Q&As: Antilock brakes — cars, trucks, motorcycles
October 2009
- 1 How do antilock brakes work?
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Antilock brakes are designed to help drivers avoid crashes. When a driver hits regular brakes hard, the wheels may lock and the vehicle may skid. Wheel lockup can result in longer stopping distances, loss of steering control and, when road friction is uneven, loss of stability if the vehicle begins to spin. The main advantage of antilocks is that they can reduce these problems on wet and slippery roads. Antilocks work with a vehicle's normal service brakes to decrease stopping distance and increase the control and stability of the vehicle during hard braking. Vehicles equipped with antilocks have speed sensors mounted at each wheel and a secondary electro-hydraulic braking circuit. The principle behind antilocks is that a skidding wheel provides less stopping force and control than a wheel that is rotating. Antilocks prevent wheels from skidding by monitoring the speed of each wheel and automatically pulsing the brake pressure on any wheels where skidding is detected. Antilocks should not make much difference in stopping distances on dry roads, although they can enhance vehicle stability and allow drivers to maintain steering control during emergency stops when conventional brakes might allow wheel lockup and skidding.
- 2 Are all antilock brake systems the same?
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Antilocks differ among vehicles, but there are some basic similarities. Each system has sensors that monitor the rotational speeds of selected wheels when brakes are applied. When one of these wheels approaches lockup, a control unit reduces brake pressure to that wheel (or set of wheels) just enough to allow rotation again. This typically happens many times per second, resulting in improved control and, on many wet and slippery surfaces, shorter stopping distances. Differences among antilock brake systems include the following:
- Cars and many SUVs have four-wheel systems with wheel-speed sensors on each wheel. In one type of system, the antilocks reduce brake pressure to both rear wheels whenever one approaches lockup. Brake pressure to the front wheels of four-wheel systems is controlled independently to maximize stopping power, which is concentrated in the front. In four-wheel independent systems, each wheel is controlled individually, so when any one approaches lockup, the antilocks reduce brake pressure to that wheel.
- Some pickups and cargo vans have rear-wheel-only antilock systems to address different braking needs when vehicles are loaded versus unloaded. The antilocks monitor the rotational speeds of rear wheels only and release pressure to both when either is about to lock.
- Tractor-trailers have separate antilock systems for the tractors and the trailers. Ideally, both the tractor and trailer of a combination rig should have antilock brakes, but putting antilocks on either component should produce improvement compared with conventional brakes. With antilocks on the tractor only, a driver can maintain better steering control even if trailer wheels lock and the trailer swings. If only the trailer has antilocks, trailer swing can be reduced even if steering control is lost.
- 3 Why don't antilocks reduce stopping distances as much on dry roads as wet ones?
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Adequate braking is easy to achieve on dry roads with or without antilock brakes. Even if wheels lock, the coefficient of friction between tires and road surface still is relatively high, so a vehicle stops relatively quickly. It is even possible on some surfaces to stop sooner without antilocks than with them, although such instances are rare. They occur, for example, when loosely packed snow or gravel creates a "dam" effect in front of locked wheels, shortening the stopping distance more than antilocks could.
- 4 Do car antilocks reduce crashes?
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Although car antilocks perform well on the test track, it is unclear whether they have made significant reductions in the number of on-the-road crashes. A 1994 Highway Loss Data Institute (HLDI) study1 and a subsequent 1995 study2 compared insurance claims for groups of otherwise identical cars with and without antilocks, finding no differences in the overall frequency or cost of crashes for which insurance claims for vehicle damage are filed. Because antilocks should make the most difference on wet and slippery roads, researchers also studied insurance claims experience in 29 states during winter months. Even here they found no difference in the frequency of insurance claims for vehicles with and without antilock brakes. A 1997 Institute study3 and a 2001 update4 reported no difference in the overall fatal crash involvement of cars with and without antilocks.
According to one federal report, "the overall, net effect of antilock brakes" on both police-reported crashes and fatal crashes "was close to zero."5 A more recent federal report concluded that antilocks reduce overall crash involvement risk by 6 percent for cars and 8 percent for pickups and SUVs, but they have no effect on fatal crash risk.6 Leonard Evans, a researcher with General Motors, reported that antilock-equipped cars were less likely to rear-end other vehicles but more likely to have other vehicles rear-end them.7 The net result was little effect on overall crash risk. In a study conducted for auto manufacturers, Failure Analysis Associates reported a net beneficial effect of antilocks on nonfatal crashes but no effect on fatal crashes.8
- 5 Why aren't car antilocks reducing crashes as expected?
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No one knows for sure why their test performance has not translated into a substantial reduction in real-world crashes. A possible reason is that the average motorist rarely experiences total loss of vehicle control, which antilocks are designed to prevent. There also is evidence that many car owners do not know how to use antilock brakes effectively. A 1994 Institute survey of drivers with antilock-equipped cars found that more than 50 percent in North Carolina and 40 percent in Wisconsin incorrectly thought they should pump the brakes.9 Another possibility is that some motorists may drive less cautiously because they believe antilocks allow them to brake better.
- 6 Are motorcycle antilock braking systems (ABS) effective at reducing crashes?
