Advertise on Bikeforums.net



User Tag List

Page 1 of 2 12 LastLast
Results 1 to 25 of 33
  1. #1
    Senior Member rumrunn6's Avatar
    Join Date
    Jul 2008
    Location
    25 miles northwest of Boston
    My Bikes
    Bottecchia Sprint
    Posts
    12,085
    Mentioned
    5 Post(s)
    Tagged
    0 Thread(s)

    Bike Computer Wheel Circumference Chart(s)

    re: Bike Computer Wheel Circumference Chart(s)

    Hmm ... scratching my head cuz the chart that came with my computer doesn't jive with Sheldon Brown's and also I don't understand why a 27 x 1" wheel tire combo would be different than a 27 1/4" wheel tire combo.

    I tried measuring my wheels' circumferences but it is too awkward with a metal tape measure, so i'd like to use the chart. I've been riding different bikes with different wheel/tire combos and I'll be using yet another tomorrow, as well as changing a tire size on a current bike. I want to get this right and also want my personal performance comparisons to be accurate. Hope that makes sense.

    OK, 1st issue chart differences:

    My computer's chart:
    27 x 1 = 2136
    27 x 1 1/4 = 2155
    700 x 32c = 2155
    700 x 28c = 2136

    Sheldon Brown's chart:
    http://www.sheldonbrown.com/cyclecomp_f.html

    27 x 1 = 2145
    27 x 1 1/4 = 2161
    700 x 32c = 2155
    700 x 28c = 2136

    So it looks like my settings for my 700 x 28 tires will be fine cuz both charts match. But I'm concerned about my 27 x 1" setting. Should I set 2145 or 2136?

    I'm reluctant to use the rollout method cuz it's dark and rainy out and I don't have any chalk.

    2nd issue - why would the circumference measurement be different between the 27 x 1 and the
    27 x 1 1/4???
    cycling is like baseball ~ it doesn't take much to make it interesting

  2. #2
    Senior Moment grinningfool's Avatar
    Join Date
    Aug 2008
    Location
    Somewhere between Bangor and Augusta
    My Bikes
    1985 Miyata 210, 2003 Giant NRS 3, 1991 Trek 7000, 1988 KHS Montana Crest, 1994 GT Outpost, 2001 Schwinn Mesa GSX, 1988 Schwinn Traveler, 2001 Schwinn Super Sport
    Posts
    266
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Put a piece of tape on the floor, and align your valve stem with it. Then roll your bike one wheel revolution, and put another piece of tape on the floor aligned with the stem again. Then measure the distance from one piece of tape to the other. That is your wheel circumfrence.
    People say I'm stupid and apathetic. I don't know what that means, and I don't care.

  3. #3
    Senior Member MudPie's Avatar
    Join Date
    May 2004
    Location
    Southern California
    Posts
    1,874
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by rumrunn6 View Post
    re: Bike Computer Wheel Circumference Chart(s)

    I'm reluctant to use the rollout method cuz it's dark and rainy out and I don't have any chalk.
    Grinningfool's method is the best, and it could be done indoors on a hard surface. Instead of indexing off the valve stem, I will put a pen mark on the side of the tread as close to the floor as possible, and use that as an index. Also, I place masking tape on the floor and place distance marks on it using the tire mark as an index. Then measure with a steel tape measure. I know this sounds geeky, but I will do the same measurement multiple times (like 10) and then take the average value.

  4. #4
    Old Fogy
    Join Date
    Oct 2006
    Location
    Murray, Utah
    Posts
    1,224
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    There is .4% difference between the two figures. I'd be surprised it you could get that close on a rollout.

  5. #5
    Senior Member Retro Grouch's Avatar
    Join Date
    Feb 2004
    Location
    St Peters, Missouri
    My Bikes
    Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
    Posts
    23,818
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    If you're that particular about being exactly right, the roll out method is the only way to go. The tire size label on the side means nothing. The exact same tire installed on different rims might be different.

    Personally, I don't care that much. I just use the numbers that come printed on the computer directions. If I can't find the right tire size, I interpolate. What are you using this data for? Sometimes good enough is good enough.

