Right side of the road

According to a study by the Washington, D.C.-based National Center for Statistics and Analysis, 58 per cent of traffic fatalities involve some type of run-off-the-road or rollover accident situation. Developers of braking systems and other safety-related components have been working on ways to control the stability of a truck in just such an event, with most of the research centring around electronically controlled braking systems (ECBS). Some of these products already are available as options on European heavy trucks. But in North America, anti-lock braking systems are the norm.

What’s in store for trucks here?

One answer comes from Bendix, which is demonstrating stability control systems that offer very nearly all the benefits of the ECBS-based products but without the need to abandon the conventional ABS air brakes found on North American heavy-duty vehicles. By adding an Automatic Traction Control feature and a few additional sensors and valves, the Bendix ABS/ATC is the basis for a stability system that will intervene in run-off-the-road and rollover accident situations. Bendix calls the system its Electronic Stability Program (ESP), and says we could see its technology on North American trucks by the end of the year.

Bendix is not unique in applying rollover warning and intervention for heavy-truck braking systems. Indeed, MeritorWABCO debuted tractor and trailer systems in 2002, also using the ABS platform. Like those products, the Bendix technology is based on the ABS sensors and controllers, but with the addition of lateral accelerometers and mass- and stability-calculation algorithms to the controller. The system measures the lateral acceleration on the vehicle, knows the wheel speed, and can calculate the mass from the torque-demand and the vehicle acceleration. It then can predict the height of the centre of gravity, compare it with thresholds built in, and, should there be a likelihood of a rollover, the system will intervene. Using the ABS brake modulators to reduce the speed of the truck, it can reduce speed and prevent a rollover (within the limits of physics).

According to Bendix engineers, this is only half the solution that’s possible using the vehicle ABS platform.

Adding a yaw sensor to the electronics package together with input from the steering angle lets the system look at the direction the tractor is heading and compare it with the driver’s input. If it shows a discrepancy, then selective braking of the front wheels and braking on the trailer axles will correct the potential accident situation.

Again, this will occur only up to a certain level, and the vehicle system’s ability to correct for a driver will still be limited by the physical limits on the truck. However, intervention by the system gives a major warning to the driver to correct his driving behaviour.

Moreover, if a driver gets into a situation where the system has to intervene, the vehicle slows to a speed less than the optimum, thereby penalizing the driver and providing an incentive to modify behaviour.

To demonstrate both its stability control system and the effectiveness of its ADB 225 air-disc brake, Bendix put together a travelling roadshow which the trucking press was invited to attend. On hand was a Kenworth T2000 with ADBs and a test flatbed carrying a representative-height test load. Most importantly, it had outriggers–training wheels–to restrain the trailer.

For the stability demonstration, two courses were set up: a so-called J-turn, a straight line leading into a reducing radius left turn; and a lane-change avoidance manoeuvre where the truck aggressively avoids an in-lane obstacle then quickly returns to its lane. This double lane change results in a crack-the-whip on the trailer that can very easily cause a rollover.

With the system inactive, the truck very quickly becomes unstable, the trailer picks up its inside wheels, and-were it not for the outriggers-would roll over. With the system enabled, the brakes come on with ABS pulsing and the trailer wheels stay firmly on the ground. The difference with Bendix ESP was the pulsing of the front wheels, which could clearly be identified by the excursions of the front axle as the brakes applied and released.

The J-turn was not especially daunting, though the first run through at rollover speeds brought me the usual sweaty-palmed response going into the curve. Once the ESP intervened, though, I found subsequent runs through the turn were easily completed, keeping a tight line on the cones and letting the brakes safely reduce speed while steer-axle braking kept the Kenworth’s nose tucked in to the line I wanted.

The lane change was an eye-opener, first because we seemed to be approaching the obstacle at a speed much too fast to even get around it and still keep within the cones (even without worrying about a rollover incident). As the outfit started to get out of shape, braking pulled down the speed.

But it was the steering control that was the most impressive. Regardless of what was happening at the rear of the trailer, the tractor kept pointing in precisely the direction I wanted it to go and we cleared the obstacle without touching any of the cones. Sure, I could feel the front end pushing, but the selective braking kept it reigned in tightly.

In both manoeuvres, the braking reduced the road speed to well below optimal and we were thankful to have the Meritor FreedomLine automated transmission to pick up the right gear to get back up to speed. Any driver with a manual shift would be suitably chagrined to have to dig around in the transmission to find a gear to get going again.

Was the lane change the most impressive manoeuvre of the day? Most certainly. But a close second was taking a fully air-disc-braked tractor-trailer around the circuit to build up to a speed of 68 mph into the braking area. It was a simple foot on the clutch, slam-on-the-brakes ABS stop. Except the truck stopped like a sports car.

Moreover, second time around, I did it taking my hands off the wheel and it stopped in a dead straight line, in-lane, no trouble at all.When a heavy commercial vehicle suffers a rollover accident, the circumstances often involve excessive speed–one reason why brake manufacturers are at the centre of rollover-prevention research. There are other common factors: rollovers usually happen on curves (more frequently in a right-hand turn), on dry pavement, and with partially or fully loaded trailers–virtually never with empty trailers.

As an accident reconstructionist in British Columbia, RCMP Sgt. Al Segstro’s job isn’t just to learn why a truck rolls, but how. Segstro can reconstruct the scene based on road marks and other evidence left behind.

There’ll be a series of tire marks, he says, as the trailer’s load shifts onto the outside tires and sends them into a radically over-deflected shape. The black marks on the highway aren’t rubber; they’re road tar, superheated by the friction from the overstressed tires.
The trailer–or the rearmost trailer in a train–will be the first to roll because it’s being forced into a smaller-radius turn. The rearmost trailer axle can be leaning at 90 degrees before the tractor even starts to roll. The trailer will slide for a bit, its edge leaving marks where the metal scraped the road, from whose length investigators can calculate at least the minimum speed at which the rig was going. They can also establish the radius of the arc by comparing those initial tire marks to the position of the scrapes from the sliding metal.

The insidious thing about rollovers, Segstro notes, is that so much happens before the driver feels a thing. All is right with the world one moment. The next, the world is upside down.