One-Wheel
Drive

I was sixteen, maybe seventeen, and I drove a silver Volvo 240 DL wagon that I had christened the Silver Bullet on the grounds that it was, in every measurable respect, not a bullet.

My mother and I were moving furniture from her friend Barbara's house back to ours. Coming out of Barbara's neighbourhood I missed the turn, spotted a grassy verge big enough to swing the wagon around in, and took it. It had rained lightly the night before. Not a storm — just enough to turn the top inch of that grass into mud.

The Bullet sank, and then it did the thing.

Right rear tyre spinning merrily, flinging mud, engine revving, speedometer indicating that we were making excellent progress. Left rear tyre: motionless. Sitting there on perfectly good ground doing absolutely nothing, like a man watching his colleague struggle with a heavy box.

My mother — who is not an engineer and has never claimed to be — looked at this and asked the single best question anyone has ever asked me about cars:

I had no answer. We called my dad, who arrived in his Grand Cherokee and pulled us out, which did nothing to improve my standing in the matter.

And then, a few years later, a courtroom

You know the scene. Two kids from New York are on trial in Alabama for a murder at a convenience store they didn't commit, and the entire case comes down to a photograph of tyre marks in the dirt outside. Marisa Tomei, as Mona Lisa Vito, is on the stand.

Her testimony is that the marks could not have come from the defendants' 1964 Buick Skylark, because the car that made them had Positraction — and the Skylark didn't offer it. Vinny, playing dumb for the jury, asks her what Positraction is.

And her answer is, word for word:

"It's a limited slip differential which distributes power equally to both the right and left tires. The '64 Skylark had a regular differential, which, anyone who's been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing."

One tyre spins. The other tyre does nothing.

That is not a similar problem to my mother's question. That is the same sentence, delivered by a fictional hair salon employee in a Hollywood courtroom in 1992, about a mud patch in Alabama, and it describes a silver Volvo wagon stuck on a verge in a suburb with a teenager at the wheel.

My mother was not confused. She had independently diagnosed, from the passenger seat, the central design compromise of the automobile — the one component that every car has, that nobody thinks about, and that has generated a century of increasingly clever engineering attempts to work around its one enormous flaw.

She just called it one-wheel drive. Which, honestly, is the better name.

The first idea: leave the differential free to do its cornering job, but add internal resistance so that when one wheel starts spinning wildly relative to the other, something inside fights that difference and shoves torque back toward the slower wheel.

This is a limited-slip differential, and General Motors sold it as Positraction — which is the single most successful piece of drivetrain branding in history, given that a movie made it a household word thirty years after the fact.

There are four main ways to build one, and the differences matter:

Clutch-type. Stacks of friction plates preloaded by springs sit between the side gears and the case. Speed difference means the clutches slip, and slipping clutches generate resistance that transfers torque to the slower wheel. Cheap, effective, tuneable — and consumable. The plates wear, and they want specific friction-modified oil.

Cone-type. Same idea, conical friction surfaces instead of plates. Simple and durable.

Viscous. Interleaved plates in a sealed housing of thick silicone fluid. Spin one set fast relative to the other and shearing heats the fluid, which thickens it, which couples the two sides. Utterly passive, no wear surfaces, and slow to react — it needs the speed difference to already be happening before it does anything.

Torsen — the clever one. Short for torque sensing, and it is a genuinely beautiful piece of thinking. Vernon Gleasman at Gleason Works patented the core idea in 1951 after noticing something about worm gears: a worm can drive a worm wheel, but a worm wheel cannot back-drive the worm. Build a differential around that asymmetry and you get a device that behaves like an open diff under normal driving, then automatically binds and biases torque toward the gripping side the instant a difference appears — with no clutches, no fluid, no sensors and no wear.

The catch with all of them, and it's the same catch: a limited-slip needs the losing wheel to have some grip to work against. Put a wheel on genuinely frictionless ice — or lift it clean off the ground, which happens constantly off-road — and even a Torsen can revert to behaving like an open differential. There's nothing to multiply.

Hold onto that, because it becomes important later.

If the differential's ability to let wheels turn at different speeds is what's ruining your day, then remove it. A locking differential mechanically ties both axle shafts into one solid unit. Same speed, no exceptions, and torque can now go wherever grip exists — up to 100% on one side if the other is airborne.

This is the most effective traction device ever built, and it is also, on pavement, actively dangerous. You have deliberately recreated the exact problem Pecqueur solved in 1827. The car will not turn properly, the tyres chirp and scrub, the driveline winds up, and in the wet a locked axle will happily rotate the car for you.

