Understanding ABS A. How ABS works B. Wheel Slip IV. Testing / Findings/Conclusion A. Testing of ABS vs. Conventional Brakes B. Test results C. Conclusion ABS (Anti-lock Brake System): WHAT THEY CAN AND CANNOT DO I. Introduction Antilock brakes are impressive performers on the test track and in television advertising. The idea behind anti-lock brakes is simple. Anti-lock brakes are designed to prevent skidding and help drivers maintain steering control during an emergency-braking situation, by automatically pumping the brakes for the driver to prevent wheel lock. Because the wheels are kept from locking up, the driver is able to better control the vehicle. Antilock brakes have become very popular with the public.
Forty-three percent of the 1993 model cars came equipped with them and the ratio for 1994 models is eighty percent. This brings the total number of cars on the road today with anti-locks to about 18,000,000. Much of the increase is because of the growing safety reputation antilock brakes forged on the test track. This reputation has been inflated by car commercials that imply anti-locks can prevent crashes because of better stopping power under all conditions. Consumers very often haven’t been shown cars with anti-locks performing on surfaces that are wet and slippery.
This is the very surface on which anti-locks should make their main contribution. II. What Anti-lock brakes do and don’t do Many car owners don’t know how to use antilock brakes effectively and the manuals that come with their automobiles offer little help. Some of the manuals don’t spell out how using antilock brakes is different from using regular brakes. Some manufacturers provide no instructions at all.
Traditionally, drivers are trained to brake gently on slippery roads or to pump their brakes to avoid a skid. But it is firm, continuous brake pressure that is required to activate anti-locks, which should never be pumped. This is exactly what the antilock feature does for the driver automatically, it pumps the brakes many times a second. With the driver pumping antilock brakes during a skid, braking effectiveness may be reduced to the point of not having brakes at all. Because of misconceptions about the circumstances in which antilock brakes can help, it’s useful to review what they can and cannot do. In normal braking, a vehicle slows as its wheels are gradually brought to a stop.
Without antilock brakes, hard or emergency braking will cause the wheels to lock before the vehicle comes to a halt. In this case, some or all of the tires skid along the road. How the vehicle skids depends on the coefficient of friction, or drag factor, between the tires and the road surface. A vehicle may skid forward in a straight line or it may skid forward and drift to the right or left if the surface of the road is not level. Then again, a car may spin out.
In any of these cases, the driver has effectively lost all control over the direction the car will travel. Antilock brakes can help with these problems but how much depends on the road surface. Anti-locks don’t make much difference in stopping distances on dry roads because maximum braking is easy to achieve on dry roads with or without anti-locks. Even if the wheels lock, the coefficient of friction between the tires and road surface is still fairly high so a vehicle stops relatively quickly.
The most pronounced improvement occurs on slippery surfaces where the drag factor is low. Here, anti-locks can significantly reduce stopping distance and prevent loss of control.. Understanding ABS There are several types of antilock brake systems but they all operate similarly. Sensors near each wheel monitor rotational speed and, as the brakes are applied and the wheel slows, an electronic control unit determines when any wheel is about to lock. The anti-locks will then automatically pump the brakes at a rate of up to 18 times per second to prevent wheel lockup.
Understanding anti-locks better requires knowing about the concept of wheel slip, or the speed of a vehicle’s wheels in relation to the speed of the vehicle itself. When a car is rolling freely, without any braking, there is no wheel slip. That is, there is no difference between the speed of the vehicle and speed of its wheels. But when the wheels are fully locked, as in a skid, slip is at a maximum.
Wheel slip increases along with brake pressure. Without anti-locks, braking force rises to a peak and then falls as the wheels lock. What anti-locks do is control the speed of the wheel to keep them away from lockup and maximize braking force. They do this by allowing brake pressure to increase until close to or just beyond the peak, then release the brakes until wheel slip is reduced and then apply them again. This pressure / release /pressure cycle happens many times per second so the amount of braking force is as great as possible. It continues until traction is restored or the driver releases the brake pedal.
IV. Testing / Findings/Conclusion The National Highway Traffic Safety Administration has conducted a series of tests to evaluate the performance of antilock brakes under conditions that have been encountered in actual accidents. These tests focused on split friction surfaces. In these instances, the two right wheels of a car would be on one surface, as when the tires have gone off the edge of the road and are now on wet grass. The left tires are still on the paved road surface, such as wet asphalt. There is a considerable difference in the effective drag factors between the two surfaces.
With normal brakes, the right side wheels would be expected to lock up first, causing the vehicle to spin counter-clockwise. With anti-locks engaged, all four wheels should continue to rotate allowing the driver to bring the car to a safe stop in a relatively straight line. The test vehicles all came to a halt in a straight line with the anti-locks engaged. When they were turned off and a panic stop was attempted, they all rotated 45 degrees before coming to a stop. During one series of tests, using dry asphalt on one set of wheels and loose gravel on the other, the total stopping distances increased, some dramatically. In one instance, the test vehicle required 62 percent more stopping distance, and 74 percent in the other.
These results were expected, as antilock brakes are less effective on loose gravel than standard brakes. The actual reduction in stopping distance when a vehicle is equipped with ABS is not that significant under most circumstances. A reduction of 10% of the total stopping distance is typical. This means a vehicle that would skid 60 feet on a given surface at 40 miles per hour without ABS would still skid 54 feet if equipped with ABS. ABS will not make a car stop on inherently slick surfaces (snow, ice, rain, etc.) as if the same road were dry. Anti-lock brake systems (ABS) are recognized by government and industry leaders as one of the most significant safety advances in modern automotive engineering.
Wheel lockup is more likely on slippery roads in straight-line skid situations; this is where anti-locks provide the greatest benefit to drivers. However, technology can’t take the place of safe driving. Your best safe driving defenses are: be alert, allow enough stopping distance, and understand your car’s safety technology.
ANNO NATIVE BIBOLGRAPHY ACT Traffic Handbook-Part D Road Craft Online. Internet. 05 April 2000.
Available at web html Provided tips and information on how to be a safe and smarter driver. Car Safety Magazine Highway Loss Data Institute. (1996).
Insurance losses of car equipped with ABS’. Online. Internet. 05 April 2000.
Available at web / This article provided the testing and results of their investigation, and of The National Highway Traffic Safety Administration. CCJ’s Magazine Ross, Chris. (1996).
March 1996 article “Anti-lock braking system’.
Online. Internet. 05 April 2000.
Available at web / This article provided information of the impact ABS had on the commercial vehicle industry. The Mad Scientist Popa, Adrian. (1998).
How does the ABS (Anti-lock Brake System) work?’ Online. Internet. 05 April 2000.