Heavy Duty: Vehicle Stability Systems

Stability and rollover systems: Can your fleet afford NOT to have these?


“We have gone for some new technology and sometimes it’s worked out great and sometimes it’s kind of burned us,” Husted says. “We have a tendency to kind of wait and see and let other people test it out.”

Husted says the ROI was definitely the cherry on top.

“The cost was so negligible, it was kind of a no-brainer,” he says. “With these two accidents, just the on-site clean-up bills were over $10,000, not including repairing damage to vehicles, freight and everything else, so it’s very easy to justify. Those two rollovers more than paid for those systems on those 25 trucks.”

STABILITY SYSTEMS

Roll stability and electric stability are the main two systems and are known variously as Roll Stability Advisors (RSA) or Roll Stability Control (RSC) systems, and Electronic Stability Control (ESC) or Electronic Stability Programs (ESP). All provide varying degrees of alerts and actions during a variety of loss of control situations.

Roll Stability Advisors are passive systems that do not provide an immediate warning of an impending rollover, but send a message within seconds after an event for future training. Sensors monitor lateral force information and determine when to issue audible/visible alerts. Length and message of the wording and the length of the audible alert depend on the risk and the system’s recommendation to reduce speed.

Roll Stability Control systems actively intervene if they detect a high rollover risk due to excessive speed in a curve, reducing throttle and applying engine and foundation brakes. Generally integrated with an anti-lock braking system, some are integrated with electronically controlled braking systems.

Electronic Stability Control and Electric Stability Programs actively intervene during a high risk of a rollover or yaw instability. The electronic control unit continuously compares the vehicle’s movement to performance models, using input from the wheel speed sensors as well as lateral, yaw and steering angle sensors. These systems can brake individual wheels to prevent spinning or plowing out and can be integrated with electronically controlled or anti-lock braking systems to help keep the truck under control during extreme maneuvers by monitoring natural steering reaction.

If the truck shows a tendency to leave an appropriate travel path or if critical threshold values are approached, the system will intervene. If it detects a potential rollover risk, it reduces throttle and applies proper brake pressure to slow the vehicle. If it detects an over-steer or under-steer, it reduces throttle and applies the appropriate individual brakes to provide counter-force. During an over-steer, it applies the outside front brake; during an under-steer the inside brake.

Both types of systems are generally installed by OEMs, but some may be installed as an aftermarket accessory. Costs vary greatly, depending on a system’s capability.

MERITOR/WABCO

Troy, MI-based Meritor/WABCO offers three stability control systems; two for tractors and trucks and one for trailers—all based on ABS. All three primarily address excessive levels of lateral acceleration caused by excessive speed.

“You don’t need to reinvent the wheel,” says chief engineer Allan Korn. “It’s just more or less an upgrade.”

The roll stability system (released in 2003, with a few minor software changes since) measures lateral acceleration, and based on the mass of the vehicle and a few other parameters sets a threshold that will turn on the system when exceeded, says Korn.

“(It) will de-throttle the vehicle, send a command to the engine to limit torque, apply the engine brake and then the drive and trailer axle brakes,” Korn says. “The whole goal is to rapidly decelerate the vehicle, because that’s how you will reduce lateral acceleration, while maintaining proper control for the driver. So if you are going too fast around a curve, this system measures lateral acceleration and when it exceeds the threshold it automatically takes action to decelerate the vehicle so lateral acceleration is reduced and thus the rollover tendency is reduced.”

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