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Guest Message by DevFuse

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Simple and exact test for tire circumferences discrepancy

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1 reply to this topic

#1 frag


    Soob shade tree mechanic

  • Members
  • 1,777 posts
  • Montréal, Québec, Can.

Posted 02 November 2003 - 03:30 PM

I've been uneasy at times tinking about the very thin (in my opinion) Subaru AWD tolerance of 1/4 inch in tire circumference from front to back.
Found it difficult to directly measure the tires circumference using a tape. You have to measure exactly at the same place on the tire, and be sure the tape goes straight around the tire, etc.
This morning I went at a college parking lot, measured that 13 parking spaces were equivalent of about 20 tire revolutions. I put a chalk mark on front and rear tires at the point of road contact and proceeded to give them the 20 revs going in a straight line.
If the tires circumferences had a difference of 1/4 inch or more I would find the marks 5 or more inches apart at the end of the experiment.
They were nor more than 1 inch from each other relative to the tire's contact patch.
My mind is now at ease: my viscous coupling is not in danger.
I might try something else next time and measure what effect a difference in pressure does to the rolling radius of a tire. T'was to cold to add that to the experiment this morning.
My contribution to car science :santa:
Please, no Nobel prize necessary.

#2 theotherskip


    Eat, Live, Breath Subaru

  • Members
  • 329 posts
  • Philly, PA

Posted 02 November 2003 - 03:38 PM

i think that the vc that is used in the manual has more give to this tolerance than the transfer clutches used in the automatic, hense the lack of a way to disable the AWD when using the spare. i just make sure that i keep them rotated and i wouldn't replace just one...

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