Which downtube friction shifters work with 2021 Campagnolo 11-speed cassettes, and how well? Another question from a few years ago asks about Shimano 11-speed derailleurs; I have a related question about current (2021) Campagnolo equipment - I'd like to understand what downtube (friction) shifters will work with a Campagnolo 11-speed derailleur.
What is the cable pull required between top and bottom gears, and is it the same as for 8/9/10-speed systems? I thought that the total derailleur travel was the same, just with narrower spacing between gears, but I see that Dia-Compe make a dedicated 11-speed shifter with a larger diameter for greater cable-pull.
How well do friction shifters work with Campagnolo 11-speed cassettes (if at all)? And which ones?
Friction shifters are infinitely adjustable - by definition they don't have stops or detents for each gear position.  If they did, they'd be called &quot;indexed&quot;
So there are only three attributes your friction shifters need to run effectively

Total range - the lever must be able to pull enough cable to actuate the mechanism across the whole cassette.
Accuracy of your fingers/ears - The rider has to be able to place the chain in-gear without rub, while riding.  On a 5 speed block there's a lot more space between cogs, and the clatter is obvious.  For a 12 speed block, the cogs are closer together so the sweet-spot is smaller.  Can you find it by feel while riding?
Resistance - the friction lever needs to be able to hold its position and not drift, which would give you a spontaneous up-shift.


An addendum - the frame/bars needs some way to hold your friction shifters.  Historically that was a set of brazed-on mounts, but a modern frame won't have them so you'll need to use a band-on set of mounts that can fit around your down tube.
Stem shifters are another option, and bar-end shifters are also available in indexed or friction mode.