Why Do Rally Drivers Use Handbrake Turns?

[YellowTaxi]

Can anybody explain what goes on when a rally driver ( off-road driver) executes a 'Handbrake Turn'.

What happens is the driver pulls on the handbrake (emergency brake), the back of the car swings around, and the driver hustles through the corner faster than he normally could...
It's a technique usually used in cars with their center of mass close to the front - Cars like front-wheel-drives.

But what I don't understand is that having the rear brakes on hard causes the car to become unstable and try to swap ends. I would have thought it should act like an anchor at the rear, and thus make the vehicle even harder to deviate from a straight line... So I don't understand it.

anyone ?

 

[mgb_phys]

It's a rear wheel skid, handbrakes don't have ABS so it relies on the handbrake locking the rear wheels up.

 

[YellowTaxi]

It's a rear wheel skid, so it relies on the handbrake locking the rear wheels up.

I don't think the rear wheels have to skid for the effect to take place.

It's not a rear wheel skid, just the effect of having most of the braking force coming from the back axle is enough to help the driver to turn.
I'm pretty sure that's true, and it's why I ask the question.

Just having most of the braking force coming from the rear makes the car more unstable but it shouldn't..I would expect it to behave like a parachute attached to the back of the car - thus making it even more stable and harder to turn ... especially on a car that's front heavy.

 

[Maccara]

This is a bit overly simplified, but the function of handbrake turn is to cause the rear wheels to lock up, instantly changing from static friction to kinetic friction causing the rear wheels to lose traction more than the front wheels. Centrifugal force takes care of the rest.

 

[Danger]

There's also the issue of being able to brake hard without loss of steering control. If you watch a lot of action movies, pay attention to the front wheels of a car skidding through a high-speed corner. Usually, they're still turning, because the stunt crews disconnect the front brakes.

 

[YellowTaxi]

This is a bit overly simplified, but the function of handbrake turn is to cause the rear wheels to lock up, instantly changing from static friction to kinetic friction causing the rear wheels to lose traction more than the front wheels. Centrifugal force takes care of the rest.

Like I said before, I don't agree about "the wheels have to lock up". The de-stabilising effect is there even if the rear wheels don't loose traction with the surface.

You get some 'handbrake turn' effect just by having slightly more braking force coming from the back wheels, and the car will turn more easily without skidding.
All it needs is a little more brake pressure fed to the rear wheels through the brake fluid - the more there is at the rear, the more unstable the car becomes.

And it seems counter-intuitive to me.

 

[chroot]

It's true that it's not necessary to completely lock the rear wheels up -- you just need them to stop generating lateral traction.

If the car is already turning when you pull the brake, the rear tires are already having to produce signficant lateral traction. When you cause them to slip, even a little bit, you suddenly give up that traction and the rear end slides around.

It works the same way on front-wheel drive and rear-wheel drive cars, since both types intentionally produce perhaps 75% of their braking force in front.

- Warren

 

[K.J.Healey]

Adding to chroot, the main reason in rally to do that is you end up swinging the back of the car around faster then the front, which puts you in line with the exit path, then you put into action your 300HP all-wheel-drive, which quickly corrects your oversteer and gets the car moving forward.

 

[TVP45]

For years, I drove a light pickup truck which, in the rain and snow, did exactly that whenever I hit the brakes too hard (pre-ABS). It's not desirable to have the rear-end break free. The momentum is still the same; only the orientation has changed. How do you "tell" your tires to go from kinetic to rolling friction when you're ready?

 

[YellowTaxi]

It's true that it's not necessary to completely lock the rear wheels up -- you just need them to stop generating lateral traction.

If the car is already turning when you pull the brake, the rear tires are already having to produce signficant lateral traction. When you cause them to slip, even a little bit, you suddenly give up that traction and the rear end slides around.

It works the same way on front-wheel drive and rear-wheel drive cars, since both types intentionally produce perhaps 75% of their braking force in front.

- Warren

Quite a good explanation. You got close, but it doesn't quite tell the full story. I'll try to add to it.:-

The car doesn't have to be already turning.

Now, because each tyre can only generate so much grip, the more fore-aft force exerted on any tyre, the less sideways grip you have left-over available from that tyre. If your brakes are on harder at the rear, you have less sideways acceleration available from that end of the vehicle, and so the rear-end will tend to swing around as soon as you enter a corner. Simple.

Obviously the exact opposite happens if your brakes are on harder at the front before you enter the turn.

Thanks chroot, you've helped me sort that out.