Originally Posted by
alcjphil
Exactly what bike are you asking about? Sure, bicycle frames flex, some more than others. My 1973 Raleigh Professional was flexy compared to some other bikes back then, but I never experienced "phantom shifting". There are steel bikes that are extremely stiff, my steel Bertrand touring bike is the stiffest bike that I own. Steel bikes are not necessarily flexy. Stop blaming the material your bike is made of. Your phantom shift problem is not the fault of frame material
I have 1985 Peugeot PGN-10. The steel flexing as a cause was mentioned more often that other when I was looking through old threads. Now, reading your and other responses, will go to back to double check cables, guides, and rear derailleur setup.