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5 minute(s) of a 612 minute read
10-22-2011
Very nice post on caster -- rarely seen discussed! BTW if you ever wonder what caster looks like, look at the 2 front wheels of Supermarket's cart next time you go shopping LOL - how the axis come at an angle and allow the front wheels to steer and straighten itself as you push the cart.
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http://www.planet-9.com/cayman-boxster-modifications/16158-installing-gt3-front-lower-control-arms-2.html
How cool! A way to progressively increase the tilt of the tire as you turn the car harder! Let's put A LOT of that on! And, if you look at the front wheels of almost any German high-end car when it's parked with the wheels turned sharply. Others are catching up, but I really am amazed at how much caster is dialed in to some of these suspensions.
But the benefit doesn't stop there. Positive caster also makes the steering wheel more self-centering and everyone likes that, right?
Well, like everything else, there's a limit. You don't get a corresponding camber change at the back, so you can really create some wicked oversteer situations on a rear drive car, especially one with a solid axle.
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http://www.ozebiz.com.au/racetech/theory/align.html
THE EFFECTS OF CASTER
Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative.
Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castering front wheels of a shopping cart (above). The steering axis of a shopping cart wheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as "trail."
Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved.
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.
Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.
Like a shopping cart wheel (left) the trail created by the castering of the steering axis pulls the wheels in line.