From the Editors of Kelley Blue Book
A financial time bomb often ticks under owners of most cars as their vehicles' odometers push past 50,000 miles: the constant velocity joint.
The vast majority of motorists have never heard of constant velocity joints (or CV joints) and have no idea of what job they perform. But once they wear out, the repair bill can easily top $500 and double that if two of the joints wear out at the same time (not an uncommon occurrence).
So what do you do if your mechanic warns that worn CVs are a danger? Such warnings are not uncommon, and mechanics often advise that the joints be replaced immediately. Yet engineers who design the joints advise that they rarely need to be immediately fixed for safety reasons.
At other times, motorists are advised to spend several hundred dollars to replace protective boots around the joints to avoid a more expensive replacement of the joints later. New boots may make sense, but it depends on many factors.
As with so many mechanical systems, it pays handsomely to know something about constant velocity joints and the wide variety of options in dealing with them as they wear out. The difference can easily amount to hundreds of dollars in savings.
All front-wheel-drive cars use constant velocity joints in the two front axles that transfer power from the transmission to the wheels.
The joints allow the wheels to turn and bounce, while the transmission remains stationary. As a car travels down the road and the wheels bounce, the constant velocity joints must allow the axles to both bend and change somewhat in length.
Each axle has two constant velocity joints-an inner one near the transmission and an outer joint near the wheel. Outer joints almost always wear out first because they are subjected to more bouncing and because they are more exposed to road debris, said Leon Valencic, director of engineering at Dana Corp., a leading manufacturing of the joints.
The joints do their work thanks to a series of steel balls (held by a cage) that ride inside grooves machined into the ends of the axle shafts that enter the joint. The entire assembly is housed inside a rubber or plastic boot, which holds in grease and seals out road grime and water.
Typically, the joints fail when the boot splits and allows contaminants in. The grease then fails and the joint overheats, damaging the balls and grooves. Car owners usually figure out that there is a problem when they hear rattling during turns.
A good mechanic will warn a car owner if the boots are about to fail.