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 |  |  | | | | In for a Shock – Part Two, Trucks & SUVs | | By Kevin Clemens | |
In our last installment we talked about shock absorbers – how they work and
when you might need new ones. This time, we are going to get a bit more
specific and discuss shock absorbers on light trucks and sport-utility
vehicles. The principles remain the same, but in these heavier-duty
applications, the loads and forces are much higher, and the hardware is
considerably different.
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| Truck Axles
Last time, we learned that shock absorbers are primarily used to "damp" the
bouncing motion of the vehicle's body after it encounters a bump. By keeping
the body motions under control, the vehicle's tires remain more constantly in
contact with the ground for improved handling and control. The vast majority
of passenger cars today have front and rear independent suspension systems.
Along with light aluminum-alloy wheels and typical original-equipment tires,
the amount of unsprung weight – that which is not supported by the springs of
the suspension – is quite low and easy to control.
Contrast this with the live axles on four-wheel-drive pickup trucks and some
full-sized vans. In addition to the significant weight of each axle, larger
wheels and tires add even more to the unsprung mass. When this hefty
axle-and-wheel combination encounters a bump or pothole, part of the shock
absorber's job is to help control the axle's motions after hitting the
obstacle. The shock absorber required for this task needs a different degree
of damping on compression (jounce) and extension (rebound) than a shock
absorber designed for a lightweight sports car with an independent suspension
system.
Generating Heat
Because a four-wheel-drive pickup truck or SUV might be expected to travel
long distances over rough roads, the shock absorber body itself needs to be
made larger to help dissipate the significant heat generated by damping the
axle's and body's motions. For this reason, shock absorbers for light trucks
are usually much larger than those engineered primarily for use on passenger
cars.
You may recall from last time that gas-pressure monotube shock absorbers
resist foaming of the oil inside the shock body and, are therefore, very
effective when used on pickup trucks and other vehicles that are designed for
high-speed travel over rough roads. In off-road competition, the use of
external oil reservoirs for the shock absorbers on trucks and SUVs not only
gives more space for the hot oil to expand into, they also provide more
cooling for the oil inside the shock absorber to maintain more consistent
damping while pounding across the desert.
Ground Clearance and Lift Kits
To make it easier to travel over rough roads, trucks have more ground
clearance than standard automobiles. This extra ground clearance can also
translate into longer suspension "travel" – up and down movement. A
long-travel suspension requires extra-long shock absorbers so that the shocks
themselves don't limit the travel of the axles. If this happens, the shocks
will be damaged, either by compressing too far and punching through their
mounting points, or by extending too far and damaging their internal valving
or pulling apart their mounting rings.
While stock shock absorbers on stock vehicles rarely encounter these problems,
the popular use of "lift kits" to gain several inches of extra ground
clearance can result in problems for the shock absorbers unless the full
extension length of the shock is taken into account. Likewise, the popular
lowering of light trucks can result in bottoming of the shock on compression
when hitting a bump, damaging the shock mounts and the shock absorber. It is
therefore important, whenever a vehicle is raised or lowered, to ensure that
matching shock absorbers of the proper length are a part of the package.
SUVs
While the role of a shock absorber on a passenger car and a pickup truck is
clear, the need for special considerations for sport-utility vehicles (SUVs)
is less apparent. Many SUVs now have independent front suspensions and some
have fully independent suspension, front and rear. Light aluminum-alloy wheels
are popular and some on-road-oriented SUVs have lightweight on-pavement tires
as well.
As a result, while some more traditional SUVs are truck-like and require
heavy-duty shock absorbers and suspension parts, others are so car-like that
they can get by with car-like suspensions and shock absorbers. It's a case
where the heavy-duty vehicles are overbuilt for carrying one or two passengers
and no cargo, while a light-duty SUV might be stressed right to its limits
when carrying a full load of passengers and gear, or hauling a heavy trailer.
It should be clear, that vehicles that are designed for light-duty use
shouldn't be modified and used to carry heavy loads or driven in extreme
off-road conditions. Heavy-duty truck-based SUVs on the other hand, respond
well to original equipment and aftermarket modifications to help them carry
heavy loads more easily.
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