At some point I have to upgrade the 7sp drive train. I can't seem to find LX or XT 7 speed shifters. Correct me if I'm wrong on the following....
When going from a 7sp cassette to an 8sp, they add an extra sprocket so that the overall cassette is wider but the spacing between the sprockets is the same. The width of an individual sprocket is the same as well.
When going from 8 to 9sp, the overall cassette width is the same, but they narrow the width of each sprocket and narrow the sprocket spacing (narrower chain).
The overall width of 7sp, 8sp, and 9sp chainring clusters are identical?? Similar to the cassette, the width of a single sprocket on a 7sp chainring set is the same as 8sp, but an indivdual sprocket on a 9sp chainring is narrower than the 7sp or 8sp.
The spacing between the sprockets in the chainring cluster is the same for 7sp, 8sp, 9sp??
Assuming those things are correct, I should be able to use limit screws to set the rear deraileure travel equal to the width of a 7sp cassette. I should be able to use an 8sp shifter and only shift between 8:2. Shifting into 1 wouldn't do anything because the the limit screw would stop the deraileure from moving the chain off the cassette.
I'm not sure if BB widths are a factor, but I'm thinking a 9sp crankset/chainring is compatible with a 7sp cassette. The 7sp chain shouldn't care that the chainring is narrow, and I assume the chainring spacing is large enough for the 7sp chain.
So how many problems would this hybrid setup create?