Noise  and ABS

The last thing that anti-lock brake system engineers want drivers to do during a panic stop is to let their feet off the brakes. Braking should be our concern, say the engineers, because the less time the driver worries about stopping the car, the more time there is to concentrate on steering it.

Unfortunately, drivers can be surprised by noise and vibration when they step hard on the brake pedals and the anti-lock brakes take over. Confronted by strange sounds and pulses during a moment of panic, many drivers react by letting up on the pedal—the worst response possible when trying to stop a car.

Over the last few years, engineers who design and test anti-lock brake systems, or ABS, have been trying to come up with ways to minimize the noise and vibration that drivers hear and feel when they stomp on the brake pedals.  By removing three elements (noise, vibration, and harshness) from brake systems, or at least minimizing them, there is hope to lessen the chance of startling drivers during panic stops, and thus keep their feet planted firmly on the pedals.

Noise in the System

What makes ABS so much noisier than everyday disc brake systems? To answer that, a short explanation of ABS operation is in order. The mechanical components in both systems are functionally identical, consisting of a brake pedal, a master cylinder and booster, hydraulic lines and fluid, wheel calipers, brake pads, and rotors. In fact, unless the system is actuated by hard braking, ABS acts just like an ordinary disc brake system. That means a piston in the master cylinder forces hydraulic fluid out to a wheel caliper. There, the fluid pushes brake pads against a spinning rotor, slowing the rotor and wheel. When ABS is actuated, several more components jump into action: a couple of valves, a hydraulic pump, electronic sensors and circuitry, and a bit of programmed logic.

For ABS "the stability of the vehicle is the primary goal." The objective is to slow the rotation of the tires as quickly as possible, with minimal skidding, so that the driver never loses the ability to steer. To achieve this end, ABS monitors wheel speeds and makes an estimate about the road surface over which the tires roll. For most surfaces, ABS-controlled vehicles will outperform a vehicle without such a system. ABS is designed to maintain stability and steering, and minimize stopping distance—in that order."

In the worst case, on ice, the wheel could go from 50 mph, say, to zero in an instant, the time in which two extra ABS valves must begin operating. The first valve shuts off the master cylinder from the brake pedal, isolating the brakes from any further foot pressure by the driver. The second valve begins dumping hydraulic pressure, relieving pressure on the wheel rotor and allowing it to start spinning again. As long as the driver keeps the pedal pressed hard, the ABS system functions automatically.

The ABS system at this point begins to home in on the maximum pressure it can apply to the brake rotor without making the wheel skid. The two ABS valves close and open in rapid succession. Johnson likens the vibration and noise stemming from the operation of these valves to water hammer, the all-too-familiar knocking in the pipes that occurs at home when the faucet is suddenly turned off. The noise that is so irritating at home is also every bit as annoying when it invades a car's passenger compartment.

Another component peculiar to ABS introduces a noise that's no less insidious. After the second ABS valve dumps hydraulic pressure, that fluid must be pumped back into the master cylinder. Otherwise, the brake pedal would gradually reach the floor. To do this, engineers specify a hydraulic pump that kicks on whenever ABS is called into action. Unlike the hammer-like vibration of the valve pulses, the noise from the pump is more of a low growl.