Wire for safety

John Baxter | July 01, 2010

A new generation of electronic safety controls can prevent accidents and reduce liability concerns.

A federal study shows savings from using rollover stability systems throughout the industry would save more than $1 billion a year.

Like the best NASCAR drivers, with their tightly honed skills, fast reflexes, and years of practice, even top truckers can quickly get into a difficult steering situation. Two major brake component makers have introduced systems that can prevent a rig from rolling over or losing control when approaching a jackknife. These systems use many of the components that make ABS successful, and tap into the know-how industry engineers have developed around tractor-trailer dynamics.


Rollover and full stability systems

There are the two basic system types, says Mark Melletat, director of trailer systems and fleet operations at Meritor Wabco. One is roll stability control. The other, full stability control, combines RSC with other technology to provide broader and more powerful corrective actions.

RSC “provides the highest value of vehicle rollover stability with the fewest components and maintenance requirements and the lowest cost,” Melletat says. “It continually monitors conditions that can lead to a rollover and can automatically de-throttle the engine and apply the engine brake, as well as the drive and trailer axle foundation brakes, to reduce tractor-trailer speed when lateral acceleration limits are about to be exceeded.”

For example, if a rig will roll over in a curve at 40 mph, but the driver is doing 46 mph, the system will slow the rig to below 40 to reduce centrifugal force.

Meritor Wabco calls its full stability control system Electronic Stability Control. It combines RSC with the added capability of yaw or rotational control. RSC includes a sensor that can tell when the tractor is turning around on its own axis, like it does when starting to jackknife.

“ESC can reduce the risk of vehicle instability while in a slippery curve or taking an evasive action, preventing jackknife and drift-out conditions through selective braking of the tractor and application of the trailer brakes,” Melletat says. “If loss of stability is detected, tractor-trailer speed is reduced through engine control and application of the engine brake, tractor and trailer foundation brakes.”

Fred Andersky, director of marketing at Bendix, describes road conditions that could activate Bendix’s comparable technology, Electronic Stability Program. “Let’s say a car cuts in front of a tractor-trailer, and the driver swerves abruptly,” he says. “This starts a chain of events leading to rollover. The system at first mitigates the situation, preventing loss of control, and prevents rollover.”

Andersky says a full stability system gives a larger margin for correcting driver error. RSC could prevent a rollover if the driver is within 7 mph of the maximum safe speed. “With full stability control, the driver could be as much as 14 mph beyond the rollover point, and the system will still be likely to be able to keep the vehicle upright,” he says. “Also, full stability helps on a slick surface when a rollover won’t occur, but a jackknife might. It will perform loss of control mitigation when a roll stability system will not react.”

Andersky says the system incorporates ABS wheel speed sensors plus a lateral acceleration sensor that measures force being generated on a curve. For full stability control, a steer axle sensor is added, as well as a yaw rate sensor. The steer axle sensor tells the system how far the steering wheel is from the center position and which way the driver is steering. The yaw rate sensor works like a gyroscope. It detects how fast the tractor is turning on its axis. That’s how the system knows if the driver is headed toward a jackknife.

“Both systems are tractor-based, though they also control the trailer brakes,” Andersky adds. “Roll only cuts throttle, and then applies the brakes on the drive and trailer axles. ESC also applies the brakes on the steer axle, which is the best way to slow as fast as possible.”

But the ability to control the steer axle brakes also allows ESC to do what even the most skilled truck or racecar driver can’t – apply the brakes selectively to straighten out the vehicle. Andersky says that in a loss of control, not just a rollover, “full stability can control which brakes apply and when. Suppose the tractor is jackknifing to the left. The system will just apply the right/front steer axle brake and the trailer brakes. It will slow the vehicle, but also redirect it to where the driver wants to go.”

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