Question about how AWD clutch could wear out on one mismatched tire

Hey guys! New member here - been lurking and reading on the forums for a while now, love all the info i have been learning.

Last week, I hit a board that fell off of a truck and chunked my front driver's tire had my 17' Legacy 2.5i Premium for about 5 months now and got it with a set of new tires in the end of November, so was pretty annoyed that i had to get a new entire set of tires.

I have about a 75 mile commute to work, and it happened on the way home. I was about 50 miles from home, so put the donut tire on the front (i know it says in the manual to put it on the rear and then swap the rear tire to the front, but i was on the inner lane of the interstate in rush hour traffic, so I just threw the donut on the front to get off the road for safety reasons.

Anyways, didn't go over 50 mph the whole way back home. Then started researching if I REALLY needed to get a set of 4 tires or if i could just have 1 tire shaved down to match the other 3. Other 3 tires were at 7/32" tread depth. I had to go out of town the next day, so I opted for an entire new set of tires just because I needed it done the next day and didn't want to pay for a tire to just have half it's life wasted anyways.

Now looking back on it and researching, trying to figure out if I had had the time, would it have been a smart move to get just 1 new tire and have it shaved down - been reading the forums and searching the internet, but still somewhat confused.

From what i have read (but i could be wrong), the problem comes from the AWD clutch when you have a mismatched tire and that it will heat up and wear out quicker if you drive with the mismatched tire on it for a long time.

So, would the AWD clutch be heating up constantly since you have the donut tire on and it's spinning faster than the other 3 at all times versus the AWD clutch only heating up when you are going around a turn with all 4 tires the same size?

That's the logic i have deduced from the research i have done, but if you have an open front diff, wouldn't the AWD clutch be fine with the donut on the front since the donut would be spinning faster but the front diff is open so it would allow that tire to spin faster without placing any unnecessary heat on the AWD clutch?

I did read somewhere that the computer differentiates between a slipping tire and one that is just spinning faster due to the car going around a curve by comparing the wheel speed of the outer tire to the inner tire and if the outer tire is spinning 20% faster than the inner tire, then the computer interprets that as the wheel slipping. Not sure if that is pertinent to this question or not, but it did shed some light on the question at hand when i read it.

Thanks for any input guys!!

 

Now looking back on it and researching, trying to figure out if I had had the time, would it have been a smart move to get just 1 new tire and have it shaved down - been reading the forums and searching the internet, but still somewhat confused.

 

From what i have read (but i could be wrong), the problem comes from the AWD clutch when you have a mismatched tire and that it will heat up and wear out quicker if you drive with the mismatched tire on it for a long time.

 

So, would the AWD clutch be heating up constantly since you have the donut tire on and it's spinning faster than the other 3 at all times versus the AWD clutch only heating up when you are going around a turn with all 4 tires the same size?

 

That's the logic i have deduced from the research i have done, but if you have an open front diff, wouldn't the AWD clutch be fine with the donut on the front since the donut would be spinning faster but the front diff is open so it would allow that tire to spin faster without placing any unnecessary heat on the AWD clutch?

For instance of the donut it will eventually wear out of the AWD clutch. 1 mismatched tire with an open differential is better 2 as the open differential with split the difference. The faster you go the worse it is for the center clutch.

I will do my math for car since I already have done it. If the fronts are worn 5/32nd more than rear, the circumference is 0.9" different. This works out be ~9.5 rev per mile, my car has 4.111:1 gear ration so the AWD sees difference of 38rev/mile. At 60 mph, your travelling 1 mile per 60 seconds. This means the AWD clutch is slipping 38/60 or more than 1 revolution per 2 seconds which going to cause a lot of heat and wear. If you had 1 tire mismatched it would be half that and more than 1 rev per 4 seconds at 60mph. You made the right choice.

I did read somewhere that the computer differentiates between a slipping tire and one that is just spinning faster due to the car going around a curve by comparing the wheel speed of the outer tire to the inner tire and if the outer tire is spinning 20% faster than the inner tire, then the computer interprets that as the wheel slipping. Not sure if that is pertinent to this question or not, but it did shed some light on the question at hand when i read it.

The computer has no way of compensate from one differential spinning faster than the other than disabling the AWD.

Some decent information here at Tirerack. Usually a person doesn't have time to wait for a shaved tire if not available locally.

They list Subaru tolerance at 1/4" circumference or 2/32" remaining tread depth.

I didn't rotate my tires as often as I should have early on and then only front to back. One tire had 1/64" inch more wear after 40,000 km (about 24K miles). After regular rotations since then (cross pattern), they are all equalized to 6/32 at 60,000 km.

FYI Subaru tolerance for tires is 0.25" which ~1.3/32nd of an inch. This works out to be 10 rev per mile or one rotation of every 6 sec at 60mph.

Where is this documented?

My previous AWD car was a BMW and they documented that a 1% deviation in circumference was acceptable. I wonder if Subaru has something similar?

Where is this documented?

 

My previous AWD car was a BMW and they documented that a 1% deviation in circumference was acceptable. I wonder if Subaru has something similar?

0.25" circumference difference is in the Factory Service Manual for my 2012. I don't know if it is in the newer FSM, but that spec has been in Subaru FSM for quite a while.