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Fundamentals Of Control

When looking for guaranteed ways to transform the handling and driving characteristics of a muscle car, most rodders go straight for suspension upgrades. While that's not an incorrect plan of attack, the issue is that far too many will stop there and forgo the upgrades to the one system that most completely transforms the feel of a car: the steering.
Early on in the invention of steering systems, most were arranged in a more or less parallel design, meaning that both front wheels turned the same amount in a turn. This works well for low-speed performance, like in the buggies and carts it was originally designed for, but it causes a great deal of problems as speed increases.
Caster and camber are the two principles most enthusiasts are familiar with since they are two of three components addressed in a standard front-end alignment. To understand caster, it's important to understand the concept of trail, which is most easily demonstrated by a shopping cart's front wheel. The steering axis is located ahead of the wheel, so when the cart is pushed forward the wheel will follow directly behind the steering axis, making it self-straitening and therefore stable and easy to control. If the steering axis were placed vertically above the wheels, there would be zero caster effect and the wheels would tend to wander. The longer the distance between the steering axis and the wheel, the greater the force, or trail, exerted.
These two aren't settings, but rather the result of all of the steering and suspension chassis choices made. Understeer, also known as plow or push, is the tendency of a vehicle to resist making a turn and "push" toward the outside of the curve. What's actually happening is the slip angle is greater at the front than the rear, due to alignment settings, center of gravity, soft suspension, aerodynamics, or a combination of these and other influences. Since understeer is inherently easier to predict and control, and prevents pushing a car to its traction limit, typically new car manufacturers err toward understeer on factory alignment settings in an effort to keep unskilled drivers safe. In racing circles where chassis are dialed-in for lap times, however, understeer is typically used to refer to the inability to follow the desired line through a corner when a vehicle has hit its traction limit.

Wheel Diameter
This is probably the most basic concept that many people overlook when upgrading muscle cars. Most classic cars have steering wheel diameters 15 inches or larger because a larger wheel offers more mechanical advantage, which makes steering easier, especially in cars without power assist. Nevertheless, it also works to decrease the effective steering ratio. Simply by dropping the steering wheel diameter, say from 15 to 13 inches with a custom steering wheel, the steering response will be noticeably quicker. It'll also be noticeably harder to turn at slow speeds, necessitating either power or "Armstrong" steering, but the benefit can be almost as transforming as changing the steering box ratio.
I found that 14 inches in my 1968 camaro was about perfect

Slip Angle
The nomenclature is a little misleading here since slip angle doesn't pertain to actually slipping or sliding of the vehicle . What it's referring to is the deformation of the tire under load, resulting in a difference of angle between the contact patch of the tire relative to the angle the wheel is steered. You're more familiar with this than you might think; imagine a standard sedan with 15-inch wheels and 70-series tires taking a fast, sharp turn. The tire's carcass and tread rolls and deforms, changing the tire's contact patch causing loss of traction.
Scrub Radius
Scrub radius is basically the centerline of the wheel relative to the steering axis inclination (SAI). SAI is easiest to explain by breaking the term up. The steering axis is the line between the top pivot point of the spindle (the upper ball joint on cars with upper and lower control arms) and the lower ball joint. The inclination of the steering axis is the angle between the steering axis and the centerline of the wheel. Now to find the scrub radius, we follow the SAI all the way to the ground, and measure the distance between that point and the centerline of the tire patch. If the tire contact patch is outside of the SAI pivot, the scrub radius is positive. If it's inboard, it's negative. Unless you're swapping to a new style suspension system, or willing to do some complicated customizing, scrub radius is set at the factory and not adjustable.
Toe Angle
At its most basic, static toe angle is just the degree the front wheels deviate from parallel to the centerline of the vehicle. On the road, near-zero toe is ideal for tire wear, but due to the flex created by the numerous suspension bushings, compensation is necessary. To get close to parallel at speed, rear-wheel-drive cars always require a slight amount of toe-in, because the forward thrust from the rear wheels causes the compliant rubber bushings in the front suspension to flex rearward slightly. Front-wheel drive is exactly opposite, and requires toe-out, but for the same reasons. Cars upgraded with Heim joints rather than bushings experience far less flex.
Most of you have felt this at some point, especially if you've ever lowered or raised a vehicle, or changed the suspension significantly without compensating for the resultant change in steering geometry. Typically, bumpsteer manifests itself as a tendency for the front end to dart or wander without driver input, especially on a less-than-ideal surface, forcing a concentrated effort to keep the vehicle in a straight line. What's actually happening is the wheel is steering as the suspension moves over the road irregularities because the length between the spindle and the rack or gearbox lengthens, but the tie rod does not. The result is the spindle rotating, or toeing, outward slightly to compensate. Depending on the severity, on the street it can be a minor hassle all the way up to dangerous; on the track excessive bumpsteer is always a liability since it will limit control and traction if a bump or dip is encountered while cornering.

Most modified cars have some small degree of bumpsteer; the real goal is simply to make it small enough that it doesn't interfere. To achieve that, the tie rod has to travel on an arc parallel to the one the spindle follows. That's why bumpsteer often increases noticeably when lowering or raising a vehicle-it changes the angle of the tie rods and consequently the arc on which they travel. The only way to combat it is to try and get everything back in line; in the case of lowered cars, by either raising the rack or steering box (usually not an option), or lowering or raising the outer tie-rod attachment point at the spindle accordingly.

This is a term that gets thrown around mostly in racing circles, but having a grasp on what it means will still help with understanding the rest of the concepts. To locate an object in space, we need three things: the X-, Y-, and Z-axis coordinates. When relating to cars, the X-axis is lengthwise, and the Y-axis is side-to-side. However, that doesn't tell us anything about how the object is oriented. Yaw is the concept that describes angular motion and how a car rotates around the Z, or vertical, axis. By definition, a car is always in yaw through a turn, simply because it is headed in a different direction than the nose is pointing-which, of course, means that yaw is related to the slip angle.