Click here to join our community Log in to access your Control Panel

 Bike Forums > Calibrating Computer - I Hate Math
 Username Remember Me? Password
 Register All Albums Elite Membership Forum Rules Calendar Search Today's Posts Mark Forums Read

 General Cycling Discussion Have a cycling related question or comment that doesn't fit in one of the other specialty forums? Drop on in and post in here! When possible, please select the forum above that most fits your post!

 Thread Tools Search this Thread
 11-18-07, 05:52 PM #1 jc808 Sassbucket. Thread Starter     Join Date: Oct 2007 Location: los angeles, ca Bikes: Posts: 113 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 4 Post(s) Calibrating Computer - I Hate Math I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25. Here's the formula I followed, as per the manual: distance from floor to hub: 13 3/8" (13.375) multiply by 25.4 to convert to mm (339.725) multiply that number by 6.28 (2 x pi) = 2133.473 I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong. Any ideas?
 11-18-07, 05:59 PM #2 late Senior Member     Join Date: Oct 2002 Location: Southern Maine Bikes: Posts: 8,499 Mentioned: 43 Post(s) Tagged: 0 Thread(s) Quoted: 5177 Post(s) And now for something to really mess with your head.... tires companies do no provide precise size measurements. Try a number, use it for a while, it will seem high or low. So change it a bit and see if you can get closer. You can also try to measure by seeing how far the tire actually rolls. I found that more trouble than it was worth. I just want to be close, and if the mileage reads a little on the generous side, I am not one to complain.
 11-18-07, 06:06 PM #3 jc808 Sassbucket. Thread Starter     Join Date: Oct 2007 Location: los angeles, ca Bikes: Posts: 113 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 4 Post(s) I actually measured my wheel with a tape measure. As you said, I know not to trust what the tire says! The reason I'd like to get pretty accurate is because I'm so bad with distances. I really have nothing to base myself on to judge if I'm too high or too low.
11-18-07, 06:30 PM   #4
deraltekluge
Senior Member

Join Date: Sep 2006
Bikes: Kona Cinder Cone, Sun EZ-3 AX
Posts: 1,195
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by jc808 I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong. Any ideas?
It's only a 1.3% difference. Brown says that his numbers are within one or two percent, and...
Quote:
 f you require more accuracy, you can do a "roll-out" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution. You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later. Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.

11-18-07, 06:50 PM   #5
operator
cab horn

Join Date: Jun 2004
Location: Toronto
Bikes: 1987 Bianchi Campione
Posts: 28,321
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 9 Post(s)
Quote:
 Originally Posted by jc808 I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25. Here's the formula I followed, as per the manual: distance from floor to hub: 13 3/8" (13.375) multiply by 25.4 to convert to mm (339.725) multiply that number by 6.28 (2 x pi) = 2133.473 I had entered 2105 based on Sheldon Brown's recommendation for a 700 x 25 wheel. But this is a pretty substantial difference. When I entered 2133, it seemed like the miles were racking up really quickly. I'll be the first to admit that I don't have the best sense of distance, so I could be wrong. Any ideas?
Don't worry too much about absolute accuracy. The important thing is consistent. You can also verify it against gmaps pedometer for distanced travel. As you ride more you'll realize that all this bike computer stuff is just fluff.

Real joy is riding.

 11-18-07, 07:48 PM #6 Bill Kapaun Senior Member     Join Date: Feb 2007 Location: Mid Willamette Valley, Orygun Bikes: 86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds. Posts: 10,122 Mentioned: 2 Post(s) Tagged: 0 Thread(s) Quoted: 403 Post(s) I take a piece of string, tie a small loop in the end and then basically "lasso" the tire. Mark the string where it overlaps and then hook the loop on a nail or something and stretch the string (approx same tension as when in "lasso mode") and measure it with the tape. Repeat until you feel you have a consistent measurement. Divide that # by .03937. As mentioned, a difference of 20+ out 2000+ is pretty small. Or you could just enter 2111.
11-18-07, 08:22 PM   #7
Retro Grouch
Senior Member

Join Date: Feb 2004
Location: St Peters, Missouri
Bikes: Catrike 559 I own some others but they don't get ridden very much.
Posts: 27,908
Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 627 Post(s)
Quote:
 Originally Posted by operator Don't worry too much about absolute accuracy. The important thing is consistent. You can also verify it against gmaps pedometer for distanced travel. As you ride more you'll realize that all this bike computer stuff is just fluff.
If you're really interested in this sort of thing here's a test for you to try:

Try riding the exact same route 3 or 4 times and see how closely your computer milages compare. Trying to program your computer to be more accurate than the variation among the repeated laps is fooling yourself.

