A CV joint, or constant velocity joint, is part of the drive shaft. It is this shaft that attaches to a car's transmission at the gear box at one end and the wheel at the other. CV joints are designed to be able to bend in any direction while continuing to turn the drive wheels at a constant velocity. CV joints are primarily used in the drive shafts of front wheel drive cars. Most shafts have two of these joints , a joint close to and providing the drive to the front hub, a second can be found close to the transmission unit. They are generally known as the outer and inner joints.
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 vertical 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 rotational 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.
A key component in the drive shaft is the rubber boot which covers the CV Joint, both inner and outer. This rubber boot has two purposes, firstly to retain the special grease which lubricates the joint and secondly keep out water and dirt or abrasive grit. The outer boot is one of the most common Mot failure items. The reason it is a failure item is that a small split, due to the centrifugal force, can throw out grease which, because of the closeness to the brakes, can contaminate the brake pads and discs with lubricant. From the point of view of general maintenance, water and grit in a CV joint will cause the joint to wear and will often require the replacement of the joint or sometimes the complete drive shaft. The fix for a split boot is to dis-assemble the shaft from the car and remove the joint from it, the joint and then slide on a new rubber boot at the same time repacking with the special molyslip grease before replacing the retaining clips. This is a fairly labour intensive job which is not for the faint hearted. In some instances, an exchange shaft is a better solution.
A worn CV joint will make an easily discernable clicking as the car is driven around corners. It is always worth repacking the joint with grease if it has become dry due to a clip coming adrift for instance but don't hold your breath.
A total joint failure, caused by a split boot is rare if caught in time, but can cause loss of drive and in extreme cases could break apart while being driven and possibly damage the braking system or puncture tyres.