According to Sheldon's page:
https://www.sheldonbrown.com/nexus7.shtml
The ratios for the nexus 7 are....... .632/.741/.845/.989/1.145/1.335/1.545. which according to my understanding, you are not even 1 to 1 ratio until you hit 4th gear.
To calculate your gearing, divide 39T in front by 18 in rear which gives 4.24 ratio times the diameter of your wheel @ 19 giving a one to one ratio of 41.16. In other words, given a single speed fixed gear, your gearing would be that.
I used 19 inches because I measured my 1.5 406 and they are exactly 19 inches in diameter. Maybe a hair more for the 1.75 tires, but definitely not 20". Anyways that's splitting hairs compared to general calculation for your gearing.
So using the one to one ratio of 41.16 and using Sheldon's gearing ratios given above for the nexus 7, I came up with these gear inches for your current setup:
26"/30.4"/34.7"/40.7"/47.12" /54.9"/63.5"
(Better count your teeth again because this seems extremely low geared!)
So if you go up to a 52 T chainring, 52 divided by 18 x 19" gives a gear ratio of one to one of 54.8
this gives you
34.6" low to 84.6" high gear inches.
Last edited by HighValleyRanch; 072417 at 07:40 PM.
