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Automotive Industry

TOTAL 4-WHEEL ALIGNMENT

Motor, Jun 2007 by Mavrigian, Mike

Modern equipment greatly simplifies the wheel alignment process. But it's still important to understand how alignment angles are measured, as well as their effect on driveability, braking, tire wear and handling.

The old method of wheel alignment, called centerline two-wheel alignment, should now be considered obsolete. This method does not consider the rear wheel positions, and it simply isn't effective, because it ignores the thrust direction of the rear axle.

A much more effective method is called thrust line or thrust angle alignment, which considers the actual location and direction of the rear wheels. This allows you to adjust the front wheel angles relative to the rear wheel angles, regardless of the geometric centerline.

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If the vehicle in question features rear wheel toe adjustment, you can achieve optimum wheel alignment using the total four-wheel approach, by referring to and adjusting the vehicle thrust angle to as close to zero as possible.

If the thrust angle is off zero, it can contribute to vehicle dog-tracking (crooked body relative to direction of travel), increased tire wear and unequal left/right turning. Total four-wheel alignment allows you to adjust and hopefully correct rear axle thrust angle, then to adjust the front wheels parallel to the rear wheels.

*If the vehicle does not allow rear wheel angle adjustment, take advantage of a four-wheel thrust line alignment approach.

*If the vehicle does allow rear wheel angle adjustment, perform a total fourwheel alignment.

Granted, a state-of-the-art computerized wheel alignment system will walk a technician through the steps, perform all necessary calculations and instruct the technician to adjust angles in order to meet an OE specification for a specific production vehicle. However, it's also important to understand wheel angles and what these angles represent in terms of driveability, braking, tire wear and handling. In other words, it's helpful to understand wheel alignment theory. Only by grasping the rudiments of wheel angles will you be able to appreciate how these angles affect a vehicle in motion.

A number of dimensional angles are involved in wheel alignment theory, but only three are considered adjustable. These are wheel toe, camber and caster. We'll explain each of these adjustable angles, then follow with an overview of how these wheel angles affect optimum handling, braking and tire life.

Wheel Toe

Wheel toe describes the relationship of the right and left wheels on the same axle, as viewed from overhead. Its measured by comparing the distance between the center of the front of the tires to a distance between the centers of the rear of the tires on the same axle.

Toe-out (negative toe) is present when the wheels are farther apart in front of the axle centerline and closer together behind it. Toe-in (positive toe) is when the two wheels on the same axle are closer together at the front and wider apart at the rear. When the measured distance between the front of the wheels (ahead of the steering axle centerline) is identical to the distance between the wheels behind the axle centerline, the condition is called zero toe.

All front suspensions, regardless of design, feature toe angle adjustment, at a location on the steering tie rods or tie rod ends. live rear axles feature no toe angle adjustment, but independent rear suspensions usually do offer the feature.

The toe angle affects the directional control of a vehicle, turning response and tire tread life. Toe-related tread wear will cause a feathering wear pattern across the tread. With too much toe-in, the feathering will angle inward, toward the center of the vehicle; too much toe-out causes feathering that angles outward.

Because of the compliance in control arm bushings and other dynamic variances in suspension and steering systems, the goal is to establish a static toe angle that will result in a zero toe condition when the vehicle is driven in a straight line.

Speaking in general terms, a reardrive vehicle would likely require a front wheel toe-in (positive) setting, and a front-drive vehicle would likely require a slight toe-out (negative) setting. The reason? Commonly, as a rear-drive vehicle moves forward, the front wheels tend to try to push outward (to crawl away from each other); and a front-drive vehicles front wheels tend to try to crawl inward.

Camber

As viewed from the front or rear of the vehicle, camber refers to the "lean" of the wheel from top to bottom. A wheel that leans outward at the top (compared to true vertical) has positive camber. One that leans inward at the top has negative camber. If the wheel is oriented in a true vertical, it's called zero camber.

Camber must always be adjusted to maximize the tread contact patch based on the driving requirements. In most cases, OE specifications will recommend a slightly positive or zero camber to maximize tire wear and traction, and to provide easier steering and greater resistance to directional darting in a straight line.

Automotive Industry

TOTAL 4-WHEEL ALIGNMENT

Motor, Jun 2007 by Mavrigian, Mike

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