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Computer calibration, tyre circumference

  • 05-07-2011 02:07PM
    #1
    dnme
    Registered Users, Registered Users 2 Posts: 3,429 ✭✭✭


    My tyres are marked with 700x35c.
    On Sheldon Browns tyre chart, he lists 700x35 as having a circumference of 117cm. However when I do a roll out test with the wheel, I get a measurement of 119cm.

    Who's right?


Comments

  • #2
    Morgan
    Registered Users, Registered Users 2 Posts: 2,481 ✭✭✭Morgan


    The measurements on the Sheldon Brown site are (good) approximations. Your own measurement will be more accurate, assuming you measured it accurately.


  • #3
    Beasty
    Administrators, Social & Fun Moderators, Sports Moderators Posts: 78,456 Admin ✭✭✭✭✭Beasty


    dnme wrote: »
    My tyres are marked with 700x35c.
    On Sheldon Browns tyre chart, he lists 700x35 as having a circumference of 117cm. However when I do a roll out test with the wheel, I get a measurement of 119cm.

    Who's right?
    Roll out is actual so should be correct if the tyre is correctly inflated and you have your full weight on the bike.

    FWIW I've found a difference of up to 300m or so on my commute (pre-Garmin) depending on how inflated the tyres were


  • #4
    dnme
    Registered Users, Registered Users 2 Posts: 3,429 ✭✭✭dnme


    I have the tyres at about 90psi (they are rated 100) and did the rollout test by marking the tyre at the valve right down to ground and marking the ground aswell, then rolling it in a straight line one revolution and marking the ground again. It's precisely 219cm. I did the test off the bike btw.


  • #5
    Beasty
    Administrators, Social & Fun Moderators, Sports Moderators Posts: 78,456 Admin ✭✭✭✭✭Beasty


    You need to do it sat on the bike - your weight shortens the radius at the point of contact, reducing the distance travelled accordingly

    And it's not simply a matter of putting the right amount of weight on the bike - its distribution will impact on the two wheels differently


  • #6
    dnme
    Registered Users, Registered Users 2 Posts: 3,429 ✭✭✭dnme


    Beasty wrote: »
    You need to do it sat on the bike - your weight shortens the radius at the point of contact, reducing the distance travelled accordingly

    And it's not simply a matter of putting the right amount of weight on the bike - its distribution will impact on the two wheels differently

    ah it'll do!


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