Volvo's Bendix EST Keeps the Business Side Up

By Tom Berg, Truck Editor | September 28, 2010

Volvo VT880
With EST turned off, the trailer begins rolling over in a tight turn, and would, pulling the tractor with it, if an outrigger didn't keep it upright.
Volvo VT880
With EST on, engine power is cut and brakes apply automatically, slowing the rig before it can roll over.

Have you ever seen a big rig rolled over on its side? Ever owned a truck that did roll? It's a mess; it costs a lot of money; and there's a good chance that the driver was badly hurt or killed in the wreck.

Such accidents typically occur on sharply curving freeway off-ramps and during sudden emergency maneuvers. They happen because drivers are going too fast or get trucks into situations that make them unstable. To compensate for such tendencies, most truck builders now offer "stability control." These products use the electronics in now-standard anti-lock braking systems to slow down the vehicle — something drivers might do if they realized trouble's afoot, but don't because the rollover's happening before they realize it.

Makers of stability-enhancement systems like 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 rig 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 are 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 engineer 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 inside 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, such as in an off-ramp, and in a lane-change maneuver, which you might do to avoid a motorist or pedestrian who's 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 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 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, but it was not visually exciting.

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, he 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 business 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."

Volvo says EST is available on trucks as well as tractors. If it prevents one rollover, the savings would pay for ordering it on an entire fleet.