You hit the big points with your questions, driveline/chainline, stay length and stay indentation. You should try to get chainstays of at least 16.75 inches, and as much indentation on the drive side stay as you can get.
The problem, of course, is that the third chainring forces the whole crank assembly outboard. The determining factor is how much clearance there is for the granny ring. If you have fat, non indented chainstays, it can take a looong spindle to get clearance.
And that, of course, messes up the chainline. In particular, the big chainring/big rear cog is often unusable, or at least very noisy. Likewise, the small chainring small cog is often tough to use. Of course usually you don't use this combo anyway, as the derailer often can't wrap enough chain.
And chainline problems are exacerbated by short chainstays. Real shortys make the chain angle on the crossover combos significant, and seems to really degrade shifting. So longer chainstays are better, with 17 inch being the minimum for optimal performance according to "conventional wisdom". I've made it work with shorter, but lost the two big cogs with the big chainring, not just one cog. Bikes with longer stays do seem to work better.
The ideal arrangement is enough indent on the chainstay to permit the middle crank ring to line up with the middle of the rear cogs with long enough stays to minimize cross over problems. The best shifting triple I ever had was my Trek 720 with the 18 inch stays, but Ohh! that bottom bracket flex! Sometimes you can improve things by using a smaller granny ring, which can give more clearance, depending on the exact shape of the chainstays, and permits a marginally shorter BB spindle.
One other option is the super wide double mentioned by Brandon Ives this week, running something like a 34 or 36 x 48 or 50 set of chainrings up front, giving lower gears without the complexity of a triple. But then you have bigger gaps in your gear ratios and the low gear isn't quite as low.
Good luck!
Tom Adams, Shrewsbury NJ
>From: rodk3d@attbi.com
>To: classicrendezvous@bikelist.org
>Subject: [CR]difference in frames for double vs. triple crank
>Date: Mon, 03 Mar 2003 14:52:19 +0000
>
>Hi all,
>What are the differences in a frame (if any) that have a double or triple
>crankset?
>
>Living here in Colorado along the "foothills", not being as young as I used to
>be, blah blah blah, I find bikes with triples to be nice. They aren't necessary
>for a lot of rides in the mountains, but when we hit em hard, it's either a
>granny gear or walk!
>
>So, will most all frames work with a triple crank? Will the chainline be ok?
>Does the chainstay need to be indented? Is there anything specific I should
>look for in a frame that I intend to build up into a triple?
>
>thanks,
>Rod Kronenberg
>Fort Collins, CO
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