Steering Geometry.pptx

ppt

1. Steering Geometry Unit 4

2. Topics: Ackerman steering principle King pin inclination Slip angle Camber angle Castor angle Toe angle Toe out on turns

3. ‘Ackerman’ Steering When a vehicle travels round a bend , the inside wheel must follow a tighter curve than the outside wheel . To achieve this, the geometry of the steering must be arranged to turn the inside wheel through a larger angle than the outside wheel . The ‘Ackerman’ steering geometry provides a simple solution to this problem.

4. Principle When a four wheeler (car) takes a turn , all its four wheels should roll without slipping laterally . This is possible only when the axes of four wheels intersect at one point . This point is the centre about which the vehicle turns at that instant. At this instant, rear wheels rotate along two circles, where the centre of two circles is at ‘O’. The front wheels have their different axes. These wheels also rotate along two other circles with same centre ‘O’. For correct steering, the centre of the wheels of the rear axles and centre of front wheels must coincide . This geometry results in the inside wheel turning through a smaller radius than the outside wheel. This allows the vehicle to travel around a curve without scrubbing the tyres .

5. Ackerman’ Steering principle

6. Steering Axis Inclination/Kingpin Inclination It is the angle between king pin centre line and vertical line when seen from the front of the vehicle. King pin inclination and castor are used to improve directional stability in cars. Because of these provisions wheels tend to return to the straight ahead position after the vehicle completes any turn (due to steering left or right). This is also used to reduce steering effort when steering a stationary vehicle. In addition to this, it reduces tyre wear. This inclination varies from 4 to 8 degrees in modern cars. It should be equal on both sides, i.e. on both front wheels.

7. Slip angle The whole force acting on the car body must be carried to the road by the tyres . Side ways forces acting on a car when cornering causes the tyres to distort, so that the tyre tread and consequently the car, follow slightly different paths than the direction the wheel is pointing. The forces and distortion is absorbed in the thread of the tyre . The difference between the direction of travel and the wheel is called the slip angle . A slip angle will always exist when sideways forces are acting on the tyre e.g. when the vehicle turns in a curve. Intuitively one can see that an improper slip angle causes the rolling resis-tance to increase causing excess tyre wear. More than 40% of fuel energy is typically used to overcome tyre rolling resistance. It follows that even a modest increase in rolling resis-tance increases fuel consumption. The Slip Angle increases through low tyre pressure .

8. Camber Angle The camber angle is the inward or outward lean of the wheel relative to the vertical reference. (or) Camber angle is the angle between the vertical line and centre line of the tyre when viewed from the front of the vehicle.

9. Camber Angle Camber angle is positive when this is outward. This happens when wheels are further apart at top than at bottom. On the contrary, camber angle is negative when angle is inward. This happens when wheels are further apart at bottom than at top. The camber, should not be more than 2degrees , because this causes uneven or more tyre wear on one side than on other side. The front wheels are usually fitted with positive camber angle . This is done to prevent tilting of top of wheels inward due to excessive load or play in the king pin and wheel bearing. The load brings the wheels to vertical position. Excessive camber is not good because it prevents proper wheel contact with the road. Unequal camber causes the vehicle towards the direction in which camber is more. This disturbs the directional stability.

10. Castor Angle The castor angle is the rearward lean of the steering axis relative to the vertical reference.

11. Castor Angle The main purpose of the castor angle is to create a self-centering effect in the steering. Tilting the steering axis in this way means that the driving force acts at the point where the castor angle intersects the road . The resistance between the tyre and the road creates an opposite force that acts along the axis of the tyre . The effect is to generate side force, pushing the tyre back in line with the driving force. The further away from the straight ahead position the greater the side force. The difference between the vertical reference and the point at which the castor angle intersects the road is called the castor trail . The larger the castor trail the greater the self centring effect. Increasing the castor angle will increase the weight of the steering. Generally, larger castor angles are used on higher performance vehicles to maximize stability at speed. The trade-off for increasing the castor angle is increased steering effort and tyre wear.

12. Toe Angle The toe describes the angle of each wheel relative to the centre line of the vehicle when viewed from above. The ideal toe angle should ensure that the front and rear wheels are parallel as the vehicle is driving along the road. To achieve this, the static toe angle will have to be set to accommodate the movement in suspension linkages and steering joints. The toe angle can be described as an angle or in terms of the difference measured between the front and the rear of the wheels on an axle. Correct toe adjustment is necessary to ensure that the vehicle drives in a straight line. Incorrect toe can lead to pulling problems and increased tyre wear.

13. Toe angle

14. Toe angle Measuring methods

15. Toe out on turns Turning angle is the steering angle that controls the amount of angle each individual wheel turns while cornering . During cornering, the inside wheel must turn at a greater angle since it is turning at a smaller radius . This is accomplished by the angles built into the steering linkage. The linkage trapezoid with the steering arm and tie rod, is designed so that the center lines of the inner and outer wheels are intersecting at a point on a line through the rear axle. This will give an ideal track for each wheel when turning. The difference in turning angle is called Toe Out on Turns . It is measured with the inner wheel at an angle of 20°. An incorrect Toe Out on Turns indicates that parts of the steering linkage is damaged . The maximum turning angle for each wheel is normally also measured. A difference in the values will indicate that the steering linkage is not centered or damaged . The measurement of all these angles is done very easily and accurately using electronic turntables.

16. Toe out on turns

17. END