Theory of Ground Vehicles, 5th Edition

3Performance Characteristics of Road Vehicles

Performance characteristics of a road vehicle refer to its capability to accelerate, decelerate, and negotiate grades in a straight‐line motion. The tractive (or braking) effort developed by the tires and the resisting forces acting on the vehicle determine the performance potential of the vehicle and is discussed in detail in this chapter. Procedures for predicting and evaluating the performance characteristics of road vehicles are also presented.

3.1 Equation of Motion and Maximum Tractive Effort

The major external forces acting on a two‐axle vehicle are shown in Figure 3.1. In the longitudinal direction, they include the aerodynamic resistance R_a, rolling resistance of the front and rear tires R_rf and R_rr, drawbar load R_d, grade resistance R_g(W sin θ_s), and tractive effort of the front and rear tires F_f and F_r. For a rear‐wheel drive vehicle, F_f = 0, whereas for a front‐wheel drive vehicle, F_r = 0.

The equation of motion along the longitudinal axis x of the vehicle is expressed by

(3.1)

where d²x/dt² or a is the linear acceleration of the vehicle along the longitudinal axis, g is acceleration due to gravity, m is vehicle mass, and W is vehicle weight.

By introducing the concept of inertia force, the above equation may be rewritten as

or

(3.2)

where F is the total tractive effort and R_r is the total rolling resistance of ...