A car’s front wheel turns to steer and bounces over every bump, yet the engine has to keep driving it smoothly. A constant velocity joint makes that possible: six steel balls in a cage carry the drive round any bend, and both shafts always turn at exactly the same speed.
A car’s half shaft, cut open: the ball joint at the wheel and the tripod joint at the gearbox. Steer, move the suspension and open the throttle, and watch the balls roll in their grooves while the cage keeps exactly half the angle. Drag to turn the view, scroll or pinch to zoom, and click a part.
Try itOpen the throttle, then steer. The outer joint bends by up to 40 degrees, and the wheel still turns exactly as fast as the shaft.
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Meet the parts
Ten parts, one half shaft
Every part of the half shaft, laid out on the studio floor: the tripod joint from the gearbox end at the back, the half shaft across the middle, and the ball joint from the wheel end at the front. Tap any part or pick a name below.
Hover or tap the picture. Copper rings mark every part.
Parts diagram
From the gearbox to the wheel
The half shaft sawn through along its middle and seen from above, steered 24 degrees. The numbers follow the drive: 1 to 5 the plunging tripod joint at the gearbox, 6 the half shaft, and 7 to 13 the ball joint at the wheel.
The upper row carries on in the lower row: the middle of the half shaft is left out so both joints can be shown large.
How it works, step by step
Thirteen steps from the gearbox to the wheel
The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.
Step 1 of 13
A tough job for a front wheel
Demo · why not a universal joint?
Feel the wobble
Bend a universal joint and its output can’t keep up with its input: it speeds up and slows down twice every turn, even though the input turns perfectly steadily. Set the joint angle and listen.
The input turns at a steady speed. At an angle, the cross in the middle makes the output run ahead for a quarter of a turn, then fall behind for the next. Two universal joints set up in step can cancel it out, but only when both bend by exactly the same angle. A constant velocity joint keeps the output steady at any angle.
Orange: the universal joint’s output.
Cyan: the constant velocity joint’s output.
–output speed, % of the input
–speeds up and slows down by
–most the output runs ahead or behind
–shudders a second
Demo · the secret
The cage takes half the angle
The ball joint cut through the plane it bends in. Bend it, and the cage tilts by exactly half the angle between the shafts. That puts every ball in the plane that splits the angle in half, the same distance from both shafts, so both turn at the same speed.
–joint angle · cage angle
–ball to the shaft’s axis
–ball to the wheel’s axis
–wheel speed this ball allows
Demo · the gearbox end
The joint that slides
The gearbox stays put while the wheel moves up and down, so the distance between the two joints keeps changing. The inner joint is a plunging tripod: its three rollers slide in and out along the housing to make up the difference.
The half shaft itself never changes length. Seen along the car, the distance between the joints is shortest when the shaft is level, and grows as the wheel rises or drops from there. Lock the inner joint and see what the shaft would have to do instead.
Brass: the three rollers on the spider.
Cyan: how far the spider has slid in the housing.
–the spider slides in the housing
–distance between the joints
–the shaft’s angle at each joint
–wheel travel
Demo · grease and the boot
A torn boot
Both joints are packed with grease and sealed in a rubber boot. Tear the boot and drive: the grease is flung out, road dirt and water get in, and the balls start to wear their grooves. Then turn hard and listen.
–grease left
–dirt and water inside
–wear in the grooves
–distance · clicks a second
More to know
Kinds, history and care
Six ways to drive round a bend
Constant velocity · fixedBall joint (Rzeppa type)The joint at the wheel end on this page: six balls in grooves, held in one plane by a cage. Typical ones bend 45 to 48 degrees, and some more than 50.
Constant velocity · plungingTripod jointThe joint at the gearbox end on this page. Three rollers on a spider slide in a three-track housing: a typical one can slide about 50 mm and bend about 26 degrees.
Constant velocity · plungingDouble offset jointA ball joint that can also slide, because its grooves run straight along the housing. Used at the gearbox end of some cars instead of a tripod.
Constant velocity, if set up rightDouble Cardan jointTwo universal joints joined by a short yoke that keeps both angles equal, so their wobbles cancel. Found in steering columns and on tough four-wheel-drive vehicles.
Constant velocity · earlyTracta jointTwo forks and two tongue-and-groove pieces, only four parts in all. Jean-Albert Grégoire and Pierre Fenaille invented it in 1926 for front-wheel-drive cars.
Constant velocity · no cageWeiss jointFour balls sit where grooves in two forks cross. Two of them carry the drive while the other two hold the joint together. Carl Weiss patented it in the 1920s.
From gimbals to front-wheel drive
Gerolamo Cardano writes about gimbals. The universal joint later takes his name: the Cardan joint.
Gaspar Schott describes the universal joint, and wrongly says its output turns at a steady speed.
Robert Hooke, who had found that its speed is not uniform, gives it the name universal joint.
Hooke suggests two joints a quarter of a turn apart, so that each cancels the other’s wobble.
Grégoire and Fenaille’s Tracta joint, and Alfred Rzeppa’s ball joint, designed while he worked for Ford.
Rzeppa applies to patent grooves cut round two centres, one each side of the joint’s centre, so the grooves themselves steer the cage into the plane that splits the angle.
The Mini arrives with Birfield’s track-steered ball joints, made by Hardy Spicer, and front-wheel drive spreads to small cars.
Michel Orain of Glaenzer Spicer in France develops the tripod joint.
Most front-wheel-drive cars have a ball joint behind each wheel and a plunging joint, often a tripod, at the gearbox.
Keeping the joints alive
Look at the boots whenever the car is lifted. A crack, a split, or grease flung round the inside of the wheel means a boot needs replacing.
Catch a torn boot early. A new boot with fresh grease and clamps can save the joint. Once it clicks, the joint, or the whole half shaft, usually has to be replaced.
Clicking on tight turns, worst at full lock and faster as you speed up, points to a worn outer joint.
A clunk as you pull away, or a shudder while you accelerate, can point to a worn inner joint.
Use the grease that comes with the boot or joint. CV joints need their own kind, usually with molybdenum disulfide in it.
Support the half shaft when the hub is taken apart. Don’t let it hang from the inner joint, or pull it out to its full length.
Things worth knowing
Constant means every momentA universal joint’s output keeps up on average: one turn out for every turn in. It is the speed within each turn that wobbles. A CV joint is steady all the way round.
Fourteen shudders a secondAt 40 degrees a universal joint’s output swings between 77 % and 131 % of the input’s speed, twice a turn. On a wheel at 50 km/h, that is 14 times a second.
Half the angleWhen the joint bends 45 degrees, the cage leans 22.5 degrees. Every ball then sits in the plane that splits the angle, the same distance from both shafts.
Rolling to and froAt 40 degrees, each ball rolls 20 degrees each way along its grooves and back once every turn: about 19 mm from end to end in this joint.
A famous mistakeGaspar Schott thought the universal joint turned at a constant speed. Robert Hooke worked out that it doesn’t, and how two of them can fix it.
The tell-tale clickA worn outer joint clicks in tight turns, and the clicks come faster as the car speeds up. A torn boot is usually where it started.
Every Mechanism
