CV Joint

A constant-velocity joint, almost always abbreviated to CV joint, is a type of coupling that transmits rotary drive through a changing angle while keeping the output shaft turning at exactly the same instantaneous speed as the input. This property is what makes it indispensable on the driven wheels of modern cars, particularly those with front-wheel drive, where each driveshaft must both steer and move up and down with the suspension yet still deliver torque smoothly and without vibration.

The problem the CV joint solves stems from the shortcomings of the simpler universal joint. A plain universal joint can carry drive around an angle, but as it does so the output shaft speeds up and slows down twice per revolution, producing a cyclic fluctuation that worsens as the angle increases. At the modest angles of a propeller shaft this is tolerable, but at the large and constantly varying angles of a steered, independently sprung wheel it would cause unacceptable vibration and wear. The CV joint maintains a constant velocity by arranging its internal geometry so the contact points always lie on the plane that bisects the operating angle.

Mechanically there are two common designs. The outer joint, nearest the wheel, is typically a Rzeppa-type ball joint in which six steel balls run in curved grooves inside a spherical housing, held in correct alignment by a cage; this can accommodate the large angles demanded by steering. The inner joint, nearest the differential, is often a tripod, or tripode, design using three roller-tipped trunnions that can both pivot and slide, allowing the shaft to lengthen and shorten as the suspension moves. A typical driven half-shaft therefore carries an inner and an outer CV joint working together.

The benefit to the vehicle is smooth, vibration-free drive across the full range of steering and suspension movement, enabling the compact transverse engine and front-drive layouts that dominate today's market. The same technology serves the rear of many independently suspended rear- and all-wheel-drive cars, and appears in driveshafts where angles or movement exceed what a universal joint can handle gracefully.

The chief vulnerability of a CV joint is its protective rubber or thermoplastic boot, which retains the special grease and excludes dirt and water. If this boot splits, grease is flung out and grit works in, and the joint begins to wear; the classic symptom is a rhythmic clicking or knocking heard when turning under power, signalling that the outer joint in particular is failing. Caught early, replacing a torn boot and repacking the grease can save the joint, but a neglected split eventually destroys it, requiring a new joint or complete half-shaft. The CV joint thus sits within the wider driveline alongside the half-shaft, the differential and the axle, quietly enabling the everyday combination of driving and steering through the same wheels.