Wheel Alignment Q&A

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Why measure at the contact patch?

Traditional wheel alignment measures the wheel (tire) location at the wheel centerline. Real Square systems measure the wheel location at the bottom of the tire. Why?

The chassis only knows where the contact patches are. Imagine a table with four legs. Now image the legs are on scales. The weights only change if you move the location of the bottom of the legs. The leg could be straight or bent in a loop.

The table top is your chassis, the bottom of the legs are your contact patches. The chassis does not know where the wheel centerline is, it only knows where the contact patches are.

We measure at the contact patch because that is the most relevant location. Traditional methods used the centerline because that was the widest point of the tire and thus, the most accurate measuring location. Modern wheel fixtures solve this problem. Automotive alignment machines continue to use the wheel centerline because they’re only concerned with wheel alignment, not rear end lateral placement or wheel weights.

To achieve maximum consistency, we measure at the contact patch. Consider this scenario: You’ve set your rear end lateral (left to right) location based off of the right side wheel centerline. Now, at the track, you determine the right front needs more camber and you add 1 degree of negative camber. Back at the shop you check your alignment and now see that the rear end is too far to the right. (Adding negative camber moved the right front centerline to the left or closer to the center of the chassis.) However, in reality the rear end should be moved to the right to maintain contact patch consistency. Although the right front moved to the left at centerline, it moved to the right at the contact patch.

Wheel Centerline vs. Contact Patch Location
Tire Diameter 1 Deg 1.5 Deg 2 Deg 2.5 Deg 3 Deg 3.5 Deg 4 Deg 4.5 Deg 5 Deg 5.5 Deg 6 Deg
28″ 0.244 0.366 0.488 0.610 0.732 0.854 0.976 1.098 1.220 1.342 1.464
27″ 0.236 0.354 0.472 0.590 0.708 0.826 0.944 1.062 1.180 1.298 1.416
26″ 0.227 0.341 0.454 0.568 0.681 0.795 0.908 1.022 1.135 1.249 1.362
25″ 0.218 0.327 0.436 0.545 0.654 0.763 0.872 0.981 1.090 1.199 1.308
24″ 0.209 0.314 0.418 0.523 0.627 0.732 0.836 0.941 1.045 1.150 1.254
23″ 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200
22″ 0.191 0.287 0.382 0.478 0.573 0.669 0.764 0.860 0.955 1.051 1.146