Call me dense, and this may indeed be one of my moments, but I just don't see the problem. If the cage arc has its center on the BB center for the expected chainwheel, and the frame is straight, how can the cage not appear concentric and require the derailleur to be shimmed for a rotation?
Ken Freeman Ann Arbor, MI USA
-----Original Message----- From: classicrendezvous-bounces@bikelist.org [mailto:classicrendezvous-bounces@bikelist.org] On Behalf Of Sheldon Brown Sent: Saturday, July 28, 2007 7:29 PM To: Bianca Pratorius; classicrendezvous@bikelist.org Subject: Re: [CR]Have you ever done this?
Garth Libre wrote:
>Apparently some front derailleurs were made with a certain seat tube
>angle in mind.
Not just "some." Almost all! The only exception I recall was the '70s
Schwinn/Huret model that had the cage secured to the mechanism by an
eyebolt. That's what I used on my Moulton Mk III to make it work with the
72/52 double.
>A touring triple front derailleur, I would guess, was designed with a
>72 -73 degree seat tube angle in mind, and a racing double was designed
>with a 74 or even 75 steep angle in mind.
Actually it's a bit more complicated than that. The angles you cite are angles from the seat tube to the horizontal, but more relevant (and an actual derailer spec) is the angle betwixt the seat tube and the chainstay.
This sometimes causes problems with small wheel bikes (folders and the like)
because the chainstay angle is different for different wheel sizes (assuming
equal bottom bracket height.)
>When you attach a derailleur and tighten down, you can get the lip of
>the fd to not match the curvature of the chainwheel.
The size of the largest chainring also gets into this. See:
http://sheldonbrown.com/
>Fortunately I have a braze on attachment so I fashioned a thin copper
>shim to go between the derailleur and the braze on tab, above the bolt
>that holds everything in place. When I tightened down on the allen bolt
>the derailleur changed it's angle and then seemed to match. This seems
>like a successful mod, (invisable, simple, reversible and cheap). Has
>anyone done this, or is there a better method I could have employed?
I've done similar things, always a bit of a kludge.
I've said it before, and I'll say it again: Front derailer braze-ons are the work of SATAN!
Sheldon "Clamp-on" Brown
+--------------------------------------------------------+
| As far as the laws of mathematics refer to reality, |
| they are not certain; and as far as they are certain, |
| they do not refer to reality. --Albert Einstein |
+--------------------------------------------------------+
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