Keeping the tire on the ground is just as important on a daily driver as it is on any race car. That's the suspension system's job — and even though it's really important, the suspension system tends to be neglected and overlooked during routine inspections.
That's understandable. Unlike other vehicle systems, such as the HVAC or exhaust systems that are very obvious when they work well and also when something goes wrong, the thing about suspension systems is that when they're working well, you won't even notice them. Problems with the vehicle's suspension system tend to happen gradually, so they're not usually noticed until there's a big issue.
That's too bad.
Suspension systems are so important that the components are included on safety inspections and certifications. One race technician told me that his team's crew takes temperature readings across the tire to make sure the wheel is contacting the track surface properly — it's that important to operating the vehicle.
Not only do the suspension components absorb bumps and shocks from the road, which makes the ride smoother and reduces driver fatigue, they keep the vehicle's wheels firmly in contact with the road surface no matter what bumps and shocks the road throws at it. This lets the wheels do their job of making the vehicle stop and steer safely — things that are important on daily drivers.
Locating and replacing worn suspension parts when they're needed can cause such a drastic improvement in ride quality and handling that customers will think you're a hero. It's worthwhile, then, to understand how the system works and how to inspect and service the components.
The springs used in vehicle suspension systems are usually pretty straightforward and simple. Leaf springs, most commonly used on trucks, are long pieces of steel that are bolted between the body and the axle. They work to absorb a vehicle's movement. Coil springs are used on most cars, and are just that: coils of metal that also absorb vibration and oscillations as the vehicle travels down the road. It's also important to be aware of vehicles that use air springs — inflatable chambers that inflate and deflate as needed to maintain ride height. Those systems usually operate even with the ignition OFF, so the system must be disabled before raising the vehicles so that the air system doesn't overinflate and permanently damage the springs.
Although broken springs do tend to cause a noticeable noise over bumps, as well as a change in ride height, some drivers may be completely unaware that there's a problem with their vehicle's springs. It's important to check them carefully during an inspection or maintenance service. It's actually quite easy to do: Just look for cracks and breaks in the component. These tend to occur near the top and bottom of the spring, and sometimes you have to look really carefully to see the break. From experience, looking for chipped or swollen paint on the spring usually helps locate the broken area. I'm told that when the protective paint gets chipped, debris and moisture get in, and the spring weakens at that point.
It's worth noting that springs are under tremendous pressure; they must be removed and reinstalled using the right compressor to avoid suddenly releasing the component and injuring someone. Even tiny springs can pack a huge punch. When replacing a spring, make sure that it's installed with the correct end at the top and that the end or edge of the bottom part is seated right up at the end of the spring seat (in other words, don't leave a gap). If there's a gap, the spring can shift and make a heckuva metal-on-metal noise over bumps.
One other thing about replacing springs: If the rubber mounts are being reused, the marks made by the old spring can help line up the new spring and can save time during assembly. Just a quick tip to make the job easier.
While a vehicle's springs absorb oscillations, shock absorbers are what stop those oscillations and reduce the other movements that occur as the vehicle bounces along the road.
On passenger cars, the shock absorbers are often combined with the spring into a strut assembly to save weight and space, but on trucks they're still usually separate components. This is because of the huge weight and stresses associated with the heavy loads and rough conditions in which trucks must operate.
How do shock absorbers work? Compare running through deep water with running on dry land — it's pretty tough to do! The water provides resistance, and you've really got to work hard to force your legs to move through that resistance. Shock absorbers work in much the same way.
For the movement of the vehicle to be transmitted to the passenger compartment (and other suspension components), that movement has to travel through a fluid-filled cylinder. Because it's tough to move through that increased resistance (like moving your legs through water), the vibrations are reduced or eliminated.
Shocks absorbers are, in their simplest form, just sealed, fluid-filled (and sometimes also gas-filled) cylinders containing a piston that travels up and down inside the cylinder as the vehicle bounces up and down. The resistive material inside the shock absorber dissipates the movement according to road conditions.
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The technology that can go on inside that shock absorber is incredible, and is getting better all the time. But the inspection and servicing procedures for the components still remains essentially the same: Check to see if anything is leaking or broken, and check that the component is mounted securely and doing what it's supposed to do.
This is easy to do. Inspect shock absorbers by bouncing the vehicle a few times repeatedly while it's on the ground. When you stop bouncing it, the vehicle should come to rest within a few cycles. Also inspect shock absorbers when the vehicle is raised by looking for leaks and obviously broken parts. Note that shock absorbers often have a very small amount of grease or oil coating the lower portion of the shock. This is normal, and it's not a reason to replace the whole part. Oil leaking out of the component and dripping on the ground is another story, though — that's definitely a reason to replace the part immediately!
Note, too, that by design, shock absorbers are often in the way of other leaking components, so the oil dripping off a shock may actually be coming from something mounted above it. In such cases, be sure you're replacing the correct part, and also check the rubber bushing or mounts at the bottom of the shock — oil or hydraulic fluid can sometimes destroy them.
