Originally Posted by
saulgoldie
I'm vexxed. I'm working on an older bike with friction shifters and derailleurs, circa 1980s. (OK, itsa Trek.) The derailleur mounts directly into the dropout without a separate hanger like you see on cheaper bikes. When I shift the rear derailleur to the low cog--the LARGE one on the inside, it shifts UP fine. But when I want to shift it DOWN one gear to the second largest cog, it hangs up.
(Generally steel frames don't have separate derailleur hangers, while aluminium and carbon frames do have them.)
(Shifting from the largest freewheel sprocket to a smaller one is "shifting up" - the direction refers to the drive ratio, not the height of the sprocket.)
One possible cause of your shifting problem is that the sprocket is slightly too large for the derailleur - is the top pulley very close to the large sprocket? Some older derailleurs were only officially rated to handle a 25 tooth freewheel. Aggravating factors could be short chain, dragging cable, sticky pivots on the derailleur or weak return spring. Clean and lube everything and see what happens.