1973-02-01

How Do You Characterize “Performance”? 730250

This paper calls attention to the multiplicity of numerical criteria in common use to characterize the ability of land vehicles to accelerate and to climb hills; these include a variety of relations between speed, time and distance from a standing start, gradability, and instantaneous acceleration. The procedure for converting from one criterion to another is not always obvious.
To accelerate or climb hills, all a vehicle needs is a tractive effort (or horsepower) at the driving wheels which exceeds the requirement for constant speed on a level road. The ratio of excess tractive effort to vehicle weight, sometimes defined as “performance,” is a particularly convenient criterion of ability to accelerate and climb hills because it is numerically almost equal to the percent grade at constant speed, divided by 100, and to the level-road acceleration expressed as a fraction of the acceleration of gravity (22 mph/s).
A convenient way to measure this ratio is with a performance meter, similar in construction to a grade meter or “accelerometer”; certain limitations of this instrument are discussed. Performance is more commonly measured by tests of level-road acceleration or speed on known grades; to perform both of these tests often involves needless duplication.
Another convenient criterion of vehicle ability is reserve horsepower per pound, equal to performance times speed, in miles per hour, divided by 375.
The paper describes procedures for converting from the relation between speed and excess tractive effort (or horsepower) to a description or the standing start acceleration, and vice versa.

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