caster

TireRack.com:[1]
The caster angle identifies the forward or backward slope of a line drawn through the upper and lower steering pivot points when viewed directly from the side of the vehicle. Caster is expressed in degrees and is measured by comparing a line running through the steering system's upper and lower pivot points (typically the upper and lower ball joints of an A-arm or wishbone suspension design, or the lower ball joint and the strut tower mount of a McPherson strut design) to a line drawn perpendicular to the ground. Caster is said to be positive if the line slopes towards the rear of the vehicle at the top, and negative if the line slopes towards the front.

diag_caster.gif

A very visual example of positive caster is a motorcycle's front steering forks. The forks point forward at the bottom and slope backward at the top. This rearward slope causes the front tire to remain stable when riding straight ahead and tilt towards the inside of the corner when turned.

Caster angle settings allow the vehicle manufacturer to balance steering effort, high speed stability and front end cornering effectiveness.

Increasing the amount of positive caster will increase steering effort and straight line tracking, as well as improve high speed stability and cornering effectiveness. Positive caster also increases tire lean when cornering (almost like having more negative camber) as the steering angle is increased.

What's the downside to positive caster? If thevehicle doesn't have power steering, a noticeable increase in steering effort will be felt as positive caster is increased. Other than that, the effects of positive caster are pretty much "positive," especially increasing the lean of the tire when the vehicle is cornering while returning it to a more upright position when driving straight ahead.

Bibliography
1. TireRack.com - Tire Tech Information - Alignment [http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=4]

Adjustments

Caster – Typically adjustable for most cars with a separate adjustment on the left and right front. Some stock or spec cars may have no adjustment or a very small adjustment range.

Tuning advice

Caster changes in road racing are best used to adjust the amount of steering wheel feedback the driver wants to feel. This effect is generally positive until instability occurs in corner exit or all the way through high‐speed corners. Because of the benefits to stability and camber, generally it is best to run as much caster as possible before steering feel or stability is negatively affected.

Most oval cars maintain somewhere around a two‐degree split in caster left to right. The left will typically have a lower caster setting than the right. Less caster in the left front will help the car turn, particularly from corner entrance through the center of the corner. Closing the split between left and right will tighten the car, while increasing the split will loosen it. More caster in the right‐front will help the car turn, particularly from the entrance to the turn through the center.

Interactions

Depending on the suspension design, caster changes often result in changes to the toe and/or camber settings.

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