Summary
Highlights
The video introduces road vehicle performance as a foundational element for highway design and traffic analysis. Key considerations in highway design include the length of acceleration and deceleration lanes, maximum highway grades, stopping sight distance, passing sight distance, and accident prevention devices, all reliant on understanding vehicle performance.
Studying vehicle performance offers insights into highway design and traffic operations, helping to accommodate diverse vehicles. It also establishes a basis for testing the impact of new vehicle technologies on existing design guidelines. The chapter aims to introduce the basic principles of road vehicle performance.
The video outlines several topics: tractive effort and resistance (2.2), aerodynamic resistance (2.3), rolling resistance (2.4), grade resistance (2.5), available tractive effort (2.6), vehicle taxation (2.7), fuel efficiency (2.8), and principles of working (2.9). These topics will be discussed in detail over several sessions.
Tractive effort (or thrust) and resistance are the two main opposing forces determining a road vehicle's straight-line performance. Tractive effort is the force available at the roadway surface to perform work, while resistance is the force impeding vehicle motion. The three major sources of resistance are aerodynamic, rolling, and grade resistance.
The video introduces symbols for aerodynamic resistance (RA), rolling resistance of front (RLF) and rear (RLR) tires, available tractive effort of front (FF) and rear (FR) tires, total vehicle weight (W), angle of grade (G), and mass (m). The basic equation for motion, summing forces along the longitudinal axis, is presented as F_total = m * a + RA + RL + RG.
Aerodynamic resistance is a significant resistive force, especially at high speeds. Proper aerodynamic design is crucial for vehicle performance and fuel efficiency. Factors like open windows or convertibles with the top down can significantly increase drag coefficients. The projected frontal area also plays a role in aerodynamic resistance.
The equation for aerodynamic resistance (RA) is given as (rho / 2) * CD * AF * V^2, where rho is air density, CD is the coefficient of drag, AF is the frontal area, and V is vehicle speed. Tables are presented for typical values of air density at different atmospheric conditions and drag coefficients for various vehicle types. The horsepower required to overcome aerodynamic resistance (HP_A) is also provided.
Rolling resistance is generated from internal mechanical friction and the interaction between pneumatic tires and the road surface. The primary source is tire deformation as it passes over the roadway. Factors influencing rolling resistance include tire and roadway rigidity, tire condition (inflation pressure, temperature), and vehicle operating speed.
The coefficient of rolling resistance (F_RL) is approximated by 0.01 * (1 + V / 147), where V is vehicle speed in feet per second. Rolling resistance (R_RL) is calculated as F_RL * W * cos(theta G), which simplifies to F_RL * W when the grade angle theta G is small. The horsepower required to overcome rolling resistance (HP_R_RL) is also provided.