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Discussion Starter · #1 ·
It's been a long time since Subaru of America has posted anything official concerning the impact of tire circumference on the AWD system, so I thought I'd note that the December 2010 issue of TechTips has the following article which, once again, confirms the need for all four tires/wheels circumferences to be within 1/4 inch of each other.

Here is the text (also in the attachment)

"The Techline continues to receive calls from Technicians who have performed all types of unsuccessful repairs to transmissions with a “binding on turns” concern from the Customer. One of the first questions we will ask is whether or not the tire circumferences have been measured. All too often the answer is no. It is extremely important when diagnosing a transfer clutch-related concern, the tires must be one of the first variables you rule out. Remember, all 4 tires should be within approximately ¼" of each other in circumference when measured at the center of the tread. Otherwise, binding issues can result (or re-occur) due to the ratio imbalance caused by mis-matched tires. A quick keyword search on STIS of “Binding on Turns” will return a list of other bulletins and TIPS articles on this subject."

Odd size tires simulate tires slipping due to their different rotational speeds, and a slipping tire condition causes the AWD system to engage the clutch more fully (to help stop the slipping). If a car is driven with this condition when the tires in fact have good traction, the clutch will be forced to slip continuously, which will cause undue wear. The result could be mechanical damage that ultimately causes the clutch to remain engaged at all times (or not to work at all). The same can also happen to the center viscous coupling used in the manual transmissions.

The TechTips article deals with tire circumference when there might already have been a failure. What it does not say explicitly, but does imply, is that driving with tires with significantly different circumferences can damage the AWD system.

Different circumference tires turn at different speeds. Tires turning at different speeds appear to the AWD system as if they are slipping. The AWD system is designed to prevent this by engaging the AWD clutch in the automatic transmissions, or activating the center viscose coupling in the manual transmissions. If a car with different circumference tires is driven when the tires in fact have good traction, the clutch plates will be forced to continuously slip one against the other and in the case of the manual transmission, the viscous coupling will overheat. In both cases, the effect is undue wear and the potential for premature failure of the AWD system.

Some have mistakenly interpreted the measurement requirement to be the diameter or radius, whereas it's the circumference that counts.

A 1/4 inch difference in circumference in a typical 17 inch tire used on Subarus translates to between 1/32 and 2/32 of an inch difference in tread depth (or radius). It's not much.
 

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I shall sticky this.
 

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Thanks for posting that. If one desires to adhere to their 1/4" recommendation, one may find he has to rotate his tires quite frequently! :)

It's recommended every 5-6K on all cars anyway. Re: AWD cars I would say yes it should be even more frequent. I agree with the every oil change philosophy, esp if you use synthetic. I do em both every 3k or sooner. I also keep not only an air pressure gauge in the glovebox but a tread depth gauge as well, helps me tell if my tires are wearing evenly or if the alignment is out. **** I even have a tire plug kit and $20 air compressor under the rear lid (not to mention the spare tire which has never hit the road). I don't want to ruin a good tire cause of a stupid nail and have to replace all four with 4-7/32 left!
Thanks to the new (gov mandated) Tire Pressure Monitoring System (TPMS), now hardly anyone even has an air pressure gauge anymore, they just wait for the idiot light to come on and "warn'' them. God I hate TPMS! I'm so glad my car doesn't have it.

189K... on my way to 300K.
 

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this is good information, especially since it comes from an official source. but this statement leads me to wonder.

Remember, all 4 tires should be within approximately ¼" of each other in circumference
these specs and the AWD design is years, decades old. and 1/4 inch of a legacy L 1990 tire, 185/70/14 is a different percentage than 1/4 inch of an 2001 outback tire, 225/60/16, or a 2010 outback premium, 225/60/17.

and since the 1/4 inch difference in circumference converts to ''rotations per mile'' and that is what causes ''issues'', is the difference actually 1/4 inch or a percentage??????????? a tiny percentage, but a percentage none the less. this would mean for a larger tire the actual number may be a little larger.

