NATIONAL Tank Truck Carriers president John Conley said the organization will continue to shine a light on rollovers because the issue is critical to its members.

“If we're having five rollovers a day throughout the country, and there are well over 100,000 tank trailers dispatched, is that a bad number? I don't know,” he said during NTTC's 2011 Cargo Tank Maintenance Seminar in Louisville, Kentucky. “Statistically from a risk analysis, five out of every 100,000 may not a bad number. But think if those were airplanes. If five of every 100,000 airplanes are going to crash, all of a sudden we'd pay a lot more attention.

“Believe me, DOT (Department of Transportation), Congress, and NTSB (National Transportation Safety Board) are looking at it, and we want to get rid of it. We've done a number of things over the years. The rollover poster program, in conjunction with JB Keller, is a good program. A number of our member companies have developed their own poster programs.

NTTC John Conley

“In 2008, the NTTC executive committee voted to approach DOT and ask them to require roll stability on any new tractor used to pull a tank trailer. The people at NHTSA said, ‘We appreciate the fact that an industry organization would do that, but please don't put that petition in.’ They are working on a broader one that's supposed to come out this year that will address all tractors.”

He said Congress gives money to DOT, which gives money to the Transportation Research Board to give to consultants, universities, and others to study issues — one of which produced a report that said 75% of cargo tank rollovers were the result of driver actions.

“We take this thing seriously,” Conley said.

He said NTSB has taken a greater interest as a result of a 2009 rollover accident 10 miles north of Indianapolis, Indiana.

A 2006 Navistar International truck-tractor in combination with a 1994 Mississippi Tank Company MC331 specification cargo tank semitrailer, operated by AmeriGas Propane LP, was loaded with 9,000 gallons of liquefied petroleum gas that escaped, formed a vapor cloud, and ignited, resulting in burns to five people.

NTSB recommended to the Federal Motor Carrier Safety Administration (FMCSA) in July 2011 that all cargo tank trailers with a gross vehicle weight rating greater than 10,000 pounds be required to be retrofitted with a rollover stability-control system.

NTSB cannot require anything, but it can make recommendations to federal agencies and private companies, and it has a lot of political clout, according to Conley.

“They put extreme pressure on DOT and FMCSA,” he said. “They would like to require a rollover stability system retrofit for any cargo tank. One NTSB recommendation is for engineers to determine what the rollover threshold is for any cargo tank and then mark the cargo tank with that threshold, and in the future come up with designs to reduce rollovers.”

Michael Tolber of Bendix Commercial Vehicle Systems said that static roll stability is when nothing is changing and the vehicle is going at a constant speed.

“In order to go around a curve, you need the tires to produce a force,” he said. “That force is counteracted on a vehicle by tire acting against the force the road is providing to the tire, and that essentially turns the vehicle around the corner. In order to navigate this turn, you need a lot of lateral acceleration, because velocity is changing the direction. Eventually, though, you're traveling forward at a constant rate of speed. You're changing direction and therefore need lateral acceleration.

“If you're going around a corner of 185 feet in radius at 25 mph, that is .23g of lateral acceleration. If you increase the speed by 5 mph, that lateral acceleration is .33g. So for a 20% increase in speed, you see a 43% increase in lateral acceleration. Five mph might not seem like much for a curve of this nature, but you see how much lateral acceleration increases. This is significant because most tank trucks will not roll over at .23g. But if you increase the speed by 5 mph, you're definitely in the realm of where most tank trucks will roll over.

“With stability-control systems, we measure the lateral acceleration of the vehicle, and when it reaches a certain threshold — in this case, between .23g and .33g — we apply the brakes and slow the vehicle down. That applies to both the tractor-based and trailer-based stability-control systems.”

He said the roll stability of the vehicle can be improved by lowering the center of gravity or lowering the speed.

