tire wear and all wheel drive

Discussion in 'General Motoring' started by Andy K, Aug 13, 2003.

  1. Andy K

    Andy K Guest

    Hi all

    Just had to get tire repaired due to nail in same('02 Forester S). Tire guy
    expressed some concern about my swapping spare for the repaired tire
    because of difference in tread depth/wear affecting all wheel drive
    performance. I remember some posts about this issue some time ago, but don't
    remember what the consensus of opinion was. I personally don't see the point
    in replacing all 4 tires if one happens to go flat, nor do I have the $$ to
    do so. Please advise.

    Andy
     
    Andy K, Aug 13, 2003
    #1
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  2. You shouldn't really be making decisions based on usenet opinions.

    The issue is a difference of rotation rates when wheels are not
    the same circumference. I think it has been shown that front
    to rear rotation rates should be kept as close as possible,
    but side to side differences are less important, as the centre
    differential has more potential for premature wear.

    There are two fairly distinct camps of thinking.

    One believes the edict of Subaru in the U.S. which
    states cirumferences should be kept within ½ inch or so.
    That's only about 0.6% difference.

    Another, of which I am a member, thinks that normal tire wear balanced
    with regular rotations will be just fine. Replacing all four tires
    is likely unnecessary when only one is damaged. Wheels travel
    different distances while driving in the city around corners than
    any small difference in circumference is likely to make.

    I have not seen any concrete examples of premature centre
    differential failure on a Subaru AWD which can be shown to be caused
    by tire sizes. Given the number of owners who talk about the hundreds
    of thousands of kilometers they have on their cars, it should
    be showing up somewhere.
     
    Dave Null Sr., Aug 13, 2003
    #2
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  3. Andy K

    GC Guest

    It all depends on the wear of the other tires. I had to replaced a tire at
    15k
    and have 97k on the car now. I do not think my next car will be AWD or 4WD
    too much of chance of screwing up the system.

    Gary
     
    GC, Aug 13, 2003
    #3
  4. The advantages far outweigh the disadvantages since the risk of
    "screwing up the system" must be quite small, or we would have lots
    of truly pissed off owners in here (or elsewhere) complaining
    about differential failures. We don't.
    Instead, we hear about how many cars are
    surviving to very high mileage with no major problems.
     
    Dave Null Sr., Aug 13, 2003
    #4
  5. Andy K

    Clive Guest

    Tyre suppliers myth..good for business.
    If it was so important it would be emblazoned
    across the appropriate section in the hand book
    in large red capitals. I don't see it.
    My Subaru dealer tells me that they've never seen
    a transmission problem due to differing sizes.
    I'm on my 3rd Subaru...got caught once by the
    rubber merchants, took advice after the event
    and now ignore the spiel when I need a new pair.
    No problems in any respect and a massive £ saving.
    Maybe there's a difference in UK/US specs
    /warranty etc but obviously no difference in tyre
    suppliers!.


    --
    Clive Norris
    Selectron (UK) Ltd
    www.espguitars.co.uk
    www.espshop.co.uk
    www.mightymite.com
    www.svetlana-tubes.com
    www.emgpickups.co.uk
    www.tube-shop.com
    www.whirlwindusa.com
     
    Clive, Aug 13, 2003
    #5
  6. Andy K

    Don Nickell Guest

    You will get all sorts of stories from dealers, as Clive says.
    I had an 88' Porsche with crap for tires, so got a set of Goodyear AT
    crap from a "locally owned and operated" dealer in Santa Fe, NM (Amigo Tire)
    instead of listening to a friend who said use $$Michelin Pilots$$.

    On the freeway going home a rear tire went flat. I went back to said
    dealer. "Did you run this tire when it was flat?" What the hell did
    he expect, when a tire lets go on the freeway at 75 there's bound to
    be some "run flat" time. Sheeze... He absolutely refused to refund
    even though I pointed to crap on the bead of the wheel that he didn't
    clean off; previous owner had used some liquid "fix flat" at one time
    or other. Also turned out to be the stem insert was the leak, found
    when I put the $$Michelin Pilots$$ on.

