Types of Wheel Alignment and Procedures.
Two of the reasons why modern cars are as comfortable as they are have to do with not only the way the suspension systems are configured, but also with how suspension systems on modern cars are attached to the vehicle. While these factors provide a smooth ride, this smooth ride comes at the cost of increased complexity that can make the process of aligning all the wheels properly, a difficult one at times.
As a practical matter, a comprehensive wheel alignment procedure on a modern vehicle consists of several distinctly separate operations, these being-
Toe- in / Toe-out adjustment
On passenger vehicles, these conditions are typically caused by word or damaged steering components, worn or damaged control arm bushing, or the over-, or under adjustment of tie rod ends. On vehicles that use steering boxes instead of steering racks, these conditions can also be caused by the above, in addition to damaged, worn, or bent drag links and/or worn idler arm bushings.
These conditions are typically caused by worn, damaged, or broken strut mountings, worn, damaged, or broken control arm bushings, or by lateral contact with obstacles.
Note that while this setting is critical for good straight-line tracking, it is generally not possible to adjust this angle. However, deformation of the body caused by an accident can disturb this setting, which can sometimes be corrected by a body repair shop that can “pull” the chassis back into shape with computerised equipment.
Four wheel adjustment
The image above shows all the adjustments required to ensure that all the wheels on a vehicle are properly aligned to each other, as well as to the centreline of the vehicle, which is commonly referred to as the “thrust line”.
Although this type of procedure is most commonly performed on powerful high-end sports cars to ensure that the car’s handling characteristics are not compromised, many “ordinary” cars can benefit greatly from this procedure as well. If the procedure is performed by a suitably qualified technician, this procedure produces the best results in terms of alignment accuracy.
Thrust line adjustment
In the panel above, the red line represents the thrust angle, which should run at a right angle through the length of the car relative to the centreline of the rear axle, and if this setting is correct, the front wheels are aligned to this line.
However, if the centreline of the rear axle is not at right angles with the thrust line, the misaligned rear axle (as shown by the green and blue lines) will force the rear wheels not to follow the front wheels, and continual counter steering is required to keep the vehicle moving in a straight line. Thrust line adjustment therefore involves realigning the rear axle with the thrust line to ensure that the rear wheels run in the same line as the front wheels without the need to apply a corrective steering input.