Actual wheel diameter

itboffin
itboffin Posts: 20,052
edited January 2010 in Commuting chat
I'm confused as to what value I should enter into my cycle comp fo wheel diameter, if I choose the pre defined 700x23 that uses 2096mm but with that setting my journey distances are wrong, I know for a fact the journey from home to the station is 19.96miles trust me I've measured it everyway. So if I use the standard setting it comes out at 19.56miles, that almost losing a mile each day FFS

yeh I know the most effective method is to measure the distance travelled by one full turn of the wheels, but really how are the standards so wildly out?
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Comments

  • wgwarburton
    wgwarburton Posts: 1,863
    itboffin wrote:
    I'm confused as to what value I should enter into my cycle comp fo wheel diameter, if I choose the pre defined 700x23 that uses 2096mm but with that setting my journey distances are wrong, I know for a fact the journey from home to the station is 19.96miles trust me I've measured it everyway. So if I use the standard setting it comes out at 19.56miles, that almost losing a mile each day FFS

    yeh I know the most effective method is to measure the distance travelled by one full turn of the wheels, but really how are the standards so wildly out?

    Wildly? It's only a 2% error... :-)

    Sheldon quotes 2097, rather than 2096 for a 23c. These things are nominal and tyres vary quite a bit according to what the maker's trying to achieve, wear, pressure, temperature, rider weight(!) etc.

    If 2096 isn't working for you, try 2097 or a little more- a 25c is about 2105, so somewhere around 2100 should give the the answer you want. The number's the circumference, rather than the diameter (your rims aren't two metres high...) so an increase of 2mm in nominal tyre size increases the diameter by 4mm and the circumference by 12- Looking at the calibration table that calculation works for 23c to 28c (+5mm radius, +30mm circumference, calibration 2097 to 2136 = 29 ) but not for 25c (+2mm radius, +12mm circumference, Calibration 2097 to 2105 = 8 ).

    The most accurate way to calibrate is to use a long, known distance and do the maths to get the right Calibration number- If you know that you're doing 19.96 miles then use that as your known quantity. You might have to increase quite a bit- try 2130 and work from there.

    Cheers,
    W.
  • always_tyred
    always_tyred Posts: 4,965
    Why don't you simply change the diameter setting to 2138?

    Sorted.
  • will3
    will3 Posts: 2,173
    Surely it depends on how much you pump up your tyres (and how fat you are) too since the actual rolled distance per revolution is( the distance from axle centre to the road contact point)*2*pi
    which will be less with a flacid tyre than a hard one.
  • wgwarburton
    wgwarburton Posts: 1,863
    will3 wrote:
    Surely it depends on how much you pump up your tyres (and how fat you are) too since the actual rolled distance per revolution is( the distance from axle centre to the road contact point)*2*pi
    which will be less with a flacid tyre than a hard one.

    Indeed, but it's surely reasonable to assume that an experienced rider like ITB will always have a hard one when it matters?

    Cheers,
    W.
  • UndercoverElephant
    UndercoverElephant Posts: 5,796
    WGWarburton wrote:
    will3 wrote:
    Surely it depends on how much you pump up your tyres (and how fat you are) too since the actual rolled distance per revolution is( the distance from axle centre to the road contact point)*2*pi
    which will be less with a flacid tyre than a hard one.

    Indeed, but it's surely reasonable to assume that an experienced rider like ITB will always have a hard one when it matters?

    Cheers,
    W.

    Too much time in the saddle, perhaps?
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  • Bassjunkieuk
    Bassjunkieuk Posts: 4,232
    Why not just wrap a bit of string/a tape measure around the tyres?

    Surely if your going to be this anal then the circumference should alter slightly as the tyres wear? I'd recommended a routine of ensuring each tyre is up to the correct pressure before every ride and checking the circumference each week :D
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  • jonginge
    jonginge Posts: 5,945
    Bassjunkieuk wrote:
    Why not just wrap a bit of string/a tape measure around the tyres?

    Surely if your going to be this anal then the circumference should alter slightly as the tyres wear? I'd recommended a routine of ensuring each tyre is up to the correct pressure before every ride and checking the circumference each week :D
    You'd need to take into account the compression of the tyre when loaded...
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  • nicklouse
    nicklouse Posts: 50,675
    put a line on the floor mark the tyre. roll bike one rev. measure. sorted.
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  • Bassjunkieuk
    Bassjunkieuk Posts: 4,232
    JonGinge wrote:
    Bassjunkieuk wrote:
    Why not just wrap a bit of string/a tape measure around the tyres?