  6. #6
    aka Tom Reingold noglider's Avatar
    Join Date
    Jan 2009
    Location
    West Village, New York City
    My Bikes
    too many
    Posts
    19,179
    Mentioned
    50 Post(s)
    Tagged
    1 Thread(s)
    If you're super-particular, you should do the rollout with you on the bike, because your weight will deflect the tire a bit. Have a friend do the marking and measuring.
    You don't read my signature anyway, do you?

    Tom Reingold, noglider@pobox.com
    Residences: West Village, New York City and High Falls, NY
    Blogs: The Experienced Cyclist; noglider's ride blog

  7. #7
    Insane Bicycle Mechanic Jeff Wills's Avatar
    Join Date
    Mar 2008
    Location
    other Vancouver
    Posts
    6,771
    Mentioned
    16 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by noglider View Post
    If you're super-particular, you should do the rollout with you on the bike, because your weight will deflect the tire a bit. Have a friend do the marking and measuring.
    And do the rollout for 10 revolutions of the wheel. Divide the result by 10 and Bob's your uncle.
    Jeff Wills

    All my bikes.

  8. #8
    Call me The Breeze I_bRAD's Avatar
    Join Date
    Aug 2006
    Location
    Corbyville Ontario
    My Bikes
    2004 Litespeed Siena, 1996 Litespeed Obed, 1992 Miele (unknown model), 1982 Meile Uno LS.
    Posts
    3,699
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    An easier way is to measure from the hub to the floor and mulitply by 2pi.

  9. #9
    Senior Member
    Join Date
    Jul 2006
    Location
    El Segundo, Ca.
    My Bikes
    '93 Performance R203, '83 Bianchi 980
    Posts
    215
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    i agree with the 2 pi method for ease and accuracy. pi equals 3.1416. measure from floor to
    axle center IN MILLIMETERS, multiply by 2 pi. round the answer to the amount of digits your
    computer requires.

  10. #10
    Life is good RonH's Avatar
    Join Date
    Jan 2001
    Location
    Between Crystal River and Hernando, Florida, 6 miles west of the Withlacoochee Trail
    My Bikes
    I've had several since 1999 but have settled on my beloved 2001 Litespeed Tuscany and my latest, a 2013 Cannondale CAAD 10.
    Posts
    13,691
    Mentioned
    3 Post(s)
    Tagged
    0 Thread(s)
    Use the chart that came with the computer.
    My bikes: 2001 Litespeed Tuscany---
    2013 Cannondale CAAD 10 2 "Racing Edition"--The bike shop owner said it's toast. R.I.P.

    Life is like a 10-speed bicycle. Most of us have gears we never use. ~ Charles Schultz

  11. #11
    Senior Member demoncyclist's Avatar
    Join Date
    Oct 2002
    Location
    Medway, MA
    My Bikes
    2011 Lynskey Sportive, 1988 Cannondale SM400
    Posts
    2,467
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    +1 on rollout if you must be dead nuts accurate. As for the circumference question- it is the same reason that the measurement is different for differing width 700c tires. The wider tire has a slightly larger circumference.
    DEMON

    Satanic Mechanic
    STAR 3 Spinning Instructor

    2011 Lynskey Sportive (Shimano Ultegra 10s)
    1988 Cannondale SM400 (Suntour XC-M 7s)

  12. #12
    Primate Metzinger's Avatar
    Join Date
    Jan 2006
    Location
    gone
    My Bikes
    Concorde Columbus SL, Rocky Mountain Edge, Sparta stadfiets
    Posts
    2,582
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by waldowales View Post
    There is .4% difference between the two figures. I'd be surprised it you could get that close on a rollout.
    0.4% of 2155 is 8.6mm. I think I could measure a bit closer than that. With a pencil.

    Quote Originally Posted by I_bRAD View Post
    An easier way is to measure from the hub to the floor and mulitply by 2pi.
    Results would be curious here.

