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Gear Inches calculator

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Gear Inches calculator

Old 05-31-21, 10:01 AM
  #1  
TejanoTrackie 
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Gear Inches calculator

I see a lot of posts regarding Gear Inches on this forum, and decided to develop a formula that can be used to calculate Gear Inches without the need to use an online calculator. Also, some rim sizes are not included in these online calculators. The formula is as follows:

GI = (CR / COG) x (BSD + 2 x TW) / 25.4

where:

GI = Gear Inches

CR = Number of chainring teeth

COG = Number of cog teeth

BSD = Bead Seat Diameter in millimeters, which is effectively the inner diameter of the tire when mounted on the rim

TW = Tire width in millimeters, which is also assumed to be the tire height

So, how does this compare with popular online gearing calculators, such as BikeCalc.com ?

Suppose we have a 700x23c tire. This fits a rim with a 622mm BSD. Let's say we are using a 46T chainring and a 17T cog. The Gear Inches per the formula is:

GI = (46 / 17) x (622 + 2 x 23) / 25.4 = 71.16 , whereas per BikeCalc.com it is 71.27 , which is a difference of 0.15 %. Interestingly, if I use 623mm for the BSD, then the results are the same. Anyways, this formula seems to give reasonable results. I you have a programmable calculator with a root solver, then you can set this up as a simple menu where you enter the known variables CR , COG , BSD and TW, and then solve for GI. Yeah, I know, pretty nerdy stuff, but hey I'm a nerd and proud of it !

Enjoy
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Old 05-31-21, 10:32 AM
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Oddly enough, 0.15% is probably less the uncertainty of how much your tire is squished by your weight.

You might be able to find out the formula that BikeCalc uses... many of these simple "calculator" pages are written in Javascript, so you can look at the page source and find the formula. You usually have to look past all of the code that's sending your personal data to a dozen different tracking sites.

What I do is measure the diameter of my wheel with a yardstick and multiply by the gear ratio.
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Old 05-31-21, 10:40 AM
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Close enough is Chain ring teeth divided by sprocket teeth, multiplied by the actual diameter of the wheel in inches.

My 700c x 25 measures around 27 inches across. If it squishes to 26.5 under my weight then that makes a difference of less than 2%.
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Old 05-31-21, 11:34 AM
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Originally Posted by Mikefule
Close enough is Chain ring teeth divided by sprocket teeth, multiplied by the actual diameter of the wheel in inches.

My 700c x 25 measures around 27 inches across. If it squishes to 26.5 under my weight then that makes a difference of less than 2%.
This is actually the true definition of Gear Inches. According to the formula, your rim + tire diameter should be about (622+2x25) / 25.4 = 26.5", so either you've got a rather tall tire or your measurement is a bit off.
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Old 05-31-21, 11:59 AM
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Originally Posted by Gresp15C
You might be able to find out the formula that BikeCalc uses... many of these simple "calculator" pages are written in Javascript, so you can look at the page source and find the formula. You usually have to look past all of the code that's sending your personal data to a dozen different tracking sites.
I just checked the BikeCalc.com webpage (Ctrl U), but it just shows the results for the page based on the selection, and not any formulas. Maybe, I'm missing something, so let me know if you can find the formulas someplace.
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Last edited by TejanoTrackie; 05-31-21 at 12:35 PM.
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Old 05-31-21, 03:48 PM
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Originally Posted by TejanoTrackie
I just checked the BikeCalc.com webpage (Ctrl U), but it just shows the results for the page based on the selection, and not any formulas. Maybe, I'm missing something, so let me know if you can find the formulas someplace.
You're right. I can't figure where the code is located, if anywhere. Maybe the server is calculating it.
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Old 05-31-21, 04:00 PM
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Here is his explanation.

BikeCalc.com - How to calculate Bicycle Wheel Size

He uses the same formula.

Otto
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Old 05-31-21, 06:16 PM
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Originally Posted by ofajen
Here is his explanation.

BikeCalc.com - How to calculate Bicycle Wheel Size

He uses the same formula.
Interesting. I wonder why his tabular values for Gear Inches are slightly different. Perhaps he rounded wheel diameter and gear ratio to two decimal digits before multiplying them together. When I do that, my results agree exactly with his.
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What, Me Worry? - Alfred E. Neuman

Originally Posted by Dcv
I'd like to think i have as much money as brains.
I see the light at the end of the tunnel, but the tunnel keeps getting longer - me
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