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I got a Ciclo Master CM 434 computer and am trying to calibrate the wheel size. I ride 700x22 Conti Sprinter tubulars. I usually pump them up to about 105-110 lbs.

The chart in the instruction booklet says I should calibrate the computer to approx 2128 mm.

When I do a roll out measurement it comes out to about 2087 mm. I did the measurement by pumping my tires up to pressure, putting a piece of white tape on my tire and rolling it next to a tape measure. All the while putting pressure down on the handlebars. I did the measurement several times and came out with about the same number.

There's quite a difference between the two numbers. Which one do I go by? Am I doing my roll out measurement wrong? :(
 

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"Cypress Gardens" Fl.
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No, I dont think your measurements are wrong, but you could try a different way just to be sure. I just did a roll-out of my front tire, and as far as I'm concerned is pretty damn accurate. With the tire off the bike I taped a business card sideways on the tire and marked the rear edge on a long table. Next I rolled the tire one full revolution and marked the rear edge again. Measure the two points in milimeters and program it into your computer. You can do the same thing sitting on your bike if you want, but the difference will be slight. According to my Blackburn manual anything less than 30 mm will not effect accuracy much.
 

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Resident Curmudgeon
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You can get even more accurate by inflating the tires to the pressure you usually use, then do your roll out by sitting on the bike & rolling along a wall. Takes 2 people, as you'll need someone to measure for you.
 

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eminence grease
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averaging the results of multiple roll-outs is the best way to go, the numbers in the booklets are notorious approximations.

I line up the valve stem at the bottom of the tire, make a chalk line on the ground, roll forward and mark the ground again. Then I repeat a couple of times.

These days, I use the number I found most often in roll outs - 2136 - in every computer I own. Don't care tremendously about the accuracy between bikes and for example, the difference between my number and yours amounts to about 2 miles in every 100.
 

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I would take the flexible tape and measure the wheel without any weight on it. My opinion, when weight is applied, the flexing of the tire is only on the bottom and that is a very small minute amount not to change the diameter. I've heard on this board from several people they feel the flexing is cumulative continuous around the wheel as you roll it. My attitude is immediately when that flex spot is no longer on the bottom, it immediately comes back to normal so you may have 359 degrees of full normal tire and only 1 degree at any one time of flex.
 

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Too much time on my hands today . . .

This probably falls into the "idle speculation" category, but I think it's still an entertaining thought about the degree of accuracy when measuring front wheel circumference:

Doesn't the front wheel almost always turn faster and travel further than the rear wheel? If you look at tire tracks left on a wet road, you'll see that the front wheel track almost constantly snakes to the left and to the right of the rear wheel track. To be able to do that and still remain connected to the frame, I think the front wheel has to turn faster and go further than the rear wheel.

So measurements of front wheel circumferences with an accuracy carried to the single millimeter might be dead-on for the wheel itself - and still give you an incorrect computer readout for the speed and the distance traveled by the entire bike.
 

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Accuracy

croswell1 said:
According to my Blackburn manual anything less than 30 mm will not effect accuracy much.
Well, a 30 mm error is about 1.4%, so 1.4 miles every 100. Whether that is acceptable depends on personal preferences. Most car odometers are around this range (I've seen as high as 2.5%). My wife's current bike computer is accurate to it's limit - 0.5% because it only lets you put in a number to the nearest cm.
 

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Very idle speculation

wim said:
This probably falls into the "idle speculation" category, but I think it's still an entertaining thought about the degree of accuracy when measuring front wheel circumference:

Doesn't the front wheel almost always turn faster and travel further than the rear wheel? If you look at tire tracks left on a wet road, you'll see that the front wheel track almost constantly snakes to the left and to the right of the rear wheel track. To be able to do that and still remain connected to the frame, I think the front wheel has to turn faster and go further than the rear wheel.

So measurements of front wheel circumferences with an accuracy carried to the single millimeter might be dead-on for the wheel itself - and still give you an incorrect computer readout for the speed and the distance traveled by the entire bike.
So, learn to ride a straight line :)
 

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Tried, and failed.

So, learn to ride a straight line
Learned that many years ago, but only with the rear wheel. Tried to learn how to keep the front wheel straight by sticking it into a streetcar track in Wassenaar, NL in 1956 and crashed instantly. Little Willem swore then and there to just keep see-sawing that handlebar. :D
 

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Discussion Starter · #12 · (Edited)
il sogno said:
I'm thinking I will go with the 2087mm calibration and test it out when riding by mileage markers.
I rode yesterday and 2087 was a bit too short. It also seemed to throw off the incline % readings. I was thinking about this on my ride and it occured to me that my metal tape measure has a little tab on the end. When I did the measurement, the tab was lifting the tape measure off of the ground by about 5-7 mm. So I did a remeasurement and came up with 2104mm.

Here's how I remeasured. I taped a blank sheet of paper on my hardwood floor, the edge of the paper lined up with the edge of the slats of wood. Approx 2+ meters on the same line of wood slats, I taped another piece of paper.

I took the wheel off the bike and put a piece of white paper tape on my tire. I then lined up the tape with the back edge of the first piece of paper. I rolled the wheel out keeping to the edge of the wood slats in the flooring and marked the point where it terminated on the second piece of paper with a pen. I rolled out twice, applying downward pressure both times.

I then measured the distance between the two points with my tape measure making sure the tab at the end of the tape measure did not come into play.

I will ride again today and check the mileage between mile markers. Ultimately I think this is the best way to check the calibration settings. Heck of a way to spend Memorial Day weekend, eh? :)
 

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Larry Lackapants
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I got around 2088 mm with a 23 mm tire (schwalbe,clincher) so i guess the measurement is about there somewhere. better to underestimate- with a smaller circumference you'll get a lower speed than the real one, so you'll push a bit harder :)
kidding
 

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I'm only about 500 miles into my riding experience but I have years in the field as a carpenter...and any carpenter worth his salt knows that you can't accurately(sp) get a reading off the hook...if you have help you "burn an inch" which means you would measure off the first centi mark and figure it out from there and deduct 10mm

just my 2 cents
 

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Discussion Starter · #16 ·
thespoonman said:
I'm only about 500 miles into my riding experience but I have years in the field as a carpenter...and any carpenter worth his salt knows that you can't accurately(sp) get a reading off the hook...if you have help you "burn an inch" which means you would measure off the first centi mark and figure it out from there and deduct 10mm

just my 2 cents
So I rode today and did more calibrating. It's now at 2118mm. I checked it over 3.5 miles and it seems to be okay down to 1/100th of an inch.
 
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