Volvo's Bendix EST Keeps The Shiny Side Up
Tom Berg
Senior Equipment Editor
With EST turned on, the rig stays on its 18 wheels because EST cuts engine power and applies the brakes – complete with anti-lock action.
Most truck builders offer stability control devices that can help keep a truck from rolling over in sharp turns and sudden maneuvers. These products use the electronics in anti-lock braking systems to slow trucks down. That's something drivers might want to do if they realize trouble's afoot, but seldom can because the rollover's happening before they realize it.
Makers of stability enhancement systems, such us ArvinMeritor, Bendix and Bosch, have been showing off their products for several years, but only recently has one of them allowed reporters with CDLs to drive a truck equipped with it. That's Bendix, which supplies a system to Volvo Trucks. Together they hosted a demonstration in Dallas during the recent Great American Trucking Show.
Volvo calls its optional Bendix system Electronic Stability Technology, or EST. It uses the existing ABS wheel sensors and adds sensors at the steering wheel and on the chassis to determine when road speed and a directional change is about to cause a rollover. EST reacts instantly by cutting engine power and applying the brakes on the tractor and trailer. It works – at least within limits – to keep drivers out of trouble.
The demo rig was a Volvo VT880 pulling a tank trailer loaded with water to bring gross combination weight to 78,000 pounds, according to Charlie Ross, a Bendix technician who acted as a driver instructor. He showed me how to go through the course, how a rollover begins without the driver realizing it, and what EST does to compensate.
He first turned EST off to show how a rollover starts. You'd think you'd get a seat-of-the-pants warning as the trailer begins to lift its outside wheels in a hard turn, but you don't. It's past the point of no return before you might even see it in the mirrors, Ross said. These are forceful maneuvers and as a passenger I had to hang on. Wheeled outriggers on the demo trailer kept it from going on its side, and we could recover to try it again.
Ross did this twice – in a decreasing radius turn and then in a lane-change maneuver. The turn is typical of a curving offramp where so many rigs roll. In fact, I had seen one the previous Sunday at Interstates 71 and 270 on the north side of Columbus, Ohio, where a tractor-trailer lay on its left side. Some of its boxed cargo, having plunged through the van's roof, was scattered onto the shoulder and grass. Police had shut down the ramp and it must have been embarrassing to the driver, if he lived through it. Quite often, rollovers kill drivers, according to accident statistics.
The lane-change maneuver is what you do to avoid a motorist or pedestrian who has blundered into your way.
"The lane change doesn't get you," Ross said, noting that the trailer follows the tractor rather well through the first quick turn, "but the recovery does." When trying to cut back into the original lane, the trailer's mass changes direction again, but with greater force, and can start a rollover.
In both situations with EST turned off, the tanker trailer would've rolled onto its side were it not for the outside outrigger, whose tire forcefully contacted the asphalt as the rig's regular tires howled. Tires don't last long during such demonstrations, Bendix and Volvo people said.
Then Ross switched on EST and we went through the turns again at the same 30 to 40 mph speeds. This time the system sensed what was happening and slowed the vehicle enough to avoid the rollovers. While standing outside during previous runs, I could hear the ABS cycling the brakes and watch the rig slow down. It did not photograph well because it was less obvious visually.
Driving it was more impressive. I got us up to the proper speeds and at Ross' direction, kept my foot on the accelerator – something that goes against a driver's instincts in such a situation – as I went into the cone-marked turns. EST realized I was going too fast and cut engine power, then applied the brakes, all in less than a second. By the time I entered the dangerous part of each maneuver, our speed was slow enough for me to complete the turns with no more trouble.
The Bendix system applies each brake individually, based on the situation, Ross explained. The outside brakes work harder because centrifugal force has put more weight on those wheels and they'd be able to apply more braking power. When the rig began stabilizing, it eased off the brakes and let me reapply power. By then the engine was lugging and I could start thinking about downshifting. It all happened so fast that I couldn't feel exactly what EST was doing – but I knew it had slowed us down and kept the rig's shiny side up.
"Now, this won't prevent all rollovers," Ross said. "If we were going 50 or 60 mph, we'd probably roll over anyway. But the system does all it can to help the driver."