  13. #13
    Member Nogyro's Avatar
    Join Date
    May 2009
    Location
    Puryear, Tn.
    Posts
    43
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by demoncyclist View Post
    +1 on rollout if you must be dead nuts accurate. As for the circumference question- it is the same reason that the measurement is different for differing width 700c tires. The wider tire has a slightly larger circumference.
    The instructions that came with my Cateye said to do the rollout method with one twist, they said to be sitting on the bike. The tire squish will effect overall distance as well.

  14. #14
    Call me The Breeze I_bRAD's Avatar
    Join Date
    Aug 2006
    Location
    Corbyville Ontario
    My Bikes
    2004 Litespeed Siena, 1996 Litespeed Obed, 1992 Miele (unknown model), 1982 Meile Uno LS.
    Posts
    3,699
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Metzinger View Post
    Results would be curious here.
    By curious, do you mean correct? Otherwise I'm not sure what you're getting at.

  15. #15
    Senior Member
    Join Date
    Feb 2005
    Location
    Oklahoma
    My Bikes
    Trek 5500, Colnago C-50
    Posts
    9,114
    Mentioned
    2 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by rumrunn6 View Post
    re: Bike Computer Wheel Circumference Chart(s)


    why would the circumference measurement be different between the 27 x 1 and the
    27 x 1 1/4???
    Because a 27 x 1 1/4 tire has a bigger circumference than a 27 x 1 ??

    The roll out measurements I've seen have been very inaccurate. I think mostly because a roll out is such a small sample of a bike ride. If you are going to go with a roll out do what some others have suggested, 10 revolutions with the full weight of the rider on the bike, then divide by 10. It will make a difference.
    When I want a more accurate calibration I ride a known surveyed mile from survey pin to survey pin then adjust the circumference setting. But I usually just go by the chart and round down slightly. They tend to be a little optimistic.

    Al

  16. #16
    I have senior moments... bikinfool's Avatar
    Join Date
    Oct 2008
    Location
    Woodside, CA
    My Bikes
    Many
    Posts
    2,153
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    There are going to be differences among tire sizes, manufacturers don't always measure the same way, so whatever tires Sheldon used might not have much to do with the ones your computer mfr used, etc. I'd think that weighted rollout with your particular setups for most accurate figures. I was never was that anal with my bike computers (and haven't used any for years). You'll still end up with differences from other ways of measuring (stated map distances, gps, your friends' computers, etc).
    suum quique
    Mountain bikes: Santa Cruz Hecklers (99, 02, 07), Santa Cruz Nomad, Moots YBB, Trek OCLV Pro Issue, American Breezer
    Road bikes: TST, Trek 2300 (Carbon/Alum)

  17. #17
    Senior Member
    Join Date
    Oct 2007
    Location
    SoCal
    Posts
    1,143
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by I_bRAD View Post
    By curious, do you mean correct? Otherwise I'm not sure what you're getting at.
    It would only be dead on if we were using solid tires that don't deform and you measure with the bike exactly upright.
    Roll out is the only way to go. I find the charts to be too high of a number. If you want to read faster speeds than you are actually riding use those.

  18. #18
    Call me The Breeze I_bRAD's Avatar
    Join Date
    Aug 2006
    Location
    Corbyville Ontario
    My Bikes
    2004 Litespeed Siena, 1996 Litespeed Obed, 1992 Miele (unknown model), 1982 Meile Uno LS.
    Posts
    3,699
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by dvs cycles View Post
    It would only be dead on if we were using solid tires that don't deform and you measure with the bike exactly upright.
    Roll out is the only way to go. I find the charts to be too high of a number. If you want to read faster speeds than you are actually riding use those.
    If you sit on the bike and have someone measure that's accurate enough for me. Hell, even without someone sitting on it. I'm happy if I know I went around 100kms on a ride. I don't need to know I went 101.3kms.

