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Motoring Q&A

April 15, 2014 Car Care

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By Paul Brand

MCT News Service

QUESTION: I have two cars with ABS — a 1999 Buick Regal and 2013 Toyota Corolla. I am wondering if ABS is really the best way for stopping on an icy road. This past winter, I had several occasions where I tried to stop as I approached a stop sign, but when applying and maintaining brake pressure the ABS engaged but did little to stop the car. Then I did the old-fashioned pumping of the brakes with much better results. Am I using ABS incorrectly?

ANSWER: Considering that antilock braking systems are capable of recognizing wheel lockup and modulating/releasing hydraulic pressure to restore rotation of the wheel many times per second — much, much faster than the human foot can accomplish — ABS should be more effective than pumping the brakes.

But there may be mitigating factors. First, if the anxiety of not stopping as quickly as the situation warrants causes the driver to continue to press harder and harder on the brake pedal, the ABS tends to cycle more slowly because of the much larger releases of hydraulic pressure required to restore wheel rotation. These much deeper modulations will increase stopping distance measurably.

Recognize what provides the “best” stopping traction on ice — it’s that moment just before and as the wheel/tire begins to lock up. At that instant, the tread blocks on the tire surface are at the edge of their maximum traction, just before sliding. Since the ABS system can recognize and release hydraulic pressure at that precise moment much faster than the human foot, ABS should stop better on ice.

Is it possible the pumping action can be more effective? Not in my experience, but perhaps in a scenario where the locked wheel/tire is actually melting the ice under the tread. This is entirely speculative on my part — just trying to envision a scenario where pumping the brakes might be more effective.

But here’s the bottom line: Neither ABS nor pumping the brakes can overcome the laws of physics. Experiencing either action is confirmation of going too fast and/or braking too late for existing conditions. On glare ice, I still want an ABS system to minimize the consequences of my mistake.

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