Rear Shifting Problem.
 

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.



I tried a different derailleur. I took out a link from the chain. I adjusted the angle of the dangle. I adjusted the inside limiter so that it just doesn't go to the largest cog, and it works fine. It is just that large cog. It has 28 teeth. I have not tried a long cage derailleur. But I am not hopeful about that. I have never seen this before after working on dozens of bikes with all sorts of configurations. I'm out of ideas.



I'm hesitant to take it to a shop, because I am not confident that the mechanics have much experience with older machinery. So...??

Saul

Saul, what do you mean that it "hangs up"?

The first thing I suspect is that the guide pulley (the top one) is hanging up on that biggest cog. If there's not enough "B" gap (space between large cog and guide pulley) to let the chain lift off the cog's teeth during a shift the chain will act as a sandwich or splint trapping the two together.

The first ways to avoid this is with components that are meant to work together. Like a rear der whose large cog capacity is the same or greater that the large cog of the set. sounds like this isn't the case... is it?

A few points-
When shortening the chain make sure you can still shift into the big/big cog/ring combo. Serious damage can happen if the chain was found to be too short and you forgot to not shift into the big/big.
A long cage rear der might not have a claimed largest rear cog size greater than it's short cage sibling. Shimano road ders on the early/mid 1990s, when they offered SS or GS cage lengths of their 105, 600 and DA models, had the same 27 tooth max cog limit. The long cage only increased the chain wrap/take up spec.
When exceeding the chain wrap capacity, it's usually "safer" to have a slightly too long chain that droops on the small/small combo than a too short chain.
Make sure any der pivot springs are working. Some ders have both the B and the A (B is the hanger axis pivot and A the cage's) sprung, some only the A pivot is sprung.
If only tiny amounts of B gap are needed more one might see if the guide pulley could be replaced with a smaller diameter version. Andy

(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.

Good idea.

I've been fixing bikes since 1971, but I have to tell you I have no idea what your problem is until you provide a better description of what you're using and the problem, or post a video or some photoes.

B screw

Nailed it. Next question!

Cheap steel frames do have a separate hanger, which is typically attached in the dropout slot with a nut/bolt.

I wonder why that doesn't seem to count, I guess it's because to me the "claw" is part of the derailleur rather than part of the frame - many bikes that use them have cheap derailleurs that have an integral hanger. But I have an old steel frame for which I had to buy a claw when I built it up. Strange.

As others have suggested, a pic or two would provide more to diagnose.

On my '72 MotoGR, going from a 28 in back to a 32 or 34 cog proved the limitations of the original Campy DR. So I found a 'donor' Fuji off which I borrowed its Suntour V-GT Luxe. That worked fine with the larger cog in back. Even with that though I got smoother shifting after I added a WolfTooth aluminum hanger & messed a little with the pesky B-Screw setting and chain length.