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CEE 320 Winter 2006 Vehicle Dynamics CEE 320 Steve Muench.

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Presentation on theme: "CEE 320 Winter 2006 Vehicle Dynamics CEE 320 Steve Muench."— Presentation transcript:

1 CEE 320 Winter 2006 Vehicle Dynamics CEE 320 Steve Muench

2 CEE 320 Winter 2006 Outline 1.Resistance a.Aerodynamic b.Rolling c.Grade 2.Tractive Effort 3.Acceleration 4.Braking Force 5.Stopping Sight Distance (SSD)

3 CEE 320 Winter 2006 Main Concepts Resistance Tractive effort Vehicle acceleration Braking Stopping distance

4 CEE 320 Winter 2006 Resistance Resistance is defined as the force impeding vehicle motion 1.What is this force? 2.Aerodynamic resistance 3.Rolling resistance 4.Grade resistance

5 CEE 320 Winter 2006 Aerodynamic Resistance R a Composed of: 1.Turbulent air flow around vehicle body (85%) 2.Friction of air over vehicle body (12%) 3.Vehicle component resistance, from radiators and air vents (3%) from National Research Council Canada

6 CEE 320 Winter 2006 Rolling Resistance R rl Composed primarily of 1.Resistance from tire deformation (  90%) 2.Tire penetration and surface compression (  4%) 3.Tire slippage and air circulation around wheel (  6%) 4.Wide range of factors affect total rolling resistance 5.Simplifying approximation:

7 CEE 320 Winter 2006 Grade Resistance R g Composed of –Gravitational force acting on the vehicle For small angles, θgθg W θgθg RgRg

8 CEE 320 Winter 2006 Available Tractive Effort The minimum of: 1.Force generated by the engine, F e 2.Maximum value that is a function of the vehicle’s weight distribution and road-tire interaction, F max

9 CEE 320 Winter 2006 Tractive Effort Relationships

10 CEE 320 Winter 2006 Engine-Generated Tractive Effort Force Power FeFe =Engine generated tractive effort reaching wheels (lb) MeMe =Engine torque (ft-lb) ε0ε0 =Gear reduction ratio ηdηd =Driveline efficiency r=Wheel radius (ft)

11 CEE 320 Winter 2006 Vehicle Speed vs. Engine Speed V =velocity (ft/s) r =wheel radius (ft) nene =crankshaft rps i =driveline slippage ε0ε0 =gear reduction ratio

12 CEE 320 Winter 2006 Typical Torque-Power Curves

13 CEE 320 Winter 2006 Maximum Tractive Effort Front Wheel Drive Vehicle Rear Wheel Drive Vehicle What about 4WD?

14 CEE 320 Winter 2006 Diagram RaRa R rlf R rlr ma W θgθg F bf F br h h lflf lrlr L θgθg WfWf WrWr

15 CEE 320 Winter 2006 Vehicle Acceleration Governing Equation Mass Factor (accounts for inertia of vehicle’s rotating parts)

16 CEE 320 Winter 2006 Example A 1989 Ford 5.0L Mustang Convertible starts on a flat grade from a dead stop as fast as possible. What’s the maximum acceleration it can achieve before spinning its wheels? μ = 0.40 (wet, bad pavement) 1989 Ford 5.0L Mustang Convertible Torque300 @ 3200 rpm Curb Weight3640 Weight DistributionFront 57% Rear 43% Wheelbase100.5 in Tire SizeP225/60R15 Gear Reduction Ratio3.8 Driveline efficiency90% Center of Gravity20 inches high

17 CEE 320 Winter 2006 Braking Force Front axle Rear axle

18 CEE 320 Winter 2006 Braking Force Ratio Efficiency

19 CEE 320 Winter 2006 Braking Distance Theoretical –ignoring air resistance Practical Perception Total For grade = 0

20 CEE 320 Winter 2006 Stopping Sight Distance (SSD) Worst-case conditions –Poor driver skills –Low braking efficiency –Wet pavement Perception-reaction time = 2.5 seconds Equation

21 CEE 320 Winter 2006 Stopping Sight Distance (SSD) from ASSHTO A Policy on Geometric Design of Highways and Streets, 2001 Note: this table assumes level grade (G = 0)

22 CEE 320 Winter 2006 SSD – Quick and Dirty 1.Acceleration due to gravity, g = 32.2 ft/sec 2 2.There are 1.47 ft/sec per mph 3.Assume G = 0 (flat grade) V = V 1 in mph a = deceleration, 11.2 ft/s 2 in US customary units t p = Conservative perception / reaction time = 2.5 seconds

23 CEE 320 Winter 2006

24 Primary References Mannering, F.L.; Kilareski, W.P. and Washburn, S.S. (2005). Principles of Highway Engineering and Traffic Analysis, Third Edition). Chapter 2 American Association of State Highway and Transportation Officals (AASHTO). (2001). A Policy on Geometric Design of Highways and Streets, Fourth Edition. Washington, D.C.


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