ANTI-LOCK BRAKES - REVISITED
The most significant automotive achievement of the 1980's was the development of anti-lock brake systems (ABS) for passenger cars and light trucks. Whereas items such as airbags are designed to reduce accident injuries, ABS is designed to help prevent the accident from occurring.
Our previous article, "Anti-Lock Braking Systems - A forensic Nightmare?" (Vol. 1 No. 4) discusses the benefits of ABS. In summary, ABS decreases stopping distance and increases control. ABS prevents wheel lock-up during braking on wet or icy roads.
To appreciate ABS, it is important to understand vehicle braking. Misconceptions can lead to unrealistic expectations and unwarranted confidence.
The friction developed between tires and the road produces traction for acceleration, steering and braking. This friction determines the distance required to stop a car. The amount of friction is dependant on the road conditions and whether or not the tires are sliding on the road (skidding).
Wheel Slip-Threshold Braking
Wheel slip (skidding) occurs when the tire breaks traction with the road surface. When this occurs, friction is reduced between the tires and the road.
Skidding results in longer stopping distances. For the shortest possible stopping distance, the friction must be kept at a maximum. This is accomplished by "threshold braking", when the tire develops maximum friction without wheel slip.
A tire that is skidding has exceeded the maximum available friction and cannot provide maximum braking or effective steering.
The accompanying figure shows the relationship between tire slip and coefficient of friction (traction). Traction is highest at 10-20% wheel slip. The range where ABS operates is shown.
There are few situations where ABS braking may require longer distances than conventional braking. These are wet ice and fresh snow.
When braking in fresh snow more than 2-3 cm deep or in loose material such as gravel, there is a higher coefficient of friction for locked/skidding tires than for an ABS braking tire. This is due to build-up of material ahead of the skidding tire, which adds more slowing force.
Remember, even if the ABS system should fail, the system reverts to the conventional braking system. You do not lose braking capability.
In general, the real benefits of ABS are realized in straight line, emergency braking. Under these conditions, ABS installations provide full steering control, complete vehicle stability and shorter stopping distances. The best performance of ABS is on icy or slick surfaces where wheels may easily lock. Complaints of excessive stopping distances on less than ideal surfaces, most notably wet ice, fresh snow or deep loose gravel should not be dismissed.
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Walters Forensic Engineering | 277 Wellington
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