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  1. #1
    Carpe Velo Yo Spiff's Avatar
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    Just a thought about tire measurements for bike computers...

    Every bike computer comes with a chart of settings for common tire sizes, which gets you in the neighborhood of the right setting, though we know a roll-out is the most accurate method of adjustment.

    But, I was wondering, why don't the tire manufacturers simply put a measurement in mm on the sidewall of the tire as an aid? This would be fairly accurate. Variables would be tire pressure and tread wear, but those are relatively minor.

    There I go thinking again...
    2000 Bianchi Veloce, '88 Schwinn Prologue, '88 Trek 900, '92 Trek T100, 2000 Rans Tailwind

  2. #2
    Senior Member Bluechip's Avatar
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    I think that would be no more accurate than the computer charts. Still to many variables- rim width, seating depth and the ones you mentioned. Just do the roll out if it matters that much to you. I've gone to gps on my main bikes and find that it's close enough for me.

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    There are three basic variables contributing to the effective circumference/diameter. 1: the size of the tire (width/diameter), 2: the pressure used, 3: the load (the total weight of the bicycle, rider, and cargo). The roll-out method (done properly) takes into account all of them. The tire diameter takes only one of them into account.

  4. #4
    aka Tom Reingold noglider's Avatar
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    njkayaker, you forgot about rim width.
    Please email me rather than sending me a private message. My address is noglider@pobox.com

    Tom Reingold
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  5. #5
    Carpe Velo Yo Spiff's Avatar
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    I didn't consider rim width as a factor. It does still seem to me that a specific measurement from the tire maker would be a more accurate measurement than the chart that comes with the computer.

    Quote Originally Posted by Bluechip View Post
    Just do the roll out if it matters that much to you.
    Actually, it really doesn't matter that much to me, as long as my measurement is close enough so that I don't miss a turn listed on a cue sheet. Mostly it was just a thought that has occurred to me and I wanted to see if it was a good thought or not.

    I just got some new tires to put on my tandem, so I'll do a rollout and see how much it differs from the chart.
    2000 Bianchi Veloce, '88 Schwinn Prologue, '88 Trek 900, '92 Trek T100, 2000 Rans Tailwind

  6. #6
    Senior Member JerrySTL's Avatar
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    Like the others said, rim width.

    Also inflation comes into play. A 150 lb guy on the same 700x25 tires and wheels at 120 psi is going to have a different reading than my 210 lbs at 100 psi.

    I inflate my tires to my normal riding pressures, and put my weight on the bike when doing the roll-out measurement.

  7. #7
    mgb
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    What is the best way to do an accurate roll-out measurement?

  8. #8
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    Quote Originally Posted by noglider View Post
    njkayaker, you forgot about rim width.
    Well, roll-out deals with that too!

  9. #9
    aka Tom Reingold noglider's Avatar
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    Quote Originally Posted by njkayaker View Post
    Well, roll-out deals with that too!
    Yes, it does. I thought you wanted a complete list of the major factors of tire circumference.
    Please email me rather than sending me a private message. My address is noglider@pobox.com

    Tom Reingold
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  10. #10
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    Quote Originally Posted by noglider View Post
    Yes, it does. I thought you wanted a complete list of the major factors of tire circumference.
    For reasonable differences in rim width, rim width might not matter. It might not be a "major" factor at all.

    Ignoring rim-width, for 23-28 mm tires, the difference in circumference is < 2%.

    ===================

    http://www.cateye.com/files/manual_d...0DW_ENG_v3.pdf

    23 mm -> 2096
    25 mm -> 2105
    28 mm -> 2136

    http://media.cannondale.com/media/up...uter_FINAL.pdf

    23 mm -> 2096
    25 mm -> 2105
    28 mm -> 2136

    http://www.planetbike.com/files/8003manual.pdf

    23 mm -> 2105
    25 mm -> 2105 (?)
    28 mm -> 2143

    http://service.specialized.com/collateral/ownersguide/new/assets/pdf/1_13_Spdzn_comp_ManualUS.pdf

    23 mm -> 2102
    25 mm -> 2113
    23 mm -> 2138

    https://www.google.com/url?sa=t&rct=...53217764,d.dmg

    23 mm -> 2133
    25 mm -> 2146
    28 mm -> 2149

    ===================

    The equation appears to be something close to:

    622 + 2 * width * pi
    Last edited by njkayaker; 09-26-13 at 12:31 PM.

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