Feb 1, 2006 12:00 PM
THE TANK truck industry increasingly expresses concerns about rollovers that can result in fatalities, as well as injuries, but equipment for better stability is available.
William Gantz of Bendix Commercial Vehicle Systems, Bryan Simoncic of Haldex Brake Products, Mike Melletat of Meritor WABCO, and Travis McCloud of Front Line Design Technologies discussed developments in vehicle stability November 7-9 at the National Tank Truck Carriers 2005 Cargo Tank Maintenance Seminar in Louisville, Kentucky.
“Milliseconds make a difference,” Gantz said, referring to the response time needed to correct a rollover situation.
Melletat noted that statistics show that 6% of combination-vehicle accidents are rollovers, and casualties occur in over 50% of these. The industry suffers about $1 billion annually in costs generated by rollover incidents.
“By the time the driver recognizes the potential for a rollover, it's too late,” Melletat said.
Simoncic said that drivers often don't realize they are rolling over until they see it happening in the truck mirror.
“It's going to take everyone in this industry to combat rollovers,” McCloud said.
He commented on certain misconceptions that often are applied to rollover concerns. He pointed out that product surge does not occur in fully loaded cargo tanks. In addition, liquid movement is not a factor until the level falls below 90%.
He also dispelled the notion that lane changes and other side movements shake the liquid, causing wild gyrations within the tank.
“If you make a quick 12-foot lane change in four seconds, the tank moves sideways at a speed of less than three miles per hour,” he added. “The side acceleration is not enough to excite the liquid.”
McCloud said tank trailers can be redesigned to reduce the risk of rollovers, but even new designs carry certain disadvantages.
The axle track width can be increased by three inches, but that would add weight. Also, 102-inch trailers are not allowed in some states on state highways.
“Manufacturers are doing what they can, given the current laws and regulations,” McCloud said. “It takes a large change in the center of gravity to affect rollover significantly. This can be costly and sometimes impractical. Even with a lower cargo tank, rollovers can still occur due to human error, such as driving too fast for conditions.”
Stability systems are available and can save lives, reduce injuries, and reduce accident expenses, the speakers agreed.
Meritor WABCO Vehicle Control Systems offers a variety of stability enhancement systems. Meritor WABCO's roll stability control (RSC) and roll stability support (RSS) for linehaul tractor/trailer combinations are now running on thousands of fleet vehicles across North America.
The newest system in the lineup, ABS-based electronic stability control (ESC), offers another level of improved vehicle control for trucks and tractors.
The RSC system focuses on a vehicle's center of gravity and the lateral acceleration limit, or rollover threshold.
Meritor WABCO engineers studied the dynamics of rollovers and developed algorithms calculating the critical lateral acceleration limit. When critical thresholds are exceeded, RSC intervenes to regulate the vehicle's deceleration functions. Track and field testing demonstrated the system's ability to slow the vehicle, giving the driver added control and maneuverability.
“ESC builds upon the already-established RSC system and exemplifies our continuing efforts to pioneer stability enhancement technologies,” Melletat said. “This system provides another level of vehicle control by sensing the tendency of the vehicle to spin around and automatically applies the brakes to reduce that risk.”
RSS is an independent system for trailers and offers assistance to reduce the likelihood of rollover. The technology is an integrated feature of the next-generation Meritor WABCO trailer antilock braking system (ABS). Similar to the tractor's RSC system, RSS focuses on lateral acceleration and wheel speed, monitoring the critical acceleration limit. If this limit is exceeded, the trailer brakes are automatically applied, helping to reduce the vehicle's speed.
Stability control, like ABS, is automatic. There's no switch or selection device. An indicator lamp on the dash lets the driver know stability control will be available when needed.
Based on government statistics defining the different type of events that result in rollover or jackknife type accidents, the Bendix ESP system could potentially mitigate up to 67% of those known causes.
The Bendix Electronic Stability Program (ESP) is part of the ABS-6 platform of products. ABS-6 offers three types of systems within the portfolio: standard ABS, ABS with traction control or ABS-6 premium, and the ABS-6 Advance System that incorporates the ESP system.
The Advanced ABS-6 product continuously monitors a variety of vehicle parameters through sensors to determine if the vehicle is reaching a critical stability threshold.
When a situation develops, the ESP system automatically intervenes to assist the driver. The key to stability is slowing the vehicle down. ESP not only selectively applies brakes at individual wheel ends to maintain stability, it also controls the throttle as needed.
The system is capable of recognizing and assisting with both rollover, jackknife, and vehicle under- and over-steer driving situations over a variety of road conditions.
It also helps to mitigate jackknives and vehicle slip situations by monitoring various vehicle parameters and automatically applying brake pressures to both the tractor and trailer.
The ESP system captures driver intent, enabling the system to capture information about the path of travel and intervene if necessary. The continuous cross-checking of the sensors that make up the system provides an early indication of potential stability problems.
The Bendix Smart automatic traction control system makes adjustments based on the vehicle orientation (straight versus curve) and the driver's throttle input.
Bendix ESP is a stand-alone system, allowing it to work independently of other systems on the tractor or truck.
The Haldex TRS (trailer roll stability system) is a new introduction for trailers. It provides vehicle stabilization in situations where there is a risk of rollover.
“Every trailer has its own rollover threshold and traction limit, based on design, construction, current condition of brakes, suspension, tires, and load,” Simoncic said.
The TRS equipment can be installed on single, tandem, or tri-axle semi trailers.
Roll stability systems automatically apply the brakes to slow down the vehicle when a rollover event is detected. “Technology now detects and measures the physical and unique characteristics of trailer motion,” he said.
A key TRS component added to the ABS system is the lateral accelerometer, which detects rollovers. The equipment receives input from the lateral accelerometer of a potential unstable vehicle condition and then sends a series of brake test pulses to determine an impending event.
Depending on the wheel speed senor response, the equipment will decide if the opposite side brakes need to be applied. Reservoir pressure is diverted to the top of the relay piston to apply the brakes without the brake pedal being applied. Test pulses are required so the system learns and adapts to the equipment, load, and environment.
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