# wheel circumference size for computer

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>

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>

0

## Posts

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

73don'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">

thats what i use anyway

Mleh Mleh Mleh

73<font size="1"><font color="purple">

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

2,475<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

HevipedalIt's not only people that are irrational; 1.4142135623730950488016887242096980785696718753769480731766797379907324784621

296761You 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.

38c = 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.

1724172 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!

1899,232Or just forget it and enter what the manual says.

<i>~Pete</i>

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.

417Sorry if it was a bit ambiguous.

23622This 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

2006 Ridgeback World Horizon Touring

???? Ridgeback MBR

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