How a shock absorber works for trucks and SUVs | Mobil™

By Kevin Clemens

Maintaining the shock absorbers on light trucks and sport utility vehicles (SUVs) is similar to that for cars, but in these heavier-duty applications, the loads and forces are much higher, and the hardware is considerably different.

As a refresher before we return to the subject of trucks and SUVs, let’s review how shock absorbers work.

J.D. Power & Associates reports that by 2025, more than one-third of passenger vehicles will be equipped with alternative powertrains and operated with alternative fuels. About 17.5 percent of the vehicles will be gas/electric hybrids and plug-in hybrids. Plug-in electric hybrids will comprise about a five percent share.

If you have ever waved your hand back and forth through water, then, in principle, you know how a shock absorber works. The resistance to motion you feel with your hand changes with speed – the faster you move your hand, the more energy it takes to push against the resistance of the water.

A shock absorber works much the same way. Inside the shock absorber there’s a piston that moves inside a tube that is filled with oil. As the piston moves, the oil is forced through tiny holes and valves within the piston, precisely controlling the amount of resistance to movement. This resistance to the motion converts theenergy into heat. (Yes, a shock absorber that has been doing its job over a rough road does get warm!)

Although there have been many different designs for shock absorbers over the course of automotive history, today there are four basic types available:

• Twin-tube
• Mono-tube
• Gas-filled mono-tube
• External reservoir

Sometimes these types of shock absorbers are incorporated into a strut-type suspension that uses the shock absorber as part of the spring support, but the basic principles still apply.

Truck axles
Shock absorbers are primarily used to “damp” the bouncing motion of the a vehicle’s body after it encounters a bump. By keeping the body motions under control, the vehicle’s tires remain 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 – weight unsupported 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 tothe unsprung mass. When this hefty axle-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 for this task requires 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 must be larger to help dissipate the significant heat generated by damping the axle and body’s motions. For this reason, shock absorbers for light trucks are usually larger than those primarily used onpassenger cars.

Gas-pressure monotube shock absorbers resist foaming of the oil inside the shock body and are therefore effective when used on pickup trucks and other vehicles designed for high-speed travel over rough roads.

In off-road competition, for instance, the use of external oil reservoirs for the shock absorbers on trucks and SUVs not only provides more space for hot oil to expand into, but they also provide more cooling for 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 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, 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 extra inches of 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’s important to ensure that matching shock absorbers of the proper length are a part of the package whenever a vehicle is raised or lowered.

SUVs
While the role of a shock absorber on a passenger car and a pickup truck is clear, the differences for sport utility vehicles might not be.

Many SUVs now have independent front suspensions and some have fully independent front and rear suspension. Light aluminum-alloy wheels are popular, and some on-road-oriented SUVs have lightweight on-pavement tires as well.

While some more traditional SUVs are truck-like and require heavy-duty shock absorbers and suspension parts, others are so car-like they can get by with car-like suspensions and shock absorbers.

Although heavy-duty vehicles have no problem carrying one or two passengers and no cargo, a light SUV might be stressed right to its limits when carrying a full load of passengers and gear or hauling a heavy trailer.

Vehicles designedfor 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 that help them carry heavy loads more easily.

Air springs
While not strictly part of a discussion about shock absorbers, air springs are a popular option on vehicles that carry heavy loads or pull heavy trailers.

Air springs are rubberized air bags or “helper springs” that usually mount alongside the shock absorber on a light truck or SUV’s live rear axle. When deflated, it has no effect on the vehicle’s ride and handling.

When a heavy load is placed in the vehicle or a heavy trailer pushes the rear of the vehicle downward, air can be added to the air springs through a valve to offset shifts in rear ride height, putting the shock absorber in its normal operating range to prevent bottoming. The vehicle will also be more stable with its rear at proper ride height.

A variety of manufacturers make add-on air springs, and some vehicle manufacturers even have air-spring rear suspensions as standard equipment on their heavy-duty SUVs.

What shocks to buy
Obviously, with such a broad range of pickup trucks, light-duty and heavy-duty SUVs, and even full-sized vans, there is no single “right” shock absorber for each type of vehicle. The following list might help.

The right choices
With the right shock-absorber for the right vehicle and application, you will end up with a vehicle that rides well, handles properly and will wear out less quickly, even if you carry heavy loads or tow a heavy trailer.