Speedometer is asking for circumference value in mm?
09-07-12, 03:33 PM
#26
Mechanic/Tourist
p.s. Mark the tire? Just use the valve stem. When you are dealing with a rollout in the 2000mm area you have a lot of leeway - a few mm's don't matter.
Last edited by cny-bikeman; 09-07-12 at 05:27 PM.
09-07-12, 03:59 PM
#27
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Are you riding a bike or running a metrology lab? Since GPS and your computer measure distance using different methods they will never agree exactly. Rough or uneven road surfaces will make the wheel travel a farther distance than what GPS measures (to great precision I might add). Both are correct within their own frame of reference. Just take cnybikeman's advice and adjust your circumference setting and ride your bike. You are already at about 5% accuracy; make an adjustment once and call it good. If you keep comparing them and readjusting you will spend all of your time obsessing over the "correct" value instead of enjoying your ride.
09-07-12, 04:30 PM
#28
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When I was young I had a part time job delivering groceries. There was another person who rode a bike, and after hours we'd have parimutual track races inside. Riding competitively on a flat linoleum track teaches bike handling, especially the day after they wax the floors.
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09-08-12, 04:16 AM
#29
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342X2X3.147 = 2152.5 on my calculator. Also Pi is 3.142 rounded to 3 places - 3.14159 to five places. 342X2X3.14159=2149. This is really insignificant but I'm and engineer and can't help it.Ha!
I think the most accurate method is the roll out method, with the rider on the bike. Even doing this though the rolling circumference will change depending on temperature and tire pressure changes. The best way is to ride a known distance and calculate the percent difference of your odometer reading. This percent could then be applied to the value entered for the circumference. Highway mile markers could be used, but they are not always accurate. No matter what is done there is always doing to be some error.
I think the most accurate method is the roll out method, with the rider on the bike. Even doing this though the rolling circumference will change depending on temperature and tire pressure changes. The best way is to ride a known distance and calculate the percent difference of your odometer reading. This percent could then be applied to the value entered for the circumference. Highway mile markers could be used, but they are not always accurate. No matter what is done there is always doing to be some error.
This will get you really close if a rollout of the wheel measurement seems inconvenient.
Measure from the center of the axle to the outer radius of the tire. Where the rubber hits the road. Maybe reduce that number by a couple of mm to account for tire squish while riding.
Multiply that measurement, times Pie, times 2.
So, here's an example.
(Radius)x(2)x(Pie)
342x2x3.147=2155
Radius=342
Pie=3.147
Measure from the center of the axle to the outer radius of the tire. Where the rubber hits the road. Maybe reduce that number by a couple of mm to account for tire squish while riding.
Multiply that measurement, times Pie, times 2.
So, here's an example.
(Radius)x(2)x(Pie)
342x2x3.147=2155
Radius=342
Pie=3.147
09-08-12, 11:37 PM
#30
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Any of the rollout tests or formulas will work plenty well.
BUT -- the lazy man's approach is to google "bicycle tire circumference"
You'll get many hits; the first is a link to Sheldon Brown's discussion on the topic, which includes a table showing the data.
I'll save you the trouble:
https://sheldonbrown.com/cyclecomputer-calibration.html
(HINT: ignore the "group" descriptions at the top of the table and instead look at the bottom of the table to interpret the columns. The circumference in mm is the far right column)
Or you can follow the other links and get to similar (but often slightly different) results. But they are all close enough.
You can tweak the setting later if you choose, but don't fret a couple of percent error...
-TH
BUT -- the lazy man's approach is to google "bicycle tire circumference"
You'll get many hits; the first is a link to Sheldon Brown's discussion on the topic, which includes a table showing the data.
I'll save you the trouble:
https://sheldonbrown.com/cyclecomputer-calibration.html
(HINT: ignore the "group" descriptions at the top of the table and instead look at the bottom of the table to interpret the columns. The circumference in mm is the far right column)
Or you can follow the other links and get to similar (but often slightly different) results. But they are all close enough.
You can tweak the setting later if you choose, but don't fret a couple of percent error...
-TH
09-09-12, 01:41 AM
#31
Really Old Senior Member
Don't forget to allow for tire pressure, temperature and moon phase.
