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  1. #1
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    For Those Who Are Better At MAth Than I: Tire Size/Speedo Question

    O-K, Poindexters, this 'uns for you!

    I recently put a 700c X 28 Gatorskin on my rear....but I still have a 700 x 25 in the front, where my bike-computer sensor is. Do I need to change the measurement input on the computer to still remain accurate? [It seemed "pretty accurate" over the course of the 21 mile ride I did T'other day...maybe off a few tenths- but I'm wondering. I keep thinking that the front wheel still spins the same number of revs per mile, etc.....but I dunno- am I missing something?)

    A few years ago I would have known this... Points are dropping off the old IQ....seriously!

  2. #2
    Senior Member Wilfred Laurier's Avatar
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    use the actual circumfrence of the wheel to which the sensor is attached

    for you it sounds like the front is the answer

    did you do a roll-out test or just estimate or use a chart to find the wheel size?

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    the comp is still measuring the distance traveled by the front tire so you are still good to go.
    I do not claim to be a doctor, scientist, genie, bike magician, good looking, or qualified in any way. The contents of my post are opinions and should be taken as such.

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    Thanks, guys! That's what I thought- just wanted to be sure.

    Wilfred, I used both a chart and actually measured the circ umfrence with a string- if memory serves. It seems to be very acurate (For some reason, I thought I got a slightly different mileage total on my first and only ride since I changed the rear tire....but it's been a while since I'd done that route, so maybe I'm just remembering wrong).

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    coprolite fietsbob's Avatar
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    Oh, so this is not about the speedo swimming suits?

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    Quote Originally Posted by fietsbob View Post
    Oh, so this is not about the speedo swimming suits?
    I was actually going to put a disclaimer about that- but I figgered[sic] the images it would conjur up to those who might not have otherwise thought about it, would be too much to inflict on my fellow cyclists!

  7. #7
    dcf
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    Given that you now have a smaller tire in front as compared to the rear (which implies that the two wheels will be rotating at slightly different RPM's), you need to be careful that the difference in rotational speed doesn't cause your frame to stretch...

  8. #8
    Senior Member BlazingPedals's Avatar
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    An actual roll-out is the best method of measurement. Mark the pavement, line up the valve stem with the mark, roll out at least once and mark again when the valve stem comes around again. Measure, divide by number of revolutions if necessary, and convert to metric if necessary. 1 inch = 25.4 mm

    Just as a ballpark number, though, a 28mm tire on a 622 rim would be 622 + (2x28) = 678mm diameter. Circumference is pi(d) so 678 x 3.14 = 2129mm. Or, if your computer wants cm, then it would be 213cm. That would assume that the tire was a true 28mm tall when you were on the bike and the tire was squished down. Various factors could affect that number by 5 or 10.

    edit: Yes, the tire should have a load on it when you're rolling it out. If you want it to be accurate.

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    Senior Member Wilfred Laurier's Avatar
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    Quote Originally Posted by dcf View Post
    Given that you now have a smaller tire in front as compared to the rear (which implies that the two wheels will be rotating at slightly different RPM's), you need to be careful that the difference in rotational speed doesn't cause your frame to stretch...
    this is crazy

    larger rear wheel will provide less force when braking relatively
    smaller front wheel provides more force while braking

    frame will be compressed not stretched
    sheesh!

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    Senior Member Looigi's Avatar
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    Quote Originally Posted by digger531 View Post
    the comp is still measuring the distance traveled by the front tire so you are still good to go.
    Yes, but because the rider sits between the front and rear tire, the distance the rider travels is the average of the distances traveled by the front tire and the rear tire.

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    I heard of this one guy who went back in time by having a smaller front wheel on a motorcicle....

    No...wait...he just dressed like he was back in time- the 60's. (And I think that was the last time he bathed...)

  12. #12
    Senior Member BlazingPedals's Avatar
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    The front tire gets there first, so it's obviously going faster than the back wheel. To get an accurate measurement, you want the average of the two, since you'll be sitting between the wheels.

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    Quote Originally Posted by BlazingPedals View Post
    The front tire gets there first, so it's obviously going faster than the back wheel..
    How could anyone argue with THAT logic? So, ipso-fatso, if I were to get a chopper, I could theoretically break the sound barrier?

    [Waits for Steven Wright to show up in this forum...or maybe Stephen Hawking]

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    Senior Member woodcraft's Avatar
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    Because the wheel first goes half way around, and then goes half of the remaining half, and then half of the remaining arc, etc., etc.,

    It never does go all of the way around!

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    When I get a flat, I just move the tire so the flat part is not on the bottom,and continue on my merry way.

  16. #16
    rebmeM roineS JanMM's Avatar
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    In a similar vein......one of my bikes has 20"/26" front/rear wheels. And rolls forward on a level road without pedaling. Honest.
    RANS V3 (steel), RANS V-Rex, RANS Screamer

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    Don't bother doing a roll out or any other measurement, just use the chart.

    How accurate do you really need to be? It might be fun for a highly technical type, but in my experience with a bunch of bikes, different tires and speedometers, just using the chart if very accurate.... and if you're off by a couple of 1/10ths every 10 miles who cares?