    So, only recourse is small claims court and argue your case. Great
    to be an American?

    Be careful; get good recomendations from club members, Sub dealer, etc.

    Then do a lot of Hail Marys.

    Don
     
    Don Nickell, Aug 13, 2003
    #6
  7. Andy K

    Dan Duncan Guest

    One of the nice things about having a full sized spare is that you can rotate
    the spare in and the most worn tire out when you rotate the tires. (You DO
    rotate the tires, right?)

    This keeps your spare at roughly the same size so you can just put it on
    and leave it on if you ever get a flat and you can go 25% more miles
    before you need to buy new tires. (woohoo!)

    If the flat can't be repaired you'll end up with a new spare that's
    less worn than the other tires, but this would also be true if you
    never rotated your spare in. I haven't had to do it, but my plan
    in this case is to replace the most worn tire with the new spare
    and then carry on as usual. By the time I rotate again, 3 of my
    existing tires may be worn down to the level of the current spare
    and continually pulling out the most worn and spreading the wear across
    those 4 will let the newest tire catch up.

    -DanD
     
    Dan Duncan, Aug 13, 2003
    #7
  8. Ditto, but my Subaru mechanic refused to "safety" the car unless the four
    tires were within a 1/4 diameter, because he said it would cause it to skid
    under acceleration in snow as the power to each wheel would be different.
     
    Dominic Richens, Aug 13, 2003
    #8
  9. Andy K

    Clive Guest

    Clive, Aug 13, 2003
    #9
  10. Andy K

    Edward Hayes Guest

    I do agree that tires of equal size should be used on all four corners. Audi
    recommends 1/4 in difference max for cars driven for max performance and
    appantly Subaru has picked up on that number. I can't find anything written
    by or any recommendations from Subaru. I would think that if 1/4 or 1/2 inch
    difference in circumference would cause a problem would state so in writing.
    Tires marked 215/60/16 from different manufacture vary by about 1/4 inch
    circumference. Any tire outlet who tells me that I MUST replace all four for
    safety or any other reason will loose me as a purchaser. I make this
    statement assuming the three tires have useable tread. Salesmen will go to
    great lengths to see you 4 tires instead of one. Just my opinion. Others
    disputing this should post data and not guesses or tire salesmen's expert
    opinion. Eddie

    You will get all sorts of stories from dealers, as Clive says.
    I had an 88' Porsche with crap for tires, so got a set of Goodyear AT
    crap from a "locally owned and operated" dealer in Santa Fe, NM (Amigo Tire)
    instead of listening to a friend who said use $$Michelin Pilots$$.

    On the freeway going home a rear tire went flat. I went back to said
    dealer. "Did you run this tire when it was flat?" What the hell did
    he expect, when a tire lets go on the freeway at 75 there's bound to
    be some "run flat" time. Sheeze... He absolutely refused to refund
    even though I pointed to crap on the bead of the wheel that he didn't
    clean off; previous owner had used some liquid "fix flat" at one time
    or other. Also turned out to be the stem insert was the leak, found
    when I put the $$Michelin Pilots$$ on.

    So, only recourse is small claims court and argue your case. Great
    to be an American?

    Be careful; get good recomendations from club members, Sub dealer, etc.

    Then do a lot of Hail Marys.

    Don


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    Edward Hayes, Aug 14, 2003
    #10
  11. Andy K

    Bill Putney Guest

    We've got some people posting about 1/4" difference in *diameter*, and some
    about 1/4" difference in *circumference*. Which is it? It makes a huge
    difference - by a factor of pi (3.1415926535897932384626433832795...).

