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Does using the exact wheel circumference as opposed to using the canned wheel/tire size setting make that big of a difference when setting up your cyclometer? I mean how anal or lazy do you have to be to use either method? :confused: Can it be that far off to make much of a difference? Please enlighten me......
 

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roll out... roll out...

kilofox said:
Does using the exact wheel circumference as opposed to using the canned wheel/tire size setting make that big of a difference when setting up your cyclometer? I mean how anal or lazy do you have to be to use either method? :confused: Can it be that far off to make much of a difference? Please enlighten me......
... a rollout gives the best overall accuracy... if you get a decent match from a chart, you were just lucky...

Inflate your tires to your preffered PSI, position the wheel with the sensor on it (generally the front) with the valve stem at BDC (bottom dead center). Make a mark at this point on the ground... weight the bike (get on it) and roll forward (in a straight line) one full revolution (valve stem is again at BDC). Make another mark and measure the distance (2.54 centimeters per inch)...

In reality, I perform a 4 times rollout... that is, I roll the wheel through 4 complete revolutions, then take the average.

Note, that proper inflation, proper weighting (might need a friend to help balance) and maintaining a straight line are important. This will yield an extremely accurate circumference that takes into account the deformation of a weighted tire on the road.
 

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Call me a Fred
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kilofox said:
Does using the exact wheel circumference as opposed to using the canned wheel/tire size setting make that big of a difference when setting up your cyclometer? I mean how anal or lazy do you have to be to use either method? :confused: Can it be that far off to make much of a difference? Please enlighten me......
I've done both (on two different bikes) and found that the canned data was sufficient. I get the same distance on the same ride with either bike. If you get real anal about it, then you would have to remeasure as the tire wore down.
 

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From the Sheldon Brown site...how to determine circumference for bike computers...

Looks like you could program the bike computer to any setting using this
formula that came from Sheldon Brown's site:

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Deriving tire diameter from ISO/ETRTO numbers

The I.S.O. tire size consists of a tire width and a bead seat diameter. Both
of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire
has a nominal width of 28mm on a rim with a bead seat diameter of 622 mm.

To get an approximate diameter (in mm) add the bead seat diameter to twice
the tire width (since the tire comes into the diameter twice: 622 + (28 X 2)
= 678.

Multiply this by pi (3.142) to get the circumference in mm (F) 2130.
Appropriate calculations will yield calibration numbers for computers in
other groups.



kilofox said:
Does using the exact wheel circumference as opposed to using the canned wheel/tire size setting make that big of a difference when setting up your cyclometer? I mean how anal or lazy do you have to be to use either method? :confused: Can it be that far off to make much of a difference? Please enlighten me......
 

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Depends

If you have a rear wheel sensor like a Cateye Astrale there is a big difference between an unweighted rollout and a weighted rollout. After several attempts I found that the Astrale chart is more accurate than an actual measured circumference or an unweighted rollout. My best results have been to calibrate from the chart and then make small adjustments to match riding known distances, like section line road intersections.
~Al
 

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hi, I'm Larry
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Can be up to 5%

And the tire you use can make a big difference. Even within the same size tires I have seen differences of 2-3% between brands. My Conti 23's run larger than my Vred 23s.

A weighted roll out or making adjustments after riding known distances is best, but I've been too lazy to adjust mine that way, especially when I swap wheel sets around. I just know it's 2% low when I run the Vred and do the math in my head to figure out avg speed when just got to know (which is rare).

It depends on your pesonality type. If you have a deep down need to split hairs, do a roll out. If your the more layed back type, leave it alone, it's close enough. Who's passing whom is more important than what the computer says.
 

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How I measure wheel circumference

I put a piece of duct tape on the front wheel and then using an ink pen put a small dot of ink on the tape. I tape two pieces of typing paper to the ground at the distance of one rollout. As you ride over the two pieces of paper the ink leaves two small dots that you can then measure the exact distance between.

I've found that when I ride with my cousin who just uses the value for wheel circumference in the manual that her mileage & avg speed are always more than mine for the same ride when we're riding side by side for the entire ride. Her mileage is about 1-2 miles further for a 50 mile ride.

kilofox said:
Does using the exact wheel circumference as opposed to using the canned wheel/tire size setting make that big of a difference when setting up your cyclometer? I mean how anal or lazy do you have to be to use either method? :confused: Can it be that far off to make much of a difference? Please enlighten me......
 

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by far the most accurate way to calibrate a cyclocomputer is to ride a measured mile a couple of times in each direction and average the results. That's the method The Athletic Congress requires for calibrating bikes to measure race courses, and some local running clubs have a measured mile just for that purpose. The roll out method is no more accurate than simply using the charts you can find at Sheldon Brown's site.
 
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