4WD, AWD and the difference that matters when one wheel is on ice

"Does it have four-wheel drive?" is close to the wrong question. Two vehicles can both answer yes and behave completely differently in the situation you bought it for.

The question that actually predicts behaviour is:

How many differentials does it have, which of them can be locked, and can you use it on tarmac?

Everything below is that question answered for each layout. It assumes you have read what a differential does — if not, the differential is the ten-minute version, and the one sentence you need from it is that an open differential splits torque equally and is therefore limited by whichever wheel has least grip.

Three jobs, not one

A two-wheel-drive car has one differential, between its two driven wheels. Driving all four means three separate speed differences have to be accommodated:

• between the two front wheels,
• between the two rear wheels,
• and between the front axle and the rear axle, because in a corner the front wheels trace a longer arc than the rears.

That third one is the whole story, and it is where the original gets confused. It says the transfer case "guarantees that every wheel turns at the same speed", calls that "the differential being locked", and then a paragraph later says power is split between the axles by a differential. Those cannot both be true, and the muddle is between the centre differential and the axle differentials.

So: a transfer case that simply couples the two propshafts together has no centre differential. A transfer case with one has three differentials in the vehicle. That single difference decides everything else.

Part-time 4WD — no centre differential

The traditional 4×4. Rear-wheel drive normally; select four-wheel drive and the front axle is bolted, rigidly, to the same shaft as the rear.

The original is right about the consequence and right to be emphatic: this must not be used on a dry hard surface. The reason is worth spelling out, because "it damages the vehicle" is not a reason anybody remembers.

In a turn the front axle wants to travel further than the rear. With no centre differential between them, that difference has to go somewhere. On mud, snow or gravel it goes into the tyres slipping a few degrees against a surface that lets them, which costs nothing. On tarmac the surface will not let them, so the difference is stored as twist in the propshafts, the transfer case and the axles — driveline windup. You feel it first as the steering going heavy and the vehicle refusing to turn tightly, and it releases either as a graunch when a tyre finally breaks away or as a broken component.

The practical rules that follow: engage it on the loose, disengage it before you rejoin the road, and if it will not disengage, drive a short distance in reverse on a loose surface to unwind it.

This layout typically offers 2WD, 4WD High and 4WD Low. Its axle differentials are usually open, which matters for the last section.

Full-time 4WD — a centre differential added

Put a differential in the transfer case and the front-to-rear speed difference is accommodated the same way the side-to-side one is. Now four-wheel drive can be left engaged permanently, on any surface, at any speed. This is what the original means by the "extra differential", and it is the single change that makes the system usable on the road.

It introduces a new weakness, though, and it is the same weakness as before, one level up. A centre differential is a differential, so it too passes only what the weaker end can accept. A vehicle with three open differentials and one wheel in the air has, in principle, no drive at all — the centre diff sends torque to the axle that will take least, and that axle sends it to the wheel that will take least.

Which is why a serious off-road vehicle has a centre diff lock as well, and why the good ones have axle locks too. Lock all three and every wheel turns at engine speed regardless of grip. The same tarmac warning applies to each lock you engage.

AWD — usually something else entirely

The original treats AWD and 4WD as the same idea under two names. They are not, and the difference is not marketing.

Most of what is badged AWD on a road car is a front-wheel-drive car with an on-demand coupling to the rear axle — a viscous unit on older designs, and on modern ones an electronically controlled multi-plate clutch of the Haldex type. Under normal driving almost all the torque goes to the front wheels. When the front axle starts to turn faster than the rear, or when the module predicts it will, the clutch closes and sends some torque backwards.

So an AWD car is a front-drive car most of the time, and it is optimised for grip on the road rather than for crawling over obstacles. There is no low range, usually no locks, and nothing the driver selects. The corpus's four-wheel-drive clutch control module is the part that runs it.

Which is better depends entirely on what you are doing, and neither source says so: AWD is for traction on a road surface; 4WD with low range and locks is for traction where there is no road. A car that is superb in snow on the motorway can be defeated by a rutted field, and vice versa.

The answer to the question in the title

One wheel on ice, everything else on grip. What happens?

The counter-intuitive row is the second one, and it is the reason this article exists. A part-time 4×4 in four-wheel drive with one wheel lifted can be as stuck as a hatchback, because bolting two axles together does nothing about the open differential inside each axle. Four driven wheels is not the same as four wheels driving.

And the last row explains why the cheapest modern car often outperforms an old 4×4 on a slippery road: the hatchback has brake-based intervention and the 4×4 has three open differentials.

Low range is a gear, not grip

Worth stating because it is widely misunderstood. Low range is an extra reduction in the transfer case — commonly somewhere around 2:1 — that multiplies torque and divides speed. It lets you crawl at walking pace with the engine turning fast enough to be controllable, and it multiplies whatever torque the engine makes.

It does not create traction. If the wheels are already spinning, low range makes them spin more easily, not less. What it gives you is control: fine throttle resolution, real engine braking downhill, and the ability to apply large torque without slipping the clutch.

What goes wrong

• It will not engage. The corpus is blunt about this and it matches the trade: on a road-used vehicle the cause is usually mechanical rather than electrical — transfer case actuators and shift mechanisms seize because the system is never used. If your vehicle has selectable four-wheel drive, engage it for a few minutes every month or two, on a loose surface. That is maintenance, not enthusiasm.
• The selector switch. The four-wheel-drive switch reports the driver's choice to the transmission, the engine module and stability control, so a failed one produces effects well beyond the transfer case.
• Mismatched tyres, which is the expensive one nobody warns about. On a full-time or on-demand system, tyres of different rolling circumference — a new pair on one axle against a half-worn pair on the other, or one replacement of the wrong size — make the front and rear axles genuinely turn at different rates all the time. The centre differential or the coupling then absorbs that difference continuously, gets hot, and wears out. Replace all four together on a full-time system, keep them the same make and pattern, and rotate them so they wear evenly. This ruins Haldex-type couplings routinely and the owner is usually told the coupling was unlucky.
• The transfer case fluid, which is a real service item on a vehicle that actually works, and is usually forgotten because the vehicle usually does not.

And one line from the original worth keeping

It advises not exceeding about 100 km/h in four-wheel-drive mode on a part-time system. The number is arbitrary but the instinct is right, and the better version is the one above: on a part-time system the question is not how fast, it is what you are driving on.

Video guides

Video, for the parts of this that are easier watched than read.