1990 legacy L 2.2, 1/4 inch = 0.329 % (about 2.75 revolutions per mile difference)
2001 outback 3.0, 1/4 inch = 0.2989 %
2010 outback 3.6, 1/4 inch = 0.288 % (about 2.10 revolutions per mile difference)

but even allowing for the percentage the circumference difference never reaches 5/16 inch. (it does reach 9/32 inch circumference for the 2010 obw) so stick with the 1/4 inch spec and keep all your tires the same size.

just a thought, or two.
 

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2012 Outback Premium Ruby Red Pearl 2.5 CVT AWP
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"Odd size tires simulate tires slipping due to their different rotational speeds, and a slipping tire condition causes the AWD system to engage the clutch more fully (to help stop the slipping). If a car is driven with this condition when the tires in fact have good traction, the clutch will be forced to slip continuously, which will cause undue wear. The result could be mechanical damage that ultimately causes the clutch to remain engaged at all times (or not to work at all). The same can also happen to the center viscous coupling used in the manual transmissions."

So if odd sized tires cause the AWD clutches to think one or more of the tires are slipping, thus engaging the clutches, would one be able to turn off the traction control in a Gen 4 to "override" this? Just wondering...
 

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05 OBXT 5eat stg1.2
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Nope, doesn't work that way. It's not the traction control that is doing it, it is the AWD system. Traction control wouldn't even notice such a small amount of variance.

The transfer clutch most likely will not lock up the clutch more with an off sized tire. The wheel speed sensor doesn't appear to be that precise. It will just slip. The transfer clutch packs are designed to deal with some slip, but if it is constant with every rotation, it will eat it up until it will eventually slip all of the time.
 

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Discussion Starter · #9 ·
"Odd size tires simulate tires slipping due to their different rotational speeds, and a slipping tire condition causes the AWD system to engage the clutch more fully (to help stop the slipping).

I wrote that piece before having the more detailed knowledge gained in experiments reported in http://www.subaruoutback.org/forums...426-freessm-complete-access-your-ecm-tcu.html. Seabass is correct -- the odd tire isn't likely to cause the clutch to lock-up. However, even when cruising, unlike some other "AWDs", the clutch in the 4-speed AT, and presumably the CVT, is always engaged to some degree. The key issue then is the constant relative rotation and wear of the clutch plate friction surfaces that would occur with an odd size tire.
 

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A question .....I just ruined a Bridgestone RE 92A tire. Tread depth is 7/32" while new tread depth is 10/32. Am I SOL as to just putting a new tire on?
 

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If you can find somewhere that will shave it down, then that's the way to go. Tirerack will do it.
Just heard back from Tire Rack........$254 for the tire $25 for the shave, $20.73 for shipping. Tire Discount doesn't carry that tire or shave them. I can get 4 new Hankook Optima H727's for $472 less a $50 rebate.......hmmmmmmm.
 

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Reading this makes me wonder about something..

If the AWD system is as sensitive as people are indicating and with even things like tire pressure causing differences in the circumference of tires, why doesn't Subaru use some of these fancy sensors to warn drivers about impending doom of one tire having a shorter circumference than the others? Subaru not doing that makes me wonder if they've built more leeway into the system than folks think and it's not as big of a deal as many think it is..? :8:
 

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Reading this makes me wonder about something..

If the AWD system is as sensitive as people are indicating and with even things like tire pressure causing differences in the circumference of tires, why doesn't Subaru use some of these fancy sensors to warn drivers about impending doom of one tire having a shorter circumference than the others? Subaru not doing that makes me wonder if they've built more leeway into the system than folks think and it's not as big of a deal as many think it is..? :8:
My guess is that it is like anything else when it comes to consumers - the less information you give them generally the fewer questions and drama that follow.
 

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Discussion Starter · #15 ·
to warn drivers about impending doom
Good question.

