wheel circumference size for computer

RichardD

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.

2080mm.


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>

Unknown

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

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MTB eijit

maddog_2cp

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........

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Drop bars are a historical accident...... discuss</font id="purple"></font id="size1">

Unknown

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_2cp

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">

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Drop bars are a historical accident...... discuss</font id="purple"></font id="size1">

hevipedal

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

jsmithfym

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.

IanTrcp

<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.

djmc

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.

RichardD

Thanks everyone for the feedback. I may try marking out a longer test and trying that.

pieinthesky

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!

kevin44

it is the diameter of the weighted wheel that you want

pbiggs

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.

<i>~Pete</i>

Unknown

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.

pieinthesky

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.

psmiffy

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

Moovie

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)

Mooves

________________________________________________________________________
2006 Ridgeback World Horizon Touring
???? Ridgeback MBR

hazeii

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.