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mismatched tread depth on AWD

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  • mismatched tread depth on AWD

    I have a set of four Michelin Premier tires on my 2015 Subaru Forester. With 15,000 miles on them, one of the rear tires needed replacing today after it ran over what appears to be the tip of a box cutter. I limped to Costco on the wimpy space-saver spare and they confirmed that they could not repair the blown tire. They replaced it with a new one, at a pro-rated warranty cost of about half the original price.

    The new tire has about 2 mm (0.080") deeper tread than the three worn tires. Is this difference enough to cause drive-train problems, or steering problems when I swap front and rear?
    Allan Ostling

  • #2
    Because of a lot of different reasons, the short answer is no.

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    • #3
      Originally posted by Arcane View Post
      Because of a lot of different reasons, the short answer is no.
      The answer must be "yes" at some value of mismatch. Can you quantify this approximately?
      Allan Ostling

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      • #4
        Ask Subaru.... you will cause some additional asymmetrical transmission rotation but as it's AWD it won't ever bind up on you.

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        • #5
          Originally posted by Arcane View Post
          Because of a lot of different reasons, the short answer is no.
          Yup. Just for the fun of it measure the other 3 tires on the car. I would not be surprised to find that you have more than .080" variation between them. Just through normal wear tires can be vary in diameter by quite a bit. People run used, mis-matched tires on cars all the time. How much difference is there between your regular tire and the dinky spare? For another experiment take two tires and inflate one 8-10 lbs. over the suggested pressure; inflate the other to 10 lbs. under. Now measure them and see what the difference is...
          Keith
          __________________________
          Just one project too many--that's what finally got him...

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          • #6
            Originally posted by aostling View Post
            The answer must be "yes" at some value of mismatch. Can you quantify this approximately?
            No I can't but I agree, at some value of mismatch the answer would be yes. I do know small variations such as a tread depth difference of .080" aren't going to be a problem. If they were, we would see a lot more problems than we do and we don't see any. When was the last time someone measured the rolling radii (not circumference) of their tires? Because that's what really matters. A difference in tire pressure will result in a different rolling radius. A difference in load on each wheel will so the same. As a matter of fact, I run the front tires on my 4X4 Chevy 3/4 ton at 5 psi higher air pressure than the rear tires to compensate for the extra weight on the front axle because I have found out by experimenting that that difference eliminates any bind in my transfer case when I am driving in a straight line in 4x4 mode.

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            • #7
              Originally posted by Arcane View Post
              ... I do know small variations such as a tread depth difference of .080" aren't going to be a problem.
              Okay, I am reassured. The tire boss at Costco also said that 15,000 miles of wear would not be too much of a mismatch.

              What is a problem is the space-saver spare. The unavailability of a full-size spare was almost a "no buy" situation when I was shopping. The spare tire well is too tight to accept the OEM tire and wheel. But a guy in Ajo, Arizona, posted a method of fitting a slightly smaller tire, here: http://www.subaruforester.org/vbulle...-spare-396521/. This necessitates giving up the rigid-foam tool organizer beneath the floor of the luggage compartment. I'll gladly forego that, to have peace of mind when I am driving on some of the vehicle tracks which are common throughout the American West.
              Allan Ostling

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              • #8
                I wonder if it is really true that differences in inflation pressure actually cause a difference in the number of rotations per mile. The effective radius decreases for a deflated (Brady-ized?) tire, which may cause a different amount of thrust for a given axle torque. But the circumference of the tire stays pretty much the same, and so each rotation should move the vehicle the same distance. This would not be the case for a "balloon" tire that expanded with added air pressure, but a belted tire constrains the dimensions of the tire to a large extent, except for the small amount of stretch that might occur in the plies and belts.

                The .080" difference in diameter amounts to only about 0.3%. There will be at least that much difference when driving on a road with a bit of a curve. If the wheelbase is 6 ft wide, a curve with a radius of 1800 feet would be about equivalent.

                http://www.subaruforester.org/vbulle...erence-125150/

                Last edited by PStechPaul; 09-19-2016, 03:41 AM.
                http://pauleschoen.com/pix/PM08_P76_P54.png

                Paul: www.peschoen.com
                P S Technology, Inc. www.pstech-inc.com
                and Muttley www.muttleydog.com

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                • #9
                  Originally posted by PStechPaul View Post
                  ... The .080" difference in diameter amounts to only about 0.3%.
                  The tread depth difference is 0.080", so the diameter difference is 0.160". This does not negate your reasoning, which is reassuring.
                  Allan Ostling

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                  • #10
                    My saloon detects tyre deflation using the ABS, presumably by different rotational speed though the manual hints it is detecting different rolling resistance. So on some vehicles you might get false alerts.

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                    • #11
                      Why don't you contact Michelin?

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                      • #12
                        Unless the tires are serially numbered, which I doubt, there's probably that much difference between brand new tires of the same line. Besides, the tires you buy from tire stores are "factory seconds" to those put on new cars at the factory. Cripes, they can't even guarantee them to be round! The tire balance takes care of that.

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                        • #13
                          Why on earth would you contact Michelin?

                          -D
                          DZER

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                          • #14
                            In an ideal world there should be no difference in sizes. Having said that, there is more than enough leeway that differences like you have are easily accommodated. If this were a specialty race or performance vehicle you would "sweat the small stuff" indeed.
                            The reason for keeping your speed down with a space saver spare is because the diameter is grossly different from the regular tire and this causes the differential gears in the axle to act as if you are constantly turning a rather sharp corner. This has the distinct result of very expensive damage/wear to the differential gears as they do not run on bushings or bearings but rely on the oil for lubrication to reduce wear, keeping in mind that they only operate on turns. Moving straight, they are essentially locked up. Getting stuck and having one wheel spinning is death to differentials as they are operating way beyond their intended limits.

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                            • #15
                              Originally posted by aostling View Post
                              The tread depth difference is 0.080", so the diameter difference is 0.160". This does not negate your reasoning, which is reassuring.
                              The tire difference is still figured from the radius of the tire which is still .080". Could the difference be partially compensated for by playing with different inflation pressures? May be a couple of pound more in the small tire? Just a thought.

                              Sarge

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