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

[Ron Frazier (ALE)]

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
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>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