Be aware that one reason you should never replace shock absorbers is to correct ride height. Shock absorbers don't control ride height, springs do. It's just something to keep in mind if you're trying to repair a customer's ride height complaint.
Most cars today have struts on at least two of their corners, if not all four. Struts combine shocks and springs into one unit. Quite often, the strut will be a structural part of the vehicle — called a MacPherson strut or modified MacPherson strut — that includes a bearing and mount so that the steering rack can turn the front wheels as required.
Because the strut is a combination of the spring and shock absorber, the inspection procedures for the unit should seem very similar: Bounce the vehicle while it's parked to see if it stops moving, check that ride height is the same on both sides, then raise the vehicle to check for obviously leaking or broken components (a light coating of oil isn't a fault).
Struts have a few more parts, though, so there's a bit more to the inspection process. If the customer is complaining about a noise from the suspension system, for example, check the strut mounts carefully. The bearings at the top have been known to seize up or bind. This shows up as a noise on turns, memory steer or even a clunk if the bearing is bad enough. Also, inspect the portion at the top that attaches to the vehicle to see whether the rubber portion has separated from the metal part — that can cause a noise, too.
One important note about replacing struts: Before you remove it from the vehicle, make sure you've got the correct side to replace it with. The right and left struts are often mirror images of each other and are not interchangeable. Just make sure any fittings for brake lines and steering components are in the right spots, to save headaches when you go to install the new part later on.
Stabilizer bars, or "sway bars," are the long bars that run across the vehicle. They work to keep both wheels on the ground as the vehicle goes around corners (resisting the tendency for one side to lift off the ground). The stabilizer bar is what keeps the vehicle's suspension feeling stiff while cornering. It is so important that on some race cars, the bars are actually adjustable. Some high-end vehicles have "active" sway bars that are controlled by a computer, while others eliminate the component completely and instead use a computer that stiffens the suspension independently, as needed, to almost eliminate body roll altogether.
The other 99.9 percent of vehicles have stabilizer bars that are held firmly in place at two spots by using clamps and bushings that allow the bar to rotate and twist as needed. The ends of the bar are fastened to a fixed link. As one side of the vehicle lifts, the force of the bar maintains pressure on the opposing wheel to resist the tendency to lift upward, keeping the wheel squarely on the road.
The parts that tend to fail are the mounts or bushings that hold the bar in place (they get dry, worn out or simply become unacceptably noisy) and the links (which break, causing a crunching noise over bumps and too much body roll in turns).
Bushings can often be lubricated with the appropriate grease if replacing the parts is not an option — but check the service manual, because some lubricants can cause the mount to deteriorate. It's also important to be careful when reinstalling them afterward, because they can squeeze out of their brackets or shift and "escape" during installation. They then either wear out prematurely, or allow metal-on-metal contact — which means they can't do their job.
Broken links are just replaced. They're usually really straightforward to do: Just remove the pieces of the old one and install the new kit. The only things to be careful of are installing the kit correctly (the same way it came out, so that the bolt doesn't interfere with other components) and being careful to not over-tighten the nut.
Whether you're inspecting the system to resolve a customer complaint about noises or ride quality, or as part of regular maintenance, it's important to be thorough. If you've established a regular, predetermined road test route that includes turns, dips, bumpy roads and smooth patches, you'll likely notice suspension problems during routine maintenance and can notify the customer if something's wrong.
A good inspection starts by checking ride height to see whether it's the same from side to side. It's a good idea to check that the tire pressures are correct and there are no tire or wheel conditions that could be causing or indicating problems. Walk around the vehicle and bounce each corner while listening for noises, and make sure it stops bouncing within a few cycles.
Road test the vehicle. Listen for noises that occur over bumps or rough roads, and observe how the vehicle handles when cornering. Make sure the steering returns to center after turning.
Then, raise the vehicle off the ground and inspect for leaking, loose or broken components. If you're diagnosing a noise over bumps, driving the vehicle onto a drive-on hoist and having an assistant bounce the vehicle while you listen to various components until you locate the source of the noise can work well and save diagnostic time.
One more note about diagnosing suspension problems: If you're inspecting the vehicle's suspension because it pulls after it bumped a curb (or worse), it helps to compare one side of the vehicle to the other to see whether one side looks bent. This definitely isn't foolproof, but it helps give you an idea of things that could be damaged — and whether further work and diagnosis will be required.
Suspension systems are important, not just because they make the driver more comfortable, but because they keep the wheels solidly in contact with the road, so that stopping and steering is optimized. The systems need to be inspected and serviced regularly to stay effective. Suspension problems can develop slowly over time and may go unnoticed until they're discovered during an inspection, so it's important to thoroughly inspect vehicles that come in for maintenance work.
The good thing about repairing suspension components is that the results are really noticeable, and usually make customers very happy. It's worthwhile to understand how the systems work and make inspecting them part of your regular routine — your customers and your bank account will thank you.
Vanessa Attwell is a Master Technician for two major manufacturers, and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards. She drinks too much coffee and spends her spare time sitting in traffic.
