Two-Wheel vs. Four-Wheel Alignments

Wheel alignments are a common requirement on many collision damaged vehicles. Although the methods for measuring the alignment of the wheels can vary, the results should always be the same. The wheels should be aligned within OEM specifications.

Many vehicles on the road today have independent rear suspension with adjustments for toe, camber, and sometimes caster. Other vehicles have a solid rear axle with no adjustments for the rear wheels. This may leave one asking, "If there are no rear wheel adjustments, do I only perform a two-wheel alignment?"

A two-wheel alignment is simply aligning the camber, toe, and caster (if adjustable). This is typically the front wheels. This would have the wheels at the correct angles with respect to each other and would usually produce a centered steering wheel.

However, we do not live in a perfect world. There are numerous variables that can cause the misalignment of the rear wheels. If the front wheels are aligned without taking this variance into consideration, the steering wheel will now be off center when driven, and the vehicle may seem to pull to one side. This is due to the front wheels not being aligned to the thrust angle of the rear wheels.

On an independent rear suspension equipped vehicle, thrust angle is a product of individual toe and can often be corrected during the four-wheel alignment process if the structure and suspension parts are not damaged. This ensures that the front and rear wheels are within specifications.

With solid rear axles, simple adjustments are not an option on most vehicles. There may be procedures to slightly shim to compensate for OEM tolerances, but other than that, adjustment is often limited or nonexistent.

With a solid axle, it is important that both rear wheels are tracking correctly; ideally parallel to the centerline of the vehicle. So even though the rear wheels do not have any toe or camber adjustments, it is important to make sure the front and rear wheels are within specifications. Also, if the rear wheels are out of specification, it may be an indication of bent suspension parts or structural damage that has not been repaired.

It is impossible to determine if the rear wheels are out of specification with a two-wheel alignment and makes performing a four-wheel alignment necessary. In the case of solid rear axle vehicles, since the thrust angle cannot be corrected, the front wheels are aligned not only to each other but to the thrust angle as well, putting all four wheels within specifications.

Failure to correct the thrust angle or to align to it (in the case of a solid axle vehicle) may result in increased tire wear, off center steering wheel, a pull to one side, and can even cause handling issues or affect advanced driver assistance systems (ADAS) features. This means that no matter what the rear suspension type or axle configuration, all four wheels and the steering wheel should be considered during the alignment process.

A thrust angle alignment (solid rear axle) references all four wheels and aligns the front wheels so they are in correct relationship to the thrust angle presented by the rear wheels where slight OEM tolerances do not position them perfectly parallel to the vehicle centerline. If the thrust angle is out of specification, it should be corrected before completing the alignment.

In conclusion, this is why all four wheels should be checked. Modern alignment equipment requires that sensor heads be attached/reference all four wheels and the steering wheel position verified.

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