A few years ago I looked at this in more detail for a technical paper I put together about the effect of an odd size tire on the AWD clutch plates. Here's a part of what I found (the calculations are based on my 07's specs, such as tire size, wheelbase, turning circle etc):

In a full-time AWD vehicle traveling on a straight, level surface with identical tires in all respects, the front and rear drive shafts and their corresponding transfer clutch plates will all turn at the same rate. Under these conditions, there will be no slippage between the driving and driven plates. Of course, in normal driving there will be some relative movement due to steering corrections, cornering, and road surface imperfections, but the impact of these on the transfer clutch is minor because the amount of slippage in each instance is limited and intermittent. To put this in some perspective, when the Outback makes a tight, 90 degree turn, the clutch plates will rotate one against the other, but by no more than one revolution, this being due to the fact that the front and rear wheels, and the inside and outside wheels, do not follow the same track.

However, with one tire of different circumference, the different rate of rotation of the front and rear drive shafts due to the different rotation rate of the odd tire will be continuous, even when driving perfectly straight. Calculations reveal that a one-quarter inch difference will cause the AWD clutch driving and driven plates to turn one against the other by over 7.2 revolutions per mile, or 72,000 revolutions in every 10,000 miles. This is 72,000 revolutions that are not meant to occur. Again, to put this in perspective, without any difference in the circumference of the tires, the same result would require the car to complete more than seven tight 90-degree turns in every mile of travel!


A 1/4-inch difference represents a rotation difference of only 0.3% (approximately). While I'm sure there's technology that could detect this difference, the cost to ensure it can discern an odd size tire from some other condition would probably be prohibitive from a car maker's point of view.

There's no "impending doom", at least not in an immediate sense. The immediate and ongoing effect is undue, additional wear and, ultimately, premature failure of the clutch. When this will happen will depend on how long an odd tire is used, and how long the car is driven by the original owner or later owners. The Subaru requirement for same-circumference tires is probably driven more by it's warranty liability than in ensuring the car functions fully for hundreds of thousands of miles. But it does speak to the fact that the odd tire does have a negative impact, and if the intent is to keep the car for a long time, this is one factor, just like regular oil changes, to keep in mind.
 

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Good question.

A few years ago I looked at this in more detail for a technical paper I put together about the effect of an odd size tire on the AWD clutch plates. Here's a part of what I found (the calculations are based on my 07's specs, such as tire size, wheelbase, turning circle etc):

In a full-time AWD vehicle traveling on a straight, level surface with identical tires in all respects, the front and rear drive shafts and their corresponding transfer clutch plates will all turn at the same rate. Under these conditions, there will be no slippage between the driving and driven plates. Of course, in normal driving there will be some relative movement due to steering corrections, cornering, and road surface imperfections, but the impact of these on the transfer clutch is minor because the amount of slippage in each instance is limited and intermittent. To put this in some perspective, when the Outback makes a tight, 90 degree turn, the clutch plates will rotate one against the other, but by no more than one revolution, this being due to the fact that the front and rear wheels, and the inside and outside wheels, do not follow the same track.

However, with one tire of different circumference, the different rate of rotation of the front and rear drive shafts due to the different rotation rate of the odd tire will be continuous, even when driving perfectly straight. Calculations reveal that a one-quarter inch difference will cause the AWD clutch driving and driven plates to turn one against the other by over 7.2 revolutions per mile, or 72,000 revolutions in every 10,000 miles. This is 72,000 revolutions that are not meant to occur. Again, to put this in perspective, without any difference in the circumference of the tires, the same result would require the car to complete more than seven tight 90-degree turns in every mile of travel!


A 1/4-inch difference represents a rotation difference of only 0.3% (approximately). While I'm sure there's technology that could detect this difference, the cost to ensure it can discern an odd size tire from some other condition would probably be prohibitive from a car maker's point of view.

