Four-wheel drive explained: 4WD vs AWD, and when it actually helps

Four-wheel drive (4WD) means the motor drives all four wheels, not just two. It helps most on hills, loose ground and slippery floors, because all four tyres can push instead of two. It does not make a car faster on a flat floor. All-wheel drive (AWD) usually means four-wheel drive with a centre differential, so the front and back axles can turn at different speeds in corners.

This page answers the questions people ask most about four-wheel drive, with a lab to test each answer and three Technic-style builds that show it in real parts. It follows on from What is a differential?, which explains the left-and-right side of the story.

What does four-wheel drive actually do?

A tyre can only push the car as hard as its grip allows. Grip depends on the surface and on how much weight presses the tyre down. With two-wheel drive, only the two driven tyres push. With four-wheel drive, all four do, so the car can use the grip of every tyre at once.

The simplest build that shows this is the Driveshaft 4×4 Pusher (level 2, about 20 minutes). It has no motor: a 7-long driveshaft runs from the back axle to the front axle, with a pair of 12-tooth corner gears at each end. Lift it and turn one wheel, and all four wheels turn. Its build page asks you to put the corner gears on opposite sides at each end, so both axles turn the same way. Put them on the same side and the car fights itself.

Is rear-wheel drive or front-wheel drive better for hills?

Rear-wheel drive, in most cars. When a car points uphill, some of its weight leans back onto the rear wheels. Rear-drive wheels get extra grip just when they need it, while front-drive wheels lose some. Try it in the lab: on a wood ramp with the weight in the middle, rear drive climbs to about 18° and front drive gives up at about 12°.

Does 4WD always climb better?

With a locked driveshaft, yes, as far as grip goes. In the lab the same car climbs to about 29° on wood, because it uses all four tyres whichever end the weight sits on. On a real build, check the motor is strong enough too: why a Technic-style car won't climb helps you tell a grip problem from a stall or a tip.

The Twin-Spring 4×4 (level 3, about 35 minutes) does it differently. It has two pull-back motors, one on each axle, so both ends push without a shaft between them. Its build puts the roll bar in the middle so the weight is shared and both ends grip. Its play idea is a ramp race against the Pull-back Racer.

What is the difference between 4WD and AWD?

Car makers use the words loosely, but the usual difference is this:

• Part-time 4WD locks the front and rear shafts together. That is great in mud and on steep tracks, but on a grippy road the tyres fight in every corner, so drivers switch it off on tarmac.
• AWD has a centre differential between the front and rear shafts. It can stay on all the time, because the shafts may turn at different speeds.

Why do the tyres scrub in corners with a locked shaft?

In a corner, the front axle goes round a bigger circle than the back axle. In the lab's tight corner, it travels about 80% further. A locked driveshaft makes both axles turn the same amount, so the tyres have to slide to make up the gap. Engineers call this driveline wind-up. On a grippy mat you can feel the car hop and hear the tyres scrub. Switch the lab to *Tight corner* to see the two paths.

So what does a centre differential do?

It does the same job between the front and back axles that an axle differential does between the left and right wheels. The AWD with Central Diff (level 5, about 90 minutes) shows it in full. Its differential lies long-ways in the middle of the car. An M motor and an L motor both push on its ring, and it sends their power to a shaft for the front axle and a shaft for the back. At each axle a 12-tooth corner gear turns a 20-tooth one, so the wheels turn slower and stronger than the shafts. A Servo swings the whole front axle on a turntable, and a universal joint lets the front shaft bend as it steers. The build page warns that the joint does not like more than about 30°.

Its build page lists the Diff-Lock Mud Truck as the build to do first. That truck's lock makes both driven wheels turn together, which is the same idea as locking a centre differential.

Is AWD with an open centre differential always better?

No, and this surprises most people. An open differential gives both axles the same push. If one axle is light, it starts to spin first, and then the other axle can push no harder. In the lab, put the weight at the back and set a 20° wood ramp: AWD with an open centre differential spins its wheels, while plain rear drive climbs. That is why off-road vehicles can lock the centre differential, and why many real AWD cars add clutches or brakes to send more push to the axle that grips.

How can I test four-wheel drive on a real build?

1. Make a ramp from a board and a stack of books, and measure its angle with a phone or protractor.
2. Raise it a little at a time. Note the steepest angle each car climbs, three tries each.
3. Change one thing only: the surface (card, wood, a rubber mat) or where extra weight sits.
4. Push the Driveshaft 4×4 round a tight circle on a grippy mat, then on a smooth table. Where does it scrub?

Real numbers won't match the lab, because real tyres and motors are messier. Compare your cars with each other, not with the lab.

Check yourself