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Old 09-25-13, 03:34 PM   #1
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Just a thought about tire measurements for bike computers...

Every bike computer comes with a chart of settings for common tire sizes, which gets you in the neighborhood of the right setting, though we know a roll-out is the most accurate method of adjustment.

But, I was wondering, why don't the tire manufacturers simply put a measurement in mm on the sidewall of the tire as an aid? This would be fairly accurate. Variables would be tire pressure and tread wear, but those are relatively minor.

There I go thinking again...
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Old 09-25-13, 04:27 PM   #2
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I think that would be no more accurate than the computer charts. Still to many variables- rim width, seating depth and the ones you mentioned. Just do the roll out if it matters that much to you. I've gone to gps on my main bikes and find that it's close enough for me.
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Old 09-25-13, 08:34 PM   #3
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There are three basic variables contributing to the effective circumference/diameter. 1: the size of the tire (width/diameter), 2: the pressure used, 3: the load (the total weight of the bicycle, rider, and cargo). The roll-out method (done properly) takes into account all of them. The tire diameter takes only one of them into account.
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Old 09-26-13, 07:49 AM   #4
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njkayaker, you forgot about rim width.
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Old 09-26-13, 08:40 AM   #5
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I didn't consider rim width as a factor. It does still seem to me that a specific measurement from the tire maker would be a more accurate measurement than the chart that comes with the computer.

Quote:
Originally Posted by Bluechip View Post
Just do the roll out if it matters that much to you.
Actually, it really doesn't matter that much to me, as long as my measurement is close enough so that I don't miss a turn listed on a cue sheet. Mostly it was just a thought that has occurred to me and I wanted to see if it was a good thought or not.

I just got some new tires to put on my tandem, so I'll do a rollout and see how much it differs from the chart.
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Old 09-26-13, 08:53 AM   #6
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Like the others said, rim width.

Also inflation comes into play. A 150 lb guy on the same 700x25 tires and wheels at 120 psi is going to have a different reading than my 210 lbs at 100 psi.

I inflate my tires to my normal riding pressures, and put my weight on the bike when doing the roll-out measurement.
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Old 09-26-13, 10:49 AM   #7
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What is the best way to do an accurate roll-out measurement?
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Old 09-26-13, 11:42 AM   #8
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Quote:
Originally Posted by noglider View Post
njkayaker, you forgot about rim width.
Well, roll-out deals with that too!
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Old 09-26-13, 11:44 AM   #9
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Quote:
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Well, roll-out deals with that too!
Yes, it does. I thought you wanted a complete list of the major factors of tire circumference.
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Old 09-26-13, 12:07 PM   #10
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Quote:
Originally Posted by noglider View Post
Yes, it does. I thought you wanted a complete list of the major factors of tire circumference.
For reasonable differences in rim width, rim width might not matter. It might not be a "major" factor at all.

Ignoring rim-width, for 23-28 mm tires, the difference in circumference is < 2%.

===================

http://www.cateye.com/files/manual_d...0DW_ENG_v3.pdf

23 mm -> 2096
25 mm -> 2105
28 mm -> 2136

http://media.cannondale.com/media/up...uter_FINAL.pdf

23 mm -> 2096
25 mm -> 2105
28 mm -> 2136

http://www.planetbike.com/files/8003manual.pdf

23 mm -> 2105
25 mm -> 2105 (?)
28 mm -> 2143

http://service.specialized.com/collateral/ownersguide/new/assets/pdf/1_13_Spdzn_comp_ManualUS.pdf

23 mm -> 2102
25 mm -> 2113
23 mm -> 2138

https://www.google.com/url?sa=t&rct=...53217764,d.dmg

23 mm -> 2133
25 mm -> 2146
28 mm -> 2149

===================

The equation appears to be something close to:

622 + 2 * width * pi

Last edited by njkayaker; 09-26-13 at 12:31 PM.
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