Gear ratio calculator

[quixoticgeek]

I've tried using sheldon's gear ratio calculator to calculate the setup that I have on my Brompton, but it doesn't give the same numbers as the Kinetics spec does. I've tried searching for other calculators, but it seems that they either don't understand wheels below 20", or in the case of this calculator It just ignores the input options I gave it, and gives me the BWR results...

I have a 35-349 tyre, the Sturmey Archer XRD8(w), a 33 chainring, and a 20T sprocket.

What gear lengths do I have?

How do they change if I change from a 33T to a 28T chainring?

Cheers

Julia

[quixoticgeek]

Managed to get this gear calculator to work. Still gives me different numbers to either Sheldon or Kinetics...

*headdesk*

J

[pdm]

Just roll your own calculator on a spreadsheet.
Measure the outside tyre diameter on the wheel. (d -  should be about 16 inches in your case - often quoted at 16.375")
Set up some columns to indicate the number of sprocket teeth (s) and a row each number of front chainring (c) teeth.

In each cell, the inches gear ratio will be: d x c / s

e.g. for d=16.375":

        21     19     17     15     14     13     12     11
39  30.4   33.6  37.6  42.6  49.6  49.1  53.2  58.1
53  41.3   45.7  51.1  57.9  62.0  66.8  72.3  78.9