Originally Posted by shane45
I need to know why Sheldon indicates that it is not OK to shift the front der. first, from the big to middle chainring - before shifting down on the rear cogs. That is my ONLY question.
If I am in the big front and small rear, why is not OK to shift to middle front without touching my rear??? My chainline is still fine, and the tension is fine. Sheldon seems to indicate that this is a no-no and we should be shifting down on the back cogs before switching the front chainring. What is wrong with going from big front / small rear to middle front / small rear using only the front derailleur?
It is the assumption that you would use that particular gear combination - ie middle-front & small-back - until shifting again. Purely shifting is fine, of course. But using that combo - you are accelarating your chain and cog wear compared to using the same gear ratio with bigger cogs due to the chain tension being higher.
[edit]:
I can only suppose Sheldon calls this shift pattern "bizarre" because experienced riders would a) downshift at the rear because this gives a smaller jump and b) know about the wear issue and avoid this. I myself shifted in this "bizarre" pattern when I got my first bike, and rapidly buggered the chain and cassette's smaller cogs, as well as wearing the middle chain ring. Lesson learned the hard way, in the hip pocket.