The yellow arrow in the drawings below denotes the direction of travel of the model car. Caster is basically used to add to or detract from the directional stability of your model car. How the caster is set depends on the the method of drive that the car has, front wheel drive cars usually have very different caster settings than a rear wheel drive car, for example. The trailing effect that is caused by the horizontal difference between the centres of the top and bottom pivot points is the most important aspect to consider because, as stated above, front and rear wheel drive cars are ordinarily configured with different caster angles. In the images below the green patch represents the contact patch area of the tyres.
Neutral caster has the upper and lower pivot points aligned vertically.
The forces that bear down on the car and the wheel have only a single vertical point of contact which is at the mercy of any external forces that may act upon it. Any car set with neutral caster will be an unstable one. Negative caster , which is sometimes referred to as leading caster, has the upper pivot points positioned in front of the lower pivot points. Because of the horizontal offset between these two pivots the forces that bear down on the car are transmitted with a forward bias. Depending on the driving configuration of the car, this either adds stability or makes it un-drivable Positive caster , which is sometimes referred to as trailing caster, has the upper pivot points positioned in behind of the lower pivot points.