The Constant Velocity Axle Shaft is a piece of the drive train that connects the transmission or differential to the wheel and transmits power from the engine to the tire and wheel assembly to make them spin. Sometimes they are called “Half Shafts” or simply “Axle Shafts”. These are typical examples:

Front Wheel Drive vehicles have two CV Axle Shafts, one connecting each of the front wheels to the transmission. All Wheel Drive vehicles have four CV Axle Shafts, two connecting the transmission to the front wheels, and two more connecting the rear differential to the rear wheels. The CV Joints allow for the axle to continue to spin while the suspension moves in response to the road and driver inputs. Here is a typical front wheel drive set up:

Each end of the Axle Shaft has a Constant Velocity Joint, which consists of various parts enclosed within a rubber boot filled with heavy grease.

The most common issue with the Constant Velocity axle shaft is that the rubber boot can rip. When the boot is torn the centripetal force of the spinning shaft causes the heavy grease to fling out of the CV Joint. Without proper lubrication, the parts inside the CV Joint wear out and eventually fail. Replacement of the axle shaft is then necessary.

Another frequent problem with CV Axle Shafts involves the ABS systems. On some vehicles, the CV Joint includes an ABS Reluctor Ring that works together with the wheel speed sensor to prevent the wheel from locking up during emergency braking. Sometimes the ABS Reluctor Ring on the axle will corrode and break as seen in the pictures below. When this occurs, the ABS light on the dash will illuminate and the Antilock Braking System will no longer function properly. To fix this problem, the ABS Reluctor Ring will need to be replaced, which on some vehicles requires replacement of the entire CV Axle Shaft.   

There are several other problems that can occur that cause the CV Axle Shaft to fail, such as the shaft separating from the bearing, or from the transmission. This will cause the vehicle to pull as the shaft will not transmit power to the tire on the side that is broken.