Road Vehicle Performance

1. Road Vehicle Performance Chapter 2 Principles of Highway Engineering and Traffic Analysis – 3rd Edition Fred Mannering, Walter Kilareski, Scott Washburn

2. Purpose of Lecture Chapter 2 involves understanding the forces that act on a vehicle Straight-line performance is reviewed Important concepts include: Understanding three sources of vehicle resistance Practical stopping distance

3. Road Vehicle Performance Two functions Insight into highway design and traffic operations To be able to accommodate a large variety of vehicle types on the roads The basis to understanding vehicle designs and their impact on performance

4. Roadway Design Roadway design is governed by two main factors: Vehicle capabilities acceleration/deceleration braking cornering (chap. 3) Human capabilities (late chap. 2, chap. 3) perception/reaction times eyesight (peripheral range, height above roadway)

5. Roadway Design & Vehicle Performance Performance of road vehicles forms the basis for roadway design guidelines such as: length of acceleration / deceleration lanes maximum grades stopping-sight distances passing-sight distances setting speed limits timing of signalized intersections

6. Tractive Effort and Resistance Two primary opposing forces Tractive effort is the force available at the road surface to do work Expressed in pounds or Newtons Resistance is the force impeding vehicle motion, 3 major sources Aerodynamic resistance Rolling resistance (originates from the roadway surface/tire interface) Grade or gravitational resistance

8. Vehicle Motion For simplification, when summing forces along the longitudinal axis, traffic effort and rolling resistance at the front and rear tires is combined, giving:

9. Aerodynamic Resistance Aerodynamic resistance can impact vehicle performance, especially at high speeds Turbulent flow of air around vehicle body (85% of total resistance) Friction of the air passing over the body of the vehicle (12% of total resistance) Air flowing through the vehicle components (3% of total resistance)

10. Aerodynamic Resistance Equations 2.3 and 2.4 can be used to determine aerodynamic resistance and the force required to overcome it based on factors such as drag coefficient of particular vehicle types; air density; frontal area of vehicle; vehicle speed relative to prevailing wind speed

11. Aerodynamic Resistance

12. Aerodynamic Resistance Air density is a function of both elevation and temperature (see text Table 2.1). ? altitude, ? density ? temperature, ? density Drag coefficient is a term that implicitly accounts for all three of the aerodynamic resistance sources previously discussed Drag coefficient is measured from empirical data, either from wind tunnel experiments or actual field tests in which a vehicle is allowed to decelerate from a known speed with other sources of resistance (rolling and grade) taken into account

13. Overcoming Aerodynamic Resistance

14. Aerodynamic Resistance As seen in equation 2.3, Ra is proportional to V 2. Thus, this resistance will increase rapidly with increasing speed. We can develop an expression for determining the power needed to overcome aerodynamic resistance

15. Rolling Resistance Refers to the resistance generated from a vehicle’s internal mechanical friction, and pneumatic tires and their interaction with the roadway surface. Primary source (about 90%) of this resistance is the deformation of the tire as it passes over the roadway surface. Tire penetration/roadway surface compression (about 4%) Tire slippage and air circulation around tire & wheel (about 6%)

16. Rolling Resistance Due to wide range of factors that affect rolling resistance, a simplifying approximation is used. Studies have shown that rolling resistance can be approximated as the product of a friction term (coefficient of rolling resistance) and the weight of the vehicle acting normal to the roadway surface.

17. Rolling Resistance Rolling resistance is the product of a friction term (coefficient of rolling resistance) and the weight of the vehicle acting normal to the roadway surface Coefficient of rolling resistance on paved surfaces is given as:

19. Rolling Resistance & Horsepower

20. Example Problem A 2500 lb car is driven at sea level (?=0.002378 slugs/ft3) on level paved surface. The car has drag coefficient CD=0.38 and 20ft2 frontal area. At max speed 50hp is expended to overcome rolling and aerodynamic resistance. What is the car’s max speed?

21. Example Problem continued

22. Grade Resistance Gravitational force parallel to the roadway acting on the vehicle Grades are generally given in percentages meaning that a 5% grade results in a 5 ft vertical rise over a 100 ft horizontal run

24. Example #2.2 A 2,000 lb car has CD = 0.40, Af=20ft2 and available tractive effort of 255 lb. If the car is traveling at an elevation of 5000ft (?=0002045 slugs/ft3) on a paved surface at a speed of 70mph, what is the maximum grade this car could ascend and still maintain 70mph speed?

25. Example 2.2 Continued Need to understand the forces acting on the vehicle:

26. Principles of Braking For highway design and traffic analysis braking characteristics are most important aspect of vehicle performance Braking behavior influences geometric design, signal timing, sign placement, accident avoidance systems, roadway surface design

27. Sections 2.91-2.94 Review on your own These sections deal more with how auto designers might approach braking principles or accident reconstruction if many variables are know (such as road adhesion, braking efficiency, air density, etc.) As highway designers, we have to generalize many of these factors in order to accommodate a variety of driver skills, vehicle types, pavement conditions and weather conditions

28. Practical Stopping Distance Assuming constant deceleration

29. Further Assumptions AASHTO recommends a deceleration rate of 11.2 ft/s2 Studies have shown that most drivers brake at rates greater than this Also that drivers can maintain control even on wet pavement at this rate

30. Accounting for Grade Equation 2.47 incorporates the effects of grade on braking distances

31. Accounting for Reaction Time Up to this point only considering the distance traveled or required to stop a vehicle from the point of brake application Need to also account for the time passage when a driver is perceiving and reacting to the need to stop This distance is referred to as perception/reaction time

32. Perception/Reaction Time

33. Total Required Braking Distance Combination of braking distance and the distance traveled during perception/reaction time

34. Example – Practical Stopping Distance Two drivers with reaction times of 2.5 sec. One is traveling at 55mph, the other at 70mph. How much distance will each of the drivers travel while perceiving/reacting to the need to stop? What is the total stopping distance for each? Assume a grade of -2.5%.

35. Example - Continued