    Bill Putney
    (to reply by e-mail, replace the last letter of the alphabet in my address with
    "x")
     
    Bill Putney, Aug 14, 2003
    #11

  12. I could be wrong, he might have said circumference, but that's such a small
    difference I have a problem believing it would make a difference, so I said
    diameter :)
     
    Dominic Richens, Aug 14, 2003
    #12
  13. Underinflation should not cause any problems. The circumference of the
    tire remains the same independent of low air pressure, so the number
    of wheel revolutions per mile remains the same.
     
    Victor Roberts, Aug 15, 2003
    #13
  14. Andy K

    Jim Stewart Guest

    I'm not so sure. I'm sure that overinflation will increase the
    circumference of the tire. So it stands to reason that under-
    inflation would decrease the circumference, at least as long
    as the tire body is contracting with the reduction of air pressure.

    Rubber is elastic and will grow and shrink with changes of
    internal pressure.
     
    Jim Stewart, Aug 15, 2003
    #14
  15. While I admit that over inflation MIGHT stretch the rubber and
    increase the circumference, note that I said *under inflation* - as in
    too little air pressure - :) and you decided to use *over
    inflation* in your example. And it does *not* stand to reason that
    just because *over inflation* might stretch the tire under some
    circumstances, that *under inflation* would make the reduce
    circumference.
     
    Victor Roberts, Aug 15, 2003
    #15
  16. Andy K

    Sympatico Guest

    Very interesting, didn't think I'd start such a storm... :)

    Found the following at Continental Tires describing how one of their
    pressure monitoring system works:

    DDS requires no sensors of its own but evaluates the data gathered via
    the
    EBS wheel speed sensors. Any loss of pressure changes the radius of the
    tires,
    and results in a specific alteration of the speed signal.

    URL:
    http://www.conti-online.com/generator/www/de/en/continentalteves/continental
    teves/themes/products/tire_pressure_loss_detection/dds_0602_en.html

    I must admit that it is confusing: The tire tread, the part in contact with
    the road, doesn't stretch or contract much so stays the same length, but the
    radius of the tire/wheel assembly does change, so something, somewhere, has
    to give...

    Nicolas
     
    Sympatico, Aug 15, 2003
    #16
  17. Andy K

    Edward Hayes Guest

    The tire radius is not a constant because the force/load on the tire is not
    symmetrical. Circumference stays approx the same because the radius front to
    back of the tire is greater than the top to bottom when loaded. At least
    that's my understanding of why the circumference stays approx constant.
    Eddie
     
    Edward Hayes, Aug 15, 2003
    #17
  18. Andy K

    Jim Stewart Guest

    Well, we have a difference of opinion. Who's going to jack
    up their car and do some tests? Might be interesting to know.
     
    Jim Stewart, Aug 15, 2003
    #18
  19. Andy K

    Mike Guest


    Time for some math (someone double check this)

    1 mile is 63360"
    the circumfrance of a 215/60/16 is approx 82.3"
    with that, it requires that wheel to turn 770 times to go a mile

    if you increase the circumfrance by 1/4 inch (.25) this wheel will
    require 767.5
    turns to go a mile, which is a 2.5 turn delta.

    At 60 MPH, the larger wheel will turn 2.5 turns less than the smaller
    wheel every minute.

    Where is this difference obsorbed and will it cause damage? Is it
    different for Autos Vs Manual?
     
    Mike, Aug 15, 2003
    #19
  20. Andy K

    John Varela Guest

    There are steel wires going around the circumference so they tend to keep the
    circumference constant. (You can't really talk about the radius of a mounted
    tire because it's not circular, being flat on one side.) Of course the steel
    wires will stretch some when the tire's inflated, but it's not like a balloon.

    At slow vehicle speeds there should be no measurable change in rpm of an
    underinflated tire. But--as I understand it: I'm only repeating what I read
    elsewhere--at higher speeds the tire develops distortions around its
    circumference including in the contact patch, so that the effective
    circumference of the underinflated tire will change, and this is what causes a
    measurable change in rpm.
     
    John Varela, Aug 15, 2003
    #20
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