Cross-section through a typical outer CV joint (Saab 96)

Constant Velocity Joints 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 cars, although some rear wheel drive cars, notably Alfa-Romeo, BMW, Porsche, and Volvo use them as part of the rear axle and all-wheel-drive Audis use them for all four half-axles and on the front-to-rear drive shaft (propeller shaft) as well.

Early front wheel drive systems such as those used on the Citro n Traction Avant and the front axles of Land Rover and similar four wheel drive vehicles used Hardy-Spicer joints, where a cross-shaped metal pivot sits between two forked carriers (These are not strictly CV joints as they result in a variation of the transmitted speed except for certain specific configurations). These are simple to make and can be tremendously strong, and are still used to provide a flexible coupling in the propeller shafts, where there is not very much movement. However, they become "notchy" and difficult to turn when operated at extreme angles, and need regular maintenance. They also need more complicated support bearings when used in drive axles, and could only be used in rigid axle designs.

As front wheel drive systems became more popular, with cars such as the Mini using compact transverse engine layouts, the shortcomings of Hardy-Spicer joints in front axles became more and more apparent. Based on a design by Alfred Rzeppa in 1928, Constant Velocity joints solved a lot of these problems. They allowed a smooth transfer of power despite the wide range of angles they were bent in. Driveshafts using CV joints are self-supporting along their length, and do not need additional supports (although very long shafts such as the right-hand driveshaft on the Citro n CX or Peugeot 205 have an intermediate bearing that supports the inboard joint).

Two different types of CV joint are used on the driveshafts of modern cars. At the "inboard" end, where the shaft only moves up and down with the movement of the suspension, a "Triax" joint is used. This has a three-pointed yoke attached to the shaft, which has barrel-shaped rollers on the ends. These fit into a cup with three matching grooves attached to the differential. Since there is only significant movement in one axis, this simple arrangement works well.

At the "outboard" of the shaft, a slightly different unit is used. This has a large steel ball attached to the end of the shaft, with grooves machined in it to take (usually six) large steel balls. These are held in place by a bronze or steel cage, and fit into a grooved cup similar to the triax joint. This joint is extremely flexible, and can accommodate the large changes of angle when the front wheels are turned by the steering system.

These joints are very strong, and are usually highly overspecified for a given application. Maintenance is usually limited to checking that the rubber gaiter (dust/weather boot) that covers them is secure and not split. If the gaiter is damaged, the MoS₂-molybdenite grease that the joint is packed with, will be thrown out. The joint will then pick up dirt, water, and road deicing salt and cause the joint to overheat and wear, and the grease can also contaminate the brakes. Damaged CV joint gaiters will usually cause a car to fail a safety inspection.

Faultfinding and diagnosis

Constant velocity joints are usually reliable and largely trouble-free. The two main failures are wear and partial seizure.

Wear in the outer joint usually shows up as vibration at certain speeds, a bit like the vibration caused by an unbalanced wheel. To detemine 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 disasemble the wheel hub and CV joint.