Steering Geometry
Casteris the angle at which the steering pivot axis is tilted (forward or rearward) from true vertical (to the ground's plane). 4x4s usually have a positive caster, where the pivot axis is tilted rearward (with the upper ball joint or kingpin being farther back than the lower one). Caster can change when you change the lengths of control arms, do a shackle reversal, or change the size of shackles (tilting leaf springs one way or another and therefore rotating the axle). Having too much positive caster on a truck will give it some funny driving characteristics and make the tires feel like they are turning under the truck, not to mention increasing steering effort. Not enough positive caster, and it will feel like the truck is wandering. Caster also helps keep the wheels returning to center and tracking straight when moving forward and no force is applied to the steering system (think about how a shopping cart keeps its front wheels straight when being pushed).
Camber is the angle of the wheel from side to side. Camber isn't really adjustable on solid-axle 4x4s, and if you have a change in it, you should be concerned because you probably have a bent axle. Having too much camber (positive or negative) can prematurely wear tires. Camber has a major effect on the handling of a vehicle because as a vehicle leans, so do its tires, so angling the tire one way or another will put either more or less tire contact patch on the ground when cornering. A tire with a small negative camber angle will have its maximum cornering force.
Scrub Radius (Kingpin Offset) is the distance between the two points derived from the intersection of the wheel centerline and the steering axis with the ground plane. This is also referred to as kingpin offset. The centerline of the wheel is taken at the center of the tire tread's contact patch. The steering axis is determined by the straight line plane passing through the upper and lower ball joints (or kingpins) and the U-joint. Positive scrub would have the wheel centerline outside of the steering axis. Zero scrub would have the two planes (wheel centerline and steering axis) meet exactly at ground level. And negative scrub would set the wheel centerline inboard of the steering axis. The scrub radius is affected by camber, the kingpin angle, wheel offset, and tire diameter.
Kingpin Inclination (KPI, or Steering-Axis Inclination) describes how much the steering knuckles are angled in, putting the top of the kingpin (or ball joint) inboard of the lower one. This is known as kingpin inclination, or steering-axis inclination if you have ball joints and not kingpins. It is a directional control angle measured (in degrees) from the centerline of the kingpin (or ball joint) to true vertical. KPI is not adjustable (a bigger tire or different wheel offset will not change the angle of inclination) and has a direct relationship with a vehicle's camber as one main purpose of it is to reduce the need for excessive camber. Have you ever noticed the outside tire will almost seem to raise the vehicle up in a turn, while the inside seems to drop it down a little? This isn't your imagination: The KPI (or SAI) is designed to move the knuckle down from its highest point when turned to the inside or leading wheel. The outside knuckle will angle the opposite way, raising the knuckle a little. This will in effect raise one side and lower the other side of your vehicle in a turn, putting more opposing force against the weight transfer of your vehicle in a turn.
Ackerman Angle In the 1800s, Rudolph Ackerman figured out a solution that corrected the problem of steering arms that kept perfectly parallel throughout a turn. Parallel steering arms do not work well during a turn since the inside tire wants to travel a smaller circumference arc than the outside tire. This produces an enormous force that causes the wheels to either scrub or "buck" as one or another wheel breaks traction. Ackerman came up with a steering arm design that allows the inside tire to turn at a greater angle than the outside tire. This steering arm angularity is directly related to the distance from the center of your rear differential to the front axle. Ideally you would want to draw a straight-lined "V" from the rear axle's centersection, through the kingpins or ball joints, directly to the tie-rod end mounting points. Obviously, not every vehicle designed has a perfect Ackerman angle (for example, the same steering and solid-axle setup is used on Blazers and Suburbans, which obviously have different wheelbases).