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Manufacturers of new commercial vehicles (CVs) sold in the U.S. must certify that the vehicle meets safety standards for braking capability via stopping performance tests (FMVSS 121). However, for the remaining 10- to 20-year service life of that CV, the carrier is responsible for ensuring that it is maintained in safe operating condition, under regulations codified in 49 CFR 393 and 396. The origin of these Federal Motor Carrier Safety Regulations (FMCSRs) for braking safety of in-use vehicles can be traced back to 1936, with the first publication of stopping performance requirements. However, due to the logistic challenges of conducting stopping performance tests, such tests are seldom performed on in-use vehicles. As such, research was conducted in the 1950s through the 1980s to find an equivalent method, via inspection of brake system components, to that of the performance-based regulations for identifying vehicles that were considered unsafe for travel on the public roadways.

Enhancements have been developed for low speed roller brake testers that make them more useful devices for testing and diagnosing braking systems. The enhancements are primarily designed for the evaluation of air brake systems but have application to hydraulic brake systems as well.

Air brake adjustment is a critical factor in achieving optimum system performance. Brake chamber stroke must be minimized if maximum brake torque is to be achieved. A significant loss of brake torque occurs over what is considered to be the acceptable brake adjustment range particularly if the brake is hot. A large number of air braked vehicles on the road have brakes out of adjustment or have brakes considered to be in adjustment that are operating at a reduced effectiveness level. Better adjustment maintenance would improve truck safety.

A vehicle test procedure for determining the adhesion utilization properties of braking systems has been developed. The procedure requires minima instrumentation and equipment. It was applied to 19 passenger cars over a range of conditions and the effects of factors such as load, center of gravity height, speed, parasitic drag, engine braking, drive configuration (front or rear), brake conditioning, and tire properties on adhesion utilization were determined. The vehicles were also subjected to a series of stopping tests on six different surfaces to evaluate the degree of correlation between predictions from the adhesion utilization curves and actual performance. Frictional properties of the tires from 11 of the vehicles were measured on the surfaces to aid in this correlation study.

This paper serves as the seventh report in a series of reports on NHTSA's Heavy Vehicle Brake Research Program and deals with the subject of tractor and trailer brake system compatibility. It provides a detailed definition of compatibility, discusses the factors that influence it and presents data and analyses which indicate the degree of compatibility in the heavy duty combination vehicle fleet at large. The paper suggests ways in which compatibility can be improved so that combination vehicle brake systems will be more durable and provide an enhanced level of safety.

Currently there are no adequate standards or regulations that address the performance of aftermarket replacement brake linings to insure that the use of these materials does not degrade vehicle braking performance from the original equipment (OE) design intent level. This paper discusses the results of an evaluation of a large sampling of aftermarket linings available for the rear brake of a specific model passenger car and shows that many of these linings have significantly different performance than the OE material. The paper also shows how this deviation can adversely affect vehicle braking efficiency or the ability of the brake system to utilize available tire/roadway friction without locking wheels and losing control.

This paper provides a synthesis of the information available describing the braking performance of heavy (on-highway) vehicles in the U.S. and contrasts the braking performance of heavy vehicles with that of passenger cars. It discusses the unique demands that are placed on heavy vehicle brake systems and shows that it is difficult to optimize braking performance for all operating conditions without adding complexity to the system. As designed, U.S. heavy-vehicles do not perform as well as cars. In addition, heavy vehicle brake performance can degrade significantly from the design-intent level if proper maintenance is not performed. The brake maintenance situation in the U.S. leaves something to be desired.

Differences in regulations and design philosophies have brought about the development of significantly different hardware in the braking systems of U.S. and European heavy vehicles. In the service braking system, for example, European heavy vehicles generally have larger front brakes than their U.S. counterparts, and they are usually equipped with load-sensing brake proportioning valves (not used on U.S. vehicles). Emergency brake systems and parking brake systems are also different. This paper discusses those differences that have a significant impact on safety-related performance and presents the results of recent tests that were run to compare the braking performance of a U.S. five-axle tractor semitrailer combination to a European vehicle of the same basic size, weight and axle configuration in various simulated accident avoidance maneuvers.