Quote:
Originally Posted by Underdrive
Isn't the diameter of the rim the size that matters? Tracking is measured at the rim edge.
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Yes and no.
Modern alignment systems use a fixture that attaches to the rim and reports the angle / toe in of the wheel relative to the vehicle center line in degrees. It is using the rim 'edge' to define the center plane of the wheel and the diameter of the rim is irrelevant. Accurate measurement requires straight rim edges and correct fitting of the fixture to the rim. If your vehicle dog tracks I believe this also bodges the measurement because it skews the centerline.
There are fixtures for home alignments that attach to the outside edge of the rim that create lines parallel to the planes of both wheels and you can measure the distances between these two fixtures to determine toe in. But, the measurements must be made at a separation corresponding to the OEM tire diameter if you want to use Volvo's toe measurement value.
Vintage Volvo predate modern alignment systems. If you check the original Volvo green book service manual, you will see that Volvo defines toe in as the difference in the distances between the front and back of the tires as measured at the height of the center of the hub, exactly as shown in that link provided by C1800. In that case, the correct diameter of the tires is absolutely important if you are using the Volvo specified measurement. If your tire diameter is smaller than OEM, you can use that calculator with the OEM tire diameter and Volvo's spec to calculate the recommended toe in in degrees. Toe in measured in degrees is a better measurement because it is invariant to the diameter of the tire. However, toe in specified in mm or inches was the norm because most alignment shops in the '60s and '70s could not measure angles.