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Introduction:
                Looking at the car side-on, imagine an axis line drawn through the steering axis of the wheel, so on a car with a McPherson-strut this line would be straight along the vertical axis of the strut. Positive castor occurs when the top of the axis line tilts more towards the rear of the car (the green line in the diagram).Negative caster is the opposite (the red line).
 

Effect of positive caster:

The benefit of positive caster is that it provides good self-centering of the steering, allowing the car to travel in a straight line without constant holding of the wheel.

 

Effect of negative caster:

Increasing negative caster will increase steering effort, weight and feel at the expense of self-centering.

Function of a king-pin:

The function of king pin is to

• Help the straight ahead recovery of vehicle, which helps to provide directional stability also.

•  When vehicle turns, king-pin causes body to move-up according to wheels.

 

Meaning of caster:

It is the backward lift of king pin and axle at top.

• It gives direction stability to travel in straight direction with minimum effort by the driver with the help of steering.