Originally Posted by
Retro Grouch
It can be done but, trust me, every single step is going to be a lot more involved than you are envisioning.
Speaking as a guy who installed a SRAM S-7 in a Motobecane Messenger it wasn't that difficult.
1) Decide what hub you want
2) Build it into a wheel or have bike shop build it into a wheel
3) Figure out how you plan to mount the shifter. (I used some scrap steel to fab up a bar end mount for the S-7 twist shifter)
4) Spread the rear triangle. (Sheldon's website can tell you how)
5) Replace BB with a new one with the correct length spindle. (in my case a 118mm did the trick)
6) Zip tie shift cable to downtube and chainstay.
7) Go ride!
