Spec'ing tank trailers for roll stability
Feb 1, 2012 12:00 PM, By Rick Weber
ROLLOVERS are among the most serious crashes of cargo tank motor vehicles carrying hazardous materials and are more likely to be fatal to the driver of the vehicle than other crashes, according to the Federal Motor Carrier Safety Administration's (FMCSA) Cargo Tank Roll Stability Study published in 2007.
Cargo tank trucks account for only 15% of all fatal crashes involving heavy trucks, yet cargo-tank rollovers account for 31% of the heavy-vehicle rollover fatal crashes, the report concluded, adding that improving the rollover performance of cargo tank motor vehicles offers significant benefits to society in terms of lives saved, environmental damage avoided, and improved traffic flow.
“You can almost use (the report) as a textbook,” said Duane Plumski, product engineering supervisor for Polar Tank Trailer LLC. “I'm not saying you need to read the whole 300 pages, but it is a good resource.”
Plumski gave a condensed version in “Spec'ing a Trailer For Roll Stability.” He delivered the presentation during the 2011 Cargo Tank Maintenance Seminar October 24-26, 2011 in Louisville, Kentucky.
Explaining the physics of a rollover, he said a side force creates a moment arm between the center of gravity and the outside wheel. It could result from a point load like a guard rail, acceleration in a sharp turn, or uneven ground.
He said most tank trailers sold still have a 96-inch overall width, which means a 71.5-inch axle track. Most states allow 102-inch-wide vehicles on highways and roads, which would make possible a 77.5-inch track width, he said. Extending the track width to 77.5 inches could reduce the statistical likelihood of a rollover by 17%, according to the Cargo Tank Roll Stability Study.
“If you're spec'ing new trailers, there are some options to lower the center of gravity,” he said.
There are trailer configurations with a lower center of gravity: double conical, as opposed to straight round; double taper, as opposed to straight bore; elliptical, as opposed to round; and tri axle, as opposed to tandem.
“That's better for stability because it's applying more weight lower to the ground,” he said.
Lowering the center of gravity by three inches can reduce the statistical likelihood of a rollover by 12%, according to the Cargo Tank Roll Stability Study.
Another dynamic of hauling a liquid product is sloshing and surging, he said, and that “can affect stability, depending on the magnitude or how empty you're running your trailer. As you're spec'ing a new trailer, work with the OEM to choose a capacity that works best to make sure the trailer is as full as possible. I recommend hauling full, when possible, rather than partial loads. If trailer is completely full, then this dynamic doesn't play a role.”
He also recommended electronic Roll Stability Devices (RSD) to prevent rollovers.
“Most OEMs offer them as a standard option on new equipment,” he said. “They're able to be retrofitted on most older equipment, even spring ride trailers. They're not available on doubles — two trailers linked by a converter dolly or turntable. That's the only application I'm aware of that doesn't allow roll stability to be used.”
Combining a three-inch lower center of gravity and the wider track option could prevent the occurrence of 27% of rollovers, according to the Cargo Tank Roll Stability Study.
“Couple this with an RSD, and there is more tolerance for driver error — the ingredient most prevalent but also most difficult to control,” he said. “If you're driving a sports car down the road, it has a real low center of gravity. Normal driving is a lot different than a rollover situation. You have a lot more forgiveness to make an error driving a sports car than a tank trailer. If you add all these up, you start to increase that tolerance. The ingredient most prevalent in rollovers is driver error, but if we do everything in our ability, we can give them a better shot of being safe on the road.”
For more information, he recommended searching “roll stability” at www.bendix.com, www.haldex.com, or www.wabco-auto.com; or reading the Cargo Tank Roll Stability Study or National Transportation Safety Board (NTSB) Safety Recommendations issued September 2, 2011.
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