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Old 11-18-07, 05:52 PM   #1
jc808
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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?
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Old 11-18-07, 05:59 PM   #2
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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.
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Old 11-18-07, 06:06 PM   #3
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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.
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Old 11-18-07, 06:30 PM   #4
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Quote:
Originally Posted by jc808 View Post
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.
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Old 11-18-07, 06:50 PM   #5
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Quote:
Originally Posted by jc808 View Post
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.
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Old 11-18-07, 07:48 PM   #6
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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.
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Old 11-18-07, 08:22 PM   #7
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Quote:
Originally Posted by operator View Post
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.
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Old 11-18-07, 08:23 PM   #8
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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.
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Old 11-18-07, 08:40 PM   #9
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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
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Old 11-18-07, 08:59 PM   #10
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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.
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Old 11-18-07, 09:52 PM   #11
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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!
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Old 11-19-07, 07:18 AM   #12
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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.
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Old 11-19-07, 09:25 AM   #13
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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?
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Old 11-19-07, 09:27 AM   #14
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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.
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Old 11-19-07, 10:30 AM   #15
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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?
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Old 11-19-07, 11:05 AM   #16
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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.
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