# Measuring Wheel Circumference



## FINEMNT (Sep 14, 2009)

I followed the instructions of my Cateye Strada Wireless and get approx 2108mm for a tire size of 700 x 23. Is this close to your measurements? I'm asking because I compared total distance on a 10mi trip to a wired Cateye and it was approx 1/4mi difference less. The manual states it's the most accurate following the instructions. Here's the manual: http://www.cateye.com/sites/cateye/upload/manuals/en/CC-RD300W_ENG+v3.pdf

Edit: I just caught something, it states with the "rider's weight on the bike", which I did not do. I just rolled the bike by myself.


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## Hank Stamper (Sep 9, 2009)

I just went with 2096 as they recommend in that chart and it seems to be right on.

I see what you're getting at but seeing as though tire pressure can change during rides and you might not pump them up the same all the time it might not be worth trying to be this exact.


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## wim (Feb 28, 2005)

You'll get different numbers for different brands and -models of a 700 x 23, but 2,108 mm is a common value for that size tire. Make sure the tire is inflated to your normal riding pressures. The "rider's weight on the bike" thing doesn't make all that much of a difference if you normally roll along with a light touch on the bars.

Best way to check the accuracy of your settings is to ride a precisely known distance like 1.0 mile (map, mile marker, on-lin ride route program at full zoom) and compare it to your mile read-out. Good to know: if your mile read-out is off by, say, 10%, your roll-out input (2,108 in your case) is also off by 10%. Makes for an easy correction.


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## FatTireFred (Jan 31, 2005)

wim said:



> Best way to check the accuracy of your settings is to ride a precisely known distance like 1.0 mile (map, mile marker, on-lin ride route program at full zoom) and compare it to your mile read-out.




...assuming the rider can ride a straight line w/o getting too squirrely. a common reason for different readouts for riders on the same ride


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## FBinNY (Jan 24, 2009)

If you carefully measure the rollout of a loaded tire ridden at normal pressure, you'll get the most accurate circumference number to use. 

If you want the most accurate reading possible, and have a tape measure long enough, roll the bike 3-5 turns of the wheel, starting and ending with the valve at 6 o'clock. This longer measurement will minimize the error from not starting and ending at exactly the same place.

That said, computer accuracy will vary according to how precisely the actual rollout distance matches your calibration. Changing tire pressure or overall diameter will change the rollout, but most people don't recalibrate when they change tires.

Also there'll be some variation based on real world riding habits. For example, on real steep hills, I'll often slalom to make climbing easier. Obviously a zigzag course will add distance over the true straight course.

When you're all done, compare the computer distance to a known true distance over a number of miles, using the roadside mile markers as a reference. If there's an error, figure the percentage, and re-enter a new "circumference" compensated for the error. For example, if your computer is reading 2% high, use a circumference 2% larger.


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## wim (Feb 28, 2005)

FatTireFred said:


> ...assuming the rider can ride a straight line w/o getting too squirrely. a common reason for different readouts for riders on the same ride


Well, that brings up the interesting question: does a squirrely rider want to know the highway distance from his start point to his end point—or does he want to know the distance he actually rode his bike? With some riders I know, the difference can be astounding.


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## Kerry Irons (Feb 25, 2002)

*Better method*



wim said:


> Best way to check the accuracy of your settings is to ride a precisely known distance like 1.0 mile (map, mile marker, on-lin ride route program at full zoom) and compare it to your mile read-out. Good to know: if your mile read-out is off by, say, 10%, your roll-out input (2,108 in your case) is also off by 10%. Makes for an easy correction.


The problem with this is that you really only have 2 significant figures on the computer readout (nearest 0.1 mile) so you could be off and not know it. With odometers that read to 0.01 miles, that last figure is "soft" and often you can watch it change even after you stop. A better way to calibrate is to ride 10 miles on a road with mile markers. You just got one more high quality significant figure. 

My own rollout on a 700x23 gives 2096. YMMV.


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## jmlapoint (Sep 4, 2008)

I do the weighted roll-out to get a starting setting and then I ride a known distance to fine tune. Tire pressure can really alter your setting. Using a different front tire can also affect your setting.
I use the same tire, and the same pressure over an exact distance to adjust my setting. Once set, I find the distance is very accurate and reproducible with my CatEye Strada Wireless Computer.

John


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## seeborough (Feb 3, 2004)

Kerry Irons said:


> The problem with this is that you really only have 2 significant figures on the computer readout (nearest 0.1 mile) so you could be off and not know it. With odometers that read to 0.01 miles, that last figure is "soft" and often you can watch it change even after you stop. A better way to calibrate is to ride 10 miles on a road with mile markers. You just got one more high quality significant figure.
> 
> My own rollout on a 700x23 gives 2096. YMMV.


Riding 10 miles with mile markers on the road still gives a lot of "wiggle room". Just for sh*ts and giggles, I have used our high school track to measure the accuracy of my settings. If you stay in the lane (or on the painted lane divider) and the odometer increases .25 miles on every lap at the exact same spot, accuracy can pretty much be guaranteed.


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## Camilo (Jun 23, 2007)

I did a rollout measurement with the first computer I got in the 90s. Since then, I've only used the tire size charts that come with the computers and it's plenty accurate, within a 1/10 or so in 10 miles as far as I can tell, compared to my Garmin 305 gps. If I ride 30 or 40 miles with a group, we always compare computers and deem the one with the highest mileage to be most accurate.

OP, rather than obsess about actually measuring the circumference, why not just adjust the number you input up or down to make it more accurate?


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## matanza (Jun 9, 2004)

much ado about nuthin' relatively...


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