1991-02-01

# Heavy Truck Deceleration Rates as a Function of Brake Adjustment 910126

When reconstructing a highway accident involving a heavy truck, an accurate deceleration rate is often an elusive piece of the puzzle. This paper presents a method for calculating the deceleration rate of a heavy truck equipped with an air-mechanical brake system. Traditionally a deceleration rate had been more an estimate of the percent of wheel lock-up based on the brake adjustment, and then proportioned to the loading on the axles. Equations for two configurations of brake chamber size and slack adjuster length were written by engineers at the National Highway Traffic Safety Administration's (NHTSA) Vehicle Research and Test Center (VRTC) to describe braking torque as it varies with pushrod stroke. These equations were developed by means of a regression analysis, to fit a curve to dynamometer data produced at the VRTC in East Liberty, Ohio. Once this braking torque is converted to a braking force, it can be compared to the braking force available at the tire/road interface. This comparison allows the reconstructionist to evaluate which wheels will lock and to calculate an accurate deceleration rate.
Another method for calculating braking force is based on a commonly used brake sizing formula. Comparing the dynamometer data to results from the brake sizing equation, allowed the brake sizing equation to be modified to account for all variations of brake component configurations. The increase of pushrod stroke due to brake heating and drum expansion is examined using another set of equations also written at VRTC. When this temperature effect is combined with a mathematical model to predict brake temperature on downgrades, as developed through projects funded by the Federal Highway Administration (FHWA), a reliable deceleration rate can be calculated for a heavy truck descending a grade of variable length and slope.
Computed deceleration rates are compared to actual measured rates from stopping tests of tractor semi-trailers. These road tests provide validation for the discussed methods and demonstrate the expected range of accuracy.

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