There's no "impending doom", at least not in an immediate sense. The immediate and ongoing effect is undue, additional wear and, ultimately, premature failure of the clutch. When this will happen will depend on how long an odd tire is used, and how long the car is driven by the original owner or later owners. The Subaru requirement for same-circumference tires is probably driven more by it's warranty liability than in ensuring the car functions fully for hundreds of thousands of miles. But it does speak to the fact that the odd tire does have a negative impact, and if the intent is to keep the car for a long time, this is one factor, just like regular oil changes, to keep in mind.
Thanks for the info! :29:

As for the expense, it seems like the wheel speed sensors and a few lines of programming code could be used to detect if one wheel is consistently "spinning" a little faster than the others.. :8:
 

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Just heard back from Tire Rack........$254 for the tire $25 for the shave, $20.73 for shipping. Tire Discount doesn't carry that tire or shave them. I can get 4 new Hankook Optima H727's for $472 less a $50 rebate.......hmmmmmmm.
I would never pay for those POS stock tires. IMO, you'd be better off replacing all of them for tires that don't suck. I only got 20k out of the potenzas anyway and I was glad to be rid of them. I forgot how ridiculously expensive they are!
 

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Discussion Starter · #18 ·
As for the expense, it seems like the wheel speed sensors and a few lines of programming code could be used to detect if one wheel is consistently "spinning" a little faster than the others..
Raw wheel speed data is available from the ABS/VDC wheel speed sensors. However, rolling circumference also changes with tire pressure, speed and load. Assuming that tire pressures are correct and speed is the same for all four, load can still be a factor.

I think one of the challenges is to avoid false positives that might occur if the car is driven with a load that temporarily alters one or more wheel speeds. I don't think any cost-effective system would be able to distinguish between a tire that is compressed ever so slightly due to load, and one that has a slightly different circumference due to it's original manufacture or wear.

In this regard, I suppose the software could look at the data over a very long time (i.e. not just that one loaded trip to the cottage) and ultimately conclude that it's likely one (or more) tires is consistently running differently. However, given that there will be many instances when the perceived differences will be due to factors other than incorrect circumference, the time over which the software would have to consider the possibilities probably would make the ultimate warning of little value.

No doubt, it could be possible to have a system that could make the distinction more quickly and reliably -- I would imagine there are many such systems on the space shuttle, but I suspect it would require additional data-gathering sensors and processors, thus adding to the cost (not to mention the maintenance costs that might arise down the road).

Also, I believe that as it is currently designed, the ABS wheel speed signal is not at a high enough rate (frequency) to distinguish such a small difference. The ABS and VDC are looking for much larger variances such as when one wheel stops turning when the brake is applied (wheel skid) or a wheel starts turning at a far faster rate (loss of traction).

Interestingly, the Owners Manual for my 07 (and indeed all the 3rd generation OBs) says that if the car is driven with a low, or odd-size tire, the AWD light will come on. Yet no-one has confirmed that this happens even with the temporary spare installed, and my inquiries to Subaru have not been explicitly answered. (See: http://www.subaruoutback.org/forums...wd-warning-light-flashing-odd-size-tires.html)

This might also be why it appears that in response to the federal low-tire-pressure-warning legislation, most car mfrs rely on pressure sensors in the tires, rather than making use of the ABS wheel speed signals, which, theoretically, could provide essentially the same warning of a low tire.
 

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I must admit that $300 seems pretty ridiculous. There are only 9,000 miles on the tires so theoretically they're good for at least that much more. The Impreza doesn't get driven that much(obviously), so if I get another 3 years out of them I really couldn't complain, but I would anyway....
 

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Just heard back from Tire Rack........$254 for the tire $25 for the shave, $20.73 for shipping. Tire Discount doesn't carry that tire or shave them. I can get 4 new Hankook Optima H727's for $472 less a $50 rebate.......hmmmmmmm.
Worthwhile to me. The 727's have generally positive reviews, and you might be able to sell your remaining 3 OEM tires on Craigslist or something. Not for much, but anything will help you recoup costs.
 
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