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  1. #1
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    Trying to find proper circumference for 700c wheel with 25 mm tires

    I am in the process of setting up my Garmin Edge 500. I have a 700c wheels and run 25 mm Continental GP4000s tires. My best effort at measuring the diameter of these wheels was 680 mm. Hence, the circumference would be 680 x 3.14... which comes out to 2136 mm for the circumference. Is that within the proper range for this setup? A few mm error on the diameter can lead to over a cm difference in circumference!!!! YIKES....after thousands of tire rotations that leads to mega error in actual mileage!!!!
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  2. #2
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    That's pretty close, though maybe a little high. I think the usual value is about 2112, but tires vary. And before you get all panicked over the centimeters adding up, keep in mind that it's the relative error that matters. A centimeter in 200 centimeters is half a percent, so you'd have to ride 200 miles before the error added up to a mile.

    Anyway, the best way if you want accuracy is to measure the circumference by rolling the wheel along the ground. Measuring diameter is hard. Measuring rollout can easily get you much close than a centimeter.

  3. #3
    tlg
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    The most acurate way is to actually measure. Fill your tires to your riding pressure. Put a couple drops of water, white out, or paint, on your tire. Sit on your bike and ride it across a flat surface. Then measure between the dots from the water or paint on the floor. There's your circumference.

  4. #4
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    Do the roll-out method. Simply position the wheel so the valve is at the bottom; mark that position on the ground (or do like me an place valve over the concrete seam on the garage floor); roll bike forward one revolution so valve is at the bottom again; mark spot on floor; then measure the distance between the starting ending spots.

  5. #5
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    Quote Originally Posted by tarwheel2 View Post
    Do the roll-out method. Simply position the wheel so the valve is at the bottom; mark that position on the ground (or do like me an place valve over the concrete seam on the garage floor); roll bike forward one revolution so valve is at the bottom again; mark spot on floor; then measure the distance between the starting ending spots.
    ^^^ This. Of course.
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  6. #6
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    But, I'm a geometry teacher and I want to show off my mad knowledge of the relationship of Circumference and Diameter!!! Ha

    Thanks for the advice guys. I'll let you know how close I was with my diameter conversion to reality.
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  7. #7
    tlg
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    Quote Originally Posted by TallCoolOne View Post
    But, I'm a geometry teacher and I want to show off my mad knowledge of the relationship of Circumference and Diameter!!! Ha
    Then you should know geometry is only practical in the classroom... not the real world.

    The problem with using simple 2πR is it doesn't take into account a rubber tire which doesn't have a precise consistant diameter.

  8. #8
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    Quote Originally Posted by TallCoolOne View Post
    But, I'm a geometry teacher and I want to show off my mad knowledge of the relationship of Circumference and Diameter.
    Be sure to make a formula that accounts for the amount of tire squish when you're sitting on the bike then.
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  9. #9
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    Calibration

    Quote Originally Posted by TallCoolOne View Post
    I am in the process of setting up my Garmin Edge 500. I have a 700c wheels and run 25 mm Continental GP4000s tires. My best effort at measuring the diameter of these wheels was 680 mm. Hence, the circumference would be 680 x 3.14... which comes out to 2136 mm for the circumference. Is that within the proper range for this setup? A few mm error on the diameter can lead to over a cm difference in circumference!!!! YIKES....after thousands of tire rotations that leads to mega error in actual mileage!!!!
    Almost everyone has access to a road with mile markers. Those markers are put in by people called surveyors, and they are accurate to better than a few feet per mile. Ride 10 miles and compare your computer reading with the actual distance. Correct your calibration from that. If you can ride longer you will get even more accurate results.

  10. #10
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    Won't the 500 auto-calibrate the tire circumference using GPS, like the Edge 705 and 800? That's what I've been relying on. However, with traditional cyclometers I do the roll-out. Don't forget that when you multiply the diameter by 3.14, you're also multiplying any error in measurement by the same amount. If you do the roll-out very carefully, you can get it to within a few mm out of ~2000 which is way more accurate than needed, of course. BTW, I've tried doing the roll-out weighted and unweighted and didn't see any significant difference, YMMV (pun intended).

  11. #11
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    as looigi said, it's a garmin...you don't need to program in the diameter. it will auto program using gps.
    i work for some bike racers...
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    oh, those belong in another forum

  12. #12
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    Quote Originally Posted by Kerry Irons View Post
    Almost everyone has access to a road with mile markers. Those markers are put in by people called surveyors, and they are accurate to better than a few feet per mile. Ride 10 miles and compare your computer reading with the actual distance. Correct your calibration from that. If you can ride longer you will get even more accurate results.
    If he wasn't already using a Garmin 500 (it auto calibrates) there is another way of doing this. You can ride any given distance and check your odometer with any mapping software. The longer the ride, the more accurate you can set your tire circumference.

