How do a CV axle and a differential work?

Why do the wheels need different speeds in a corner?

Because they follow different circles. In a turn the outer wheel runs on a bigger circle than the inner one, so it has to cover more ground in the same time. In the example in the diagram above, at 20 km/h in a turn of 4.5 m radius, the inner front wheel turns at 147 rpm and the outer at 188 rpm.

If both wheels were locked to one shaft, one of the tyres would have to slip to make up the difference. The car would fight the steering, the tyres would scrub and wear, and the drive would wind up stress in the shafts. A differential lets each wheel find its own speed while still driving both.

How does an open differential work?

The gearbox’s final-drive pinion turns the ring gear, and the ring gear is bolted to the differential case. Inside the case, a cross shaft carries two small spider gears with 10 teeth each. They mesh with two larger side gears, 16 teeth each, one splined to each half-shaft.

Driving straight, the spider gears don’t turn on their shaft. They are carried round with the case and push both side gears at the same speed. In a corner, the spider gears start to turn on their shaft, rolling round the side gears: one side gear speeds up and the other slows by the same amount. The case always turns at the average of the two wheels, 168 rpm in the example, and the spider gears spin at 33 rpm.

Why does one wheel spin on ice?

An open differential always splits the torque equally between the two wheels. That is fine when both have grip, but it means the drive to both wheels can only be as large as the weaker wheel can take. Put one wheel on ice and that wheel needs almost no torque to spin, so the other wheel gets almost none either.

Switch the diagram to the icy road: the car stops, the wheel on the ice spins at 334 rpm, twice the speed of the case, and the wheel with grip doesn’t turn at all. Limited-slip differentials resist that, and traction control does a similar job by braking the spinning wheel so the differential sends more torque to the other one.

What makes a CV joint constant velocity?

A simple universal joint, the kind on a rear-wheel-drive propshaft, speeds up and slows down twice every turn when it runs at an angle. A front wheel steers through big angles, so that would make the wheel surge as you turn.

The outer joint in the app’s model is a Rzeppa joint: six hardened 11/16 inch balls running in curved tracks between an inner and an outer race. A cage holds all the balls in one plane, and because the two sets of tracks are offset, by 3.5 mm each way, that plane always bisects the angle of the joint. The result is that the output turns at exactly the same speed as the input at any angle, up to 46°.

Why are there two different joints on one shaft?

They do different jobs. The outer joint has to bend a long way for steering but doesn’t need to change length. The inner joint, at the gearbox end, has to let the shaft get longer and shorter as the suspension moves the wheel up and down.

The inner joint is a tripod: three rollers, each running on 26 needle rollers, slide along three parallel tracks in a housing. It allows up to 15 mm of plunge each way and up to 23° of angle. At an angle its centre wobbles slightly, 1.2 mm at full angle, three times every turn. A worn tripod joint turns that wobble into a shudder you feel under hard acceleration.

Why do CV boots matter so much?

Every CV joint lives in a rubber or thermoplastic boot packed with special grease. A split boot lets the grease fling out and lets water and grit in, and the joint wears out within weeks. That is the most common reason a CV axle is replaced, and the classic symptom is a clicking from the outer joint on full lock. Checking the boots at every service is the cheapest way to make the joints last.