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What Is ASR on Cars?

by Rob Wagner

Anti-Slip Regulation, or ASR, is a secondary safety feature that works with the Anti-Lock Braking System, or ABS, on cars, trucks and sport utility vehicles. Also called Traction Control System, or TCS, the ASR helps prevent the vehicle’s wheels from losing traction by using an electro-hydraulic system that controls the engine and brakes in adverse road conditions or if the driver uses excessive acceleration and the wheels slip on the pavement.

Origins

ASR is common in most cars and motorcycles since about 1992. It can trace its history to the early 1930s when Porsche developed the limited slip differential that allows one wheel to spin slightly faster than the others to improve traction. ASR is closely related to ABS, which originated in aircraft landing gear to shorten landing distances. ABS first appeared in 1966 in the British-made, four-wheel drive Jensen FF Grand Tourer. An early user of ASR, which in effect complements ABS, was BMW in 1979. Another early user to employ ASR was the Saab 9000 in the late 1980s. By 1992, the Chevrolet Corvette began using ASR. ASR has since evolved to feature systems that may retard ignition timing, cut fuel flow or reduce the number of engine cylinders operating. Other systems apply brakes to an individual wheel instead of manipulating engine power. Contemporary versions of ASR combine both braking and engine power to supply traction when necessary.

Purpose

ASR helps correct driver error in adverse road conditions. Drivers may use excessive acceleration when road conditions are poor, causing the wheels to slip and the vehicle to fishtail or hydroplane on wet pavement. It may cause tractor-trailer rigs to jackknife, particularly on steep grades and when carrying lighter loads. ASR helps the driver maintain control of the vehicle. ASR stops the spinning wheel by maintaining tractive power and limiting acceleration. Most vehicles have an indicator light to warn the driver of slippery conditions.

Novice Driver’s Friend

ASR is a safety device. Although professional drivers complain ASR compromises performance, it’s standard equipment in high-performance vehicles because the throttle is very sensitive. Likewise, novice or casual drivers often overestimate their ability to control a vehicle in adverse weather conditions by reasserting driver control in unanticipated circumstances.

Function

An engine control unit (ECU) monitors the wheels’ rotation after the ignition is turned on and after the vehicle begins to move. The ECU monitors and compares the acceleration and speed of the powered wheels to the non-powered wheels. The ECU activates the ASR when the wheels’ rotation exceeds the slip threshold. The ASR activates the differential brake valve to control the brake cylinder, and the engine’s driving torque is applied to the braked wheel. This causes the propulsion power on non-powered wheels to increase. ASR then shifts from differential brake control to engine control to reduce engine power. In some systems, ASR will retard the ignition or reduce fuel flow to specific cylinders to reduce power at speeds above 50 mph.

About the Author

Rob Wagner is a journalist with over 35 years experience reporting and editing for newspapers and magazines. His experience ranges from legal affairs reporting to covering the Middle East. He served stints as a newspaper and magazine editor in Saudi Arabia and the United Arab Emirates. Wagner attended California State University, Los Angeles, and has a degree in journalism.

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