AUTO TECH AND ROAD SAFETY

Understanding Anti-lock Braking Systems

Understanding Anti-lock Braking Systems image

Text: Vince Pornelos / Photos: Bosch, Brent Co | posted May 28, 2008 09:38

ABS - more than just your alphabet soup

It's hard to appreciate the value of a vehicle's Anti-lock Braking System (ABS) in bumper-to-bumper, stop-and-go city traffic, but it?s an essential safety feature worth every penny on the price tag.

To better understand the system, here are the fundamentals: the purpose of a brake is to apply frictional force on a disc (or drum) in order to stop the wheels and as a result, stop the car. The harder you step (or stand) on the brakes, the quicker you stop.

Sounds simple enough? Not really.

When one just steps on the brake pedal with as much force as possible, there is a great risk of lock-up (a wheel being fully stopped while the vehicle is still moving), hence, the loss of steering control. To further complicate things, a car is exponentially prone to a lock-up during braking in a low-traction environment like rainy weather, sandy roads, or snow, and is further multiplied the faster the car is moving. ABS was designed to counter these, allowing one to brake as hard as possible yet maintain the ability to control the vehicle.

Car with ABS braking

Imagine this scenario: a one-tonne or heavier vehicle, cruising at 80 km/h on a stretch of road at night, when the headlights shine upon an obstacle on the road. On a car without ABS, if one just stomps on the brake pedal at this speed (emergency braking), chances are the wheels will completely stop as maximum braking force is applied to the wheels, overcoming the grip of the tires. The car will continue forward due to the momentum, leading to a dangerous, uncontrollable skid and slamming onto the obstacle. Or worse, a pedestrian.

ABS seeks to prevent that particular accident by massaging (continuously and quickly applying and bleeding pressure, similar to a pulsating heartbeat) the brake discs so as to prevent the tires from locking up. The system senses when one or more wheels are rotating at very different speeds (i.e. when the left front wheel rapidly decelerates while the others are still spinning), and as a result will cadence the force applied onto the brake calipers, giving the driver enough time to steer out of a costly situation.

ABS also has the added advantage of saving tires. When the wheels lock-up, they are prone to flat-spotting: shaving/burning enough rubber, flattening a portion of the tire, rendering them unsafe and useless for further driving. Like driving on hexagonal wheels.

ABS, however, is not without trade-offs, the most notable being increased braking distances. Thus, over the years ABS has received some notable enhancements such as Electronic Brakeforce Distribution (EBD) and Brake Assist (BA). EBD uses sensors to accurately allocate appropriate braking force to each wheel, maximizing the effectiveness of ABS. BA, on the other hand, was developed to counter the tendency of most drivers to under-brake (not apply enough force to the brake pedal), hence, is basically a multiplier; automatically detecting emergency braking maneuvers and applies more force on the discs (under the guidance of ABS) to shorten stopping distances.