    Frankly, I use the same computer on my 700X23 road bike and my 700X32 commuter and I doubt there's more than a 1/4 mile difference in my normal 13 mile commute.... that's more than accurate enough for me. Probably it's a little short for one and a little long for the other, I have no idea because even my GPS doesn't give me the exact same reading every ride.

    Now mind you, I'm not an engineer or computer scientist and I guess that these forums are biased in that direction. I work in a world where darn close is good enough and you move on!

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    Senior Member Wilfred Laurier's Avatar
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    Quote Originally Posted by Camilo View Post
    Don't bother doing a roll out or any other measurement, just use the chart.
    You are right that it does not make a big difference, but a rollout test is so easy and so quick that avoiding it is only necessary if you just installed your computer and are late for work.

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    Quote Originally Posted by BlazingPedals View Post
    Just as a ballpark number, though, a 28mm tire on a 622 rim would be 622 + (2x28) = 678mm diameter. Circumference is pi(d) so 678 x 3.14 = 2129mm. Or, if your computer wants cm, then it would be 213cm. That would assume that the tire was a true 28mm tall when you were on the bike and the tire was squished down. Various factors could affect that number by 5 or 10.

    edit: Yes, the tire should have a load on it when you're rolling it out. If you want it to be accurate.
    I think you are wrong on this. Even though squishing the tire deforms it, it shouldn't affect the overall circumference and thus shouldn't affect the overall distance traveled per revolution.
    Last edited by thomasbrent; 02-16-13 at 09:44 AM. Reason: Used "over" too many times.

  20. #20
    Senior Member Wilfred Laurier's Avatar
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    Quote Originally Posted by thomasbrent View Post
    I think you are wrong on this. Even though squishing the tire deforms it, it shouldn't affect the overall circumference and thus shouldn't affect the overall distance traveled per revolution.
    It probably does have a very small effect. The bottom of the tire is straight between the front and rear of the contact patche, and a straight line is shorter than an arc between the same points.
    I would guess the difference is pretty minor, though. Like 1 or two mm in total.

  21. #21
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    Quote Originally Posted by Camilo View Post
    Don't bother doing a roll out or any other measurement, just use the chart.

    How accurate do you really need to be? It might be fun for a highly technical type, but in my experience with a bunch of bikes, different tires and speedometers, just using the chart if very accurate.... and if you're off by a couple of 1/10ths every 10 miles who cares?

    Frankly, I use the same computer on my 700X23 road bike and my 700X32 commuter and I doubt there's more than a 1/4 mile difference in my normal 13 mile commute.... that's more than accurate enough for me. Probably it's a little short for one and a little long for the other, I have no idea because even my GPS doesn't give me the exact same reading every ride.

    Now mind you, I'm not an engineer or computer scientist and I guess that these forums are biased in that direction. I work in a world where darn close is good enough and you move on!
    Exactly!

    I did measure when I installed a bicycle computer on my motor scooter....'cause the size tire on the scooter wasn't listed in the chart...so it was the only way- and I guess the degree of precision would matter a little more on a much smaller diameter tire, like on the scooter; and one that goes at a higher speed....than a millimeter or two would matter on a c. 28" tire.

    I notice, sometimes on the bike, I'll get slightly different mileage readings over the exact same route on different days- Maybe one day I didn't ride as straight; or cut the corners wider; or had to swerve to avoid things, etc. so what would be the point of worrying about a few millimeters?

  22. #22
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by BlazingPedals View Post
    The front tire gets there first, so it's obviously going faster than the back wheel.
    Actually, it's worse than you think because the front tire really does go farther.

    Ride through a puddle and then make a turn. Now go back and examine your tire tracks. The back tire cuts the corner and travels a little less distance at every turn.

  23. #23
    Senior Member BlazingPedals's Avatar
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    Quote Originally Posted by DayGloDago View Post
    I notice, sometimes on the bike, I'll get slightly different mileage readings over the exact same route on different days- Maybe one day I didn't ride as straight; or cut the corners wider; or had to swerve to avoid things, etc. so what would be the point of worrying about a few millimeters?
    That's kind of like my line when the group finishes a long ride and I'm reading lower mileage than someone else. "Your wobbling made you ride an extra mile today."

    Just a quick math check says that it makes about 1% difference if you don't load the tire while doing the roll-out. Some may see that as close enough, I'd rather take a little extra time to do it right.

  24. #24
    Senior Member woodcraft's Avatar
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    OK, I had set the computer by doing a roll-out. Recently, my buddy explained it to me this way: "Imagine that you had a larger and less inflated tire. The flat area at the contact patch will be bigger when loaded, and the axle will be lower- say 1/2". That, then is the radius used to calculate the circumference- shorter radius= shorter circumference."

    I tried it just now. 700/23, pressure a little low- 85 lbs, I weigh about 165. Unweighted: 212cm, weighted:211cm

    A little less than .5%. About a 1/4 mile in 50?

  25. #25
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    I'll save the mental math gymnastics for driving.... I have to remember that my truck, in reality, is going 12% faster than what the speed-O-meter says, 'cause I gots bigger than stock tires on it..... (I just figure 10% and add 1 or 2 MPH). Maybe I should do a roll-out test with the truck?

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