07-09-10, 05:02 PM
I now have several thousand miles on my home made 39 tooth cog http://share.ovi.com/media/currentresident.bicycle/currentresident.10233 I had to do a bit of filing on the derailer to get it to shift from the 28 tooth cog. The derailer shown has a maximum jump of 10 teeth so I may have one made with 40 teeth and use a 30 as the next largest. I would like to have it offset like the 33 tooth cog that Ritchey made for santana. The derailer shown a shimano megarange works fine on this setup however regular shimano derailers won't. I think you can use a 39 if you have a 160 mm rear spacing. I also made a spacer out of plexiglass it is as large as possible without interfering with the chain. The largest cog always deflects toward the next largest cog so the spacer is in compression and reduces the flex on the largest cog it also runs truer. With antiflex spacers I believe the largest cog could be made out of aluminum though care would need to be taken to prevent the splines from shearing. This could be prevented by bolting the largest cog to the next largest cog. The 28 to 39 jump is a very practical one and I see no need of using a 34 between them.