Steering Geometry and its types
Definition: - Camber is the tilt of the car wheels from the vertical. Camber is positive if the tilt is outward at the top. Camber is also called ‘wheel rake’.
Effect: - It is always desirable that tyres should roll on the ground vertically so that the wear is uniform. If while running, the tyres are uninclined from the vertical either inward or outward, they wear more on one side than the other.
Amount: - Camber should not generally exceed 2°. However, the exact amount of camber is specified taking into account the king pin inclination.
KING PIN INCLINATION (STEERING AXIS INCLINATION):-
Definition: - Inclination of the king pin from vertical is called the king pin inclination or king pin rake. In modem cars where the king pin has been replaced by the ball joints, this term has also been renamed as ‘Steering Axis Inclination’ and is defined as the inclination of the ball joint-axis from the vertical. Steering axis is an imaginary line drawn through the lower and the upper steering pivot points.
Effect: - King pin inclination (or steering axis inclination) helps the straight ahead recovery, thus providing directional stability. When the vehicle takes a turn, the inclination of king pin causes the vehicle body to move up, in relation to the wheel. So as soon as the steering wheel is left after the turn is completed, the weight of the vehicle tends to return the wheels to the straight position.
Amount: - About 7 to 8 degrees. However the exact amount is decided considering the wheel rake value
COMBINED ANGLE AND SCRUB RADIUS:-
Definition: - Combined angle or included angle is the formed in the vertical plane between the wheel centre line and the king pin centre line (or steering axis). Combined angle is equal to camber plus king pin inclination (or steering axis inclination).
Effect: - The effect of combined angle variation on scrub radius and hence on the forces acting to turn the wheel in case of a rear-wheel drive. It is seen that unless scrub radius is zero a torque acts to turn the wheel away from the straight ahead position.
Amount: - Combined angle may be 9-10 degrees and the scrub radius should be up to about 12mm.
Definition :- The angle between the king pin centre line (or steering axis) and the vertical, in the plane of the wheel is called the castor angle. If the king pin centre line meets the ground at a point in form of wheel is called positive castor while it is behind the wheel centre line it is called negative castor.
Effect:- In rear-wheel drive vehicle, the steering axis pulls the front tyres, whereas the tire drag on account of the vehicle weight is on the vertical line at the centre of the footprint. Since in positive castor steering axis would meet the ground ahead of the centre of tire print, the later would always follow the former. Thus positive castor on the car wheels provides directional stability.
Castor has another effect also, when the vehicle having positive castor takes a turn, the outer side of the vehicle is lowered while the inner one is raised i.e. positive castor help the centrifugal force is rolling out the vehicle. Negative castor tends to ‘roll-in’ the vehicle i.e. the effect of centrifugal force is counteracted.
Amount:- About 3° of castor gives good result.
TOE-IN OR TOE-OUT :-
Definition :- Toe-in is the amount by which the wheels are set closer together at the front than at the rear when the vehicle is stationary. On the other hand, the wheel may be set closer at the rear than at the form in which case the difference of the distances between the front wheels at the from and at the rear is called toe-out.
Effect:- There is usually an inherent tendency for the wheels to toe-out because of purposeful deviation from centre point steering and also due to errors in steering angles of the inner and outer wheels on moderate bends. To offset this tendency a small amount of toe-in is initially provided so that the wheels move perfectly straight ahead under normal running conditions. However in case of some front wheel drive cars, initial toe-out has been provided to counter the tendency to toe-in present therein. The toe-in or toe-out as provided is sometimes called wheel alignment.
Amount:- Toe-in initially provided generally does not exceed 3mm.
Other Important Steering Geometries Terminologies
Slip Angle: The angle between direction of the motion of the vehicle and the centre plane of the tyre is known as Slip Angle. It ranges from 8º to 10º.
Under steer: When the front slip angle is greater than that of rear, the vehicle tends to steer in the direction of side force. Then it is known as under steer. This provides greater driving stability, especially when there is a side wind.
Over Steer: When the rear slip angle is greater than that of front slip angle, the vehicle tends to move away from the direction of centre path. This is known as over steer. This is advantageous when the vehicle moving on the road having many bends curves.
Steering Gear Ratio or Reduction Ratio: It has been defined as the “number of turns on the steering wheel required to produce on turn of steering gear cross shaft to which the pitman arm is attached. Generally it varies between 14′.1 and 24′.1.
Turning Radius: It is the radius of the circle on which the outside front wheels moves when the front wheels are turned to their extreme outer position. This radius is 5m to 7.5m for buses and trucks.
Wheel Alignment: It returns to the positioning of the front wheels and steering mechanism that gives the vehicle directional stability, reduce the tyre wear to a minimum.
This post is under Steering System. All topics under Steering System are:
4. Steering Geometry