A CV joint, or constant velocity joint, is part of a drive shaft, the shaft that attaches to a car's transmission at one end and the wheel at the other. These joints are designed to be able to bend in any direction while continuing to turn the drive wheels at a constant velocity. They are primarily used in the drive shafts of front wheel drive cars.
Due to bumps and uneven surfaces in the road, a car's wheels tend to move up and down continuously while driving down the road; as a result, drive shafts cannot be made up of a solid shaft. The CV joint's precursor, the universal joint, was used in the drive shafts of rear wheel drive cars because of its ability to bend in any direction. With the advent of front wheel drive cars, however, car manufacturers had a new problem: the joints in the drive shafts needed to account not only for the up-and-down motions of the wheels, but also for the back-and-forth motions of steering. The CV joint is used in front wheel drive cars because of its ability to maintain a constant drive force to the wheels despite the many different kinds of movements in the front end of the car. It is often used in rear wheel drive and four-wheel drive cars, as well.
CV joints should be inspected periodically and may require replacement as a car ages. Each one is covered with a bulbous rubber boot that tends to deteriorate over time. When a CV boot cracks or tears open, the CV joint is left exposed to the elements, which will quickly damage the joint. If the CV axles are inspected periodically, torn boots can be replaced as needed, potentially extending the life of the joints; however, if torn boots are left unattended, the joint or the entire axle may soon need to be replaced.