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Yes. Results from recent studies by the Institute and HLDI compared crash rates for motorcycles equipped with optional ABS against the same models without the option. The rate of fatal crashes per 10,000 registered vehicle years was 28 percent lower for motorcycles equipped with optional ABS than for those same motorcycles without ABS. In crashes of all severities, the frequency at which collision claims were filed was 19 percent lower for the ABS models.10,11
- 7 Are antilocks a new idea? Are they widely available?
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The idea of antilocks has been around for years. They first were used on airplanes in the 1950s. A rear-wheel system was developed for the 1969 Ford Thunderbird, and the 1971 Chrysler Imperial had four-wheel antilocks. Availability has grown steadily in more recent years. Antilocks were standard on 1985 S class Mercedes models and standard or optional on about 30 domestic and foreign car models during the 1987 model year. Availability soared to 90 models the next year. Currently, antilocks are on about 72 percent of all new cars sold and 94 percent of light trucks.12
- 8 Are antilocks required on big truck rigs?
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In March 1995, the National Highway Traffic Safety Administration issued a rule requiring antilock brakes for heavy trucks, tractors, trailers, and buses. All new truck tractors were required to have antilocks after March 1, 1997, and they were mandatory on new air-braked trailers and single-unit trucks and buses after March 1, 1998. New single-unit trucks and buses with hydraulic brakes had to be equipped with antilocks after March 1, 1999. This was not the first antilock standard for US trucks. A federal brake standard took effect in 1975, but its antilock and stopping distance requirements were suspended after litigation in 1978. Antilock brake systems have been required on all new trucks, buses, and trailers in Japan and the European Union for several years.
Antilocks are important for big trucks because of the poor braking capabilities of these vehicles compared with passenger cars. On dry roads, big trucks take much farther to stop — 47 percent farther in Institute tests. On wet and slippery roads, the stopping distance disparity is even worse. Tractor-trailer combinations also have the potential for loss of control and jackknifing on both dry and, especially, slippery roads. (Jackknifing occurs when the rear wheels of a tractor lock up, allowing the tractor to skid and spin so that it folds into the trailer. This also can happen when trailer wheels lock and cause the trailer to swing around the tractor.) Antilock brakes not only reduce stopping distances on wet and slippery roads but also help drivers maintain control.
The standard for tractors requires antilock control on the front axle and at least one rear axle. On at least one of the tractor axles, each wheel must be independently controlled by an antilock modulator. This ensures that a wheel provides shorter stopping distances and optimal braking force on all surfaces, especially on roads where one side is slipperier than the other. For semi-trailers, at least one axle must have antilocks. Full trailers must have antilocks for at least one front and one rear axle.
The real-world crash effects of antilocks on large trucks have not yet been established.
- 9 Are anti-jackknifing devices a substitute for truck antilock?
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No. Some devices marketed for trucks purportedly would prevent jackknifing in emergency braking situations. One device on the market mechanically limits the amount a trailer can swing around the pin that connects it to the tractor. This kind of device is less effective than antilock brakes. It does not prevent a truck's wheels from locking up, nor does it provide equivalent handling stability on wet or dry roads.
- References
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1Highway Loss Data Institute. 1994. Collision and property damage liability losses of passenger cars with and without antilock brakes. Insurance special report A-41. Arlington, VA.
2Highway Loss Data Institute. 1995. Three years' on-the-road experience with antilock brakes: an update. Insurance special report A-47. Arlington, VA.
3Farmer, C.M.; Lund, A.K.; Trempel, R.E.; and Braver, E.R. 1997. Fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 29:745-57.
4Farmer, C.M. 2001. New evidence concerning fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 33:361-69.
5Kahane, C.J. 1994. Preliminary evaluation of the effectiveness of antilock brake systems for passenger cars. Report no. DOT HS-808-206. Washington, DC: National Highway Traffic Safety Administration.
6Kahane, C.J. and Dang, J.N. 2009. The long-term effect of ABS for passenger cars and LTVs (DOT-HS-811-182). Washington, DC: National Highway Traffic Safety Administration.
7Evans, L. and Gerrish, P. 1996. Antilock brakes and risk of front and rear impact in two-vehicle crashes. Accident Analysis and Prevention 28:315-23.
8Padmanaban, J. and Lau, E. 1996. Accident experience of passenger vehicles with four-wheel antilock braking systems. Proceedings of the 40th Annual Conference of the Association for the Advancement of Automotive Medicine, 111-25. Des Plaines, IL: Association for the Advancement of Automotive Medicine.
9Williams, A.F. and Wells, J.K. 1994. Driver experience with antilock brake systems. Accident Analysis and Prevention 26:807-11.
10Teoh, E.R. 2008. Effectiveness of antilock braking systems in reducing fatal motorcycle crashes. Arlington, VA: Insurance Institute for Highway Safety.
11Highway Loss Data Institute. 2008. Motorcycle antilock braking system (ABS). Highway Loss Data Institute Bulletin, 25(1). Arlington, VA.
12Ward's. 2008. Automotive Yearbook 2008. Southfield, MI: Ward's Communications.