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Discussion Starter · #1 ·
Quick question: would just having alignment out somewhat causing one tire to wear in one part more than the others be enough to create damage to the AWD system? Seems this could occur and go undetected for at least some time. I know I cannot always detect alignment issues right away.
 

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Quick question: would just having alignment out somewhat causing one tire to wear in one part more than the others be enough to create damage to the AWD system? Seems this could occur and go undetected for at least some time. I know I cannot always detect alignment issues right away.
No. The issue with the AWD system is that you don't want one part of the system (e.g. left rear tire) rotating at a different speed than some other part of the system (e.g. right rear tire, or even right front tire).

Since there are multiple differentials involved (left-to-right, front-to-rear), Subaru specifies a maximum size difference among tires and specifies where the donut can be used in case of a flat and even provides a removable fuse to disable AWD. And towing is out of the question.

But if you follow the forum, more than a few posters suggest that Subaru is ultra conservative in this area - perhaps to the point of silliness. Conventional differentials have been in use for years - maybe a hundred years - they are a robust component. Vehicles with oddly sized tires on each side may not be the norm, but they are hardly newsworthy. Differentials rarely fail (at least not for this reason).

If your tires are out of align, you might get pre-matue or odd-ball tire wear. You might get 'pulling' to one side or another. or have an out-of-align steering wheel. You might have some slight reduction in handling. The differentials can take care of themselves.
 

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Discussion Starter · #3 ·
No. The issue with the AWD system is that you don't want one part of the system (e.g. left rear tire) rotating at a different speed than some other part of the system (e.g. right rear tire, or even right front tire).

Since there are multiple differentials involved (left-to-right, front-to-rear), Subaru specifies a maximum size difference among tires and specifies where the donut can be used in case of a flat and even provides a removable fuse to disable AWD. And towing is out of the question.

But if you follow the forum, more than a few posters suggest that Subaru is ultra conservative in this area - perhaps to the point of silliness. Conventional differentials have been in use for years - maybe a hundred years - they are a robust component. Vehicles with oddly sized tires on each side may not be the norm, but they are hardly newsworthy. Differentials rarely fail (at least not for this reason).

If your tires are out of align, you might get pre-matue or odd-ball tire wear. You might get 'pulling' to one side or another. or have an out-of-align steering wheel. You might have some slight reduction in handling. The differentials can take care of themselves.


OK, thanks for clarifying. When I read on another question about Subaru's strict guidelines (and I think the Tirerack link someone posted said 1/4 of an inch was a problem), that got me wondering. Don't get me wrong - I try to avoid alignment issues - but I would hate to think something like my failure to catch an alignment issue and some minor uneven wear could significantly damage my AWD system.
 

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OK, thanks for clarifying. When I read on another question about Subaru's strict guidelines (and I think the Tirerack link someone posted said 1/4 of an inch was a problem), that got me wondering. Don't get me wrong - I try to avoid alignment issues - but I would hate to think something like my failure to catch an alignment issue and some minor uneven wear could significantly damage my AWD system.
Misalignment is most unlikely to cause any damage. Of course, be sure to rotate your tires.

That's 1/4" rolling circumference. In practical terms, you will hit the 1/4" threshold in about 15,000-17,000 miles on a set of tires. This is about 2/32" of tread, although checking by circumference (and not tread depth) is strongly recommended. FWIW, tires seem to wear quicker when they're new - that first and second 32nd of an inch goes pretty quickly. Then the wear rate seems to slow down.

About the only issue would be if you had a good set of tires part-way through their life, and then had a catastrophic failure on one tire (highway blow-out that trashed the tire, wife/ex-wife/girlfriend drills holes in the sidewall). You're faced with some hard choices:

- Get one new tire and have it shaved or ground down? There are companies in most larger cities that will do this (for a price, of course, and it's not that cheap).

- Or, get four new tires and waste the tread remaining on the original tires?

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I did a bit more reading on the 1/4" matter and, while what I said was once correct, it seems Subaru may have briefly visited the real world and moderated this requirement. In another thread bbottomley points out:

Subaru did once specify the 1/4" max difference in rolling circumference, but the general view here is that they have now backed away from that. The original spec did indeed work out to somewhere between 1/32" and 2/32" difference in tread depth, and that was an awfully tight spec. It was probably written by the lawyers.

Quite coincidentally the spec seems to have gone away, and at about the time when Subaru introduced the donut spare, which is a major violator of the 1/4" difference! Subaru is now counseling moderation. Keep them close, if they aren't close it's OK to temporarily drive gently, and so forth.
Got all that? Okay, take out a piece of paper for a brief quiz.
 
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