Performance Perspectives

Motor,  Nov 2006  by Mavrigian, Mike

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All of the familiar rules apply when aligning a performance vehicle. But take note of any unusual vehicle characteristics or modifications that may affect a wheel alignment.

Maintaining correct wheel alignment angles, important with regard to any vehicle, becomes even more critical for performance vehicles. Performance vehicles, in the broad reference, are vehicles that at least have the potential to be driven at higher speed than production vehicles. In addition, tire wear is an issue that becomes more important due to the use of wide, low-profile (and expensive) tires, or the use of expensive vintage/classic reproduction tires. Also, most street performance and collector-car enthusiasts are perfectionists, and demand proper handling and braking. So they're not likely to accept any form of directional pull or lack of steering response.

Here, we'll concentrate on wheel alignment as it applies to street (roadworthy) applications. The issue of wheel alignment angles that are required for dedicated racing applications represent a host of different and more complex challenges.

Wheel Alignment Types

There are three wheel alignment types-two-wheel (also known as geometric centerline), thrust line and total four-wheel.

The geometric centerline (viewed from above the vehicle) is an imaginary line drawn from the center of the rear axle to the center of the front axle. This line simply follows the centerline of the vehicle chassis, from the halfway point between the rear wheels to the halfway point between the front wheels.

The thrust line denotes the actual direction of the rear wheels (a right-pointing thrust line or a left-pointing thrust line). This is the real-world direction in which the rear wheels are aimed, irrelevant of the geometric centerline. You can also view the thrust line as a line that divides the left and right rear wheel toe.

Thrust angle is the angle formed by comparing the chassis geometric centerline to the rear wheel thrust line. This angle is measured in degrees. A right-aimed thrust angle that deviates from the centerline is referred to as positive, while a left-aimed thrust angle is negative.

Centerline steering simply refers to a level steering wheel position when the vehicle travels straight ahead. A noncentered steering wheel is an indication of a probable dirust angle problem.

The goal, regardless of what type of vehicle is involved, or what alignment approach is used, is always the same: to create a parallel direction of travel for both front and rear wheels when the vehicle travels in a straight line.

In order to achieve this wheel-parallel state, there are three types of alignment approaches, as mentioned earlier. Let's examine each.

A geometric centerline alignment involves aligning only the two front wheels responsible for steering, using the vehicle's geometric centerline as the only reference. This is the old-school approach to wheel alignment, and should now be considered obsolete.

When you align the wheels on a vehicle, you're adjusting the wheel direction based on a point of reference. That point is critical, since it provides the entire basis of your alignment work. If you refer only to the geometric centerline (the center of the rear axle), you're not considering the thrust direction of the rear wheels at all-a huge mistake.

A thrust condition ahvays causes the front wheels to steer opposite the direction of nie thrust line in order to maintain vehicle direction. If the front wheels were not adjusted parallel to the thrust line, constant steering input is needed by the driver, premature tire wear (both front and rear) will result and poor directional control is a constant.

The center of the rear axle (the halfway point between the two rear wheels) can be located, with total disregard to the rear axle setup. The rear axle may be offset from center, or it may be "crooked," causing the rear wheels to point right or left. When you consider only the geometric centerline of a vehicle as the point of reference for a front-wheel alignment, you're assuming that the rear axle is located where it's supposed to be. That's a big assumption.

A thrust line alignment considers the actual location and direction of the rear wheels. Never assume that the thrust line is parallel to the geometric centerline. When adjusting the direction of the front wheels, the frontwheel alignment should be set parallel to the direction of the rear wheels, in reference to the actual thrust une.

When aligning the front wheels on a vehicle that offers no rear-wheel adjustment, setting the front wheels according to the thrust line is the only accurate method of front-wheel alignment. The thrust line may not be parallel to the vehicle body, but at least the alignment technician can adjust the front wheels parallel to the rear wheels.

A total four-wheel alignment is possible on a vehicle that features both front- and rear-wheel adjustments. Once you know the thrust line of the rear axle, you can determine the thrust angk. Thrust angle is found by comparing the geometric centerline to the thrust line. If the rear wheels are pointed to the right, the thrust angle is referred to as positive. If the rear wheels are pointed to the left, the thrust angle is negative. On a vehicle that offers rear-wheel adjustment for toe and camber, you can use the thrust angle information to adjust the thrust angle to zero (or at least near-zero).