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June 8, 2014

Car Talk

Busting a myth about pumping the brakes

Syndicated columnists


Dear Tom and Ray:
Please settle a disagreement I'm having with my wife. I told her that we were taught never to pump the brakes on snow or ice if the vehicle has anti-lock brakes. She claims that the "MythBusters" are saying that it's safe and effective. Help! — Phil

Tom:
She's wrong, Phil. So enjoy this rare opportunity to gloat.

Ray:
The reason you pumped the brakes in the days before anti-lock braking systems was to try to prevent your wheels from locking up and skidding. Once a tire is skidding (i.e., sliding along the ground and no longer sticking to it), you lose the ability to steer the car.

Tom:
You probably can figure out for yourself how that could cause trouble in certain situations.

Ray:
The ABS is designed to pump the brakes for you — and to pump them better, faster and more accurately than any human ever could do it.

Tom:
The ABS is able to constantly read the speed of each wheel. If it detects that one wheel is going much slower than the others, it concludes that the wheel is skidding, and it releases the brake on that wheel only, until that wheel stops skidding. Then it reapplies the brake on that wheel.

Ray:
And it does this for every wheel individually — applying and releasing the brakes when necessary — in milliseconds, over and over, until the car is stopped.

Tom: That allows you to keep steering the car during a panic stop, even on ice and snow.

Ray:
It doesn't necessarily make you stop any faster, but it does allow you to keep the car pointed in a straight line, or steer around something if you need to.

Tom:
And since the ABS is already pumping the brakes for you, there's no need to for you to do any pumping. The proper procedure, with ABS, is to step on the brake pedal, hold your foot there until the car is stopped, and keep steering.

Ray: If the ABS engages, you'll feel a rumbling in the pedal. Don't worry about it. That's the ABS pump quickly applying and releasing individual brakes. Just keep your foot planted and steer the car.

Tom:
And under no circumstances should you look over at your wife during an emergency stop and say, "See, hon, I told you I was right."

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)

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