Originally Posted by
Fissile
I take it your frame doesn't have brazed-on shifter bosses, since you seem to be looking at clamp-on downtube shifters? I've got some bad news for you, the clamp on shifters will probably not work either. The old bikes that used clamp-on downtube shifters had a small tab (usually triangular in shape) brazed to the downtube. The clamp-on shifters were installed so the edge of the shifter clamp was in contact with the edge of the of the tab brazed to the downtube. Without this tab, the clamp-on shifters will slowly work themselves down from a combination of road vibration and shifter cable tension. The result will be ghost shifting.
Don't think you can just tighten the crap out of the clamp either, you'll probably end up crushing the downtube.
If your bike lacks both the downtube shifter bosses, as well as the clamp-on shifter tab, here are your options:
1) Strip the paint off the frame and have a welder braze on downtube bosses and then repaint the frame.
2) Use brifters.
3) Use bar-end shifters. I love bar-end shifters, I've got them on two road bikes.
Based on your description of the problem, #2 and #3 won't work either. Both brifters and barends require cable housing stops on the downtube and the most effective of these rely on downtube braze-on bosses as attachment points. There are non-adjustable clamp-on cable stops but they won't stay put any better than clamp-on dt shifters if the frame doesn't have the little locater tab.
I agree with Tim and fuzz2050, even without the tab, dt shifters will remain in place if you tighten the clamp firmly. It's not necessary to crush the downtube to do so and most frames cheap enough to lack braze-on bosses will have a pretty thick-walled downtube anyway.