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Chapter 18

Chapter 18. Wheel Alignment Fundamentals. Introduction. Correct wheel alignment provides for safe and comfortable driving while reducing tire wear. This chapter discusses the principles of wheel alignment and alignment angles. Alignment Angles. Toe Camber Caster

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Chapter 18

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  1. Chapter 18 Wheel Alignment Fundamentals

  2. Introduction • Correct wheel alignment provides for safe and comfortable driving while reducing tire wear. • This chapter discusses the principles of wheel alignment and alignment angles.

  3. Alignment Angles • Toe • Camber • Caster • Steering axis inclination (SAI) • Turning radius

  4. Toe • The most important tire wear angle. • Toe compares the measurement between the front and rear of the tires on the same axle.

  5. Toe(continued) • Toe in — tires are closer at the front. • Toe out — tires are closer at the rear. • One inch of toe would cause a tire to be dragged sideways 182 feet/mile!

  6. Toe(continued) • During an alignment, toe is set to a point that provides for the tire to be as close to zero as possible while driving. • Toe is always adjustable on the front. • Toe is adjustable on the rear of some vehicles.

  7. Toe(continued) • Front wheel toe does not cause a vehicle to pull to one side. • Rear wheel toe angles can cause a pull.

  8. Camber • The inward or outward tilt of the top of the tire. • A tire that tilts out at the top is said to have positive camber. • A tire that tilts in has negative camber.

  9. Camber(continued) • Camber is a tire wearing angle. • Excess positive camber causes wear on the outside edge. • Excess negative camber causes wear on the inside edge.

  10. Camber(continued) • Unequal camber on opposite sides will cause a pull to the most positive camber angle side. • Most vehicles are designed for some positive camber.

  11. Camber(continued) • A cambered tire will tend to roll in a circle. • Front wheel camber is usually adjustable.

  12. Caster • Caster is the forward or rearward tilt of the spindle support arm. • When the top is tilted to the rear, caster is positive. • When the top is tilted to the front, caster is negative.

  13. Caster(continued) • Caster does not normally cause tire wear. • On the front wheels, differences in the caster cause a vehicle to pull to the most negative angle. • Rear-wheel drive cars are more sensitive to caster angle differences.

  14. Caster(continued) • Positive caster helps maintain vehicle stability. • Positive caster also aids steering wheel return. • Think of bicycles — they always have positive caster.

  15. Steering Axis Inclination • SAI is the amount that the spindle support arm leans in at the top. • SAI is the angle most responsible for vehicle stability.

  16. SAI/Included Angle • SAI results in the lifting of the vehicle when the wheels turn, and gravity tends to make them return to center. • SAI and camber added equal the included angle.

  17. Scrub Radius • Scrub radius is the intersection between true vertical at the center of the tire tread and the SAI line extended. • If the intersection is below the road surface, the scrub radius is positive. • If the intersection is above the road surface, the scrub radius is negative.

  18. Scrub Radius(continued) • Scrub radius is important for steering, safety, and tire wear angle. • Raising or lowering the height, or the offset of the tires, changes the scrub radius.

  19. Turning Radius • When a vehicle turns, the inside wheel must turn at a sharper angle than the outside. • This angle is called the turning radius or the Ackerman angle.

  20. Tracking • Tracking refers to the way the front wheels follow the rear. • A vehicle that “dog tracks” means the front wheels do not follow the rear, causing excess tire wear.

  21. Setback • Setback refers to a condition causing one wheel to be behind the one on the other side of the car. • Can cause a pull, and incorrect toe. • Usually the result of a collision.

  22. Slip Angle • Tendency during a turn for the tire to continue to go in the same direction it had been going in after the rim had turned. • Affected by tire construction, pressure, and alignment angles.

  23. Oversteer and Understeer • Understeer — the vehicle does not seem to respond in a hard turn. • Oversteer — the vehicle seems to overreact to steering in a turn. • The best situation is called neutral steer.

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