Why do tires have to match?

[BLITZEN]

I’ve been told that it’s important for the tires on our cars to match due to the Quattro system. But I don’t understand this ‘cause my thinking is that if the right side tire has a worn tread or something causing a difference in overall wheel diameter from the left, wouldn’t that be fine? It would just be like a long right hand or left handed turn. Right? So anyone who can explain why or why not tires have to match, please weigh in. Thanks!

:addict:

[mrdave]

You have 3 differentials: a front, rear, and a fancy center Torsen diff. My understanding is that any difference in tire diameter puts undue stress on the center diff - the greater the difference, the greater the stress. I believe spec is only a 2-3mm tire diameter difference allowed.

Diagram for an A4, but same idea: https://drivebywirecf.files.wordpress.com/2011/07/audi_quattro_torsen_operation.jpg

[hahnmgh63]

You spend a lot of time going straight than in a turn but you've got the right idea. The engineers designed in enough longevity and as always a buffer when designing diffs to wear a normal life. When you put different size tires on they have to compensate 100% of the time. They'll probably last for years just fine but will see increased wear.
Same goes for different rolling diameter tires front to rear, then the center differential wear increases. Like I said, probably last for years with no problems but there will be increased wear that is why the engineers specify the tires to be the same and within 3/32". Other manufacturers are around that ballpark.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18