Caster Angle is an important setting to achieve desired handling characteristics.
Race cars almost universally employ positive caster. The steering axis is "leaned back" at the top, which places the tire contact patch behind the point where the steering axis intersects the road.
Positive Caster create Five (5) effects:
1) Steering Effort and Straight-Line Stability. Because the contact patch is behind the steering axis's intersection with the road, increasing positive caster increases the force necessary to drag the tire sideways. This occurs on both front tires and is greatest at beginning of a turn when the wheels are starting out in the straight ahead position.
2) Camber Change. With positive caster, the negative camber of the outside tire is increased while the negative camber of the inside tire is decreased, or the positive camber of the inside tire is increased. This is usually beneficial as it compensates for chassis roll and generally increases the grip of the front tires.
3) Cross Weight. Because the steering axis in inclined, in a turn, the outside tire is lifted away from the road, and the inside tire is pushed down toward the road. When the inside front tire is pushed down, it also increases the weight of the outside rear tire, essentially creating a negative change to cross weight. The more steering input, and the more positive the caster, the more effect. This essentially makes the car have less understeer or more oversteer as steering input is added.
4) Auto Steering. The same principle described regarding Cross Weight change also can be used to make the car turn more easily on oval tracks. On oval tracks, the inside tire is usually set to have less positive
caster which actually causes the car to turn left with little or no
steering effort.
5) Effect of Steering Corrections. The same principle regarding Cross Weight when turning produces the exact opposite effect when making a steering correction to an excessive oversteering situation. Turning the steering the opposite direction of the normal turn will push the outside tire down, placing more weight also on the inside rear. The more positive caster, the more exaggerated and rapid this change in handling occurs--sometimes large positive caster settings results in the driver over-correcting.
So, on a race car, caster essentially is a tool to change the handling characteristics of the car during turning. More positive caster makes the car looser in mid corner and more stable in a straight line.
Many, many years ago, my Father taught me about caster by using a pencil and a pin. He took the pin and stuck it into the side of the pencil at a 90 degree angle. Then he leaned the pencil back and rotated it. The pencil was the steering axis (king pin in the old days) and the pin was the wheel axle. Rotating the pencil counterclockwise with the pin on the right illustrated the outside tire in a left turn--the pin went up. Rotating the pencil counterclockwise with the pin on the left illustrated the inside tire in a left turn--the pin went down. My first lesson was 56 years ago when I was 8, but I have used the concept ever since.