 11-18-07, 08:23 PM #8 StephenH Uber Goober     Join Date: Sep 2007 Location: Dallas area, Texas Bikes: Posts: 11,473 Mentioned: 2 Post(s) Tagged: 0 Thread(s) Quoted: 85 Post(s) Put a piece of masking tape on a tire at the bottom, matching piece on the floor. Sit on the bike, roll it forward one full revolution, measure from there back to the first tape. Removes the uncertainties about what the actual rolling diameter is, avoids using pi at all, etc.
 11-18-07, 08:40 PM #9 rm -rf don't try this at home.     Join Date: Jan 2006 Location: N. KY Bikes: Posts: 4,397 Mentioned: 5 Post(s) Tagged: 0 Thread(s) Quoted: 355 Post(s) The rollout method works. If you don't have a metric tape measure, you can use google to convert it. For instance, entering 83 1/8 inches in mm in google search, you get this answer: (83 1/8) inches = 2 111.375 millimeters google will convert a lot of units. 4 lbs in grams returns: 4 pounds = 1 814.36948 grams
 11-18-07, 08:59 PM #10 Fredmertz51 Senior Member     Join Date: Oct 2004 Location: Iowa Bikes: Actual 10-speed Olmo road, Bianchi BUSS, Kona A-Ha, Schwinn Moab 2 rain bike Posts: 335 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I use the 5th thru 7th mile of the Quad City Marathon Course. I just ride back and forth, and adjust until the computer clicks off on the mile marks. I hear they measure those things pretty accurately for the runners.
11-18-07, 09:52 PM   #11
jc808
Sassbucket.
Thread Starter

Join Date: Oct 2007
Location: los angeles, ca
Bikes:
Posts: 113
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Quote:
 Originally Posted by Fredmertz51 I use the 5th thru 7th mile of the Quad City Marathon Course. I just ride back and forth, and adjust until the computer clicks off on the mile marks. I hear they measure those things pretty accurately for the runners.
Next time I'm in Iowa, I'll bring my bike

I guess I don't really want to get into splitting hairs. I'll just split the difference and call it a day. Thanks for all the help!

 11-19-07, 07:18 AM #12 eubi No Rocket Surgeon     Join Date: Jan 2005 Location: Corona and S. El Monte, CA Bikes: Cannondale D600, Dahon Speed T7 Posts: 1,648 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I agree with rm-rf, the rollout method is the only way to go. Pump up your tires. Put a tape measure on the ground. Get on the bike. You may need a "spotter" for that step. Ride your bike so the front wheel goes one full revolution (I'm assuming your pickup is on the front fork). Your valve stem is a good indicator. Just measure how far your bike travels in one wheel revolution. Convert to mm (25.4 mm/inch), and enter this value into your computer. If you are really anal (as I am) you will do this a few times and average the results. I use this converter a lot: http://www.convertit.com/Go/ConvertI.../Converter.ASP Simple! Have fun. __________________ Fewer Cars, more handlebars! Last edited by eubi; 11-19-07 at 08:13 AM.
11-19-07, 09:25 AM   #13
RonH
Life is good

Join Date: Jan 2001
Location: Not far from the Withlacoochee Trail. 🚴🏻
Bikes: 2001 Litespeed Tuscany and 2014 Cannondale SuperSix EVO Hi-Mod
Posts: 17,674
Mentioned: 8 Post(s)
Tagged: 0 Thread(s)
Quoted: 382 Post(s)
Quote:
 Originally Posted by jc808 I just picked up my first computer. I need to figure out the circumference of my wheel to calibrate it, and It's giving me a bit of trouble. My wheel is 700 x 25.
What kind of POS computer did you get that doesn't provide a chart with all the numbers you'll need?
__________________
The LORD is merciful and gracious, slow to anger, and plenteous in mercy. - Psalm 103:8

 11-19-07, 09:27 AM #14 biknbrian Senior Member     Join Date: Jun 2007 Location: Pittsburgh, PA Bikes: BiknBrian brand custom 26 inch commuter trekker, Cannondale F600 Single Speeded MTB, Nashbar Cro-Mo CX, some other bikes and parts that could be made into bikes. Posts: 352 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) You can't just measure the circumfrence or diameter, you have to weight the tire. Your effective circumfrence will be significantly smaller with weight on it. Then you can just drive a couple mile route and make sure it closely mathches what the computer says.
 11-19-07, 10:30 AM #15 deraltekluge Senior Member     Join Date: Sep 2006 Bikes: Kona Cinder Cone, Sun EZ-3 AX Posts: 1,195 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) But first, you should ask yourself if you really care if the figures are off by 70 feet in a mile, and if you do care, why?
 11-19-07, 11:05 AM #16 sknhgy  Dirt Bomb     Join Date: Aug 2006 Location: Illinois Bikes: Posts: 2,717 Mentioned: 18 Post(s) Tagged: 0 Thread(s) Quoted: 1594 Post(s) If you put a dab of paint on a smooth road, then rode over it, then measued the distance between the spots you could get an accurate measurement.

 Thread Tools Search this Thread Search this Thread: Advanced Search

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

All times are GMT -6. The time now is 01:13 AM.

 -- BikeForums -- Mobile One Contact Us - Bike Forums - Archive - Advertising - Cookie Policy - Terms of Service - Top

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.

• Ask a Question
get answers from real people!
Click to start entering your question.