing
Why is it so important to keep good tire contact? All actions a vehicle
performs – acceleration, cornering, and braking are transmitted through
the tires. The tires perform all these actions through a phenomenon
called friction. The most important rule of friction is that the more
two objects are pressing together the higher the frictional forces between
them. Therefore, it is necessary for the tire to be pressing firmly
on the road at all times if the vehicle is going to brake and turn.
Even shocks that are only half as good as new can significantly impact
stopping distances. The Cologne Institute for Traffic Safety tested
a vehicle with partially worn shocks and then tested the same vehicle
with new ones. The used shocks increased the vehicle stopping distance
by 21 feet, a 23% increase! (See chart.) Twenty one feet is about 1
1/2 car lengths – more than enough distance to prevent serious injury
and property damage.
The same rules apply to cornering. In an emergency situation you need
as much contact between the tire and road as possible. Without maximum
contact the vehicle might not be able to perform the emergency maneuver
necessary to prevent injury and property damage.
The average age of vehicles on the road today is 8 1/2 years and is
continuing to increase. Thus, an increasing number of vehicles are going
to need shock service.
Unfortunately, customers do not often identify the need for shock replacement
because the change has been so gradual that they have accustomed themselves
to the degraded ride. Repair facilities can do their customers and themselves
a service by identifying those worn safety components.
Shock and strut work is also very profitable. Modern Tire Dealer (1997)
reports an average ticket of $169.00 at a profit of 45%. Those figures
do not include the associated services that might be required, such
as alignments after strut replacement.
Suspension and Shock Inspection Techniques
How do you know when to replace shock absorbers? Traditional methods
for determining shock replacement have always been very subjective.
In other words, it was entirely left to the astuteness, biases, and
preferences of the individual evaluating the shock.
The first reason for shock absorber replacement is physical damage.
Damage is checked visually and could be:
• Pitted, bent, or broken rod
• Severely dented dust shield (such that shield restricts shock movement)
• Severe leaking from shock
• Worn or missing bushings (if not individually replaceable)
• Abnormally worn or cupped tires
While a few items on this list are easy to spot, others are much more
difficult to quantify. Imagine a shock with oil coating it. This would
seem a perfect candidate for replacement, but what if the oil was coming
from somewhere else, like the power steering pump. Unless oil can be
seen leaking from the shock itself, replacement is difficult to justify.
Because of the difficulty in determining the need for shock replacement
with traditional methods there has long been a desire for accurate,
repeatable, and scientific suspension and shock performance testing.
Until recently the technology was not available to test suspensions
in a quick, user friendly fashion. There are two different types of
suspension testing used today. The types are: low frequency, which tests
the suspension’s resistance to chassis motion; and high frequency,which
measures the suspension’s resistance to excessive wheel hop as well
as chassis movement.
Low frequency techniques check the suspension reaction only up to 5
vibrations per second. The disadvantage of evaluating shock absorbers
at single or low frequencies is that shocks have valves that change
the damping at different vibration speeds. That means that low frequency
tests only check the shock and suspension in a small part of their operating
range. Low frequency techniques are also much less accurate due to the
combined effect of the other three shocks when the whole vehicle bounces.
Low Frequency Techniques
Manual Bumper Push Test – traditional shock testing technique.
Advantages – No equipment cost for this method of testing. Since
it is a dynamic test, the technician observes operation.
Disadvantages – Results are based on the technician’s opinion,
experience, and skill. It is not a quantitative test.
Quantitative Bumper Push Testers – a portable,
simple, self-powered instrument which attaches to bumper and measures
rocking of the car body at low frequency.
Advantages – Portable and self-powered.
Disadvantages – To take a measurement, the user has to push down
on the fender or bumper with 100 to 200 pounds of his body weight. Because
each push is going to be different, these testers have difficulty with
consistency and repeatability.
Automatic Drop Test – A device that drops each
suspension corner from a height of about 4 inches and measures chassis
rocking by weight changes at tires.
Advantages – Currently the highest evolution of low frequency
testing because measuring technique is consistent.
Disadvantages – Checks only narrow range of shock performance,
does not measure adhesion where it is most likely to fail, and still
can’t separate the performance of one shock from the performance of
all shocks.
High Frequency Techniques
High frequency testers vibrate the suspension through the entire range
of oscillations that it can react to. Currently, all high frequency
testers have a plate that each corner of the car is parked on. The plate
then vibrates through a range of speeds and sensors in the plate measure
how much the tire is pressing on the plate at all times. During the
high frequency measurement, the body does not move, therefore the other
three shocks do not effect the results. The tester is simulating a bumpy
road and measuring how well the suspension keeps the tire in contact
with the tester. The result is given in a percent adhesion. The higher
the percentage the more the suspension maintained good contact on tester.
The adhesion number that is used is the lowest obtained in the test
sequence.
