Hello,

Very boring I know but can anyone help me find a circumference for my wheels so I can put it in my Topeak Panoram (yes sorry boring I know, I'll add a thread about how I Won the Giro next time I promise!)

I've got a 2009 Kona Smoke with Continental City Contacts. Kona say they're 700x47 and the tire says they're 28 x 1.75 ( i realise one is metric and one imperial).

I really want to avoid the roll out method if poss as I'm a bit OCD and know I'll get an ever so slightly different reading each time and will end up doing it about 300 times!

I think the only real good way to get a good measurement is to actually measure the thing. It's not that hard, and you'll get a good value to use.
I used the lines in my kitchen floor to roll the tire along. Helped a lot.

Just measure it! Of course then you won't be able to blame inaccuracies on someone else.

Sheldon Brown's website has a universal calibration chart of sorts. I am assuming you need the circumference in millimeters. So if you go to that chart you see there is no 700X47. You need to split the difference between 700X44 (2224) and 700X50 (2293). That will get you (2259)

The answer ... 2259

https://sheldonbrown.com/cyclecomputers/index.html

It's interesting to compare the Sheldon Brown's table to the table in the Sigma's 1606L manual
-- there are significant differences in the numbers (circumference in millimeters) for the same wheel sizes.

Sheldon Brown's table:
700x18C 2102
700x20C 2114
700x23C 2133
700x25C 2146
700x28C 2149
700x32C 2174
700x35C 2205
700x40C 2224

Sigma's table:
Tire Size Miles Km
700 X 44 1382 2224
700 X 38 1355 2180
700 X 35 1347 2168
700 X 32 1339 2155
700 X 28 1327 2136
700 X 25 1308 2105
700 X 23 1302 2097
700 X 20 1296 2086

Why?

Sigma uses a formula that takes wheel circumference in mm and divides it by 1.6093. So even if you only look at Sheldon Brown's chart, the difference between sigma's numbers and any other computer will differ. Take a look at Sheldon's chart for 700X35. If you use circumference in mm only (group F), you get 2168. If you were to divide that by 1.6093 you would get 1347. Now look again at Sheldon's chart for sigma (group E) and you see that for 700X35 you get 1347. Sheldon's chart does not really differ too much, if at all. It's just that Sigma uses an "adjustment" to get an accurate readout for computers that will be using MPH. This only applies to their older computers. I noticed that the newer models do not use this adjustment.

BTW .. That 1.6093 is the number you would use to convert KPH to MPH.

Sorry, wrong table titles. The first one is Sigma's table, the second one is Sheldon Brown's table:

So, let's present the problem from the beginning ...

Sigma's table:
700x18C 2102
700x20C 2114
700x23C 2133
700x25C 2146
700x28C 2149
700x32C 2174
700x35C 2205
700x40C 2224

Sheldon Brown's table:
Tire Size Miles Km
700 X 44 1382 2224
700 X 38 1355 2180
700 X 35 1347 2168
700 X 32 1339 2155
700 X 28 1327 2136
700 X 25 1308 2105
700 X 23 1302 2097
700 X 20 1296 2086

Now, in the case of 700x35c tires, Sigma's instructions for BC 1606L computer are to enter the number "2205", while Sheldon Brown's instructions are to enter "2168". Both numbers are supposed to relate directly to the wheel's circumference in millimeters.

Why the discrepancy?

Best to just measure your actual wheel.
Pump it up to pressure mark the wheel or use the valve stem mark your starting spot roll one revolution and measure it, or roll 3 revolutions and divide it by 3 and use that it will be even more accurate.

There are two difficulties with roll out tests.

1) A loaded wheel will deform and decrease the effective radius at the contact point. This effectively reduces the circumference of the wheel. If you do an unloaded test, your number will be two high.

2) You have to roll a very straight line. Any curve will result in a number that is too low.

The "toothpaste" trick helps a lot, especially if you have a pitched driveway. Higher speed will get you straighter. Take the last to visible toothpaste splotches. And measure. Repeat the test 3 or more times and average the results.

My new method is to get out a GPS and just compare my computer readings to GPS readings and make changes till I get them to agree to the tenth of a mile. GPS speed is VERY accurate so long as you are going in a straight line.

For the OP with OCD. Realize that ANY method is going to be inaccurate. Changes in temperature will very pressure in the tire which will effectively increase the circumference of the wheel. The loading of a wheel will vary during your trip. Even GPS logging will under-report distance if you are on a curvy route as it's method is to periodically sample and measure distance between points.

Ultimately what is important is to compare performance from one day to another. As long as you keep the setting the same for the wheel, you can be sure you are comparing apples to apples.

#1 True but its more likely more accurate than a pre-measured chart as tire manufactures and model are all diffrent.
#2 Stright line is it that hard?
Most likely OP dosen't have a GPS

To OP good luck with it many ways to get it close at the least.

I honestly don't know. I took a look at your link to the 1606 and I couldn't help notice that for 700X35C they also list an ERTO size of 37-622. So that number could be used for either a 35 or 37mm tire. I went one step further and looked at cateye's calibration chart and they have 2170 for the same tire size. When these charts are made by the various manufacturers I am not sure what method they use to come up with these number. I doubt they buy all sorts of tire sizes and do actual measurements. My guess is they obtain some manufacturing specs and do the math. Given the descepency in tires sizes I am not surprised there are different numbers for calibrating a computer.

Let's not get too wrapped around the axle here. This sport is supposed to be fun and getting yourself twisted out of shape over a few millimeters of calibration just is not worth it. Even if you use a number that is off by several MMs you still have more accuracy than is reqired by law in an auto's speedo/odo. In the end the difference between Sheldon and sigma is only 37mm (1.4 inch). Is that really something to worry about?

I stopped sweating over this kind of thing years ago and accept the charted data. I feel a lot better and have more time to play with my son.

It's so easy to just measure the distance. I've found it to be very accurate. Just pump up your tires, lay out a tape measure along the floor near a wall, get on your bike and slowly push yourself along the wall for just one revolution of your tire. Try it a couple of times to make sure you get the same result ( you will ). MY bikes computer is far more accurate than my cars odometer.