Quote:
Originally Posted by Shiny
I don't think it was me that introduced Subaru's 1/4" circumference limit but a search online has multiple repetitions of this claim. May be folklore but is the legend nevertheless. I agree this difference feels like noise but I don't know the truth.
Here's an example:
https://www.subaruoutback.org/forums...tread%20depth. |
As I previously said, .3 % is not a realistic engineering goal to achieve (1/4 inch in circumference variation) is not enough to be concerned about. I've seen brand new tires same make, same size vary 3/4 of an inch in circumference after mounted and inflated. Now someone assumes that because they're made in the same mold by the same manufacturer, and the same size, that they're identical, many times they're not. Tires aren't all that uniform.
Race tires are all measured when new to see how much variance there is from side to side on oval track cars. The taller larger circumference tire is mounted on the right side and the shorter smaller circumference tire is mounted on the left side (stagger). Now if no one measured those tires and the taller one was mounted on the left side the car, the would push through the corner (not turn into the corner), and handle terribly.
With a locked rear axle, the short tire helps turn in on a left handed turn by traveling less distance per revolution. Circumference is much easier to measure accurately than diameter is, so circumference is the standard to check variance in 2 tires, not diameter. I've measured many tires to check how much they vary, and never did we use diameter to check variance, always circumference. 1/4 inch in circumference is negligible, but 1/4 inch in diameter is is roughly 13/16/.785 inch in circumference, or 3.14 times, also known as Pi.
Mike's explanation clarified how much FCA (chrysler/Jeep) allows in variance front to rear as well as side to side, his explanation has much more clarity than the subaru link you referred to. I know Mike fairly well, and he teaches for the local automotive technical school in my area. I have more faith in his specifications for AWD vehicles than someone posting on a subaru forum.
I read your link, and I believe, from what Mike has said about tolerances from Jeep, that someone has interchanged circumference, and diameter measurements on that subaru forum. They're apples and oranges. Using the wrong term makes a big difference in the actual tolerance that the manufacturer allows.
