A brief review of air brake wheel lock control systems

The stopping distances prescribed in Federal Motor Vehicle Safety Standard 121 effective March 1975 through March 1976 required that all wheels on the vehicle develop near peak retarding force. Many trucks and buses can experience very large changes in load distribution between empty and full load. An adaptive type brake system is required to achieve near peak retarding force at each wheel regardless of load distribution. This requirement was satisfied by the installation of electronic wheel lock control systems on all axles of air-braked vehicles; these systems are also referred to as antiskid systems. They employ a wheel speed sensor on each wheel, an electronic logic module to make the decision that a wheel is tending to lock up, and control valves to regulate the air pressure to the individual axles. It is a well-known fact that lateral vehicle control is difficult to maintain when rear wheels lock up and steering is ineffective when front wheels lock up. The truck and bus industry recognizes the outstanding potential safety benefits associated with wheel lock control; however they have had considerable trouble implementing the system in a reliable manner.