Which is why the interesting engineering is all in when it locks:

Automatic lockers (the Detroit Locker being the archetype) are locked by default and ratchet apart when cornering forces demand it. Enormously effective, and they announce themselves with bangs and clunks that alarm passengers.

Selectable lockers let the driver choose. Air-actuated units like ARB's, and factory electric lockers such as those in the Jeep Wrangler Rubicon, sit open for the drive to the trail and lock on a switch when you get there. This is the right answer for almost everyone.

Spools are permanently locked — a solid piece with no differential action at all. Drag racing and dedicated competition only.

The count matters too. One locker (rear) is the common truck setup. Two (front and rear) is serious off-road hardware — Rubicon, Power Wagon. Three, adding a locking centre differential, is Mercedes G-Class territory and means all four wheels are mechanically forced to the same speed regardless of what any of them are touching.

If you've owned anything older with four-wheel drive, you remember getting out in the rain to twist a dial on each front wheel.

Manual locking hubs physically connect or disconnect the front wheels from the front axle shafts. Unlocked, the wheels spin free and the entire front driveline — shafts, diff, front driveshaft — sits still, saving fuel, wear and noise. It is a completely sound piece of engineering whose only flaw is that it requires a human being to stand in the weather.

The replacements came in stages: automatic hubs that engaged on torque or vacuum and were reliably unreliable; then centre-axle disconnect, which uncouples the axle inside the housing instead; and finally today's electronically actuated systems, where a button or a dial does what your cold hands used to.

The honest trade: manual hubs are more durable, more predictable and repairable with hand tools; electronic systems are vastly more convenient and can engage while moving. Overlanders still fit manual hubs on purpose. Almost nobody else misses them.

Everything so far has been about restoring balance: stop that wheel spinning, get torque back to the wheel that can use it. Torque vectoring asks a more ambitious question. Why stop at equal?

In a corner, the outside wheel has more load on it, more grip available, and is travelling further. Send it more torque than the inside wheel — not equal, deliberately more — and the resulting thrust imbalance pushes the car's nose into the turn. You are steering with the drivetrain.

Two ways to do it. Brake-based vectoring nips the inside wheel to create the imbalance — cheap, uses existing hardware, and scrubs off a little speed doing it. True mechanical vectoring uses clutch packs or a step-up gearset to actively overdrive the outer wheel, adding torque instead of subtracting it. Honda's SH-AWD, Mitsubishi's AYC on the Evo, and the systems in cars like the Focus RS and modern M and AMG products work this way. Far better, far more expensive.

And then the answer that makes the previous four look quaint.

If each wheel — or each axle — has its own electric motor, there is no shared torque to divide. No spider gears, no side gears, no equal-split rule, nothing to lock, nothing to trick with a brake caliper. Torque at each wheel becomes an independent software variable, adjustable thousands of times per second.

Two hundred years of increasingly clever mechanical devices existed to work around one consequence of one shaft feeding two wheels. Remove the shared shaft and the entire problem, along with its entire solution industry, simply evaporates.

Which is a strange thing to celebrate. The Torsen is a lovely object. So is a Detroit Locker, in its violent way. They are the kind of engineering that solves a hard problem with shapes and materials rather than instructions — and they're being replaced by a control algorithm you will never see, hear or need to service.

The centre differential, briefly

Everything above concerns two wheels on one axle. Drive all four and you have the identical problem one level up — front and rear axles also need to turn at different speeds in a corner, or the driveline binds. So you fit a centre differential, and it has the same flaw for the same reason.

The two dominant answers split along familiar lines. Torsen centre diffs — the Audi quattro approach on longitudinal-engine cars — are purely mechanical, always active, and typically bias rearward (around 40:60) with the ability to shift much further toward whichever axle has grip. Clutch-based systems like Haldex run a multi-plate clutch, usually on a front-drive platform, sending torque rearward when sensors detect or predict slip. Lighter, cheaper, packages into transverse layouts, and historically felt more like front-wheel drive doing its best.

And here is the loveliest detail in the whole subject. The Torsen has a known weakness called the zero-torque problem: because it works by multiplying available torque, a wheel with genuinely zero traction gives it nothing to multiply, and the mechanical marvel briefly behaves like a plain open diff. Audi's fix is EDL — electronic differential lock — which applies the brake to the spinning wheel to manufacture artificial resistance so the Torsen has something to bite against.

The finest mechanical differential ever built needs the brakes to bail it out. The 1971 idea rescues the 1951 one.