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is it possible to combine an indexed shifter with old friction derailleurs?
As the cable is moved by the shifter, I understand that derailleurs should work by default and by moved with mm that the cable is pull.
Is problem related to how much cable pull the derailler requires for each shift of sprucket? Is there restrictions/constraints for mixing index and friction?
This probably won't work. If it does, it means you've found a lucky combination of parts and also gotten lucky setting the cable tension.
You are correct that the derailleur's cable pull ratio will be critical to getting good shifting, and you would need to find an old derailleur that had the correct ratio for the rest of your setup. Perhaps someone on the Internet has put up a web page showing the ratios for old derailleurs, but I've never seen it.
Derailleurs designed for indexed shifting also have a barrel adjuster that lets you fine-tune where the derailleur pulleys sit below the sprockets when under tension. Older derailleurs didn't have barrel adjusters at all. It would be extremely difficult to set the cable tension so that the pulley position was perfect.
Just to clarify what Adam wrote:
The OP is correct that an indexed shifter will pull a fixed1 amount of cable per click. That's usually called the cable pull or similar.
The derailleur has an actuation ratio or pull ratio: it moves laterally by several mm per mm of cable pulled. For example, this post by Art's Cyclery documents that Campagnolo 10s shifters pulled (an average of, see footnote if interested) about 2.8mm per shift. Their derailleurs had an actuation ratio of about 1.5. Multiply those numbers and you should get the sprocket pitch, or the distance between sprockets, which is reported as 4.15 for 10s Campagnolo cogs. In any case, we don't know the actuation ratios for older friction RDs. Basically, there's a good chance they would not work with an indexed shifter. Better to scrounge a bar end or downtube shifter.
In some cases, I understand that it is possible to do the reverse of what the OP asked: take an indexed RD, and pair it to a friction shifter. You just keep pulling the shifter until a shift occurs, although above 10 cogs, the more precise you have to be.
Footnote 1: Campagnolo Ergo shifters do pull a variable amount of cable per shift.
