Thanks Thomas and everybody else for the input! Somebody I ran into (and can't find again) mentioned having a 19t chainring with a 50.4mm BCD for direct mounting to the inside of the TA flange. You could then set up something like a 44/20 to make a dandy touring double.
I'm pretty partial to the range I got with my 44/32/20 and modified straight 8 (13-17, 20, 26, 34), I did need/appreciate the 20/34 more than a few times; it was fabulous in some small choppy mountains I encountered. Lately though it's occurred to me that I could do pretty much the same thing with a double - and the one thing that I didn't like with my triple was the very wide tread. This would solve it, so I'm looking.
Thanks again
John
Tallahassee, Florida
USA
> From: Thomas Adams <thomasthomasa@yahoo.com>
\r?\n> Subject: Re: [CR] TA Pro5, Stronglight 49 Crank/Chainring (and shifting) Questions
\r?\n> To: "John Clay" <jmedclay@yahoo.com>, "John Wood" <braxton72@gmail.com>
\r?\n> Cc: "Classic Rendezvous" <classicrendezvous@bikelist.org>
\r?\n> Date: Monday, December 28, 2009, 6:54 PM
\r?\n> Dear John:
\r?\n>
\r?\n> The smallest normal inner ring on a TA is 26
\r?\n> teeth. So I presume you're imagining two outer
\r?\n> rings. One, I don't know of a supply for the micro
\r?\n> outer rings, and two, the TA arms are not set up to clamp
\r?\n> two outer rings to the arms. So you would have to set
\r?\n> up everything from scratch: get a longer set of the 5
\r?\n> fixing bolts, find some kind of spacers to space the inner
\r?\n> ring, and then try several bottom bracket spindles to get
\r?\n> the sizing right. I have no experience with this kind
\r?\n> of set up, but I wonder if a vintage frame will have
\r?\n> clearance/chainline issues with such a tiny chainring.
\r?\n>
\r?\n>
\r?\n> It's also hard to envision a riding situation
\r?\n> where a 32 or 34 tooth rear cog and a front 26 would not be
\r?\n> sufficient. Perhaps you are trying to micro size
\r?\n> you're rear cluster? I tried this once with a
\r?\n> short chainstay bike with a 13-24 rear and a 52/42/24 front
\r?\n> triple, and never was happy with chainline or
\r?\n> shifting. A better set up to emulate might be Sheldon
\r?\n> Brown's wide range double with the TA cranks on his
\r?\n> Hetchins: 50x28 with a 12-28 cluster for a 27 inch low
\r?\n> gear.
\r?\n>
\r?\n> http://sheldonbrown.org/
\r?\n>
\r?\n> A 32/34 rear cog would get you a serious low gear
\r?\n> with decent chain line. I suspect though that longer
\r?\n> chainstays make this work better.
\r?\n>
\r?\n> A 20 tooth front chainring would be an interesting
\r?\n> project, but I'd be skeptical of how well it would work,
\r?\n> and I suspect the 20 tooth ring would wear very quickly
\r?\n> indeed. And getting a derailleur to shift it would be
\r?\n> tough, especially as the TA arms don't like modern
\r?\n> knobby caged front derailleurs. The arm sits so tight
\r?\n> to the chainrings that vintage straight up and down front
\r?\n> cages are the rule.
\r?\n>
\r?\n> Good luck.
\r?\n>
\r?\n> Tom Adams
\r?\n> Manhattan, KS
\r?\n>
\r?\n> --- On Mon, 12/28/09, John Wood
\r?\n> <braxton72@gmail.com> wrote:
\r?\n>
\r?\n>
\r?\n> From: John Wood <braxton72@gmail.com>
\r?\n> Subject: Re: [CR] TA Pro5, Stronglight 49 Crank/Chainring
\r?\n> (and shifting) Questions
\r?\n> To: "John Clay" <jmedclay@yahoo.com>
\r?\n> Cc: "Classic Rendezvous"
\r?\n> <classicrendezvous@bikelist.org>
\r?\n> Date: Monday, December 28, 2009, 2:56 PM
\r?\n>
\r?\n>
\r?\n> On Mon, Dec 28, 2009 at 11:34 AM,
\r?\n> John Clay <jmedclay@yahoo.com>
\r?\n> wrote:
\r?\n>
\r?\n> >
\r?\n> > The context of all of this is that I'm thinking
\r?\n> about making a 44-20ish
\r?\n> > touring double. Have any of you ever set up anything
\r?\n> like that? If so, did
\r?\n> > it shift reliably? I'd expect the shifting to be
\r?\n> anything but crisp and
\r?\n> > quick, but it needs to be reliable, non-jamming. Do
\r?\n> any of you have any
\r?\n> > insight or experience with similar arrangements?
\r?\n> >
\r?\n>
\r?\n> Hi John,
\r?\n> The closest I have come to the above is a 46/29, and that
\r?\n> shifted fine, but
\r?\n> I did have to play around a bit with front
\r?\n> derailleurs. I wound up using a
\r?\n> mid to late 90's Campy Chorus FD.
\r?\n> I would think the ideal FD for the set up you describe
\r?\n> would be the new SRAM
\r?\n> XX, assuming you can make it work with whatever shifter
\r?\n> you're using.
\r?\n>
\r?\n>
\r?\n>
\r?\n> --
\r?\n> John Wood
\r?\n> Missoula, Montana, USA