Vehicle Dynamics

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# Vehicle Dynamics - PowerPoint PPT Presentation

Vehicle Dynamics. CEE 320 Steve Muench. Outline. Resistance Aerodynamic Rolling Grade Tractive Effort Acceleration Braking Force Stopping Sight Distance (SSD). Main Concepts. Resistance Tractive effort Vehicle acceleration Braking Stopping distance. Resistance.

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Presentation Transcript

### Vehicle Dynamics

CEE 320Steve Muench

Outline
• Resistance
• Aerodynamic
• Rolling
• Tractive Effort
• Acceleration
• Braking Force
• Stopping Sight Distance (SSD)
Main Concepts
• Resistance
• Tractive effort
• Vehicle acceleration
• Braking
• Stopping distance
Resistance

Resistance is defined as the force impeding vehicle motion

• What is this force?
• Aerodynamic resistance
• Rolling resistance
Aerodynamic Resistance Ra

Composed of:

• Turbulent air flow around vehicle body (85%)
• Friction of air over vehicle body (12%)
• Vehicle component resistance, from radiators and air vents (3%)

Rolling Resistance Rrl

Composed primarily of

• Resistance from tire deformation (90%)
• Tire penetration and surface compression ( 4%)
• Tire slippage and air circulation around wheel ( 6%)
• Wide range of factors affect total rolling resistance
• Simplifying approximation:

Composed of

• Gravitational force acting on the vehicle

θg

For small angles,

Rg

θg

W

Available Tractive Effort

The minimum of:

• Force generated by the engine, Fe
• Maximum value that is a function of the vehicle’s weight distribution and road-tire interaction, Fmax
Maximum Tractive Effort
• Front Wheel Drive Vehicle
• Rear Wheel Drive Vehicle
Diagram

Ra

h

ma

Rrlf

h

Wf

W

Fbf

θg

lf

Rrlr

lr

Wr

L

Fbr

θg

Vehicle Acceleration
• Governing Equation
• Mass Factor

(accounts for inertia of vehicle’s rotating parts)

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

Braking Force
• Front axle
• Rear axle
Braking Force
• Ratio
• Efficiency
Braking Distance
• Theoretical
• ignoring air resistance
• Practical
• Perception
• Total

Stopping Sight Distance (SSD)
• Worst-case conditions
• Poor driver skills
• Low braking efficiency
• Wet pavement
• Perception-reaction time = 2.5 seconds
• Equation
Stopping Sight Distance (SSD)

from ASSHTO APolicy on Geometric Design of Highways and Streets, 2001

Note: this table assumes level grade (G = 0)

SSD – Quick and Dirty
• Acceleration due to gravity, g = 32.2 ft/sec2
• There are 1.47 ft/sec per mph
• Assume G = 0 (flat grade)

V = V1 in mph

a = deceleration, 11.2 ft/s2 in US customary units

tp = Conservative perception / reaction time = 2.5 seconds

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.