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wheel circumference size for computer

RichardDRichardD Posts: 172
edited June 2007 in Workshop
So, last night, I put a new tyre on the front wheel and thought that
it must be time to check the circumference of the wheel and program
it in to the computer, rather than relying on the default (2124mm)
it assumes. I've done this before, no problem (Ha!)

I'm rolling the bike along next to a tape measure, using the brand
name on the side of the type as a marker at the start of the tape.

One revolution one the name lines up with 2105mm.

Hmm, what if there's a bit of weight on the wheel, as in normal
riding, if thought. Wheel back, line up, roll forward putting a
little weight on the front.

Now its 2090mm.

So I try this again, straddling the bike and trying to replicate the
weight distribution of a normal ride.


Back and forwards a few times with weight on and off and these numbers
seem to be reliable- weight shortens the distance the wheel travels
in one revolution.

BTW, the surface was level block paving, tyre was 700x23 inflated to
6 bar.

I'll put my hand up and say yes, I'm a pedant, and give or take a
percent or two won't do. <b>What number should I use?</b>


  • iirc, the last cateye i bought was for the roadie it was programmable by centimetres, and by default it was set for 210 . . . which on the little chart was meant to to be 23x700 . . . so going by your figures that does roughly line up with 210(0) as an average based on your tyre.

    At the end of the day, your not talking a significant distance difference ... this was the list of tyre sizes and centimetre settings on the computer...

    Tyre Size CM

    14 x 1.50 102
    14 x 1.75 106
    16 x 1.50 119
    16 x 1.75 120
    18 x 1.50 134
    18 x 1.75 135
    20 x 1.75 152
    20 x 1-3/8 162
    22 x 1-3/8 177
    22 x 1-1/2 179
    24 x 1 175
    24 x 3/4Tubular 178
    24 x 1-1/8 179
    24 x 1-1/4 191
    24 x 1.75 189
    24 x 2.00 192
    24 x 2.125 196
    26 x 7/8 192
    26 x 1(59) 191
    26 x 1(65) 195
    26 x 1.25 195
    26 x 1-1/8 190
    26 x 1-3/8 207
    26 x 1-1/2 210
    26 x 1.40 200
    26 x 1.50 201
    26 x 1.75 202
    26 x 1.95 205
    26 x 2.00 206
    26 x 2.10 207
    26 x 2.125 207
    26 x 2.35 208
    26 x 3.00 217
    27 x 1 215
    27 x 1-1/8 216
    27 x 1-1/4 216
    27 x 1-3/8 217
    650 x 35A 209
    650 x 38A 212
    650 x 38B 211
    700 x 18C 207
    700 x 19C 208
    700 x 20C 209
    700 x 23C 210
    700 x 25C 211
    700 x 28C 214
    700 x 30C 217
    700 x 32C 216
    700C Tubular 213
    700 x 35C 217
    700 x 38C 218
    700 x 40C 220

    MTB eijit
  • maddog_2cpmaddog_2cp Posts: 73
    I'd use the first one. Then it looks like you're going a tad faster than you actually are [;)]

    don't forget the circumference drops when you go round corners too........

    <font size="1"><font color="purple">
    Drop bars are a historical accident...... discuss</font id="purple"></font id="size1">
    Drop bars are a historical accident...... discuss
  • I didn't think it was the circumference of a weighted wheel, just the overall circumference of the wheel?

    thats what i use anyway

    Mleh Mleh Mleh
  • maddog_2cpmaddog_2cp Posts: 73
    if that was the case then all the 700c entries would have the same number in the table above...

    <font size="1"><font color="purple">
    Drop bars are a historical accident...... discuss</font id="purple"></font id="size1">
    Drop bars are a historical accident...... discuss
  • hevipedalhevipedal Posts: 2,475
    Mine measured at 2114, new wheels and new tyres re measured at 2114.....................

    <b><font color="red"> Hevipedal </font id="red"></b>
    Phrase of the week - <font color="red"><font size="3"><b> I've got a bike. You can ride it if you like.
    It's got a basket, a bell that rings and
    Things to make it look good.
    I'd give it to you if I could, but I borrowed it.
    </font id="red"> </font id="size3"> </b>

    51yrs old and Proud of it - Made it to 87kg 2 more to go for the target.
    Pedal to Paris Sept 2007
    It's not only people that are irrational; 1.4142135623730950488016887242096980785696718753769480731766797379907324784621
  • jsmithfymjsmithfym Posts: 296
    I have found that different tyres are different and always re calibrate. You need to do at least 3 or 4 revolutions to get an average.
  • IanTrcpIanTrcp Posts: 761
    <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jsmithfym</i>

    You need to do at least 3 or 4 revolutions to get an average.
    <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

    I think that this is the right answer. Better still if you have a long tape measure (as used by builders) and a flat piece of road. You can then count multiple revolutions pushing the bike along and divide this into the total distance covered to get a 'good' number to use. This method helps minimise the additional errors introduced by guessing where a single revolution stops and starts precisely.
  • djmcdjmc Posts: 38
    There is a well known ratio of circumference of a circle to radius/diameter so that
    c = pi * d.
    while pi is irrational I would have thought that the accuracy one can get this way assuming a reasonable value for pi (try 3.14159) would be a lot greater, since measuring the diameter is much easier. If the tyre is properly pumped up then the amount of deflection is trivial.
  • RichardDRichardD Posts: 172
    Thanks everyone for the feedback. I may try marking out a longer test and trying that.
  • pieintheskypieinthesky Posts: 417
    1 Pump your tyres to the correct pressure
    2 Put a small dot of ink or paint on your tyre
    3 Stretch a piece of string out on a flat surface
    4 Ride (get someone to hold you up if necessary) along the string to make sure you are going straight.
    5 You will now have a series of paint blobs on the ground (the more the better)
    6 Measure between the blobs and you have your wheel circumference.

    I can just manage one revolution in our kitchen but have to use ketchup not paint!
  • kevin44kevin44 Posts: 189
    it is the diameter of the weighted wheel that you want
  • pbiggspbiggs Posts: 9,232
    If you want to get it eactly right, inflate the tyres to usual pressure and measure the distance with your full weight on the bike.

    Or just forget it and enter what the manual says.

  • the trouble with using things like string is that they stretch and will give a different reading depending on the force used the stretch it out...

    My two preferred methods (3m cloth) tape measureround the wheel minus 2mm or when I cant find the tape measure diameter times pi minus 2mm.
  • pieintheskypieinthesky Posts: 417
    bigdawg the string is just so you ride in a straight line. Measurements are taken from marks on the ground.

    Sorry if it was a bit ambiguous.
  • psmiffypsmiffy Posts: 236
    I always get a range if doing it over a short dist so normally use a longish measured distance - marker posts on french roads on day 1 or check with gps over 10k and work out correction factor
  • MoovieMoovie Posts: 22
    I would suggest you take a GPS on a reasonable length ride alongside your Computer, resetting the trip on both first.
    This should give you a pretty good idea of the level of accuracy of your computer (I think it is a reasonable bet that the GPS will be right within a few hundred feet)


    2006 Ridgeback World Horizon Touring
    ???? Ridgeback MBR
    2006 Ridgeback World Horizon Touring
    ???? Ridgeback MBR
  • hazeiihazeii Posts: 233
    I find I get at least 1% variation day to day using the same bike (same tyres, same cycle computer setting) over the same 24 mile rural route.
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