The shocks control the movement of the springs, keeping them steady and in control. It's this set up that allows a driver to accurately "feel the road" as they drive.
As the shocks start to wear out, they can't adequately control the springs. As the springs bounce all over the place, so does the rest of the vehicle.
- This article appears in the Feb. 27 print edition of Tire Business.
It causes a loss of consistency in the tires' contact with the road, the work of the brakes and the response the driver feels from the vehicle.
The extra vibration allowed by bad shocks is distributed throughout the load to places that are not meant to handle it. It can cause damage to cargo, lighting components, onboard electronics, refrigeration units, etc. Anything that can be damaged by excessive vibration suffers when the shocks wear out.
Effect on tires
Bad shocks also cause interesting wear patterns on tires. Once the shocks start to go and the springs start to bounce out of control, the tires can't maintain consistent contact with the road.
As they bounce and vibrate while rolling over the ground it causes unusual scuffing on the treads of the tires. When tires display these bald spots the phenomenon is sometimes called "cupping."
A tire that has started to cup is definitely one that should soon be replaced. Otherwise the loss of traction will start to compromise driving safety.
We recommend that whenever a vehicle is taken in to replace the tires, the shocks are changed as well. If tires are already showing signs of cupping, the chances are the shocks have started to wear down.
Any time a vehicle is in the shop is a good time to perform other maintenance checks. If old shocks were the reason for the tire wear in the first place, replacing the tires only will be a temporary fix.
The worn shocks will continue to put unnecessary strain on the tires and the frame of the vehicle, causing damage faster than before and shortening the lives of brand new tires. Leaving the old shocks on the vehicle ends up being a waste of repair costs in the long run.
When to replace
So how often should you replace your shocks? Waiting until the very end of their lifetime as listed is inadvisable, since excessive vibration already can start to cause damage or wear by then. But replacing them too often is itself an unnecessary maintenance cost.
The frequency of replacement can depend on a lot of factors, including what kind of load a vehicle is transporting, where it usually has to drive, and what kind of drivers are usually driving it. If the vehicle is transporting liquid goods, sloshing and instability of the load can give the shocks extra work.
Driving on rough and rocky terrain such as on construction sites puts extra strain on the shocks as well. And reckless or less than careful driving puts unnecessary strain on the whole stabilization system. Shocks that are consistently subjected to extra strain should be replaced more often.
Additionally, if tires already have started to cup due to extra bouncing from worn shocks, replacing the shocks will not halt the damage. Studies have shown if the treads have begun to display bald spots, replacing the shocks only will slow the wear and tear, but not stop it.
In the end it makes the most sense to replace the shocks when you replace the tires. Even if the shocks aren't responsible for the tire wear, they are much cheaper to replace than tires, air springs or other expensive hardware that can be damaged by driving with bad shocks.
It is better to foot the bill for some preventative maintenance than wait for more obvious signs of damage that will be harder and more expensive to fix down the line.
Jeff Schlump has been the OEM account manager at Gabriel North America, located in Detroit, for the last five years. His responsibilities include Gabriel North America Ride Control products in the OEM and specialty markets and industry.