4WD and braking (show #1502)

A caller asks (show #1502) if 4WD helps braking. Tom & Ray said ‘no’, I say ‘yes, sometimes’.

With rear-wheel drive, if a front wheel starts to slip, there is nothing to keep it turning.

With 4WD if a front wheel starts to slip, then sometimes the other wheels will keep it turning until it regains traction.

If the front wheel starts to slip, without ABS, it will lock and slide until it retains traction with rear wheel drive just as you describe with 4WD. It turns because of the car’s motion and the traction at the locked tire overcomes the braking applied.

In your example, you are assuming the other 3 wheels have traction. Real 4WD as opposed to All Wheel Drive, does not have a center-of-the-car differential allowing speed differences between front and rear wheels. If one front wheel slips, the other wheel will try to speed up so that the average speed across the axle matches the rear. At that point the other front wheel will be very likely to slip, too, but by speeding up not slowing down. If that tire has more traction than the 2 in the rear, it could cause them to spin up and slip. The truck will then cause a spin around the front wheel with more traction.

With ABS, all this is goes away since the system won’t allow the wheels to lock and will help prevent the spin.

So the easy answer is; Click and Clack are right and all the 4WD’s seen in the ditch in a snowstorm is the proof.

It really depends upon what you are asking the vehicle to do. It 's just to slow down on a slippery road, your abs does a better job. But, if it’s to keep your speed moderate going down steep hills before you have to use your brakes, then yes, Engine braking on all four wheels is safer then two.

“Engine braking on all four wheels is safer then two.”

Yes…twice as safe if you do the math.