The differential, and the several ways one can be locked

Both sources for this article describe the differential as intelligent. One calls it a smart distribution box that works out which wheel can take more speed. The other says it divides the engine's power between the wheels according to their need.

It does neither, and it decides nothing. An open differential is a purely mechanical arrangement with one rule, and the rule is this:

It splits torque equally, always, and lets the two wheels turn at whatever different speeds they like.

Equal torque, unequal speed. Everything a differential does well, and the one thing it does catastrophically badly, follows from that single sentence — and once you have it, the famous failure stops being a mystery.

Why the thing exists

Go round a corner and the outer wheel travels further than the inner one. Bolt both to a solid shaft and one of them has to scrub — hopping, tyre squeal, and a large load fed back through the driveline. The differential lets the two ends of the axle turn at different rates while both keep driving.

That is all it is for. The original adds a second job that is not the differential's:

"The differential's task is to transfer rotation from vertical to horizontal — a ninety-degree break in the power transfer."

That is the final drive, not the differential. On a rear-wheel-drive car a crown wheel and pinion do turn the drive through ninety degrees, and they live in the same casing, which is where the confusion comes from. But on a transverse front-wheel-drive car — most cars — there is no ninety-degree turn at all, and the differential is still there doing exactly the same job. Two devices, one housing, different purposes.

The failure, and why it is not what people think

Jack one driven wheel off the ground, leave the other on tarmac, and drive. The raised wheel spins and the car does not move. Everybody has seen it. Both sources explain it by saying the differential sends all the power to the free wheel.

It does not. It is still doing exactly what it always does — splitting torque equally. The problem is that the torque it can split is set by whichever wheel can accept least. A wheel in the air can accept almost none, so almost none goes to the wheel on tarmac either. The free wheel spins because that trickle of torque is more than enough to accelerate a wheel with nothing to hold it back.

So the useful way to hold it:

An open differential's traction is the worst wheel's grip, doubled. Not averaged — doubled. One wheel on ice halves the car's usable traction and then halves it again.

That is also why a car that is stuck can sometimes be moved by gently applying the handbrake, on a rear-drive car with a cable handbrake: adding drag to the spinning wheel raises the torque the differential is able to pass, and some of it reaches the wheel with grip. That trick is the whole idea behind the electronic answer below.

The ways of getting round it

A diff lock does the crude, complete thing: it bolts the two output shafts together so they must turn at the same speed. Now the wheel with grip gets whatever torque it can use, regardless of the other. Engaged by compressed air, by a cable, or electrically, depending on the vehicle.

The rule that goes with it is absolute, and the original states it with an arbitrary number attached — do not corner above 40 km/h with the lock engaged. The real rule is not about speed. It is about surface. A locked axle physically cannot corner: the two wheels are forced to the same speed while the corner demands different ones. On loose ground the difference is absorbed by the tyres scrubbing through gravel or mud, which is free. On tarmac there is nowhere for it to go, so it winds up as stress in the axle shafts, the crown wheel and the tyres until something releases it — usually with a bang. Unlock it the moment you are back on a hard surface, at any speed.

A limited-slip differential is the civilised version and needs no driver. Clutch packs or a viscous coupling inside the unit resist a speed difference between the two sides, so the free wheel can still turn faster than the other — just not without dragging the other along with it. It is permanently active, needs no thought, and is gentle enough to fit to a front axle.

A Torsen is a limited-slip that works on gears rather than friction, and it is worth understanding because of what it cannot do. It multiplies the torque going to the gripping wheel by a fixed bias ratio — three or four times whatever the slipping wheel is taking. Excellent on a surface where the poor wheel still has some grip.

But zero multiplied by four is still zero. With a wheel completely in the air, a Torsen has nothing to multiply and behaves exactly like an open differential. This surprises people who have paid for one, and it is why cars with a Torsen usually also have the brake-based system below — a touch of brake on the spinning wheel gives the Torsen something to work with.

What most cars actually have

The original says that most cars today are fitted with this kind of differential lock, meaning the pneumatic, cable and electric ones.

They are not. The overwhelming majority of cars leave the factory with a plain open differential and no lock of any kind. What they have instead — and this is the thing a modern reader needs and neither source mentions — is a brake-based electronic system, sold under names like electronic differential lock, EDL or XDS.

It is not a lock. The wheel speed sensors that already exist for ABS notice one driven wheel accelerating away from the others, and the stability control system applies that wheel's brake, briefly and precisely. Adding drag to the spinning wheel raises the torque the open differential can transmit — the handbrake trick above, done automatically, several times a second.

It works remarkably well on the everyday case of one wheel on a wet drain cover, it costs nothing to fit because the hardware is already there for ABS and stability control, and it explains something that puzzles people: a modern hatchback with one wheel on ice will often simply drive away, and it has no diff lock at all.

Its limits are worth knowing too. It works by turning traction into heat in a brake disc, so it is for getting going, not for sustained low-grip work — and it will back off if it gets hot.

What goes wrong with one

Neither source covers this, and the symptoms are distinctive.

And the fluid: a limited-slip differential needs the friction modifier its maker specifies, exactly as a manual gearbox does — the clutch packs are a friction pair and the oil is half of it. Fill an LSD with plain hypoid oil and you get chatter and judder on tight turns. It is the same lesson as the manual gearbox, for the same reason, and it catches people just as often.

One safety note about jacking

With an open differential and one driven wheel raised, running the engine in gear spins the raised wheel and the car stays put. That is what makes the demonstration safe, and it is why people do it.

With a locked differential, a limited-slip one, or brake-based traction control that decides to intervene, the wheel still on the ground is driven — and the car will pull itself off the jack. Chock the wheels, use axle stands under both sides, and do not run a car in gear on a single jack because a demonstration you have seen before came out differently.

Where the differential sits in the chain is the last thing worth stating plainly, because the original gets it right and it is a genuinely useful map: engine → clutch → gearbox → differential → driveshafts → wheels. Every article in this group is one link in it, and four-wheel drive is what happens when there is more than one chain.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.