A large percentage of on-highway tractors today have air suspensions. Air suspensions require some type of control system to adjust the ride height. This system is usually referred to as a Load Leveling System. These systems come in a variety of different configurations but all basically have the same functions. When designed correctly, the system can reduce driveline vibration, reduce air consumption (improving compressor life and fuel efficiency), provide an accurate 5th wheel height and improve the ride quality.
This paper explores how the characteristics of the leveling system affect the roll stability. One and Two-valve systems are considered, as well as, the position of the valve, response times, valve deadband and the systems response to an off-center load.
Notably not every conceivable condition has been considered. The situations chosen for analysis are meant to provide an overview of how the characteristics of the leveling system can have an influence on the rollover threashold. The fact that some of the situations may suggest one leveling system configuration has a better performance than another is not sufficient to conclude that that system is universally better than the other system in practical applications.
With that said, the results of the analysis do suggest that under specific conditions, certain configurations may have some influence on the rollover threashold. When the vehicle is loaded correctly and the slow response of the valve and valve deadband are considered, the one-valve system used by most OEMs today results in slightly better rollover threashold than a two valve system. If the response times of the valves are ignored or the duration of the roll event lasts a substantial amount of time, the two-valve system provides a slightly better result.
If the cargo is loaded significantly away from the center of the vehicle track, a two-valve system will provide a better rollover threashold. However, the two-valve system may hide the problem from the driver where a one-valve system would allow the initial roll angle of the chassis to be non-zero, increasing the odds that the off-center load will be detected and corrected. The lateral position of the leveling valve will very slightly have an influence on the system. A system that is design not to change the air volume during roll conditions would be preferred if packaging allows this configuration. Overall, the contribution of this parameter is minimal even ignoring the slow response of the leveling system.