Wheel circumference help

TriGiraffe
TriGiraffe Posts: 7
edited June 2012 in Road beginners
My Spedcialized Transition has 700 x 21c tyres on it. This size isn't on any of the charts for setting my cycling computer up with, so I measured the circumference by rolling the wheel along the ground (gave me 2100mm). An inaccurate radius measurement doubled and multiplied by pi gives me around 2130mm...

I'd quite like accurate readings on the computer; does anyone have an exact measurement for these tyres?

Comments

  • topdude
    topdude Posts: 1,557
    No point asking other people to guess, you won't get more accurate than rolling the tyre along the ground for several revolutions and using that measurement :?
    He is not the messiah, he is a very naughty boy !!
  • Herbsman
    Herbsman Posts: 2,029
    They are exactly 2109.642mm according to the website
    CAPTAIN BUCKFAST'S CYCLING TIPS - GUARANTEED TO WORK! 1 OUT OF 10 RACING CYCLISTS AGREE!
  • smidsy
    smidsy Posts: 5,273
    Just run a tape around them - or am I missing something here?
    Yellow is the new Black.
  • snoopsmydogg
    snoopsmydogg Posts: 1,110
    mark the tyre with a piece of chalk and roll until you have 2 chalk marks on the ground. measure between the 2 and you have your answer
  • team47b
    team47b Posts: 6,425
    snoopsmydogg wrote:
    mark the tyre with a piece of chalk and roll until you have 2 chalk marks on the ground. measure between the 2 and you have your answer

    This is the most accurate, and specific to your bike, way as the charts don't take into account tyre variations.
    my isetta is a 300cc bike
  • Pross
    Pross Posts: 43,181
    team47b wrote:
    snoopsmydogg wrote:
    mark the tyre with a piece of chalk and roll until you have 2 chalk marks on the ground. measure between the 2 and you have your answer

    This is the most accurate, and specific to your bike, way as the charts don't take into account tyre variations.

    Surely the most accurate method is what Smidsy suggests? Rolling a bike in a completely straight line is very difficult, it won't be far out but given that the OP is concerened about a 30mm difference. If you want the ultimate accuracy just take the wheel off and run a tape around the circumference (this won't take account of the tyre deformation with the bike loaded though ;)
  • slowondefy2
    slowondefy2 Posts: 348
    topdude wrote:
    No point asking other people to guess, you won't get more accurate than rolling the tyre along the ground for several revolutions and using that measurement :?

    There is no substitute for doing this. Make sure the tyres are at their usual pressures and you're sat on the bike when you're doing it!
  • Gizmo_
    Gizmo_ Posts: 558
    Pross wrote:
    If you want the ultimate accuracy just take the wheel off and run a tape around the circumference (this won't take account of the tyre deformation with the bike loaded though ;)
    Or if he's rolling it along the ground, he should do it south to north or east to west depending on which direction he's going to be riding, because the curvature of the earth is different.
    Scott Sportster P45 2008 | Cannondale CAAD8 Tiagra 2012
  • madtam
    madtam Posts: 141
    How do allow for the differences caused by variations in air pressure, temperature and altitude ?
  • MichaelW
    MichaelW Posts: 2,164
    Does the wheel radius change when you load the bike with a rider?
  • ugo.santalucia
    ugo.santalucia Posts: 28,310
    TriGiraffe wrote:
    My Spedcialized Transition has 700 x 21c tyres on it. This size isn't on any of the charts for setting my cycling computer up with, so I measured the circumference by rolling the wheel along the ground (gave me 2100mm). An inaccurate radius measurement doubled and multiplied by pi gives me around 2130mm...

    I'd quite like accurate readings on the computer; does anyone have an exact measurement for these tyres?

    In fairness, it's a bit of a waste of time, as computers are not accurate by nature... traffic and mobile phones interfere with them, plus the odd missed spin of the wheel and the error can be + or - 5%. The error you make by setting a 20 mm tyre instead of 21 is less than that, so insignificant
    left the forum March 2023
  • klep
    klep Posts: 158
    Keep the position of the moon in mind.
  • Herbsman
    Herbsman Posts: 2,029
    The wheel circumference decreases the harder you pedal.
    CAPTAIN BUCKFAST'S CYCLING TIPS - GUARANTEED TO WORK! 1 OUT OF 10 RACING CYCLISTS AGREE!
  • desweller
    desweller Posts: 5,175
    The radius to the contact patch is smaller when you lean for corners too.
    - - - - - - - - - -
    On Strava.{/url}
  • andy_wrx
    andy_wrx Posts: 3,396
    It will decrease as the tyre wears, so you'll need to re-measure on a weekly basis and keep adjusting the computer.

    (or alternatively, like me you can get a computer with two wheelsize settings, allowing you to swap between them when you change from race wheels to training wheels with different size tyres on them...assuming you remember to switch the computer setting :roll: )

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