Originally Posted by grolby
Hi there. I just happened to notice on the internal gear calculator on Sheldon Brown's webpage, and the fact that you can enter up to three different chainring values! That seemed a little crazy, but I entered some values, and holy cow! Combining a 46/36/26 crankset with an 18 tooth cog on the Shimano Nexus 8-speed hub would give a range of 22.8 to 101 gear inches on a bike with 26-inch wheels. Thats 443%! That's roughly comparable to using the same crankset with a Megarange freewheel or cassette, and has obvious benefits in a touring application.
Now, unless you're getting really high-zoot derailers and cogs, it's probably more cost effective to go with the derailer gearing option. The use of integrated brake/shift levers could easily push the cost the other way...
There would be no reason to use such fancy shifters. With a system like this, you would do most of the shifting with the Nexus shifter, and only use the derailer to select high-medium-low range. This could be handled with a simple $12 thumb shifter or down tube shifter.
I've been seriously toying with doing something like this myself, probably using a 52-42 double and an 18 or 19 tooth sprocket, giving me a 2.5-9.3
Gain Ratio (33-125 inch) or 2.3-8.8 (32-118 inch) range.
Note that you
must have a rear derailer as well as a front, to take up the slack. A typical overpriced singlespeed chain tensioner won't have sufficient take-up capacity to handle even a 10 tooth jump, much less the 20 tooth you propose. On the other hand, since ther'll only be one rear sprocket, you don't need a wide range one. I've got an old Hurét Jubilee set aside for this project, the lightest rear derailer ever made (and, by all accounts a pretty poor shifting one as well, pretty as it is.)
Sheldon "Sometimes Uses Gears, But Rarely Uses Derailers" Brown
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| --William Blake |
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