    Surely if your going to be this anal then the circumference should alter slightly as the tyres wear? I'd recommended a routine of ensuring each tyre is up to the correct pressure before every ride and checking the circumference each week :D
    You'd need to take into account the compression of the tyre when loaded...

    True, not really an issue for us..........not sure I can say the same for ITB ;-)
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  • Runoutofgears
    Runoutofgears Posts: 348
    Or, get a long strip of paper, stick to smooth floor, make a narrow mark on the tyre with lipstick, ride bike along paper, measure between the leading edges of the two pink smudges!
    I ache, therefore I am.
  • Runoutofgears
    Runoutofgears Posts: 348
    Actually it works well without the paper if you have a plain lino or laminate wood floor
    I ache, therefore I am.
  • Bassjunkieuk
    Bassjunkieuk Posts: 4,232
    Runoutofgears wrote:
    Actually it works well without the paper if you have a plain lino or laminate wood floor

    Oh god, this sounds like a recipe for disaster, ITB with his wifes lipstick (presumably) and red marks all over the bike and floor.......
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  • Runoutofgears
    Runoutofgears Posts: 348
    I've found worse things on my floor and my tyres! :lol:
    I ache, therefore I am.
  • fenboy369
    fenboy369 Posts: 425
    Flacid, hard and Lipstick. Hahahahahaha.

    Anyway, if you are that bothered get a Garmin or Satmap GPS thing-me-bob. That must be the best way to measure distance cycled? (Could you keep it on during a train ride section and count that too?)
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  • itboffin
    itboffin Posts: 20,052
    Thanks buns very interesting stuff, I measured the distance covered by the front wheel during one full rotation as 2115 - seems odd! anyway i'm guessing the displacement caused when i'm actually on the bike might bring that down to say 2112.
    Always Tyred wrote:
    Why don't you simply change the diameter setting to 2138?

    Sorted.

    er AT what's your reasoning behind 2138? I don't understand.
    Rule #5 // Harden The Feck Up.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.
    Rule #12 // The correct number of bikes to own is n+1.
    Rule #42 // A bike race shall never be preceded with a swim and/or followed by a run.
  • itboffin
    itboffin Posts: 20,052
    Bassjunkieuk wrote:
    Why not just wrap a bit of string/a tape measure around the tyres?

    Surely if your going to be this anal then the circumference should alter slightly as the tyres wear? I'd recommended a routine of ensuring each tyre is up to the correct pressure before every ride and checking the circumference each week :D

    Natch

    I check my tyre pressures every morning and top up to the exact same settings every day :wink:
    Rule #5 // Harden The Feck Up.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.
    Rule #12 // The correct number of bikes to own is n+1.
    Rule #42 // A bike race shall never be preceded with a swim and/or followed by a run.
  • itboffin
    itboffin Posts: 20,052
    fenboy369 wrote:
    Flacid, hard and Lipstick. Hahahahahaha.

    Anyway, if you are that bothered get a Garmin or Satmap GPS thing-me-bob. That must be the best way to measure distance cycled? (Could you keep it on during a train ride section and count that too?)

    I have two GPS devices both of which lose signal at several places along the route pretty much everytime, I live in the country don't you know

    Seems GPS isn't so keen on thick wooded countryside and valleys.
    Rule #5 // Harden The Feck Up.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.
    Rule #12 // The correct number of bikes to own is n+1.
    Rule #42 // A bike race shall never be preceded with a swim and/or followed by a run.
  • always_tyred
    always_tyred Posts: 4,965
    itboffin wrote:
    Thanks buns very interesting stuff, I measured the distance covered by the front wheel during one full rotation as 2115 - seems odd! anyway i'm guessing the displacement caused when i'm actually on the bike might bring that down to say 2112.
    Always Tyred wrote:
    Why don't you simply change the diameter setting to 2138?

    Sorted.

    er AT what's your reasoning behind 2138? I don't understand.

    (19.96/19.56) * 2096

    :D

    Btw, not all computers give the same estimates of wheel diameter. For example http://www.vetta.com/Tech%20Support/DP/C-16.1_uk.pdf quote 2130mm for a 23c.

    I think that Sheldon was a bit off in his calculation. He isn't/wasn't always right, in my (limited) experience.
  • MonkeyMonster
    MonkeyMonster Posts: 4,628
    Just a thought here, but perhaps you need to get out more - figuratively not literally.

