| -- End Ad Box ---> | | | | steering axis is tilted to the point where it is horizontal. |
| Caster is the angle between the steering axis and the | | | | When you turn the steering wheel in this case, the |
| vertical axis as viewed from the side of the car. | | | | front wheels would stand up on their edges. If you turn |
| Caster affects straight-line stability and "camber gain". | | | | left, the left tire will stand on its outer edge, and the |
| Positive caster is when the top of the steering axis is | | | | right tire will stand on its inner edge. If you turn right, the |
| tilted back (steering axis intersects the ground in front | | | | left tire will stand on its inner edge the right on its outer |
| of the tire contact patch). Negative caster is when the | | | | edge. The same type of camber gain, only on a |
| top of the steering axis is tilted forward (steering axis | | | | smaller scale, takes place with less caster. This |
| intersects the ground behind the tire contact patch). I | | | | camber gain is exactly what you want in a corner. |
| have never seen negative caster used in suspension | | | | Read my accompanying on camber to see what it is |
| setup, and I do not believe it is beneficial in automobile | | | | and why it’s beneficial. |
| suspension geometry. Therefore, for the rest of this | | | | When the tires stand up on their edges, the front of |
| section, when I refer to caster, I am talking about | | | | the car is actually raised up. This is why the wheels |
| positive caster. | | | | "center themselves" when you let go of the steering |
| To visualize caster, think about the wheels of a | | | | wheel. The weight of the car pushes the wheels flat |
| shopping cart. The steering axis of the wheel | | | | on the ground, which resets the steering. This improves |
| intersects the ground far ahead of where the wheel | | | | high-speed stability because it keeps the steering firmly |
| touches the ground. As a result, the wheel is essentially | | | | in the center position. However, it is difficult to turn a |
| dragged behind the steering axis. This keeps the wheel | | | | car with a large caster setting because, while turning, |
| moving straight. If the steering axis intersected the | | | | you are actually lifting the front of the car with the |
| ground at the same spot that the wheel touched the | | | | steering. This effect is most visible in luxury sedans, |
| ground, then there would be no caster effect. The | | | | where high-speed stability is important and |
| wheel would be free to spin around the steering axis | | | | sophisticated power steering makes up for the extra |
| as long as it was not held in place by some other | | | | steering effort. If you watch one of these cars as the |
| force. | | | | wheels turn to full lock (maximum steering angle), you |
| Unlike in a shopping cart, the steering axis on a car is | | | | will see the front end of the car rise slightly. |
| placed close to the hub of the wheel. Therefore, the | | | | Increased caster is advantageous for racing and, in |
| only way to make the steering axis intersect the | | | | some cases, street driving. The only disadvantage is |
| ground ahead of the tire contact patch is to tilt the | | | | the added steering effort. While camber gain due to |
| steering axis back. The more the axis is tilted (in the | | | | caster is generally good for increasing the grip of the |
| positive caster direction), the greater the caster effect. | | | | front tires in a corner, too much camber gain will cause |
| Large caster settings increase the tendency of the | | | | the tires to heat up, lose grip, and wear out |
| front wheels to center themselves. This tendency is | | | | prematurely. Therefore, do not use more than a few |
| mainly due to the camber gain that occurs when the | | | | degrees of caster. If your car uses a MacPherson |
| steering axis is tilted and the wheels are turned. | | | | Strut suspension, it may be necessary to modify the |
| Camber gain involved with caster is not easy to | | | | strut tower mounts or tension rods to be able to |
| visualize. Think about the extreme case where the | | | | adjust caster. |