Fall 2004 Forward to a Friend






Where did ABS come from?
ABS was first developed for fighter aircraft – those landing on carriers, in particular. Sliding, skidding wheels offer no control during a landing. Just watch an auto race to see examples of both control under braking and uncontrolled slides!

The first automobile with ABS was the British-made Jensen Interceptor FF built between 1966 and 1971. Its Dunlop Maxaret ABS was based on the aircraft system.

Expensive and somewhat unreliable at first, ABS has evolved to become valued safety equipment now available in many contemporary vehicles. Today, every new Subaru vehicle sold in the United States features standard ABS.

How does ABS work?
At the heart of ABS is an electronic control unit (ECU) – a small computer. It determines when the brake of one or more of the four wheels is about to lock up based on input from wheel-speed sensors and other sensors.

Wheel-speed sensors measure the rotational speeds of the wheels, and the ECU calculates potential lock-up under braking by comparing those speeds. The type of sensor depends on the vehicle. Some sensors are measuring the rotational speeds of toothed wheels called “tone wheels” that are mounted on the axles. Other wheel sensors – those on the 2005 Legacy and Outback, for instance – measure speed from electromagnets sealed onto the axles.


Additional sensors also contribute information that indicates emergency braking situations. These include yaw-rate (sideways motion) and/or G (the force of gravity acceleration and deceleration) sensors, depending on the Subaru.

When the ECU determines that a wheel is locking, it electronically causes the hydraulic system to modulate the braking force on that wheel to prevent skidding. Modulation takes place in milliseconds.

If you learned to drive before ABS was available, you learned to pump the brakes for the same effect. However, it’s impossible to pump or modulate the brake pedal as quickly with your foot as the computer can electronically, and ABS also can localize modulation to the affected wheel or wheels.

Subaru equips all of its vehicles only with the more sophisticated four-channel ABS, which offers more control than the three-channel systems offered in many competitive models. Four-channel systems modulate the brake for each wheel separately for more precise control. Three-channel systems modulate the rear wheels as a single unit.


So when the brake pedal pulsates during an emergency braking maneuver, you’re feeling the effects of computer-generated modulation. Never pump the brake pedal in an ABS-equipped vehicle! Keep your foot down, and let the pedal pulsate!

Why is ABS in Subaru vehicles?
When brakes lock up, the tires skid. As they slide, they leave the driver with no control. Dry or wet pavement, snow or ice – the vehicle goes in the direction of its inertia, regardless of the direction in which it is steered. (ABS isn’t just for wet or snowy conditions!)

So, ultimately, the safety aspect of ABS has more to do with steering control than with stopping distance or time. Skidding front tires cannot turn the vehicle. Tires with adhesion to the road can, allowing the driver to steer. Greater control improves active safety.

And that answers the original question.


Note: Always use the utmost care in driving – over-confidence because you are driving with an ABS-equipped vehicle could easily lead to a serious accident.