“By lowering the speed, I lower the lateral acceleration,” he said. “In most cases, we have certain design restrictions of the center-of-gravity height, so that's not easy to change. Speed is pretty easy to change. If your driver is well-educated on the limits of the vehicle or you have a stability-control system, that can reduce the velocity for him when it senses that lateral acceleration has gotten too high.

“As I add more complexity to the model and look at tank trucks, it's important to consider specific tank concerns. There's sloshing of liquid. When you apply lateral acceleration to an actual mass of fluid inside the tank, that causes it to roll off the side of the tank. You're shifting the center of gravity to the outside of the vehicle, which ends up having a line of action of force on the outside of the tire.

“What happens when you start adding some dynamic steering input to the vehicle? As the driver turns back to the right, cornering forces have shifted to the right of the vehicle, and the vehicle is starting to turn back to the right. The fluid doesn't instantaneously end up on the right side of the vehicle. It has energy in it, and that takes some time to transfer over to the side of the vehicle. Once the fluid reaches a peak on the other side of trailer, the center of gravity is higher after this transient maneuver. So the right side shows the center of gravity higher than on the left.”

He said the shape of the tank also plays a role, with a rectangular tank at 50% load having a higher roll threshold for a static load.

“That makes sense because a rectangular tank has a squatter shape and the center of gravity is lower,” he said. “At 100% load, the rectangular tank has a higher threshold because it's sitting down lower and product inside isn't moving. When you shift to a static liquid load, we're going around a curve at a constant speed and radius, and now we have liquid in the tank. For a circular tank with a 50% load, there's a .5g rollover, while a rectangular tank is at .42g. The reason is that the way the load shifts inside the round tank causes it to have a higher center of gravity than a circular tank.

“The moral is that if you are going to have a rectangular tank, you want it to be full. If you're going to have partial loads, it's best to have compartments where you can keep some of them full while some are empty, and you can also add baffles that keep the liquid from sloshing from side to side.”

Trailer roll stability

Dave Engelbert, chief engineer of the air controls business unit for Haldex Brake Products, said TRS is a trailer ABS system with the added function of roll stability for trailers with constant power and air suspensions.

It operates independently of the towing vehicle system and does not require the driver to apply the brakes. The system automatically applies the brakes to slow the vehicle down when a roll event is detected. It will reduce the chance of a rollover.

“But it's not guaranteed 100%,” he said. “It's an aid. A driver can still put himself in a situation where he can overdrive the system and go beyond physics and what the system is capable of controlling.”

Current available applications: single, tandem, or tri-axle semi trailers, full trailers, Canadian B-trains, and LCVS; disc or drum foundation brakes; air suspension or spring suspension; new equipment (OEM installed/standard at several OEMs); and used equipment (retrofitted).

He said there is new technology, with Haldex adding one lateral accelerometer, one brake-apply solenoid, one port to connect the air bags, and five pressure transducers.

“Under TRS braking, the ECU (electronic control unit) receives input from the rollover stability accelerometer of an unstable vehicle condition,” he said. “The ECU then looks at the trailer's wheel slip to determine if there is an impending roll event. Depending upon how the wheels are responding, the ECU will decide if the trailer's brakes need to be applied to slow down the vehicle combination.”


Matthew Williams, manager of the trailer business unit for Meritor Wabco, said there are opportunities for dealers to work with fleets and install retrofit systems out in the field.

“There are kits for non-ABS trailers versus ABS-equipped trailers versus competitive systems,” he said. “There is the ability for cost savings if you already have an ABS system equipped on the vehicle. You can utilize some of the components on a trailer, and that reduces the cost for a retrofit for a trailer. Many of you are hauling for Chevron or Sunoco or fuel companies. We've been notified there have been requirements from those companies that your trailers have to have some sort of roll-stability system.

“Meritor Wabco has been very successful with retrofits. It's been happening and occurring out in the field already. That platform is already established, so it should be a smooth transition.” ♦

Find the Cargo Tank Maintenance Seminar archive with articles from 2010 to 2013