| CASTER BASICS |
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| In a nutshell, caster is the angular change of the front steering axis. Positive caster is when the upper ball joint and resultant steering axis is tilted clockwise (toward the back of the wheel, and negative caster is when the steering axis is angled forward or counter-clockwise. The good news is we don't have to concern ourselves with a negative caster angle since we'll never use it. |
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| As you can see from the diagram above, caster is measured by the difference in the angle generated by the 'pivot line' (which is an imaginary line that runs from the center of the upper ball joint and the center of the lower ball joint) and vertical. So for example, when going for an alignment if you ask for 6 degrees of caster, the pivot line is angled back 6 degrees. |
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| WHY CASTER? |
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| The caster angle improves handling of the race car in a couple of ways: |
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| 1- Increases dynamic camber gain on the front wheels while turning - The benefit here is that we can gain extra camber in the corners without having to resort to added static camber on the front tires. Static camber negatively affects straight line acceleration since the tilting of the wheels reduces the contact patches of the tires. By utilizing caster, we can run less camber on the front wheels for better straight line speed, yet gain the benefits of cambered wheels when we turn in. |
| To provide a good explaination of how we gain camber in the turns by increasing the caster angle, I'll quote the following article: |
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| "The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it." |
| http://www.ozebiz.com.au/racetech/theory/align.html |
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| 2- Increased straight line stability - if we look at the diagram above and follow the red dotted line through both upper and lower ball joints (red dots) to where the line intersects the road, you see that the steering axis is located ahead of the tire's contact patch. Because the contact patch actually trails behind the steering axis, positive caster provides us with steering that is self centering when we drive in a stright line. |