Heavy Duty: Vehicle Stability Systems

It is a nightmare come true. A fully loaded rig is rolling down a steep hill corner. It’s 3:30 in the morning. Suddenly lights sweep across the road from an oncoming car, swerving toward the middle of the road.

Instinctively, the driver quickly whips the wheel to the right, barely avoiding a collision as road side rumble strips shake the cab. But she is not out of danger. As she spins the wheel left to get back on the road, her fully-loaded trailer has begun to shift precariously, threatening to roll the truck on its side.

She tenses up, expecting the worst, but controls all around her have already sprung to action. Sensors monitoring the vehicle’s yaw, steering direction and lateral acceleration have sent instant commands to individual brakes, quickly creating opposite forces where needed to keep the truck’s heading from changing quickly enough to lose control. Her tanker full of propane is heading safely down the road again, and seemingly it is all over before it began.
Or is it?

These days, the difference between a close call and a deadly, multi-million dollar accident can be whether or not the truck has a vehicle stability system.

Accidents are the bane of a fleet’s existence. According to NHTSA, 368,000 large truck accidents killed 4,321 people and injured 77,000 in 2006. The financial damage from just a single incident can be staggering as well. According to FMCSA, a fleet must generate an extra $1,250,000 in revenue (assuming an average 2 percent profit) just to pay for a $25,000 accident.

Rollovers are among the worst because of the high probability of damage to the truck, cargo and any people involved. According to NHTSA, the proportion of vehicles that rolled over in fatal crashes was four times as high as the proportion in injury crashes and 16 times as high as the proportion in property-damage-only crashes. About half of the 700 heavy-duty truck drivers killed on the road each year die in rollover accidents, according to DOT.

The most effective way to reduce deaths from rollovers? According to NHTSA, the answer is preventing single-vehicle loss-of-control crashes, since the vehicle leaves the roadway, dramatically increasing rollover probability. This is where stability and rollover systems come in.

They automatically sense a loss of stability and can work actively to help prevent rollovers, using anti-lock braking system technologies like electronic controls and wheel motion sensors. In some cases, they automatically slow speeds by de-throttling the engine and applying individual brakes.

Purchasing vehicle stability systems can be expensive—especially for larger fleets—but the consensus among those in the industry is they are well worth the cost. Available in Europe for some time, they are still relatively new on this side of the pond, but already many fleets have seen positive returns on their investments.


James Husted of Findlay, OH-based Garner Trucking had rollover stability systems installed on 25 of his 85 2005 Freightliner Columbias a few years back after two drivers had rollovers within a week and half—thankfully while he was still spec’ing.

“I’ve been with the company 20 years and those were the second and third, so obviously that was something we wanted to look at,” Husted says.

What literally drove home the point for Husted was a test drive where he made sure the rubber met the road. Now, he’s a believer. It was an important move for his company, as he spent much of January spec’ing for 20 new trucks.

“From now on, that is part of our specs, no doubt,” Husted says. “We wouldn’t even consider ordering a new truck without one. And for the cost, it’s an absolute must.”

Small fleets don’t have much margin for error these says and equipment for two dozen trucks is expensive, so Husted says it was important to personally get a taste of what he was getting into.

“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.”


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.


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.”

The electronic stability control system effectively builds off both ABS and roll stability control, but as a full stability control system also addresses roll and directional instabilities, using additional sensors.

“When you’re going around a slippery curve and the vehicle starts to under-steer, the system will automatically intervene and apply individual foundation brakes to try to generate a counteractive force to kick the vehicle around so it’s going in a direction that the driver is pointing it,” Korn says.

As the vehicle is being driven, the turn angle sensor lets the system know where the driver is pointing the vehicle, the yaw-rate sensor and lateral accelerometer tell the system where the vehicle is going, and when there is a mismatch, the system will automatically intervene. During an under-steer, it will automatically activate one of the tractor drive axle brakes to create that counteractive force. During an over-steer, it will activate one of the steer active brakes, to try to correct the vehicle directionally.

The trailer-based Roll Stability Support system also helps minimize rollover tendencies, says Korn.

“It works very similar to the tractor system except when it senses a high level of lateral acceleration, it will only apply the trailer axle brakes,” Korn says. “The goal of that intervention is to slow the combination vehicle down to reduce lateral acceleration, which will reduce the rollover tendency.”

In December, Meritor introduced RSSplus, an updated system for trailers. It can be easily retrofitted, and according to the company, new diagnostics have made troubleshooting easier, and mechanics are likely to be more “hands on” with this system because they can install it themselves. The system will be available later in the first quarter.


Elyria, OH-based Bendix Commercial Vehicle Systems offers both roll and full stability systems. Electrics marketing manager Fred Andersky says while the rollover system can help prevent trucks from going around a turn too fast, the full system (ESP) adds a steer angle and yaw rate sensor to sense the spin on the axis of the vehicle, which helps out with loss of control-type situations.

Rollovers are really the secondary event when an accident occurs, he says, and the full stability system is better equipped to keep the truck headed in the right direction.

“Something happens that the driver has to make a maneuver that starts as a loss of control and then results in a rollover,” Andersky says. “The ESP system helps mitigate loss of control in those situations and goes a step further, and in slide-skid jackknife situations, this helps as well. A roll-only system doesn’t have that yaw rate sensor and steer angle sensor to figure out what’s going on, so it can’t help. With a full stability system, we’re braking the vehicle on all the axles, because the best way to get a vehicle under control is to slow it down as quick as possible, and the best way to slow it down is to cut the throttle and hit the brakes.”

