Originally Posted by
VegasJen
Wow. Y'all got way more into the weeds than I even imagined. I admit it would be far easier to simply change the RD and add a cassette with a larger final gear. And that may end up being an option. The only reason I'm even considering this front chain ring scenario is I intend to change the crank to get shorter crank arms. The bike currently has 170s and I want to go with 160s on this bike. I have done some experimentation already on crank arms and I've learned a little in that regard. For one, I have learned I'm more of a grinder than a spinner. I tried 150s on another bike and absolutely hated them. I feel like I gave up all my leverage. I have 165 on my Shiv and really like that length. I think ideal for me is 160-165 range. This bike currently has 170s, which aren't too bad, but I have short legs and I get both rocking in my hips and close up my hip angle with that long a crank. That 10mm doesn't seem like it would make that much of a difference but I can't help feeling like the 170s are just a wee bit outside my comfort range.
Studies have confirmed, folks can tolerate crank arms a bit too short, but even a bit too long causes ergonomic problems. Thus, crank calculators (formulas, online apps) now calculate cranks based on height and inseam for a bit shorter arms than they used to. But sounds like you really have it dialed in on what length arms you need, that's good. And you can always go shorter arms, on a bike designed for longer, you just have better pedal clearance; The opposite is not always true.
Bigger low cog on the cassette with RD that can handle it, is the easy route these days.
If you are simply "adding a gear" in back, no seeming downside. But if you are increasing your rear cog spacing (in teeth) to get that lower-low, the question is whether you will miss the finer spacing.
So your instinct to go with "wider" chainrings, is not off. But 4 teeth in front will make less difference than 4 teeth in back. So you should play around on a gear calculator with changes at crank and cassette, to see where that goes. With gear-calcs online, no more crunching the numbers by hand and endless tables; You can explore different scenarios in a few minutes and have solid conclusions.
Another note: Some folks like a big overlap in gearing, so on a 7 speed cassette with 2X crank, you have 6 duplicate gears between chainrings, this minimizes shifts up front. I'm the opposite, to me, too many duplicate gears is wasted, a loss of potential total gear range; I want at most, 5 dupes out of 7, and preferrably 4 out of 7, so I go for as "wide" a crank as I can find. I wanted to mount a triple, wouldn't fit on my bike due to fat seat tube, FD adaptor, etc. So I was happy to discover 50/34, that's almost as wide as the 52/42/30 I had wanted, but actually fits, and way superior to old 52/42.