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  1. #1
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    Universal tire sizing?

    Why can't manufactures list a better / accurate tire measuring system? I can buy 5 different tires in the same listed sizes and come up w/ different measurable sizes (same rim & tire pressure). I would probably be more apt to purchase tires if I really knew their true size. I think this is quite important in the 650b realm due to 26" conversions. Here is a link to the spec's for the last automotive tire I purchased : BFGoodrich*Radial T/A

  2. #2
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    Universal tire sizing?

    Quote Originally Posted by keen View Post
    Why can't manufactures list a better / accurate tire measuring system? I can buy 5 different tires in the same listed sizes and come up w/ different measurable sizes (same rim & tire pressure). I would probably be more apt to purchase tires if I really knew their true size. I think this is quite important in the 650b realm due to 26" conversions. Here is a link to the spec's for the last automotive tire I purchased : BFGoodrich Radial T/A
    Some try, but there are too many variables and marketing concerns.
    http://mtbtires.com/site2/tech/38-ge...and-other-lies
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  3. #3
    www.derbyrims.com
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    This would be great. But even car tires marked as the same size are different. The tread profile is one area of big differences with the same sized tire labeling, even within the same manufacture's brand of two models having the same size markings, more comfortable riding round casing and narrow across the tread for easier rolling and higher mileage on pavement, or more cornering performance traction with squared casing and wider tread. Besides tread design differences, the differences in profile of the same sizes tires affect bike tires in similar ways, cornering vs. rolling ease and comfort. Also rim width difference changes the profile and handling characteristics of the same tire.

  4. #4
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    Here's an email to a writer I recently sent on the matter:

    James,

    I appreciate the article you wrote regarding tire sizing. Like you, I was occasionally frustrated by tires not measuring their stated width.

    Your article was accurate in that identical tires mounted on rims of differing widths will yield different section widths. Keith Bontrager also brought up a good point in that the inflation pressure plays a role in the inflated tire’s width. But unfortunately these are not the only two factors influencing a tire’s inflated dimensions, so let’s take a look at them all. They are:
    - Internal Rim Width
    - Bead Hook / Rim wall Height
    - Inflation Pressure
    - Time
    - Tire Construction
    - Manufacturing Tolerances

    As you can see from the above list, the problem is quite complex. During my 5 years as the Senior Design Engineer for Maxxis I strove to improve the accuracy in our stated tire section widths. If you’re familiar with Maxxis mountain bike tires, you may know that the ‘older’ tires are undersized. Continental's current mtb tires seem to be undersized as well. So the actions I took included designing the tire on a rim width that would likely be used by a majority of our customers, and holding our production engineers to tighter tolerances. Twice I insisted that the sizing of prototypes I received for testing was not acceptable, and as a result those molds had to be scrapped and new molds ordered.

    Regarding time and manufacturing tolerance, below is a graph of the section width of two tires measured over a 24 hour period. You can see that even after 24 hours, the tire is still growing. Most carcass ‘swell’ has already occurred, however. But if the tire was only inflated to 30 psi, for example, it might take a full week for the ‘casing swell’ to stabilize. And manufacturing variation is higher than what you see below. I’ve seen mtb tires that vary over 1.5mm tire to tire.


    Regarding ‘Tire Construction’, sometimes tires with different constructions are cured in the same mold. For example, many of the Maxxis “Exo-Protection” and non-Exo tires are cured in the same mold. The casings of the non-Exo tires grow more with inflation pressure, and as a result they generally measure a little wider.

    So you can see that the problem is quite complex. There is no standard for rim design (apart from the UST specification), and resistance would likely come from the tire manufacturers for a number of reasons, one of which would be added cost in tire development.

    Let me know if you’d like to learn more about anything I’ve touched on, and I’ll be happy to oblige.

    Here’s another graph describing the relationship between a bicycle tire’s casing height and width and the rim width for your reference.






    W. Bryan Holwell

    Carlisle Transportation Products
    R&D Engineer - Power Sports
    Clinton, TN
    Universal tire sizing?-section-width-vs-time.jpgUniversal tire sizing?-casing-dimensions-vs-time.jpg
    Tire Design & Development Engineer. The opinions expressed in this forum are solely my own.

  5. #5
    Nouveau Retrogrouch SuperModerator
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    Universal tire sizing?

    Quote Originally Posted by bholwell View Post
    Here's an email to a writer I recently sent on the matter:


    Click image for larger version. 

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ID:	803598Click image for larger version. 

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    Thanks for posting this, Bryan. What would be the nominal size for the tire in the second graph? Seems like a huge section width change through the range.
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  6. #6
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    The longer the production..

    Quote Originally Posted by keen View Post
    Why can't manufactures list a better / accurate tire measuring system? I can buy 5 different tires in the same listed sizes and come up w/ different measurable sizes (same rim & tire pressure). I would probably be more apt to purchase tires if I really knew their true size. I think this is quite important in the 650b realm due to 26" conversions. Here is a link to the spec's for the last automotive tire I purchased : BFGoodrich*Radial T/A
    Maybe they could start revising/updating the actual measurements for popular tires if a number of users are reporting the same findings for a same make tire over a certain period of time?

  7. #7
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    Quote Originally Posted by shiggy View Post
    Thanks for posting this, Bryan. What would be the nominal size for the tire in the second graph? Seems like a huge section width change through the range.
    No problem.

    The second graph is a theoretical section height and width as it relates to internal rim width. The data was generated using SolidWorks.

    The "Casing Arc Length" is also commonly referred to as the inner tire periphery. It's roughly a standard 2.1" tire (52mm or 2.05" on a 19mm internal width rim).

