Rear derailleur compatibility for friction shifting

Question

I have a mid-1980s road bike with downtube friction shifters. I'd like to install a freewheel with a larger cog (something like 32 or 34T), but my current rear derailleur won't accommodate that. So I'm looking into replacing my rear derailleur with one that has a greater max capacity.

I've been reading about derailleur pull ratios, and it sounds like if you install a new derailleur with a different pull ratio than the old one, then you also need to install new shifters. However, most of the comments I've seen have been related to index shifters, where compatibility seems to matter a bit more than it does for friction shifters.

Does derailleur pull ratio matter for friction shifters? Can I install a derailleur with any pull ratio and have it work with my current setup, or will I need to swap out other components as well (shifters, cables, chain, etc.)?

If pull ratio is important, how do I figure out the pull ratio for my current derailleur so I can find another one with the same ratio?

Answer 1

It basically doesn't matter among rear derailleurs you might reasonably use in this situation.

It is possible to contrive a situation where the derailleur you chose needs more total pull than your shifter has to give. Basically to do that it would need to be a SRAM 1:1 RD, because they need enough more cable pull than everything else to be in a class of their own in terms of total cable movement.

Choose something like a normal Shimano-compatible nominally 8 or 9-speed mountain or hybrid/urban RD with the cog clearance needed and it will be fine.

If one wants to answer this definitively, it's important to point out that friction shifters as a category have a long history dating back to the beginning of bikes having shifters. Since the whole point of asking the question is to do weird things, it cannot be assumed one is only talking about commonly encountered models. Most friction shifters have enough total pull that they can shift anything, but if you're doing something sufficiently weird then yes you do need to measure the total pull of the shifter and the total pull needed into the RD. In most cases on the RD end you can abstract it by doing nominal cog pitch * number of cogs / nominal actuation ratio to get to a total needed amount of pull, but if what you come up with is borderline then taking a firsthand measurement may be needed since RD movement is nonlinear despite the popular notion of actuation ratios.