Posted by Janicholson on February 02, 2010 at 09:29:43 from (199.17.6.222):
In Reply to: Re: NF wedge-plate posted by rustyfarmall on February 02, 2010 at 07:51:50:
Clipped from a Honda Forum, the following explains it as clearly as I have found. The issue is centered (no pun) on the idea that on a car the pivot axis is thriugh the spindle. on a Narrow front tractor (with a bolster set perpendicular to the ground) the axis angle is fixed, and the only caster adjustment is in Trail as described below. The more trail, the more positive caster. The Wedge plates installed with thin to front, reduce caster and make steering easier, compensating for the positive caster that the IH engineers built into the tractor. Turning it around Fat in front increases trail, and effort to steer, and resistance to wobble (but not much) With the wheels straight ahead, on concrete, parallel to and near a building, measure from the front bolster to the wall. Now turn the steering full toward the wall and remeasure. The change in the distance is a reflection of caster. Zero caster will be no change, Positive will be Shorter distance to the wall, and negative will be greater distance. (the same as turning a caster wheel by hand and seeing the cart move.) JimN
“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.”
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