All-wheel drive is not one thing: the designs compared

In a corner every wheel covers a different distance. The inner wheels run on a tighter radius than the outer ones, and the front axle describes a different arc than the rear. The differential in each axle absorbs the difference between left and right. The difference between front and rear remains, and has to be dealt with separately.

Link the two axles rigidly and you force them to the same speed. On a surface that allows slip — snow, gravel, wet grass — the difference simply scrubs away. On grippy tarmac it cannot. Wind-up then builds in the driveline, showing itself as juddering during tight manoeuvres and loading shafts, joints and gear teeth.

Every all-wheel-drive design is an answer to that one question: how far may the front axle turn relative to the rear, and when may it not? The answers differ in complexity, weight and fuel consumption — and in which parts cost money as the car ages.

The oldest design has a transfer case with no centre differential. On the road only one axle drives, usually the rear; the driver engages the second through a dog clutch or a chain, after which the two are rigidly coupled. A low range often comes with it — a second set of ratios for slow, steep or heavy work.

From that follows the operating rule peculiar to this design: engaged four-wheel drive does not belong on dry tarmac. Drive it there and you create the wind-up described above, and it lands on whatever part has the least reserve — often the transfer case itself. That is not a design flaw but a rule of use.

On a used car, then, the design is not a drawback but a question about the previous owner. Engage and disengage it stationary on level ground and watch whether it locks in cleanly and releases again. Dogs that no longer catch, and actuators that click without engaging, are this design’s characteristic signs of age.

Put a third differential between the axles and the front may turn freely relative to the rear. Drive is then permanently present at both axles with nothing winding up. How torque is split follows from the design: a bevel-gear differential divides it evenly, a planetary set in a chosen fixed ratio.

An open differential, though, has a property that becomes a problem off the road: it sends torque wherever resistance is lowest. Put one wheel on ice and it spins while the other axle gets nothing. That is why this design almost always carries a lock or a self-locking arrangement — a multi-plate clutch in parallel with the differential, or worm gearing that binds of its own accord as the speed difference rises.

It is the most complex and heaviest solution and at the same time the one that asks least of the driver. In upkeep it shows up as extra oil fills: centre differential, rear axle and, where fitted, transfer case are three circuits nobody sees, whose intervals live in the maintenance schedule — not in the listing description.

The solution now most common in passenger cars has no centre differential at all. One axle — with a transversely mounted engine usually the front — always drives, while the second hangs on a plate pack squeezed together by an electrically controlled pump. How hard is decided by a control unit reading wheel speeds, throttle position, steering angle and lateral acceleration.

The advantage is packaging, weight and fuel use: in normal running the car carries little more than a front-wheel-drive system. The drawback is that a friction pack remains a wear item. It runs in its own oil, which wants changing along with a filter, and if it slips continuously — because the tyres on the two axles are worn differently, say — it ages quickly.

An older, passive relative is the viscous coupling: two sets of plates in a silicone fluid that stiffens when the speeds differ and so transmits torque. It needs no control and is therefore unobtrusive — including when it fails. A viscous coupling that has leaked or hardened either transmits nothing any more or permanently too much, and the driver notices either late.

In an electric or hybrid car, all-wheel drive can exist without any mechanical connection between the axles. One motor works at the front, a second at the rear, and what a centre differential solves mechanically is here solved by control: each axle gets exactly the torque the software assigns it.

Mechanically that removes a good deal — propshaft, centre bearing, transfer case and one oil fill. The control is also faster than any clutch, because an electric motor changes its torque in milliseconds. Other components arrive in their place, and their faults are electrical: power electronics, cooling circuits, and power limitation under sustained load or high temperature.

For a viewing that means there is no transfer case to inspect and no clutch to listen to. What gets checked is the fault memory, the behaviour under load and whether the cooling is doing its job — questions answered with a diagnostic tool rather than by ear.

Almost every modern car brakes a spinning wheel deliberately and so routes torque across the open differential to the other side. That substitutes for a mechanical cross-axle lock remarkably well, and it is why many cars without locks get further than their specification suggests.

The limit is thermal. The brake turns the energy into heat, and the heat has to go somewhere. On a long climb on loose ground, or pulling away repeatedly in deep snow, the system gets hot and backs off its intervention. A mechanical lock does not have that problem — but it does require the driver to open it again before returning to the road.

And the most important sentence about all-wheel drive: it helps you pull away and accelerate. Stopping distance and cornering limit depend on the friction between tyre and road, and the number of driven wheels does not change that. An all-wheel-drive car on summer tyres gets further in snow and stops worse — which is the situation most accidents of this kind come out of.

Every design with a mechanical link between the axles reacts to differing rolling circumferences. A new tyre has more tread than a half-worn one, so it turns slightly more slowly at the same road speed, and the clutch or centre differential reads that difference as continuous slip. What is wanted on snow becomes a permanent condition here.

In practice: a single new tyre after a puncture is rarely the right answer on an all-wheel-drive car. Whether you replace in pairs per axle or all four, and what deviation the manufacturer still allows, is in the vehicle’s own documents. That is not a cautious formula but the difference between one tyre and a clutch pack on the invoice.

Conversely, an all-wheel-drive car with four differently worn tyres tells you something beyond the tyres. Whoever economised here has probably been paying for the clutch for a while without knowing it — and the next owner pays the remainder.

Two checks, and the first calls for restraint. On an open surface with a little slip — wet cobbles, gravel, an empty yard — drive a tight circle at walking pace on full lock. Judder or hopping points to wind-up. On a part-time system that very behaviour is normal on dry tarmac, which is why you do not carry out the test there at all.

The second is listening. A drone that rises with road speed and changes note on and off the throttle often comes from a final drive. A vibration that starts at a particular speed and disappears above it points to the propshaft or its centre bearing. Clicking on full lock is a constant-velocity joint at the front.

And a look from underneath. Propshaft, centre bearing, driveshaft boots and the gearbox casings are either dry or they are not. A damp film on a casing is something to watch; a dripping shaft seal is a quote.

For a steep driveway in winter, for a trailer on wet grass and for pulling away on loose ground, all-wheel drive is a real and noticeable help. For stopping distance, cornering speed and fuel consumption it is not — there it sits on the other side of the ledger.

Anyone who genuinely goes off road needs more than driven wheels: low range, ground clearance, usable approach and departure angles and at least one lock. Anyone who means snow and hills in daily use is usually better served by a clutch-based system and good winter tyres than by a heavy off-roader whose tyres come in an expensive size.

And the sober part: more drive means more mass, more joints, more bearings and more oil. That appears in fuel use, in tyre wear and in the service bill. An all-wheel-drive car is the right choice when you can name the conditions under which you need it.