Thorn Cycles Forum
Community => Rohloff Internal Hub Gears => Topic started by: Thomas777 on August 13, 2019, 05:15:13 pm
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Considering changing my wife's bike to belt drive but not sure how to calculate gear inches.
Advice would be great!
Thanks!
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Same way as with a chain
Chainring teeth divided by sprocket teeth, multiplied by wheel diameter.
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Considering changing my wife's bike to belt drive but not sure how to calculate gear inches.
Use the same formula as usual for calculating gear-inches: Ft/Rt * Wd [Number of teeth on chainring divided by number of teeth on rear cog or sprocket multiplied by the outside diameter of the mounted and inflated tire (or a nominal equivalent if you aren't fussy about accuracy).
It all works out the same. For example (with a nominal 27in wheel)...
40t x 20t = 54 gear-inches
20t x 10t = 54 gear-inches
10t x 5t = 54 gear-inches
As for Gates' take on ratios compared to gear-inches, they provide a useful comparison here: https://www.gatescarbondrive.com/~/media/files/gcd/oemfactoryresources/gatesshimanomanualen.pdf?la=en
There are not direct equivalencies on belt drive to chain drive, but pretty close. For example (again calculated with a nominal 27in outside wheel diameter):
46 x 22 = 56.45
Gates offers the close equivalent: 38 x 18 = 57
50 x 22 = 61.36
Gates offers the close equivalent: 40 x 18 = 60
55 x 22 = 67.5
Gates offers the close equivalent: 44 x 18 = 66
60 x 22 = 73.63
Gates offers the close equivalent: 50 x 18 = 75
You'll note that like gear-inches, the ratios are also only approximately equivalent but still close. For example, a 50 x22 in chain drive gives a ratio of 2.27 (F/R) vs a belt-drive equivalent (40x18 F/R) of 2.22. This is because there are not so many tooth options available in belt drive as in chain drive.
Best,
Dan.
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Daň,
Thanks for your reply.
I must be having a brain freeze because I don't get it!
Also trying to figure out the range of gear inches in her current setup but I can't seem to find a gear inch calculator. Currently 16/36.
Thanks!
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Hi Tom,
Here's a web-based calculator: http://www.gear-calculator.com/ (http://www.gear-calculator.com/)
Here's Sheldon Brown's calculator: https://www.sheldonbrown.com/gear-calc.html (https://www.sheldonbrown.com/gear-calc.html)
The drill for the Rohloff is this:
Chainring teeth (yours, 36) divided by sprocket teeth (yours, 16) = 2.25
2.25 x wheel diameter (inflated tire, outer edge to outer edge) - let's say 26" = 58.5
58.5 wd thus be your gear-inch value in 11th gear, the Rohloff direct drive.
For the gear-inch values of the individual gears, multiply 58.5 by the Rohloff gear ratios:
1st: 0.279 2nd: 0.3.16 3rd: 0.360 4th: 0.409 5th: 0.464 6th: 0.528 7th: 0.600
8th: 0.682 9th: 0.774 10th: 0.881 11th, of course, 1.00
Then, the "overdrive" gears: 12th: 1.135 13th: 1.292 14th: 1.467
Hope that does the trick,
Cheers, John
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Thanks! I will play with it.