  13. #13
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    I converted to the edge 500 about a year ago and can tell you that its auto-calibration matched my roll-out measurements to 1/100th of a mile on 40 mile rides. I checked it for several rides and was very impressed. For the record, my roll-out measurements were made with tires inflated, me on the bike, and two tire revolutions averaged. I would advise you to just let the garmin auto cal unless you live where there are so many trees it can't accurately track your location.

  14. #14
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    Quote Originally Posted by maximum15 View Post
    I converted to the edge 500 about a year ago and can tell you that its auto-calibration matched my roll-out measurements to 1/100th of a mile on 40 mile rides. I checked it for several rides and was very impressed. For the record, my roll-out measurements were made with tires inflated, me on the bike, and two tire revolutions averaged. I would advise you to just let the garmin auto cal unless you live where there are so many trees it can't accurately track your location.
    Do I just set the wheel size to auto??? I am curious how this works.
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  15. #15
    'brifter' is a lame word.
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    Quote Originally Posted by TallCoolOne View Post
    Do I just set the wheel size to auto??? I am curious how this works.
    yes...
    i work for some bike racers...
    2013 Trek Madone 5.9 w/ '12 SRAM Red
    2010 Cervelo T1 sprint bike
    Ruger 10-22TD
    Smith&Wesson M&P 15-22
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    oh, those belong in another forum

  16. #16
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    I recently switched to 700 x 25 tires for my bike and my wife's. Her basic bike computer manual said to use 2124, When I rode my Garmin 500 on an open course (i.e. good GPS reception) in auto, the calculated circumference came out to 2119 mm. I'll continue to refine working around these numbers when I travel on a know course.

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    Omg. .

    Quote Originally Posted by TallCoolOne View Post
    ...after thousands of tire rotations that leads to mega error in actual mileage!!!!
    Take it out of the book, as it's plenty accurate for a bikeIMO. The rollout method is just absoulte overkill at it's best. Your pressure might change sometimes, new tires, new rims, your fluctuating body weight. . All this will make small changes to a rollout. Your speedmeter only reads XX.X not XX.XXX. Or really like others have said, you can let the 500 auto calibrate. It's under ->settings ->bike settings->bike details -> Bike X. At the bottom of the screen it says auto or user.
    Last edited by QQUIKM3; 05-01-2012 at 05:50 AM. Reason: text

  18. #18
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    Quote Originally Posted by QQUIKM3 View Post
    Take it out of the book, as it's plenty accurate for a bikeIMO. The rollout method is just absoulte overkill at it's best. Your pressure might change sometimes, new tires, new rims, your fluctuating body weight. . All this will make small changes to a rollout. Your speedmeter only reads XX.X not XX.XXX.
    "The Book" says the proper measurement for a 700x23 tire is 2133. Roll-out tells me that it's actually 2087 or so.
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  19. #19
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    Quote Originally Posted by PlatyPius View Post
    "The Book" says the proper measurement for a 700x23 tire is 2133. Roll-out tells me that it's actually 2087 or so.
    I don't know what book that is. The manuals I've seen, and the measurements I've done, say that's pretty close for a 28mm tire, not 23. Every 23 I've measured is somewhere between 2090 and 2100. Sheldon Brown's chart says 2097.

  20. #20
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    Quote Originally Posted by JCavilia View Post
    I don't know what book that is. The manuals I've seen, and the measurements I've done, say that's pretty close for a 28mm tire, not 23. Every 23 I've measured is somewhere between 2090 and 2100. Sheldon Brown's chart says 2097.
    Sigma says 2133 for a 700x23.

    Many new computers default to 2133, although some go to 2000.
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  21. #21
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    Quote Originally Posted by PlatyPius View Post
    Sigma says 2133 for a 700x23.

    Many new computers default to 2133, although some go to 2000.
    That seems strange. I just checked a Cateye manual online, and it says 2096 for 23, 2136 for 28, which matches my own experience. It also pretty closely matches the theoretical number you get from the nominal sizes with a 622mm bead seat diameter and 23mm tire height. 622 + (2 x 23) = 668mm. multiply by pi and you get about 2098.5.

    Not that it matters to be that accurate, but it's odd that Sigma's manual is that far off from what everybody else seems to get.
    Last edited by JCavilia; 05-01-2012 at 06:12 PM.

  22. #22
    Scott in Maryland
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    23mm Conti 4000s for me is 2096.

  23. #23
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    I've always done the loaded rollout method. Your weight and tire pressure will affect the actual rolling radius of the tire, unless someone else is using the same tires at the same pressure and weighs about the same as you ... your results may vary. The good news is that it is pretty easy to get into less than .5% error.

  24. #24
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    Quote Originally Posted by tlg View Post
    The most acurate way is to actually measure. Fill your tires to your riding pressure. Put a couple drops of water, white out, or paint, on your tire. Sit on your bike and ride it across a flat surface. Then measure between the dots from the water or paint on the floor. There's your circumference.
    This is the easiest most accurate way. People have different pressures which can effect the rolling circumference.
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