The Universal joint (U-joint) connects the drive shaft to the transmission output shaft and the differential pinion gear shaft. This joint must be flexible enough to allow changes in the driving angle (road incline) and the drive shaft. This way, the torque is constantly transmitted when the rear axle is moving up and down. Smaller U-joints are used to route the turning motion of the steering wheel through the steering column to the steering box. There are two types of U-joints, the cross and roller type and the ball and trunnion type. The cross and roller type is used the most; it allows the drive shaft to bend. The ball and trunnion type less frequently used; it allows the drive shaft to bend and also permits backward and forward motion of the drive shaft.
Constant Velocity Joints or homokinetic or CV joints allow a rotating shaft to transmit power through a variable angle, at constant rotational speed, without an appreciable increase in friction or play. They are mainly used in front wheel drive and all wheel drive cars. However, rear wheel drive cars with independent rear suspensions typically use CV joints at the ends of the rear axle halfshafts. Audi Quattros use them for all four half-axles and on the front-to-rear driveshaft (propeller shaft) as well, for a total of ten CV joints.
CV Joint Problems
Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel. To determine if the joint is worn, find a big empty car park and drive the car slowly in tight circles, left and right. Worn joints will make a rhythmic clicking or cracking noise. Wear in the inner joints shows up as a "clunk" when applying power, or if severe, when lifting off the throttle.
Partial seizure causes a strange "pattering" sensation through the suspension. It is caused by the joint overheating, which in turn is usually caused by the outer joint gaiter having split, allowing the joint to throw out its grease. If caught in time, you can clean the joint carefully, repack with grease and replace the gaiter. Kits which include the grease, gaiter and retaining clips are available from most motor factors. Some universal gaiters are split lengthwise enabling them to be fitted without having to disassemble the wheel hub and CV joint.