Originally Posted by 79pmooney

It is verboten to mention the word "drag". (Florida will be banning all public university engineering textbooks that mention it next year. )

And "seriously" - why is a triple such an impossibility? 34-52-70? All existing rings (I am assuming you have a source for that 70). An 18 tooth jump is a lot but has been done. (I have a 24-42 jump on one bike. An ordinary SunTour AR derailleur.)

Another consideration - the rear derailleur. I know you said you wouldn't use many of the cassette cogs while on the big ring. I've heard that a few times before. I've also seen what happens with a momentary brain fart when gassed at the top of big hills and starting the roll into that wonderful descent. (That is why you are doing this, isn't it?) That one little pedal stroke while you remember "Oops! Gotta shift in back first!" will only cost you a rear derailleur, a couple of spokes (maybe enough to make the wheel unridable), probably the derailleur hanger, maybe the dropout, probably chainstay paint and maybe deeper. I know, just incidental stuff.

My advice? Get a rear derailleur that can handle that entire range from the biggest ring to biggest cog all the way to the small ring and the smallest cog; at least without destroying itself. If you are smart, you won't push the gear limits beyond what your rear derailleur can do. (You can consider inventing/making derailleur modifications or buying them such as a longer derailleur cage arm.)