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Vehicle Dynamics Session 3 Tire Fundamental Characteristics and Dynamic Performance

Session 3 -- Outline. Tire Friction characteristicsLateral Force GenerationTractive/Braking ForcesSlip AnglesSlip ratiosAligning TorqueCamber Thrust. . Forces on a Vehicle. Tractive forcesThe forces acting at the tire footprint to either maintain velocity or change velocity.Road Load Forces

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Vehicle Dynamics Session 3 Tire Fundamental Characteristics and Dynamic Performance

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    1. Vehicle Dynamics Session 3 Tire Fundamental Characteristics and Dynamic Performance Dr. Richard Hathaway, P.E. Professor Mechanical and Aeronautical Engineering

    2. Session 3 -- Outline Tire Friction characteristics Lateral Force Generation Tractive/Braking Forces Slip Angles Slip ratios Aligning Torque Camber Thrust

    3. Forces on a Vehicle Tractive forces The forces acting at the tire footprint to either maintain velocity or change velocity. Road Load Forces The forces acting at a given velocity that oppose vehicle tractive forces. Road load forces usually are simply rolling plus aerodynamic forces.

    4. Tire Characteristics Traction Limits Friction Circle Slip Angle Cornering Stiffness Carpet Plot Self-Aligning Torque Camber thrust

    5. Tire Axis System

    6. Tire Characteristics Traction Limits Production tires on a typical paved surface can generate a coefficient of friction of up to about 1.0. Usually ? 0.7 - 0.8 Race tires can easily generate friction coefficients greater than 1.0 Traction is temperature dependent, as well as influenced by many factors.

    7. Tire Friction Characteristics

    8. Slip Velocity The difference between the angular velocity of the driven (braked) wheel and the angular velocity of the free rolling wheel. Slip Ratio Slip ratio is defined as the slip velocity as a percentage of the free rolling velocity. Tire Friction Characteristics

    9. Slip Ratio Slip ratio is defined as the slip velocity as a percentage of the free rolling velocity. Since then Tire Friction Characteristics

    10. Slip Ratio If spinning is arbitrarily assigned a slip ratio of 1 then at spinning This implies the peripheral speed is twice that of the free rolling tire and twice the forward velocity. The onset of spinning is usually much earlier usually ? 0.10-0.15 Tire Friction Characteristics

    11. Tire Slip Ratio (acceleration)

    12. Tire Slip Ratio (braking)

    13. Slip Angle Slip angle is defined as the angle between the rotational plane of the tire and the tire heading direction. Lateral loads on a tire introduce a slip angle.

    14. Slip Ratio definitions SAE J670 Calspan TIRF definition Tire Friction Characteristics

    15. Slip Ratio definitions (alternatives) Good Year Pacejka definition Tire Friction Characteristics

    16. Slip Ratio definitions (alternatives) Sakai Dugoff, Fancher, Segel Tire Friction Characteristics

    17. Friction Circle Assume friction between the tire and road is independent of the direction of the force. Tractive force and lateral load can be combined in vector form to determine cornering and or braking/acceleration capacity at any point. Lateral loads decrease braking/acceleration limits

    18. Friction Circle

    19. Tire Characteristics Analysis of the Slip Ratio and Lateral Force behavior at given slip angles. At a given slip angle as the slip ratio increases the tires capacity to support a lateral load diminishes. Peak tractive forces are present at slip ratios that differ slightly between braking and forward traction. At any given slip ratio, the lateral force capacity increases with increasing slip angles.

    20. Tire characteristics

    21. Tire characteristics

    22. Lateral Force Capacity

    23. Slip Angle vs Lateral Load

    24. Slip Angle vs Lateral Load

    25. Tire Cornering Stiffness Cornering stiffness is the change in lateral force per unit slip angle change at a specified normal load in the linear range of the tire.

    26. Cornering Stiffness Comparison

    27. Tire Carpet Plot

    28. Tire Carpet Plot

    29. Slip Angle vs Lateral Force Coefficient

    30. Self-Aligning Torque Self-Aligning Torque is derived from a combination of caster trail and the tires own pneumatic trail. If the mechanical (caster) trail is small the tires aligning torque (Pneumatic Trail) will dominate the steering effect. If Pneumatic trail effects dominate the limits of traction are more obvious to the driver of the vehicle. Pneumatic trail is derived from the shear force distribution in the tire footprint.

    31. Self-Aligning Torque

    32. Self-Aligning Torque

    33. Tire Self Aligning Torque

    34. Tire Self Aligning Torque

    35. Camber thrust is the lateral force generated as the tire is inclined from the surface normal. A cambered tire generates a slip angle to maintain a lateral force = 0. As a result a cambered tire has a rolling resistance greater than a zero cambered tire. A cambered tire will generate a lateral force in a turn reducing the slip angle (if proper direction) required to maintain the same lateral force. Camber Thrust

    36. Camber Thrust

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