SingleSpeed Gear Ratio Calculator
This little page was created for those of you who would like to turn your bike with vertical dropouts into a singlespeed or fixed gear without a chain tensioner. In the case of a singlespeed, this creates a much cleaner and more reliable setup than a tensioner. In the case of a fixed gear (no freewheel) it is actually IMPOSSIBLE to use a chain tensioner, because the tensioner will not work when you are resisting the motion of the pedals.
Alright, enough of that. More than likely, if you're here you know all that stuff above. So, on to the meat.......
How To Use the Calculator
To use this calculator, you will need to provide 2 different numbers, and then you will get an output of possibilities. The two numbers are the Number of Teeth on the Front Chainring and the Center-to-Center Chain Distance.
The number of teeth on the front chainring is a pretty simple. Feel free to try out a bunch of different values if you have the option (for instance a 32, 34, or a 36 if you're using a middle front ring). Just make sure you can actually get a chainring for the size you put in..... (don't put in 32.7, the calculation will work, but it won't help you much).
However, the Center-to-Center Chain Distance is a bit more complicated. This is a VERY IMPORTANT measurement. You need to mount a back wheel in your frame, and preferably at least a bottom bracket, if not a set of cranks as well. You will then need a tape measure which has divisions down to at least 1/8 of an inch, preferably 1/16 of an inch. Very carefully measure the distance from the center of the rear wheel to the center of rotation for the cranks. Be as exact as possible. If your tape measure only has 1/8 inch divisions, and your centers are in between divisions, use the 1/16 measurement in between.
Alright, now just put the two numbers you have into the calculator, and click the "show me the money" button. This page will re-load with a list of possibilities for a rear gear size, along with the gear ratios that this particular combination will use.
Hints for accurate center-to-center distance
-Use a quick release in your rear wheel, but make sure that the end with the lever is on the opposite side from the chain, and it is very easy to find the center-Use a sharpie and mark the center of the cranks while spinning them to find the center of the bottom bracket
