Auto Service World
Feature   February 1, 2009   by CARS Magazine

Pop Quiz: Suspensions

Are you smarter than an apprentice? Find out how much, or how little, you remember from trade school.


Suspension systems are easy to understand, haven’t changed much in years, and are so simple even rookie technicians can understand them – right?

Well, see if you’re as smart as you used to be with these very basic questions about very simple systems, and see how you do.

1. What’s a typical specification for play in a ball joint?
2. Is the lower ball joint in a MacPherson strut assembly loaded or unloaded?
3. True or false: a light coating of oil on the body of a shock absorber is acceptable?
4. True or false: worn shocks can cause premature brake wear?
5. Shock absorbers convert kinetic energy (motion) into _______?
6. Which is tougher: extending or compressing a shock absorber?
7. True or false: tire pressure should be checked with the tires hot.

Now let’s see how you did.

1. Answer: 0.5 mm (0.020 inch).
That’s a general specification, so always refer to service information and be sure. True, sometimes the specification is "no play at X force," or a "turning torque" spec is given.

But if the spec provided is "0.20 inch," you may want to double check, as was the case for a recent bulletin from a major vehicle manufacturer that had put the decimal point in the wrong place. Having an idea of what to expect is a good idea to prevent problems.

2. Answer: Unloaded.
In a typical MacPherson strut suspension, the lower ball joint is an "unloaded follower," and the strut carries the load. Free play in these lower ball joints can be checked by raising the wheel off the ground, letting the lower control hang free with the strut fully extended, and then checking for movement. There should be no horizontal play between the control arm and knuckle if the joint is good.

3. Answer: True.
Thanks to Aaron Shaffer from KYB America, for the answer to this question.

"A small amount of oil on the shaft is normal and doesn’t necessarily mean the shock is bad," he explains, "But obviously an excessive amount of oil indicates a worn upper seal."
Bill Dennie, ride control channel management director at Tenneco agrees.

"Oil seals inside the shock or strut are made to keep the oil and nitrogen gas inside the unit," he explains, "But the seals are designed to allow a very small amount of oil to lubricate the oil seal to prevent drying and cracking of the seal. This also allows the piston rod to move smoothly as the unit cycles up and down. Oil seals are made far superior to what they were a few years ago which is the reason technicians don’t see the same number of leaking units, which was the main reason the units were replaced in the past."

So while a light coating of oil is acceptable, a severe leak indicates that the component must be replaced.

4. Answer: True.
If the front units are worn out, they’ll compress more easily during hard braking and allow the front of the vehicle to dive downward.

If the rears are worn out, the rear of vehicle rises up and transfers weight from the rear to the front, creating more work for the front brakes since they’re trying to stop all that extra weight. Plus, worn shocks can increase the actual distance needed to stop a vehicle in the first place.

There’s a good reason why inspecting the shocks or struts is part of a safety inspection.

5. Answer: Heat.
Bill Dennie provided the answer to this question. "Shocks and struts work by converting kinetic energy into heat energy," he explains, "And this heat build-up causes ride control units to wear out gradually over time and the consumer gets used to the bad ride."

Bill also points out that the largest degradation of the ride control units occurs at around 80,000 km (but the actual results depend on factors like driver ability, vehicle type, and road condition) – something to keep in mind during routine maintenance inspections.

6. Answer: It takes less effort to push a shock together than it does to extend it.
Try it for yourself and see. Also remember that conventional shock absorbers don’t support vehicle weight and don’t control ride height. They’re there to control spring and suspension movement.

7. Answer: False.
Tires inflation should be checked cold, ideally after the vehicle sits overnight.

Air expands as it heats up, increasing internal pressure in the tire and possibly causing inaccurate readings.

Also, remember that even though the correct inflation pressure appears on the vehicle’s tire placard, replacement tires may have different PSI ratings than the originals that came with the vehicle.

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