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.
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.
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.
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.