Dear John:
The smallest normal inner ring on a TA is 26 teeth. So I presume you're imagining two outer rings. One, I don't know of a supply for the micro outer rings, and two, the TA arms are not set up to clamp two outer rings to the arms. So you would have to set up everything from scratch: get a longer set of the 5 fixing bolts, find some kind of spacers to space the inner ring, and then try several bottom bracket spindles to get the sizing right. I have no experience with this kind of set up, but I wonder if a vintage frame will have clearance/chainline issues with such a tiny chainring.
It's also hard to envision a riding situation where a 32 or 34 tooth rear cog and a front 26 would not be sufficient. Perhaps you are trying to micro size you're rear cluster? I tried this once with a short chainstay bike with a 13-24 rear and a 52/42/24 front triple, and never was happy with chainline or shifting. A better set up to emulate might be Sheldon Brown's wide range double with the TA cranks on his Hetchins: 50x28 with a 12-28 cluster for a 27 inch low gear.
http://sheldonbrown.org/
A 32/34 rear cog would get you a serious low gear with decent chain line. I suspect though that longer chainstays make this work better.
A 20 tooth front chainring would be an interesting project, but I'd be skeptical of how well it would work, and I suspect the 20 tooth ring would wear very quickly indeed. And getting a derailleur to shift it would be tough, especially as the TA arms don't like modern knobby caged front derailleurs. The arm sits so tight to the chainrings that vintage straight up and down front cages are the rule.
Good luck.
Tom Adams
Manhattan, KS
From: John Wood <braxton72@gmail.com> Subject: Re: [CR] TA Pro5, Stronglight 49 Crank/Chainring (and shifting) Questions To: "John Clay" <jmedclay@yahoo.com> Cc: "Classic Rendezvous" <classicrendezvous@bikelist.org> Date: Monday, December 28, 2009, 2:56 PM
On Mon, Dec 28, 2009 at 11:34 AM, John Clay <jmedclay@yahoo.com> wrote:
>
> The context of all of this is that I'm thinking about making a 44-20ish
> touring double. Have any of you ever set up anything like that? If so, did
> it shift reliably? I'd expect the shifting to be anything but crisp and
> quick, but it needs to be reliable, non-jamming. Do any of you have any
> insight or experience with similar arrangements?
>
Hi John, The closest I have come to the above is a 46/29, and that shifted fine, but I did have to play around a bit with front derailleurs. I wound up using a mid to late 90's Campy Chorus FD. I would think the ideal FD for the set up you describe would be the new SRAM XX, assuming you can make it work with whatever shifter you're using.
--
John Wood
Missoula, Montana, USA