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Old 11-07-2012, 12:11 PM   #31 (permalink)
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I believe unless the tire tread actually folds up, like in an extreme flat tire. the rolling circumference will actually be the same. The belt does not change length.
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Old 11-07-2012, 12:29 PM   #32 (permalink)
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Agreed, there is no dimensional bearing on psi (under normal conditions).
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Old 11-07-2012, 12:40 PM   #33 (permalink)
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So what happens if the tire sizes are off? Does the AWD system just blow up? Or does it groan? Catch fire? Why is the system built with such small tolerances?
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Old 11-07-2012, 12:49 PM   #34 (permalink)
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So what happens if the tire sizes are off? Does the AWD system just blow up? Or does it groan? Catch fire? Why is the system built with such small tolerances?
You destroy the clutch pack that drives power to the rear of the car. All full time AWD systems work this way - they expect the tires to be very similar in size and to all roll at the same rate otherwise you end up with an unbalanced system.
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Old 11-07-2012, 03:24 PM   #35 (permalink)
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The manual states 2/32 on gen 4 Outbacks. At issue perhaps should be the published spec. And so if one tire is inflated to 32 lbs and another only has 22 lbs, what is the difference in rolling circumference? Being a little snarky here, but the Gen 4 does not have any viscous differentials, manual tranny excepted, as the specs you quote are clearly intended for.
No problem with being a little snarky, especially if it makes me think twice before replying.

As I noted in a previous post here, I haven't found a 2/32 spec in the 2011 Subaru Owners Manual or the 2010 FSM, and therefore, without prejudice, would appreciate you letting us know exactly where that is stated so we can look at it for ourselves.

The reference to the viscous coupler in the MT was not being exclusive. The same applies to the ATs. See Peaty's second attachment.

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[FONT=Verdana]My question, is psi is a determining factor.. . .
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Agreed, there is no dimensional bearing on psi (under normal conditions).
There have been laboratory tests of "rolling circumference" which found that it changes with load, pressure and even speed. I agree it is counter-intuitive when we consider the steel belt, which has a fixed length. However, the studies showed that the observed changes in rolling circumference are accommodated by flexing (compression and stretching) of the tire rubber.

If we look at tire manufacturers' specs, it will usually show the tire diameter, and revolutions per mile. If we calculate the circumference from the tire diameter we get one figure, but if we calculate the circumference from revs per mile, we get another, the latter being lower. How is this possible?

There's (at least) three different circumferences that relate to tires:

The first is "static" circumference of an unloaded, but mounted and inflated tire. This could be based on measurement, or from the mfr's specs (diameter, radius etc). This will not change significantly with small differences in psi. [Some time ago I measured the circumference at three different psi levels, 25, 30 and 35, and found a 1/16-inch change over the full 10 psi range.]

The second, is the "implied" circumference that could be calculated if we measured the distance from the center of the wheel to the ground of a tire that's properly inflated and installed on the car. This will be less than the static circumference because the tire is compressed at the bottom by the weight of the car and therefore the radius is lower than in the first case.

The third is the "rolling" circumference, which will fall between the other two.

The static and implied circumferences do not take into account possible changes due to load, pressure and speed. However, if four identical tires are on a car, and are inflated properly for their application, it's presumed that the effect of the load, psi and speed variables will affect all four equally, so that any change in circumference between when the car is stationary and when the car is being driven will be the same. As the concern here is differences in rolling circumference between the four tires, measuring static circumference should be adequate, and that is reflected in the attached articles from Subaru.

If you want to take this further, find a flat (not sloped), paved lot that will allow you to drive the car in a perfectly straight line for at least 10 revolutions of the tires. Inflate the tires to spec, set the car at one end of the lot, then mark the bottom-most point of the visible sidewalls with a chalk line and mark corresponding lines on the pavement. Then move the car forward until one wheel has gone exactly 10 revolutions and its sidewall mark is again at the bottom-most point. If all four wheels have the exact same rolling circumference, the marks on the other three tires should also be at the bottom, or very close. Mark a line on the pavement corresponding to sidewall mark on the first (reference) wheel. Measure the distance between that mark and the original mark on the pavement for that same tire. Divide the distance by 10 and that will be the rolling circumference (at that speed, load, and psi). Compare it to the measured circumference when the tire is lifted off the ground and not under weight.