  19. #19
    Primate Metzinger's Avatar
    Join Date
    Jan 2006
    Location
    gone
    My Bikes
    Concorde Columbus SL, Rocky Mountain Edge, Sparta stadfiets
    Posts
    2,582
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by I_bRAD View Post
    By curious, do you mean correct? Otherwise I'm not sure what you're getting at.
    I meant varied.
    DVS covered one reason.
    In addition, you'd have to measure over 6 times more precisely to equal that of a roll-out. (2XPi)

  20. #20
    Bianchi Goddess Bianchigirll's Avatar
    Join Date
    Apr 2009
    Location
    Camp Hill, PA
    My Bikes
    Too many to list here check my signature.
    Posts
    20,163
    Mentioned
    31 Post(s)
    Tagged
    1 Thread(s)
    it sounds funny that you are concerned about the circumfrence difference between 27x 1, 27x1 1/4 a whole 1/4 difference. but dont seem alarmed that a 700x23 and 700x25 are different by 3MM same as a 700x28 is different from a 700x32 by 4MM. since there is roughly (sorry I am not exact enough here) 25MM to an inch that make the difference between a 700x28 and 700x32 about 4 or 5 MM OMG!!!
    Bianchis '87 Sport SX, '90 Proto (2), '91 Boarala 'cross, '93 Project 3, '88 Trofeo, '86 Volpe, '89 Axis, '79 Mixte SOLD, '99 Mega Pro XL Ti, '97 Ti Megatube, , '90 something Vento 603,

    Others but still loved,; '80 RIGI, '80 Batavus Professional, '87 Cornelo, '86 Bertoni (sold), '09 Motobecane SS, '98 Hetchins M.O., '09 K2 Mainframe, '89 Trek 2000, '?? Jane Doe (still on the drawing board), '90ish Haro Escape

  21. #21
    Perineal Pressurized dobber's Avatar
    Join Date
    Oct 2003
    Location
    In Ebritated
    Posts
    6,556
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Metzinger View Post
    0.4% of 2155 is 8.6mm. I think I could measure a bit closer than that. With a pencil.


    Results would be curious here.
    And on a 1 mile bike ride you'd be off 21 feet.

    Theres a limit to the required accuracy in some cases. This is one of them.
    This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.

  22. #22
    Senior Member coldfeet's Avatar
    Join Date
    Dec 2007
    Posts
    2,118
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by I_bRAD View Post
    If you sit on the bike and have someone measure that's accurate enough for me. Hell, even without someone sitting on it. I'm happy if I know I went around 100kms on a ride. I don't need to know I went 101.3kms.
    +1 if you're worried about being totally upright, measure on both sides without moving and divide by 2.

    the problem with the rollout method is you would have to keep a totally straight line.

    And yes, I think this is a little bit of overkill, the chart with the computer is probably close enough.

  23. #23
    Gear Hub fan
    Join Date
    Nov 2008
    Location
    Reno, NV
    My Bikes
    Civia Hyland Rohloff, Swobo Dixon, Colnago, Univega
    Posts
    2,830
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    IMO the reason the charts tend to be off compared to rollout is that most tires are smaller than their marked size.

    Using rollout or, better yet, a measured mile/kilometer should be the most accurate method if you want to be dead on but even most car odometers are off by more than the inaccuracy caused by using the chart with your bike computer.
    Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro

    Visit and join the Yahoo Geared Hub Bikes group for support and links.
    http://groups.yahoo.com/group/Geared_hub_bikes/

  24. #24
    Senior Member Retro Grouch's Avatar
    Join Date
    Feb 2004
    Location
    St Peters, Missouri
    My Bikes
    Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
    Posts
    23,818
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by tatfiend View Post
    Using rollout or, better yet, a measured mile/kilometer should be the most accurate method if you want to be dead on but even most car odometers are off by more than the inaccuracy caused by using the chart with your bike computer.
    Even car odometers? Car odometers suck!

    Car odometers have much more tire tread wear differential than a bicycle tire. They also are affected by every curve. Most certified running courses are measured using a calibrated bicycle odometer. They won't even consider anything that's measured using a car odometer.

  25. #25
    `````````````` CaptainCool's Avatar
    Join Date
    Feb 2007
    Location
    san jose
    Posts
    765
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    If you want to be dead on, calibrate your computer against a short course measured by a GPS unit or gmap-pedometer.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •