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  1. #1
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    29er tire measurement in mm for computer

    I have a Cateye Micro Wireless computer that I want to put on my 29er, although the chart that came with the computer doesn't have a conversion to mm for 29er tire sizes. Does anyone have a tire size chart that lists 29ers, or does anyone have experience with this? Has anyone ever done an accurate rollout? I'm running 29x2.2 tires.

  2. #2
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    I actually found a tire sizing chart somewhere else on the forum that shows:
    29x2.1 = 2288mm
    29x2.3 = 2326mm

    It didn't list anything for 29x2.2, but is it safe to assume that 29x2.2 would be right half way between those numbers, at 2307? I'm not familiar with how these measurments work.

  3. #3
    meh....
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    Quote Originally Posted by socalguy
    I actually found a tire sizing chart somewhere else on the forum that shows:
    29x2.1 = 2288mm
    29x2.3 = 2326mm

    It didn't list anything for 29x2.2, but is it safe to assume that 29x2.2 would be right half way between those numbers, at 2307? I'm not familiar with how these measurments work.
    the manual for the computer will tell you how to measure your own tire/wheel, that's the easiest way to do it if you want it fairly accurate. you can do it in the kitchen.

    or just guess using the numbers given.

  4. #4
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    I prefer accuracy. Air your tires, mark your wheel, mark the floor, sit on it and roll it one revolution, mark the floor, and then measure.

  5. #5
    Hmmmmm
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    The conversion is 25.4mm per inch. Plus, .125" = 1/8"

    Inches X 25.4 = mm.

    Or you can go to your local hardware store and buy a metric tape measurer.
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  6. #6
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    Cateye also has a much more comprehensive chart on their website. It's in the tech section I believe.

  7. #7
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    Quote Originally Posted by socalguy
    I have a Cateye Micro Wireless computer that I want to put on my 29er, although the chart that came with the computer doesn't have a conversion to mm for 29er tire sizes. Does anyone have a tire size chart that lists 29ers, or does anyone have experience with this? Has anyone ever done an accurate rollout? I'm running 29x2.2 tires.
    Do the roll out. Not all 29 x 2.2 tires are the same diameter.
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  8. #8
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    Agreed...

    I've found quite a variation in actual tire diameter between tires of the "same size". As an example a Kenda Nevegal 2.1 has a significantly larger diameter than a Panaracer Fire XC Pro 2.1. Not a huge difference, but significant enought to make a difference in the readings that I get from the computer. This is due to the volume differences between the tires. Higher volume tires will usually have a taller side wall that will give you a larger effective outside diameter. So it can vary quite a bit between tires of the same supposed size. The charts that are provided with cyclo computers are calculated using either a single tire, or are an "average", or are calculated using "the standard", which no manufacturer in the bike tire industry sticks to anyway. They aren't very accurate.

    Use the method that gjenkins@ and Shiggy mentioned and roll the tire out. It's more accurate than using a generic chart.

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  9. #9
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    We need a emoticon for someone smokin a spliff, cause whoever posted that surely was my avg size I use is 2270 and that's between Xmarks, Ignitors, RR 2.25", Bonti XR1s, Nanos. I don't even worry to change it when I go to the bigger tyrs like Rampage, WW LT, Mtn.King 2.4", Ardent 2.25" because the difference isn't even 1% and who needs to be that accurate. Definitely nothing is over 3000mm if you do the roll out properly with riding pressure in the tyre and you sitting on the bike.

    Quote Originally Posted by socalguy
    I actually found a tire sizing chart somewhere else on the forum that shows:
    29x2.1 = 2288mm
    29x2.3 = 2326mm

    It didn't list anything for 29x2.2, but is it safe to assume that 29x2.2 would be right half way between those numbers, at 2307? I'm not familiar with how these measurments work.
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  10. #10
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by Squash
    I've found quite a variation in actual tire diameter between tires of the "same size". As an example a Kenda Nevegal 2.1 has a significantly larger diameter than a Panaracer Fire XC Pro 2.1. Not a huge difference, but significant enought to make a difference in the readings that I get from the computer. This is due to the volume differences between the tires. Higher volume tires will usually have a taller side wall that will give you a larger effective outside diameter. So it can vary quite a bit between tires of the same supposed size. The charts that are provided with cyclo computers are calculated using either a single tire, or are an "average", or are calculated using "the standard", which no manufacturer in the bike tire industry sticks to anyway. They aren't very accurate.

    Use the method that gjenkins@ and Shiggy mentioned and roll the tire out. It's more accurate than using a generic chart.

    Good Dirt
    An actual example:
    Both Conti 29 x 2.2
    Mountain King 728mm diameter
    Race King 740mm
    mtbtires.com
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  11. #11
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    I'm having the same issue & found this thread by searching. (go figure!) Here's what helped me out:

    LINK

    I can see, however, how a roll-out would be helpful. I may do that just as a comparison. I ended up with a 2250mm diameter using the ETRTO #'s off my tire & link above.

  12. #12
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    You could also just measure the distance from ground to the center of the axle (radius), multiply by 2 to get the diameter, multiply by pi (3.1415926 is close enough for pi), and you have your circumference measurement.
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  13. #13
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    My computer (Cateye Enduro 8) takes only whole number entries in centimeters, so that kind of precision isn't necessary.

    The ETRTO # from my Bontragers is is 48-622. To obtain the diameter, you take 3.142 x (2 x 48 + 622) = 2255.956, in millimeters. I enter 225.
    Last edited by DeltaNu1142; 01-04-2010 at 09:33 AM.

  14. #14
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    The pi decimals were meant to be humorous, based on some of the previous discussion. Maybe I'll have to start being a little more explicit in my posts. No harm, no foul.

    Quote Originally Posted by DeltaNu1142
    My computer (Cateye Enduro 8) takes only whole number entries in centimeters, so that kind of precision isn't necessary.

    The ETRTO # from my Bontragers is is 48-622. To obtain the diameter, you take 3.142 x (2 x 48 + 622) = 2255.956, in millimeters. I enter 225.
    Hey everybody, ride my wheels! They ride good, real good.
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  15. #15
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    I'm new, gimme a break. And this font doesn't lend itself well to sarcasm
    Last edited by DeltaNu1142; 01-04-2010 at 11:17 AM.

  16. #16
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    Quote Originally Posted by DeltaNu1142
    My computer (Cateye Enduro 8) takes only whole number entries in centimeters, so that kind of precision isn't necessary.

    The ETRTO # from my Bontragers is is 48-622. To obtain the diameter, you take 3.142 x (2 x 48 + 622) = 2255.956, in millimeters. I enter 225.
    I took a measurement of 3 revolutions of this tire; it came out to 266.25 inches. Multiply by 2.54, divide by 3, and I get 225.425--pretty close! This ETRTO figure might be the ticket.
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