At an impromptu shop discussion the other day, the subjects of caster angle, steering axis inclination, and scrub radius came up. Suspension geometry is a subject that most backyard mechanics don't really understand, so perhaps it should be this month's topic of discussion. (We could even get in deeper with instant centers, roll steer, and camber curves, but for now we'll limit the discussion to the first three.) In reality, these apply to both solid-axle and IFS systems.
Let's start out with caster. Mentally draw a vertical line between, and extending beyond in both directions, the upper and lower ball joints as viewed from the side of the vehicle. When this line is directly up and down, perpendicular to the ground, it-or actually the axle-is in what's called a zero-degree caster angle. We could also call this the "vertical center line.'' Rotate the axle so that the top of the line is rearward five degrees off center from the vertical center line. Now the axle is said to have five degrees "positive'' caster.
Caster is what causes the steering wheel to self-center after a turn and keep the vehicle going straight down the road. The more caster your truck exhibits-that is, the farther the caster line is away from the vertical center line-the longer the theoretical "movement arm'' is, which in turn exerts leverage to keep the wheels straight because the tire's center of drag is behind its steering pivot point. The next time you're pushing a shopping cart at the supermarket, note how the front "caster wheels'' work, and your confusion should instantly end. The more caster we put into a front wheel, the straighter it will run. However, the drawback is more tire wear, poorer cornering, and harder steering. Why? We'll get into that in a minute.
Next, take a look at your rig from the front. Note that the upper ball joint is not directly in line with the lower one, but is offset. This is called "steering axis inclination.'' Draw a line downward, through and extending past these ball joints to the road surface. With this in mind, draw another line directly downward, through the center of the tire to the road surface. These two lines should meet at the road surface, which is then referred to as zero "scrub radius.'' This is the point where the tire pivots when turning. The wider apart these two lines are, the more scrub radius you have. When the tire doesn't pivot exactly in the center, it also has to move sideways as it turns, and consequently it "scrubs.'' Large scrub radius causes hard steering, steering kickback, and excessive camber change. Not only does the tire wear when it scrubs, but the camber change causes excessive shoulder wear.
Whoops! I guess I should explain "camber.'' This is the tilt of the tire inward or outward from a vertical plane as viewed from the front or rear. When the top is tilted inward, it's called "negative'' camber; tilted outward, it's "positive'' camber. It's easy to visualize that if a tire's flat tread surface doesn't make even contact with the ground or road surface, excessive wear in one area can take place.
Let's go back to scrub radius for a bit. What happens when you go a tire and wheel combination that is wider, or has a different offset and/or backspacing than what OEM engineers designed? The scrub radius naturally changes. That's why it's really important to maintain the same backspacing, even when going to a wider rim. Generally speaking, with a wider rim, backspacing must be increased by an amount equal to half the additional rim width to maintain the same scrub radius. However, this is usually not possible because of interference problems when turning, so we do the next best thing and place the additional width outward, leaving backspacing to OEM specifications. We'll increase our scrub radius, but only by half the amount had we moved the center an equal amount.
One of the worst examples I can think of is using wheels designed for a solid-axle frontend on an IFS system. In this case, not only do we have a scrub radius problem and a major camber change when turning, but extra leverage exerted on the steering and suspension components as well.
On a racing vehicle with its independent-arm suspension, steering axis inclination can be radically altered through design as well as changes to camber and caster. But on our 4x4s, we're limited to minor caster and camber adjustments.
Let's go back to caster again. With positive caster, when the steering is turned, the spindle on the side being turned will move downward in a arc and the opposite side will turn upward. This causes a positive camber effect, and the more scrub radius there is enhances that camber change. This also shifts weight to the outside edge of the tire, at the same time slightly raising the vehicle's ride height due to suspension unloading. Now you can see why it's harder to turn and the wheels want to resist, trying to remain straight.
What it all boils down to is, you want just enough caster to maintain steering stability and the proper balance of scrub radius. It's not always possible with wide aftermarket tires and wheels, so we do the next best thing by rotating the tires more often to prevent irregular wear patterns and live with the problem.