The total bill for a tractor-trailer rollover can cost a half million dollars, according to one fleet maintenance manager. A roll stability system, which senses when a rig is taking a curve too fast, then cuts the throttle and applies selective braking, can cost under a thousand dollars.

A roll stability system consists of the truck’s stock ABS and two main additional components: a lateral accelerometer measures a rig’s sideways acceleration in a turn (think of it as the rig’s desire to continue going straight instead of taking the curve), which is what, in excess, makes it roll over. A special Electronic Control Unit (ECU) with software that takes that lateral acceleration data and translates it into braking and throttle commands, replaces the stock ABS ECU. If the ECU senses that the lateral acceleration is high enough that the rig could roll over, it cuts the throttle and puts on the brakes.

A driver and a roll stability system react differently on a curve. First, the lateral accelerometer and ECU detect sooner than the driver that the rig is approaching the point where it could roll over. Second the ECU commands the brakes on selected wheels to brake, rather than braking all wheels simultaneously.

Meritor WABCO Vehicle Control Systems sells two roll stability products: one for the tractor (but which will also lightly apply trailer brakes for additional retardation), called Roll Stability Control (RSC) and a stand-alone trailer system called Roll Stability Support (RSS).

RSC is an optional feature of the Meritor WABCO anti-lock braking/automatic traction control system and has been available from Freightliner on new vehicles since January, 2003.

RSS is an integrated feature of the next-generation Meritor WABCO trailer anti-locking trailer systems. It became commercially available on all trailer OEMs in January 2004.

RSC and RSS rely on a conservative model for the tractor and trailer centre of gravity (CoG) which reflects the fact that the higher and heavier the load, the more easily it will roll. This model, combined with a given lateral acceleration, gives a theoretical roll threshold, or point at which the tractor or trailer will most certainly roll.

A typical event RSC can handle occurs on an exit ramp, explains Rick Romer, director of electronics products with Meritor WABCO. “Lateral acceleration is sensed by an accelerometer and the rollover threshold is calculated. If the lateral acceleration [causes the tractor and trailer to] reach that threshold, then the first thing to occur is that engine power is decreased. If the vehicle is engine retarder-equipped, the datalink will send out a command to turn it on. If needed, the brakes are applied, but if we have to do it quickly, these steps are sequenced, although the driver will feel them all occur at the same time.”

In April 2004 Challenger Motor Freight ordered RSC units as production systems on Freightliner tractors for trials; RSC and RSS are not available as after-market add-ons. “If you are bobtailing and go into a corner too fast, you lose throttle control because the roll stability system will not allow you to go any faster. If you give the steering wheel a tug the jakes cycle on,” explains Challenger maintenance manager Wayne Scott. “I have had no reports back on complications or driver feedback that [the RSC is] causing any problems. I road tested it, checked it. As far as going forward goes, I am going to make it a standard spec if I can get it OE-installed.”

RSS, which also uses a lateral accelerometer, measures lateral acceleration and compares wheel speeds on either side of the trailer. “If the [rollover] threshold is being approached, the check brake applications will be made to see if the RSS system [should] fully intervene with more aggressive braking. The RSS algorithm allows itself to learn, according to Bob Sibley, director of trailer products for Meritor WABCO. “When it senses what it thinks is an unstable condition it makes check brake applications.” If the results of the check brake indicate there was actually no instability, the RSS algorithm raises the threshold at which the check brake applications will be made, thus eliminating unnecessary braking.

If a rollover was not imminent, RSS discontinues the check braking application and takes no further action. But if RSS senses wheels off the ground, the RSS will intervene, making a more aggressive brake application relative to the trailer load. “More brake pressure is put on the wheels that are on the ground so that when the trailer stability is restored and the wheels come back down the tires will not be damaged,” Sibley explains.

The data book list price for RSC, including traction control, is about US $650-700. RSS is priced variously across OEMs, but is three and a half to four times the cost of a comparable Trailer ABS system.

Another manufacturer, Bendix Commercial Vehicle Systems, offers roll stability as a feature of its ABS-6 Advanced braking platform. ABS-6 Advanced copes with two truck control problems: rollovers in turns and the tractor-trailer jackknifing and skidding that occurs on low-friction surfaces. ABS-6 Advanced is installed in the tractor, but controls both the tractor and any trailer it hooks up to.

ABS-6 Advanced has yaw control capability – collectively what Bendix calls the Electronic Stability Program (ESP) – which selectively applies tractor and/or trailer brakes to help keep the vehicle under control on slippery surfaces. Since ESP can sense the steering wheel angle, it may, for example, apply left steer wheel braking in a left-hand turn to help the tractor turn.

If ESP senses that a rig might jackknife, say on a right-hand curve, data from its yaw rate detector (yaw is the tractor rotation that occurs when, for example, the trailer pushes it into a jackknife) tells it to apply braking to the left steer wheel and the wheels on the two rear trailer axles.

“Trucks are less susceptible to rollover in winter, but need a different solution for slick, icy pavement. ESP helps in rollovers and maneuverability in steering situations,” says Paul Waszkowski, Bendix product manager for trailers.

The ABS-6 Advanced Roll Stability Program (RSP) functionality, which is part of the ESP, reduces vehicle speed, and helps prevent rollovers. Its lateral accelerometer can tell if a truck is going straight or is turning. “With ESP, before trouble is even sensed in the vehicle, it can tell if you are turning too fast,” says Waszkowski. “The ESP can, because it reacts to the steer wheels, which enter a curve first, react a full second before a trailer system can react.

“The Bendix Trailer Roll Stability Program (TRSP) is strictly based on the lateral accelerometer. When it verifies that a rollover condition may be occurring [that is to say “believes” this, based on how the TRSP is set up], the system will apply a small burst of air as a test braking, to the wheels that may be lifting. The driver won’t sense this and the truck is still under power. If those wheels do not lock because of that burst of air, the system will take no [additional] action. The system learns that, for example, X level of acceleration with this vehicle will not cause a rollover, and adapts. If the wheels do lock, the unit will jam the brakes on the non-lift side to slow the vehicle and bring it back under control. The unit brakes more than it needs to in order to send a message to the driver to drive more safely.”

ABS-6 Advanced also has a learning procedure where, by measuring the torque the driver is asking for to accelerate, it can infer the weight of the trailer. If it senses that a trailer is empty it will know that a certain amount of correction is required to prevent a rollover. If it senses that the trailer is heavier, it reacts more aggressively.

To help fleets determine whether they should purchase ABS-6 Advanced, Bendix has a questionaire fleets can fill out on their own to evaluate their accident history. Based on the types of accidents they have had, they can decide whether to buy a non-Bendix RSP or Bendix’s full ESP system.

Should a fleet buy a roll stability system for the tractor-only, trailer-only or for both? A dual system is reportedly the ideal, but since roll stability products are currently only available as factory options, equipment life cycle is a consideration. Challenger, although interested in the trailer product, notes that it is easier to make a tractor system a standard spec, since tractors are on a four-year purchase cycle, versus ten years for trailers.

On the other hand, fleets who want roll control, but which use many owner-operators, can best control their assets by installing independent trailer roll control systems that work with any tractor.