    So on a 29mm internal width rim the formula generates a casing width of 55.54mm (an increase of 3.54mm per 10mm of rim width increase). That doesn't seem like a huge section width change to me.

    An interesting side note: Sometimes when measuring inflated ATV tires we don't have the correct (design) rim on hand. If the rim is 0.5" too narrow, TRA guidelines say to add 0.2" to the section width. So that's a 40% growth in the tires section width compared to increase in rim width. Very similar to my calculated 35.4% growth.
    Tire Design & Development Engineer. The opinions expressed in this forum are solely my own.

  8. #8
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    Quote Originally Posted by Deerhill View Post
    Maybe they could start revising/updating the actual measurements for popular tires if a number of users are reporting the same findings for a same make tire over a certain period of time?

    This isn't impossible, but it would require reworking all of the molds in order to change the tire size molded onto the sidewalls. I'm not convinced some sizing inconsistencies aren't intentional, either.
    Tire Design & Development Engineer. The opinions expressed in this forum are solely my own.

  9. #9
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    Universal tire sizing?

    Quote Originally Posted by bholwell View Post
    No problem.

    The second graph is a theoretical section height and width as it relates to internal rim width. The data was generated using SolidWorks.

    The "Casing Arc Length" is also commonly referred to as the inner tire periphery. It's roughly a standard 2.1" tire (52mm or 2.05" on a 19mm internal width rim).

    So on a 29mm internal width rim the formula generates a casing width of 55.54mm (an increase of 3.54mm per 10mm of rim width increase). That doesn't seem like a huge section width change to me.

    An interesting side note: Sometimes when measuring inflated ATV tires we don't have the correct (design) rim on hand. If the rim is 0.5" too narrow, TRA guidelines say to add 0.2" to the section width. So that's a 40% growth in the tires section width compared to increase in rim width. Very similar to my calculated 35.4% growth.
    Up to about the 30mm rim width I have seen. Having the section width continue to increase linearly to 50 seems off from my experience with smaller casings on rims up to ~40mm (internal) rims. Width growth slows and height decreases.
    http://mtbtires.com/site2/tech/38-ge...-on-tire-width
    I also mounted, but did not record, the 1.75 tire on a 45mm (external) rim and the rim was wider than the casing and VERY "flat".

    The greater the casing arc length, the more rim width seems to affect the mounted size.
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  10. #10
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    Universal tire sizing?

    Quote Originally Posted by bholwell View Post
    This isn't impossible, but it would require reworking all of the molds in order to change the tire size molded onto the sidewalls. I'm not convinced some sizing inconsistencies aren't intentional, either.
    Hutchinson did that. Years ago their 2.0" tires actually measured close to 2.0" (50-559) and were much larger than most of the other 2-inch tires on the market. They were losing OEM spec and aftermarket sales to "wider" and lighter 2.1" models from other companies (48 to 50-559). Hutchinson changed the nominal size to 2.10" and nothing else, and gained spec and sales.

    Seen many tires with one nominal size molded in and a different width on the hot stamp (Surly comes to mind).
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  11. #11
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    Quote Originally Posted by bholwell View Post
    This isn't impossible, but it would require reworking all of the molds in order to change the tire size molded onto the sidewalls. I'm not convinced some sizing inconsistencies aren't intentional, either.
    Lets say they know final dimensions are consistently smaller on a popular model... question that should be asked, how long of a time is an ethical amount to allow production to keep on advertising and selling the out of date specs ?

  12. #12
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    Universal tire sizing?

    Quote Originally Posted by Deerhill View Post
    Lets say they know final dimensions are consistently smaller on a popular model... question that should be asked, how long of a time is an ethical amount to allow production to keep on advertising and selling the out of date specs ?
    Forever, because it is a known quantity as is.
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  13. #13
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    Quote Originally Posted by shiggy View Post
    Forever, because it is a known quantity as is.
    The end user only knows the dimensions they advertise though, if it's consistently smaller than advertised, doesn't it seem right to revise it so the end user is happier with the product? A lot of options now

  14. #14
    Doesntplaywellwithmorons!
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    Schwalbe tires are equally all over the map... when they first started in with dumping tires onto the US market around 2002.. they were across the board claiming sizes much wider than reality (easily 10% under the listed size for every model). More recent models were (racing ralphs for example around 2010) hitting the claimed size, but not when first inflated.... I've VISIBLY watched the casing swell happen on them over a matter of minutes, growing around 5mm in overall width. I got a 2012 Nobby Nic 26 x 2.25 Evo TLr snakeskin... and its smaller in width on the same rim and pressure, than a 2011 example of the regular Evo version (so no TLr or SS).
    I don't post to generate business for myself or make like I'm better than sliced bread

  15. #15
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    Asking manufacturers to adopt a system that lets you compare actual sizes and weights across the range is asking a lot. Even if they tried to be honest (and many probably do), the accuracy would still be questionable.

    It's a battle I'd rather avoid, so when tire-shopping I look for reviews that state what the actual width was on their choice of rim and pressure. Much less headache this way.

    In the end a few mm or grams doesn't matter that much. If the tire rubs the stays, you were playing with too little clearance anyways. If the tire is not precisely as you wanted, chances are it's still good enough for casual riding. If you're more serious, just sell it and buy another tire until you find your one, true love.

  16. #16
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    Universal tire sizing?

    Quote Originally Posted by Deerhill View Post
    The end user only knows the dimensions they advertise though, if it's consistently smaller than advertised, doesn't it seem right to revise it so the end user is happier with the product? A lot of options now
    You stated on a popular model. Changing the nominal size could piss off and confuse more users than not changing it.
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  17. #17
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    With a garage full of partly used tires I wouldn't be interested in mfg's coming to a common std. My knowlege bank in the garage would be worthless.

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