Open Diff vs Limited-Slip vs Welded Diff: What's the Difference? (Redlined 101, Ep 18)

Open diff, limited-slip, or welded? Give all three the same throttle and you get three different results. One spins a tire and goes nowhere. One pulls with both. And one chirps through every parking lot. The difference is a box of gears in the middle of your driven axle, and it decides where your power actually goes. (If you're not sure which axle that is, start with FWD vs RWD vs AWD.)

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Why a car needs a differential at all

In a turn, the outside wheel travels a longer path than the inside wheel. If both were bolted to one solid axle, one of them would have to skid to keep up. The differential sits between the two driven wheels and lets them turn at different speeds while still both getting drive. Every car has one on its driven axle; the question is what kind.

The open diff: the one going nowhere

The one going nowhere is the open diff, the type most everyday cars come with.

Inside are the spider gears, small bevel gears that let the outside wheel spin faster than the inside wheel in a turn. On dry pavement, that's exactly what you want: smooth, quiet turning, no scrubbing.

The problem: an open diff always splits torque equally between the two wheels. Equal speed isn't required, but equal torque is. So the wheel with the least grip sets the limit. Whatever that wheel can take before it spins, the other wheel gets the same amount, and no more.

Put one tire on ice, and the torque that tire can hold is tiny. The tire on dry pavement gets that same tiny amount. That's all the push you get. In the video's example, the ice tire can only hold 20 lb-ft, so the dry tire gets 20 lb-ft too: 20 plus 20 is all you get, however hard you press the pedal. The ice tire spins up, the spider gears spin happily, and the car sits there.

The test: one wheel in the air

So here's the test. Jack up one wheel and floor it. Does a limited-slip still move the car?

A wheel in the air is the extreme version of the ice tire: it has almost no grip at all. An open diff obviously fails this test. The limited-slip answer is more interesting.

Clutch-type limited-slip: it moves

It depends which kind. A clutch-type limited-slip has stacks of friction plates inside that tie the two axles together. When one wheel tries to spin faster than the other, the plates resist it and hand some of the drive across to the slower wheel.

Because the plates are always pressing together to some degree, they don't need the lifted wheel to have any grip. Even with one wheel in the air, the plates still feed the other one, and the car moves.

The trade-off is that clutch plates wear, and many clutch-type diffs need a specific fluid or additive to work properly. Follow the maker's service schedule.

Gear-type Torsen: open diff again

A gear-type Torsen works differently. Instead of friction plates, it uses helical gears that resist turning against each other. The effect is that it multiplies torque: it can give the wheel with grip up to four times what the slipping wheel gets.

On the road, that's brilliant. It's smooth, it doesn't wear like clutch plates, and the moment one wheel starts to slip, it sends more of the drive to the wheel that can use it.

But four times zero is zero. A wheel in the air takes almost no torque, so four times almost nothing is still almost nothing. Lift a wheel, and it's an open diff again. The same goes for a tire on sheer ice: a Torsen needs a little grip on both sides to do its job.

Welded: the cheapest locker there is

Now, welded. Drifters open the diff and weld the spider gears solid, so they can't turn against each other. Now both wheels always turn together, at the same speed, no matter what. That's what a locker does, and welding is the cheapest locker there is.

For drifting, it's ideal. Kick the clutch, and both tires break loose at once, so the slide is predictable. There's no inside wheel suddenly finding grip and straightening the car out, and no outside wheel spinning alone. (For how the slide itself works, see how drifting works.)

The catch with welding

Remember why differentials exist? In a tight turn, the inside wheel has to go slower, but it can't. It's welded to the outside wheel. So one of the tires has to give, and it does it loudly:

• It scrubs. The inside tire is dragged around faster than it wants to roll.
• It chirps. That's the sound of the tire slipping and catching, over and over.
• It hops. The rear end can skip across the pavement at full lock.
• It pushes wide in the rain. With both wheels fighting the turn, the car wants to go straight, and on a wet road it does.
• It eats tires and axles. All that fighting has to go somewhere, and it goes into the rubber and the driveshafts.

That's fine on a drift lot. In a parking garage, it's a lot of noise and wear for no reason.

Which diff suits you?

• Open: cheap, smooth and quiet. Perfect for daily driving, until one tire is on ice or in the air.
• Limited-slip: puts the power down through corners and on uneven grip. Clutch-type still drives with a wheel lifted; a Torsen is smoother but needs some grip on both sides.
• Welded: both wheels locked together. Predictable slides, terrible manners everywhere else.

The verdict

Open for the daily, limited-slip for the canyon, welded for the drift lot.

And that friend who welded his daily? Every parking garage in town hears him coming.

Recap: open vs limited-slip vs welded

1. Open diff: lets the wheels turn at different speeds, but splits torque equally, so the least-grip wheel sets the limit.
2. Clutch-type LSD: friction plates tie the axles together. One wheel in the air, the car still moves.
3. Gear-type Torsen: multiplies torque up to four times toward the wheel with grip. But four times zero is zero.
4. Welded: both wheels always turn together. Predictable slides; scrubbing, chirping and hopping in tight turns.

What's in yours: open, limited-slip, or welded?

Next episode: how a rotary engine works.

Frequently asked questions

What does a limited-slip differential do?

It limits how much one driven wheel can spin faster than the other. When one tire loses grip, a limited-slip sends more of the drive to the tire that still has traction, instead of letting the slipping wheel set the limit like an open diff does.

Why does an open diff get stuck with one wheel on ice?

An open diff always splits torque equally between the two wheels. If one tire is on ice and can only take a tiny amount of torque, the other wheel gets the same tiny amount, so the car barely moves.

Does a Torsen work with one wheel in the air?

Not on its own. A Torsen multiplies the torque of the slipping wheel, up to about four times for the wheel with grip, but a wheel in the air takes almost zero torque, and four times zero is zero. With a wheel lifted, it behaves like an open diff.

Is a welded diff OK for a daily driver?

Not really. Both wheels always turn together, so in tight turns the inside tire scrubs, chirps and hops, the car pushes wide in the rain, and it wears tires and axles fast. It is a drift-lot part.

What is the difference between a clutch LSD and a Torsen?

A clutch-type limited-slip uses friction plates to tie the axles together, so it still drives the wheel with grip even when the other is off the ground. A gear-type Torsen uses gears to multiply torque toward the wheel with grip, which works smoothly but needs some grip on both sides.