Now, you can go one step further. Reduce the pressure in the tires by 10 psi, and run the same 10 revolution test. We would expect the distance traveled to be the same, but it won't be. (A change of just 1/16 inch in circumference will translate to a 10/16 inch difference in distance traveled after 10 revolutions, and that is readily observed.)

Subaru specs for tire psi generally have the fronts slightly higher because of the weight distribution. The higher curb weight at the front is compensated by the higher psi so that the rolling circumference of the front tires will be close, if not the same, as rear tires. Of course, if we load down the rear cargo area, the rear tires will now be under greater load and their rolling circumference will change. This is not a problem if the load is temporary, and this is taken into account in the design of the AWD system. But if it's the way the car is to be driven for an extended period, the rear tire pressures should be increased to compensate. (This also relates to tire wear and safety for most cars, not just AWD type.)

A small difference in rolling circumference among the four tires won't cause the transmission to blow up, but as subiesailor has already noted, it can cause undue wear on the multi-plate clutch (AT, CVT) or the viscous coupler (MT) or stress on the other drive train components, and that could eventually lead to premature failure.
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Old 11-07-2012, 03:40 PM   #36 (permalink)
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it dosent take much to tick off an AWD system, i am dealing with permanent differential damage in my 99' OBW, and am nearly certain it had a little something to do with discount (used) tires i was putting on it to keep me mobile during harsh times. im not sure how this will affect you guys in gen4's, but all i have heard since i've joined the subaru community is....

"unequal size tires, wear, and pressures are bad mmmk."
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Old 11-15-2012, 01:42 AM   #37 (permalink)
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wife had a blowout in our Tribecca. we replaced all four tires. sucks but thats part of owning awd.
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Old 11-20-2012, 06:22 PM   #38 (permalink)
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Sorry for the bump but I don't like to leave threads hanging once my issue is resolved. The dealer, after a bit of prodding, "went to bat" for me with Subaru (after I called them, lol) and decided to replace all four tires under warranty. I believe the primary reason they did so is because the service adviser told me 1/2" when it was really 1/4" and they probably never measured the tires in the first place since he didn't have measurement to give me.

They also did a vibration kit including steering column and control arms. I haven't actually driven the car yet but my mom has and told me it's wonderful. I never felt like I had a "shaker" but I wasn't going to stop them.

So, four free tires, hoping they make it another 20k.
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Old 11-20-2012, 06:37 PM   #39 (permalink)
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Wow, that is terrific! I thought the tires were warranted by Continental and basically there was no ride/noise/balance coverage after the first 2/32 of tread was gone. I wish my Dealer would have gotten my tires covered. (Maybe that's where I got the wrong info on 2/32's difference.) What does the vibration kit do, and is there a tsb number or part number? I do not have a shaker, but some road surfaces do cause the wheel to slightly vibrate, does this kit fix that?
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Old 11-20-2012, 06:57 PM   #40 (permalink)
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Wow, that is terrific! I thought the tires were warranted by Continental and basically there was no ride/noise/balance coverage after the first 2/32 of tread was gone. I wish my Dealer would have gotten my tires covered. (Maybe that's where I got the wrong info on 2/32's difference.) What does the vibration kit do, and is there a tsb number or part number? I do not have a shaker, but some road surfaces do cause the wheel to slightly vibrate, does this kit fix that?
Green - is there anything wrong with your car now? Do you really want non factory shop guys screwing with your car? Some advice if nothing is wrong with your car don't create issues by having techs work on it. You have something wrong with it then the risk of a tech making it worse is not really an issue but having them make it slightly better or fixed is a much better option.

Make sure your tire pressure is spec per the sticker on the door that might help with the road vibration
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