New Tire Diameter Variance

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CapriRacer, in another thread you wrote:

Originally Posted By: CapriRacer
………. In fact, when tire manufacturers measure the diameter, width, etc. of tires, they commonly refer to those measurements as "24 hour measurements" - because those measurements take place 24 hours after the tire is mounted. A tire will continue to grow long after 24 hours, but 24 hours gets most of the growth out and is a convenient point in time to record the measurements………………..


How much variance in tire diameter do tire manufacturers typically allow for a single passenger tire size in a single model? As a hypothetical example, how much variance in diameter do you think will be allowed in a production run of Michelin MXM4 in size 215/45R17 ?

How do tire manufacturers measure diameter? With a $100,000 machine? Is the loaded rolling radius being measured, or the actual diameter?
 
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More importantly, how much tire-to-tire variation is there in revs per mile? I would bet that number would change less than overall diameter as the tire breaks in, per CR's description in the quote.
 
I think that's the whole point... some carmakers (Fuji Heavy Industries - I'm looking in your direction) are incredibly hung up on 'rolling diameters' matching each other exactly. There's got to be some variability from tire to tire, even within the same production lot.
 
Originally Posted By: HerrStig
I think full time four wheel drive is less tolerant of wheel to wheel variations.


on hard surface with good traction.

doesnt matter as much on dirt or even gravel.
 
Saab 9-7x awd is very sensitive to diameter variation. Even with same make and model tires, the clutches whine when one of the 4 is not nearly the same wear level as the others. Forget mixing brands all together. Ford Explorer awd doesn't seem to be nearly as sensitive.
 
Subaru wants all tire diameters to be within 0.25" of each other. But they don't specify an accurate method for measuring diameter. Using a tailor's tape and wrapping it around the tire and getting a constant measurement is challenging.
 
Originally Posted By: SubLGT
Subaru wants all tire diameters to be within 0.25" of each other. But they don't specify an accurate method for measuring diameter. Using a tailor's tape and wrapping it around the tire and getting a constant measurement is challenging.


circumference within 1/4"
tread depth within 2/32 - all 4 tires.
 
Originally Posted By: Rand
Originally Posted By: SubLGT
Subaru wants all tire diameters to be within 0.25" of each other. But they don't specify an accurate method for measuring diameter. Using a tailor's tape and wrapping it around the tire and getting a constant measurement is challenging.


circumference within 1/4"
tread depth within 2/32 - all 4 tires.


Oops! Bad brain fade on my part! I mean't circumference.
 
Originally Posted By: SubLGT


Oops! Bad brain fade on my part! I mean't circumference.


yep I've never done that...
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I knew what you meant so I just posted it up
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Originally Posted By: SubLGT
CapriRacer, in another thread you wrote:

Originally Posted By: CapriRacer
………. In fact, when tire manufacturers measure the diameter, width, etc. of tires, they commonly refer to those measurements as "24 hour measurements" - because those measurements take place 24 hours after the tire is mounted. A tire will continue to grow long after 24 hours, but 24 hours gets most of the growth out and is a convenient point in time to record the measurements………………..


How much variance in tire diameter do tire manufacturers typically allow for a single passenger tire size in a single model? As a hypothetical example, how much variance in diameter do you think will be allowed in a production run of Michelin MXM4 in size 215/45R17 ?

How do tire manufacturers measure diameter? With a $100,000 machine? Is the loaded rolling radius being measured, or the actual diameter?


First, the diameter is the diameter. There is no tolerance - no good/bad. If there is a problem with the diameter variation, it is a symptom of something else.

But to answer the question you are trying to ask: What is the production variation? I'd guess plus or minus a tenth of an inch or so. I know it is well within the tolerance published by The Tire and Rim Association. (OK, so I looked up the tolerance and it's about 1/2" depending on size)

How do they measure diameter? With a simple pi tape - a tape measure that instead of measuring the circumference, uses 3.14159" to the inch, so it measures diameter.

Or at least that is what they used to use. If someone has developed a sophisticated laser measurement machine, I wouldn't be surprised, but I would have to wonder why, because that would be money spent for very little gain.

Tire manufacturers measure the freestanding diameter and calculate the rolling diameter (often expressed as Revs/Mile so there isn't any confusion.) What I remember is that the calculated value would be periodically verified by using a counter on a pulley wheel (a tire test machine), but that the calculation was so good, they didn't do it very often.

Additional thoughts after I read the rest of the thread:

I think it is obvious that tire to tire diameter variation within a given make/model/size would be less than the tolerance for the worst of the AWD systems - BUT - could be larger between make/model even of the same size. Which is why tire dealers want to install 4 tires when they encounter an AWD or a 4X4.
 
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