[ale] way OT - used car buying tips - yea or nea - PT 2

Thu Nov 29 07:31:25 EST 2012

Maybe it's just me but it seems like all of the data about uneven tire
wear causing premature drive line failure is coming from TIRE SALES.

I seriously doubt that the engineers that designed the Subaru AWD
system considered "slip" to be a whopping 4 full tire revolution per
mile incrementally. I would consider, and expect they would too, slip
to be a sudden change in rotation difference. As the hydraulics
involved use pressure differential valving, it's pretty clear to me
that small differences, like those between the inner tire and outer
tire on a curve (a similar difference as between wear levels) would be
the normal differential action. It should only be the action of a tire
suddenly spinning that causes a pressure drop across a circuit that
allows a pin valve to close off power to that wheel.

I can see if someone put different SIZE tires causing an issue. But
tire wear is all within the realm of turning differentials.

On Wed, Nov 28, 2012 at 10:55 PM, Ron Frazier (ALE)
<atllinuxenthinfo at techstarship.com> wrote:
> I found this link.  I think I'll stop researching the awd for now and pick it up again later.
>
> http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18
>
> Ron
>
>
> "Ron Frazier (ALE)" <atllinuxenthinfo at techstarship.com> wrote:
>
>>Hi Sean,
>>
>>I'm not an expert on AWD, just reporting what I've been told.  I'll try
>>to do some research when I can and post anything I find that looks
>>useful.  Here's the situation as I understand it.  I think you're right
>>that most Santa Fe style AWD systems are FWD most of the time.  Many of
>>the new cars have traction control or stability control.  If the system
>>detects slippage, the rear wheels are engaged.  If all tires are the
>>same size and none are slipping, the system is happy.  Let's say you
>>replace one tire with a used smaller one because an original one was
>>damaged.  Then, that new tire will be spinning faster.  The system may
>>think that one is slipping.  Then the viscous couplings, etc. try to
>>route additional power to the other three wheels and reduce power to
>>the "slipping" wheel.  Which, of course, they cannot do because the
>>other three are turning as fast as they can.  So, the traction control
>>system becomes engaged full time and wears things out.  If you had put
>>a larger!
>>tire on to replace the damaged one, then the system might think all
>>three others are slipping, and try to route power to the one new one,
>>which would again cause problems.
>>
>>This forum post says Subaru has a system which requires no more
>>variance in tire size than 1/4" in circumference, which is 1.25 / 32"
>>of tread depth, a goal which is almost impossible to meet.
>>
>>http://townhall-talk.edmunds.com/direct/view/.f1b9005/26
>>
>>In another forum post, I found out that tirerack.com has a tire shaving
>>service that they use to equalize the size of a set of tires for
>>people.
>>
>>In the case of the Mazda I almost bought, I was having a pre purchase
>>inspection done at the dealer, but was not buying the car from them.
>>The service manager was very emphatic that I could not just replace the
>>tire with the nail and screw in it.  I don't think he was trying to
>>defraud me.
>>
>>Sincerely,
>>
>>Ron
>>
>>
>>Sean Kilpatrick <kilpatms at gmail.com> wrote:
>>
>>>I am very suspicious of this.  There are significant differences
>>>between
>>>four-wheel drive and "all-wheel drive."  The latter, which most
>>>light-duty
>>>SUVs have today, is for temporary use only on slippery surfaces as the
>>
>>>drive electronics lock the differentials. That is, the wheels on the
>>>left
>>>side are turning in lock step with the ones on the right. When the
>>>diffs
>>>are locked, then the left/right tire pairs have to be very close to
>>the
>>>
>>>same circumference or the constant drag/slippage almost certainly will
>>
>>>cause problems. On snow, ice, or mud, not really a problem for small
>>>differences.
>>>
>>>The problem, such as it is, is real for vehicles (think older Land
>>>Rovers)
>>>that have the ability to lock any of the three diffs independently.
>>At
>>>
>>>that point the tires need to be the same size to avoid problems.
>>>
>>>Today almost all "AWD" vehicles are full time front-wheel drive and
>>>have
>>>only two diffs: One for the front and one for the center.  When the
>>>center
>>>one is "engaged" the back wheels turn as one and the front diff also
>>is
>>>
>>>locked. Significant differences in tire circumference will put stress
>>>on the
>>>drive train.
>>>
>>>But a thirty-second or two is probably not significant.  The tire on
>>my
>>>SUV
>>>has a current diameter of 28.25 inches (more than 30k miles.)  Suppose
>>>I
>>>replaced it with another used tire with an additional 2/32" of tread.
>>>The
>>>rolling diameter of the smaller tire would still be 99.78% of the
>>>larger.
>>>If this were a real problem, there would be many more drive train
>>>failures
>>>caused by installing the spare tire and continuing down the
>>interstate.
>>>(And, no, I wasn't willing to haul out the spare tire and measure its
>>>actual diameter.)
>>>
>>>Sean
>>>
>>>--------------------------------------------------------------------------------------
>>>
>>>
>>>
>>>On Wednesday, November 28, 2012 05:16:11 pm Ron Frazier (ALE) wrote:
>>>> Re: awd - All wheel drive can be very handy.  I almost bought an AWD
>>>> Mazda Tribute.  However, it had an unpatchable screw AND nail in one
>>>> tire.  They said I had to replace all tires at once to keep them the
>>>> same circumference.  I did some research and it appears to be
>>>> true.  You have to keep the tread depth of all tires within 1 or
>>2/32
>>>> of an inch or it screws up the drive train and can cause premature
>>>> failure.  I never knew that before
>>>
>>>
>>>------------------------------------------------------------------------
>>>
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>>
>>
>>--
>>
>>Sent from my Android Acer A500 tablet with bluetooth keyboard and K-9
>>Mail.
>>Please excuse my potential brevity.
>>
>>(To whom it may concern.  My email address has changed.  Replying to
>>former
>>messages prior to 03/31/12 with my personal address will go to the
>>wrong
>>address.  Please send all personal correspondence to the new address.)
>>
>>(PS - If you email me and don't get a quick response, you might want to
>>call on the phone.  I get about 300 emails per day from alternate
>>energy
>>mailing lists and such.  I don't always see new email messages very
>>quickly.)
>>
>>Ron Frazier
>>770-205-9422 (O)   Leave a message.
>>linuxdude AT techstarship.com
>>
>>
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>
>
> --
>
> Sent from my Android Acer A500 tablet with bluetooth keyboard and K-9 Mail.
> Please excuse my potential brevity.
>
> (To whom it may concern.  My email address has changed.  Replying to former
> messages prior to 03/31/12 with my personal address will go to the wrong
> address.  Please send all personal correspondence to the new address.)
>
> (PS - If you email me and don't get a quick response, you might want to
> call on the phone.  I get about 300 emails per day from alternate energy
> mailing lists and such.  I don't always see new email messages very quickly.)
>
> Ron Frazier
> 770-205-9422 (O)   Leave a message.
> linuxdude AT techstarship.com
>
>
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