    Worrying about losing maybe mile a day when cycling 40 as a commute... Otherwise I point thee at the "racing" section with all the OCD chaps there.
    Le Cannon [98 Cannondale M400] [FCN: 8]
    The Mad Monkey [2013 Hoy 003] [FCN: 4]
  • itboffin
    itboffin Posts: 20,052
    MonkeyMonster wrote:
    Just a thought here, but perhaps you need to get out more - figuratively not literally.

    Worrying about losing maybe mile a day when cycling 40 as a commute... Otherwise I point thee at the "racing" section with all the OCD chaps there.

    :lol:
    Rule #5 // Harden The Feck Up.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.
    Rule #12 // The correct number of bikes to own is n+1.
    Rule #42 // A bike race shall never be preceded with a swim and/or followed by a run.
  • rolf_f
    rolf_f Posts: 16,015
    MonkeyMonster wrote:
    Just a thought here, but perhaps you need to get out more - figuratively not literally.

    Worrying about losing maybe mile a day when cycling 40 as a commute... Otherwise I point thee at the "racing" section with all the OCD chaps there.

    Indeed - the poor thing has only done twice the mileage I've done this year so far :lol:

    As for checking tyre pressure every morning :shock: :)

    Seriously - I'd just calibrate against a plotted route - or if you are really bonkers, find a measured mile and use that!
    Faster than a tent.......
  • redddraggon
    redddraggon Posts: 10,862
    itboffin wrote:
    fenboy369 wrote:
    Flacid, hard and Lipstick. Hahahahahaha.

    Anyway, if you are that bothered get a Garmin or Satmap GPS thing-me-bob. That must be the best way to measure distance cycled? (Could you keep it on during a train ride section and count that too?)

    I have two GPS devices both of which lose signal at several places along the route pretty much everytime, I live in the country don't you know

    Seems GPS isn't so keen on thick wooded countryside and valleys.

    IME GPS Works better in the Countryside than the City.
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  • itboffin
    itboffin Posts: 20,052
    redddraggon

    pah!
    Rule #5 // Harden The Feck Up.
    Rule #9 // If you are out riding in bad weather, it means you are a badass. Period.
    Rule #12 // The correct number of bikes to own is n+1.
    Rule #42 // A bike race shall never be preceded with a swim and/or followed by a run.
  • Ian.B
    Ian.B Posts: 732
    i don't quite see why tyre pressure or rider weight would make a difference to the circumference of the tyre? It means that more of they tyre is in contact with the road at any one time, but I wouldn't have thought the molecules of teh rubber get squished together longitudinally (ie around the circumference) so as to make any measurable difference. ie the circumference of the tyre doesn't diminish because it's pumped up less or has a heavier weight on it.

    Not sure I've explained that very well. Any scientists around?
  • MonkeyMonster
    MonkeyMonster Posts: 4,628
    On a vaguely linked note - Rolf, you should get a track pump and give your knobblies a good hard seeing too. The tourer too - hard tyres make a hella difference.

    oh and regarding distance travelled on a softer tyre. Imagine on a sold tyre - the circumference is a proper circle but on a softer tyre - the bottom part is a straight line when it has weight on it - this changes the overall circumference.
    Le Cannon [98 Cannondale M400] [FCN: 8]
    The Mad Monkey [2013 Hoy 003] [FCN: 4]
  • Clever Pun
    Clever Pun Posts: 6,778
    I guess when somone wants to nail 8000 miles in a year a 2% difference could be the difference between achieving a massive goal of being a bit of a failure... better put in at least another 160 miles just to make sure :lol:
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  • rolf_f
    rolf_f Posts: 16,015
    MonkeyMonster wrote:
    On a vaguely linked note - Rolf, you should get a track pump and give your knobblies a good hard seeing too. The tourer too - hard tyres make a hella difference.

    I have and I do (though I'm on slicks during the week) - but I can't be bothered to check pressures on a daily basis. I think if my tyres are a bit soft some of the week, it is only making me harder :lol:
    MonkeyMonster wrote:
    oh and regarding distance travelled on a softer tyre. Imagine on a sold tyre - the circumference is a proper circle but on a softer tyre - the bottom part is a straight line when it has weight on it - this changes the overall circumference.

    The ultimate answer to this is to imagine the tyre is completely flat - your true radius then becomes the radius of your wheel (plus twice the thickness of your sidewalls!)
    Faster than a tent.......
  • always_tyred
    always_tyred Posts: 4,965
    Clever Pun wrote:
    I guess when somone wants to nail 8000 miles in a year a 2% difference could be the difference between achieving a massive goal of being a bit of a failure... better put in at least another 160 miles just to make sure :lol:
    One option would be to weave around more during your commute.

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