Derailleurs for bent tube frames...was [CR]Bent tube frame and Jack Taylors in general

(Example: Framebuilding)

Date: Mon, 25 Apr 2005 20:02:28 -0400
From: "HM & SS Sachs" <sachs@erols.com>
To: dartley@co.ba.md.us, classic Rendezvous <classicrendezvous@bikelist.org>
Subject: Derailleurs for bent tube frames...was [CR]Bent tube frame and Jack Taylors in general


Dan Artley wrote, but I <snip>

Another issue I've heard about with the [bent tube] road bike version is the problem with using a front derailleur. The Super LJ? thats mounted on it, I guess because of its long cage still barely reaches below the chain and the photo shows that the chain is on one of the larger cogs in back. I'm not sure if the angle of the derailleur because of the curved tube compromises shifting as well. I've heard that most owners keep it a single chainring in front.

We went through the bent tube front derailleur issues with Schwinn Town & Country tandems, and the field is fraught with ironies. First, bent tube bikes have short chainstays (duh). The T&C was maybe 16" = 41.5 cm. This makes crossing, the use of large front with the inner rears and vice versa, rather problematic with the fairly stiff chains of the era. Second, and more obvious, the seat tube angle at the point where the derailleur mounts is maybe 85 degrees. So the tail of the cage points to outer space. As important, the cage is much too far forward relative to the chainset.

A partial fix is returning to the days of yesteryear with a "plunger" front derailleur, like the Campy GS/Valentino or the plastic Simplex. With most of these, it is possible to persuade the cage to mount at an unnatural angle relative to the body by pivoting on the shaft. Not with a triple, but triple on a bent tube is (shall we say) weird. A couple of times, inspired by Bill Boston, I did something much more radical. I "butchered" Shimano Titlists. Unlike most fronts, these had a separate cage and attachment piece/bottom of the parallelogram. So, I could disassemble, drill out the spot welds, and separate the two pieces. Then I would braze them back together at the correct angle, further forward on the cage. Indeed, with the wide BB of the T&C, I would also put in a short cylindrical spacer to move the cage out further. Once, I even extended the cage with a piece from another one.

I don't know if I still have one of these, but I eventually decided that it made better sense to mount a standard FD on the front seat tube, and run a long chain from there to the rear wheel. Fixed the chain line problem and the need for butched derailler in one fell swoop. Option not available on singles...

harvey "still rusty with the torch" sachs mcLean va