Auto Tech: Bumps, bangs and knocks

By Jim Kerr

Winter roads and colder temperatures are taking their toll on our vehicles. One of the areas where I see lots of problems is with shock absorbers: if they are working properly, your vehicle rides better and handles safely. When they wear out or fail, even small bumps in the road can feel like the Grand Canyon and your car feels like it has a hinge in the middle of it. You may think your vehicle’s shocks are fine, but because the wear takes place over a long period of time, drivers are seldom aware there is a problem until it gets really bad, and it’s only when you install new ones will you notice how worn your existing ones were.

Shock absorbers and struts are essentially the same thing; both are used to damp the action of the vehicle suspension. The difference between them is that struts also support the wheel hub and hold the suspension in the proper location. Shock absorbers are typically not used to support the suspension but act as dampers only. When you are looking at controlling suspension movement, both do the same job, so if we are talking about shock absorbers, it applies to struts too.

To understand how they wear, we need to take a look at how they work. Inside a shock there is an oil-filled chamber. Inside that chamber, a piston is connected to a shaft. The shaft is connected to the body of the vehicle while the chamber and housing of the shock are bolted to the suspension. As the suspension moves up and down, the piston moves too and the oil is forced through small holes in the piston. This slows the movement of the suspension.

If all they needed were a fixed small hole in the shock piston, designing shocks would be easy. Instead, a shock absorber must allow the suspension to move rapidly upward when the tire hits a bump and then slowly allow the suspension to move back down again. If we didn’t have shock absorbers, our tires and suspension would act like a basket ball – bounce it once and it would continue to bounce several times. The shock absorber’s purpose is to prevent the tire from bouncing and to do this there must be a big passage in the piston that will allow oil to flow quickly when the tire hits a bump and a smaller passage that slows the flow of oil as the suspension spring moves the tire back down to the road.

One of the simplest ways of controlling oil flow is to have some one-way check valves in the piston so oil can flow quicker in one direction only. Much of the design work in building a shock goes into determining what size of check valves and spring pressure they open that matches the suspension design on your vehicle.

There are vehicles that have variable shock absorber rates. This is often referred to as “Ride Control” or “Real Time Damping,” but there are many other marketing names used as well. These shocks use either solenoids or electric motors that move a shaft to vary the flow rate through the piston. A computer monitors suspension movement, steering inputs and even throttle and braking to determine how the shocks should operate.

When a shock absorber operates, it moves most of the time through a very small range of travel. The piston seals and shock housing wear in that range, so shock performance decreases because fluid can bypass the piston. It may still work well on big bumps, but it is the small ones that cause handling problems. If you can feel a flutter in the body or steering wheel when passing over small dips or bumps, you likely need new shocks.

Oil can also leak from the shock so that there is no more damping. There is a seal where the moving shaft that connects to the piston passes out of the shock body. It is normal for there to be “weeping” or a slight oil film by the seal, but if oil is dripping from the shock body, you will need new shocks. Always replace them in pairs, for one end of the vehicle.

Gas-filled or gas-charged shocks cost more, but are a better buy. An inert gas is pressurized inside the shock body to reduce oil foaming and it provides better control. If your vehicle shimmies or goes bang when you hit a bump, it’s time for new shocks.