Acceleration Slip Regulation

Acceleration Slip Regulation, almost always shortened to ASR, is a form of traction control that stops a vehicle's driven wheels from spinning uselessly when more engine torque is applied than the tyres can transmit to the road. The abbreviation comes from the German Antriebsschlupfregelung, literally drive-slip control, and reflects the system's origin with German manufacturers and suppliers. Although ASR is in effect a brand name, it has become a widely recognised label for traction control in general.

The problem ASR addresses arises whenever a driver demands strong acceleration on a surface with limited grip, such as wet, icy, loose or split-friction ground. If the torque at a driven wheel exceeds the available traction, the wheel breaks away and spins; this wastes power, can swing the rear or pull the front off line, and on a corner exit can provoke a slide. ASR detects this incipient wheelspin and intervenes to keep the tyres operating within their grip limit.

It does so using the same wheel-speed sensors that the anti-lock braking system relies on. By comparing the rotational speeds of the driven wheels against the undriven wheels or vehicle speed, the control unit recognises when a driven wheel is turning too fast relative to the road. It then reduces the slip by two complementary means: it cuts engine torque, by easing the throttle electronically, retarding ignition or trimming fuelling, and it can apply the brake to the individual spinning wheel. Braking one wheel of a driven pair also acts like a limited-slip differential, transferring torque to the wheel with more grip.

For the driver, the benefit is more secure and confident acceleration, particularly on slippery surfaces and when pulling away or overtaking. The car puts its power down cleanly instead of squandering it in wheelspin, directional stability under power is preserved, and the burden of delicately metering the throttle on a low-grip surface is largely removed. ASR also reduces unnecessary tyre wear from spinning.

There are nuances worth understanding. Because braking and cutting torque both shed forward thrust, ASR can blunt acceleration when maximum drive is wanted, which is why many vehicles allow it to be partly or fully switched off, for example to rock free of deep snow or mud, or for track use. The system also depends on consistent tyre sizes and properly functioning wheel-speed sensors, and like all such aids it cannot create grip that the tyres and surface do not provide.

ASR is best seen as one of the foundation stones of modern chassis electronics. It is functionally a traction-control system, it builds directly on the hardware and logic of anti-lock braking, and together with steering and yaw sensing it forms part of electronic stability control, which extends the same principle of selective braking and torque control to keep the whole car on its intended path. It is complemented by engine drag-torque control, known as MSR, which conversely prevents the driven wheels from locking when the engine brakes too sharply on a slippery surface.