The way trailers are specified and maintained can be critical factors in preventing one of the deadliest kinds of truck accident: rollovers. During a presentation at the fall meeting of The Maintenance Council of the American Trucking Associations recently, Chris Winkler, chief research scientist for the University of Michigan's Transportation Research Institute, emphasized how dangerous rollovers are to tractor-trailer fleets and drivers.
According to Winkler, more than 15,000 rollovers of commercial trucks occur annually in the United States, or one every 10 million miles. Some 9,400 of those rollovers involve tractor-trailers - about one every four million miles - which is why over-the-road fleets must pay closer attention to their trailers.
"While 4% of all truck accidents involve rollovers, they are responsible for more than 12% of all truck fatalities," he said. "Nearly 60% of truck driver fatalities are a result of rollover."
Winkler said that truck trailers naturally possess a high center of gravity, making them very susceptible to "untripped" rollovers, meaning that the vehicle does not have to hit an object in order to flip over. While less than 4% of car rollovers are untripped, nearly 25% of tractor-trailer rollovers are.
Many factors can add up to make a trailer more susceptible to rollover, which is why fleets must pay closer attention to how they spec and maintain their equipment, Winkler said.
Typically, a trailer reaches its rollover threshold at about 0.45 times the force of gravity, or G. If a trailer's tires are underinflated, the threshold is lowered to 0.40 G. If the suspension is soft, the threshold drops to 0.35. Add in too much give from the fifthwheel, springs, suspension mismatch, and trailer chassis, and the level drops to 0.30. Finally, shifting or off-center cargo can peg the rollover threshold at 0.25 G - giving truck drivers a far smaller margin for error, Winkler said.
By contrast, cars, and even SUVs, have a higher rollover threshold: 1.0 to 1.4 G for cars and 0.8 and 1.2 G for SUVs.
To lessen the chance of rollovers, Winkler and the panel urged fleets to investigate using lower ride heights, widening axles from 96' to 102' lateral track rides to eliminate lateral movement, and remembering the important role tires play with a trailer's suspension system.