The real problem with top of the bb cable guides is that, even if they are properly brazed on, they can eventually pull off. The corner of the front derailleur guide is particularly subject to this problem. It is a natural stress riser with all the force of the cable pull concentrated on one point. This is the primary reason framebuilders stopped offering them sometime in the early 80's I think. As a painter, I have a little box with these braze-ons I have kept that have come off of a variety of frames. I could see when they were off that the brazing was complete. So it is not an issue of brazing quality but of faulty design. A lot of tension can be put on the cable if the derailleur isn't inclined to move. I've also seen a lot of bent ones from strong cable pull. My guess is that about 5% have failed.
A customer is going to demand a redo which might involve a repaint and reassembly - which can be several days of lost work. I switch all top mounted guides on my frames if they came in for a repaint. Fortunately there were only a few I did that way in the 70's. I was influenced by my 69 and later Hetchins (that used under bb guides) and I really liked the clean lines of a cable going underneath. As already mentioned, it is impossible to get paint properly between the top mounted boss and the tube and if it is a candy color, that looks really awful.
Doug Fattic, not wanting to do free work because of a faulty design in Niles, Michigan, USA
On 10/20/08 8:37 AM, "classicrendezvous-request@bikelist.org"
<classicrendezvous-request@bikelist.org> wrote:
> Date: Sun, 19 Oct 2008 22:59:23 -0400
> From: marcus.e.helman@gm.com
>
> Why did framebuilders start running cables under the bottom bracket? It
> seems to offer no advantage. I have one bike with the cables run on top
> of the BB, and one with the cables running underneath. There is no
> difference in shifting that I can tell. Maybe it's just esthetics.
> Personally, I prefer the more traditional top of BB treatment.
>
>
> Wondering,
> Marcus Helman
> Detroit, MI