The full system is also able to use specific brakes, rather than hitting them all at the same time.

“In a rollover type of scenario, you want to slow it down quick, so you hit all the brakes, and so the more brakes you’re hitting the better,” Andersky says. “But in a loss of control scenario, you may only want to hit the right front steer axle brake and the trailer brakes to help get the vehicle back in line and help the driver keep that control.”

Andersky said he has seen building interest in the full system from fleets in the last two years, especially when officials get to experience it first-hand.

“When you get them in the seat of the vehicle and they go through the maneuvers, that’s when they really see the value of it,” Andersky says.

Still, no system is fail-safe, he points out.

“It doesn’t replace a good driver or good driving practices,” he says, “And there is the basic physics—if a driver goes into a 25-mile per hour turn at 60 miles per hour, the system is going to intervene, but the laws of physics don’t get violated, and he’s going over.”


Kansas City, MO-based Haldex Commercial Vehicle Systems offers a Trailer Rollover Stability System for heavy-duty trucks. Haldex Brake Systems product manager Duane Stocksdale says having a system on a trailer is more important than on the tractor.

“If you’re going to get the best performance, you’re probably going to put it on both, (but) we believe the roll event begins at the trailer and if you look at videos of a vehicle going into a roll event, you always see the back wheels on the trailer start to lift off before you ever see anything in the tractor,” Stocksdale says. “We’ve had several demos and people sitting in the tractor are just amazed when they look in the mirror and actually see the trailer pivoting up on the outriggers and they aren’t feeling a thing in the tractor itself.” He says the system received a new tweak in January, providing added safety features.

“When you go into a roll event and the system automatically applies the brakes without driver intervention, (it) applies the brake lights so a vehicle behind the trailer can see there is a stopping event happening,” Stocksdale says.

The key to preventing rollovers is simple—reduce speed rapidly, Stocksdale says.

“Once you start to approach that roll threshold, you’ve got to get that vehicle slowed down and apply the brakes as quickly as you can and help him avoid that,” Stocksdale says.


Husted said he has not heard of a single maintenance issue with the systems on Garner Trucking’s 25 Columbias, which have remained accident-free since the install. He says the training benefits became immediately apparent, though, thanks to the information the units store for download during preventative maintenance.

“It will tell us every time the system has been activated (and) gives you two points,” Husted says. “One, the unit was approaching the point where the system needed to be activated and (two) how many times the systems had to physically intervene. And obviously if we have reading where it had to intervene, we bring the driver in and have a talk with him.”

Being able to sit down with a driver with data “in hand” has proven to be a very proactive and effective way to reinforce safety.

“There are times where the driver is actually unaware that anything had occurred,” Husted says. “Even though the vehicle decelerated, they have a tendency to believe they got the unit back under control themselves. Where when we see that the system intervened, we show him, ‘Look, this happened, do you recall this?’ I have never had a driver come in to me and admit to knowing the system saved his butt—I think that’s just human nature—but now they are aware we have seen this, we are monitoring this and they need to be more cautious.”

According to FMSCA, fleet safety managers point to inadequate training and experience as one of four basic factors that cause rollovers, along with excessive speed, driver inattention, driver fatigue. Korn says since many fleets want to know how their vehicles are being driven, the information these systems can provide is critical.

“A lot of fleets have found that to be very effective, because this increases the safety margin,” Korn says.” The best situation would be if they never intervene, because that way the vehicle is being driven very conservatively.”


As one fleet boss at a recent SAE technical session on stability control systems put it, “If you can get ‘em, get ‘em.” There is a growing variety of systems out there, and the vast majority opinion of industry professional is they provide fleets a good, fairly quick return in investment and serve as good training tool to boot. If you are one of the fleets still sitting on the sideline because you can’t afford them or have a spotless safety record, a startling revelation may just be waiting around the next corner.


If your techs are adept at handling ABS and electrical issues, learning to install, troubleshoot and maintain roll stability systems should be an easy transition. For instance, Haldex Brake Systems product manager Duane Stocksdale says there are only two differences between the Haldex system and a typical ABS.

“One is an extra air line going to the suspension air bags—so you’re monitoring the leveling valve, so the maintenance of that is the same as with any other air line,” Stocksdale says. “And there’s an internal accelerometer, and the only thing you’re going to get if it fails is a diagnostic code, so there’s really no maintenance. It installs pretty much the same; has to have a little bit more rigid surface to install because of lateral accelerometer performance, but as far as air line hookups and things like that, it’s pretty simple.”

Bendix Electrics marketing manager Fred Andersky said maintenance of their systems is also basic.

“We’re using the same wheel speed sensors, the modulators, traction control valve—so the components you would be repairing for your ABS system are the same components used in the ESP,” Andersky says.

Technicians will need to recalibrate the steer angle sensor any time they do a front-end alignment, though, and the yaw sensor must also be recalibrated if moved.

“You cannot re-position that yaw rate sensor,” Andersky says. ”Not so much an issue on tractors, but on construction vehicles, a body builder will get the chassis in and put on their mixer body and they say, ‘Hey, what’s this black box doing here in the way of my plumbing?’ so they move it, and then can’t understand why they are getting these brake interventions. Do not touch, do not move, and there are labels around it.”