Rolling circumference query

caradale
caradale Posts: 34
edited May 2014 in Workshop
I`m a recent convert to 25mm tyres and liking everything so far but one thought keeps nagging at me.
Apologies if this subject has been covered before but what i`m thinking is this
Surely different tyre sizes have different rolling circumferences so one wheel revolution on a 25mm tyre will actually cover more ground than one revolution of a 23mm, If that is true then it also seems to follow that the bikes gearing will be a bit harder ie for any given gear the bike will travel further over the ground on the larger tyre.
As the larger tyres are run at a lower pressure does this negate the difference ?
In my case I have changed from 23mm Durano to 25mm Durano S
Anybody know or got the math to give us some numbers of what the difference (if any ) there is in the real world and actually how much difference in distance over the ground covered there is

Thanks

Comments

  • drlodge
    drlodge Posts: 4,826
    Not much in it. 23c = 2096, 25c = 2105 according to Cateye. Not even 0.5% difference.
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  • ai_1
    ai_1 Posts: 3,060
    It'll depend on too many variables to cover in a simple calculation. It's not like the "rolling circumference" only changes when you change nominal tyre size either! Any time your tyre pressure changes (leakage, ambient temperature, heating from brakes/rolling, etc etc), you change brand/model of tyre (even if its the same nominal size), you carry more/less weight on the bike or just lean further forward/backward the "rolling circumference" will change slightly. It's never a precision measurement. I don't think these variations will ever be enough to notice as a gearing change unless you're making a much bigger tyre change than 23c to 25c. As per drlodge's post you're talking about a variation that's likely <1% for most combinations of the variables above.
  • arthur_scrimshaw
    arthur_scrimshaw Posts: 2,596
    The old fashioned way before all that maths got involved was to mark the a line on the tyre wall and corresponding line on the ground, sit on the bike and get a friend to wheel you forwards one revolution, mark the finishing point on the ground. Use a tape to measure the distance. That way any effects of the tyre compressing under load are accounted for.

    No need to thank me :wink:
  • ai_1
    ai_1 Posts: 3,060
    Arthur Scrimshaw wrote:
    The old fashioned way before all that maths got involved was to mark the a line on the tyre wall and corresponding line on the ground, sit on the bike and get a friend to wheel you forwards one revolution, mark the finishing point on the ground. Use a tape to measure the distance. That way any effects of the tyre compressing under load are accounted for.

    No need to thank me :wink:
    I thought that was how everyone still did it!
    Doing the maths is the less accurate method in this case.
    The question is whether to bother re-doing it if you change tyres, pressure, etc....
  • sandyballs
    sandyballs Posts: 577
    Do you also have to re-do it if you put on weight, I mean loose weight? :oops:
  • ai_1
    ai_1 Posts: 3,060
    Sandyballs wrote:
    Do you also have to re-do it if you put on weight, I mean loose weight? :oops:
    It'll make a difference but it'll be tiny unless you're using big volume tyres and have gained, I mean lost, a vast amount of weight. To be honest the difference might be less than the tolerance of your measurement anyway!
    I wouldn't bother.
  • dennisn
    dennisn Posts: 10,601
    FWIW I have found that what the computers say is the circumference is not the same as rolling the bike(with you on it) say three tire revolutions, taking the measurement and then dividing by three. Although the difference is not huge by any means.

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