So here's how I currently understand things, let me know if any of this is wrong, because I'm trying to (over)simplify the veritable profulgence of options here.

1. The number of speeds of a derailleur is an upper limit of the number of speeds it can actually run, like a 10 speed RD would be fine shifting across a 7 speed cassette, and so on.

2. The "number of speeds" of an indexed shifter is the number of clicky clicks in it, that's all. They are not compatible among different numbers of speeds.

3. A friction shifter can run anything.

I'm especially curious about number 3 because I'm thinking of using an old school friction shifter to control a modern 11 speed RD.

Cheers

So I've got a rather uninformative thread going on in Mechanics about some of your questions.

Here is my understanding, at a certain point, Maybe 9 speed and up, there started to be specific derailleur movements corresponding to specific cable pulls (indexing). Below that threshold, the ratios were the same, above that, you need to match systems. There is some mixing and matching that can be done but don't ask me.

As for friction, it can do anything but may be limited to how precise your hands are.

#3 is fine, #2 is generally good, but #1 needs some tweaking.

All drivetrain systems are originally designed to be run with compatible speed components. So it's assumed you'll be using a 10 speed RD with a 10 speed cluster, shifters and so forth.

You can't take a 10 speed RD and expect it to shift across a 7 speed cluster in indexed mode, the indexing will be off. You can do it in friction mode if the shifter has one, however.

To your questions from easiest to hardest.

2. Yes, the clicks are the indexed positions built into the shifter and typically they're only compatible with parts from the same manufacturer for the same number of speeds.

3. In theory a friction shifter can work with literally any drivetrain - but in practice there are limitations because your shift lever may not able to pull enough cable to swing the derailer all the way through its range, and with 9 or more speeds the gap between gears may be so small that the shifting will be impractically finicky to use. Note that SRAM rear derailers in particular are not going to play nicely with friction shifters, as they require more cable pull per shift than other brands.

1. Derailers don't just vary according to 'speeds' as there are variations how much chain different derailers can wrap, and the derailer for a system with a given number of 'speeds' will be optimized for a particular set of chains and cassettes. If you wanted to use a 10-sp derailer with a 7-sp freewheel you'd need to use a 10- or 9-speed chain that would be narrow enough to fit through the derailer and this chain might give you some odd shift issues depending on which freewheel you were using, so given how expensive these parts are it would make more sense to use an old derailer and 7-sp chain instead.

1. No, actually the upper limit is determined by the amount of travel the RD can handle. As long as it can cover the full width of the rear cluster, you can use it. With a compatible shifter, that is.

2. Yes and no. The number of available clicks is the maximum, but you can use them for fewer speeds. I use 9-speed shifters with 5 and 6-speed rear clusters on some of my touring bikes. The remaining clicks are blocked out by the RD limit screw. As long as the shifter's cable pull matches the RD and cluster combo you are good to go.

3. Yes, but only so far as it can pull a cable. Especially older shifters with a smaller diameter "cable drum" will often reach the 180 degree position before the RD reaches the most inner cogs.

Actually the number of clicks is one less than the drive train speed. An 8 speed shifter, for example, will click seven times. That being said, there are some downtube shifters, without a cable installed, that will click all the way around to the other side of the stop

Here are my basics...

1. Every rear derailleur has a shift ratio, which means how much the derailleur will move for the amount of cable the shifter pulls. For the sake of argument, original Shimano SIS would move 1.7mm for every 1mm of cable pull.

2. Every manufacturer has a different shift ratio than the other manufacturers. Campy, Suntour, Shimano all move a different amount for every 1mm of cable pull. There are some other mfg's made their products compatible.

3. As already stated, derailleur have a range (distance) that they can move that will limit how many cogs can be used.

Based on this, the shifter will pull a certain amount of cable. If it is a friction shifter you can manipulate the amount of swing to shift. The only caveat is a possible limit to the overall throw, 180 degrees, the shifter can move; i.e, Simplex Retrofriction.

An index shifter will pull a certain amount of cable, click, to move a compatible derailleur from one cog to another. The derailleur doesn't care as long as the shift ratio is compatible. So a 6 speed shifter shifts 6 and a 7 speed shifter shifts 7, etc.

I haven't been able to re-space a freewheel, but I have re-spaced cassettes to that a 7 speed shifter works with 6 speed cassette, 8 speed shifter with 7 cassette, etc.

There are work-arounds between shifters and derailleurs and cable attach points to hack the system. Here are a couple of sites...

https://www.cyclinguk.org/cyclists-l...gears/shimergo

Science Behind the Magic | Drivetrain Compatibility ? Art's SLO Cyclery

John

Edit Added: If you really want to get knee deep into this, you can measure the actual amount of cable pull to derailleur movement. There are those who have done this and believe the amount is not consistent (identical) from one cog shift to the next. I have never been so inclined, especiually since I used friction shifters for years, to do this. I'd rather just ride and take it for granted.

If a friction shifter can pull enough cable it can shift anything. But an 11 speed cassette will have closely spaced cogs and may be a bit finicky to friction shift smoothly.

Having said that, I'm such a cheapskate I'm going to test run a mish-mash of Dura Ace 8-speed and 10-speed components by using a bar-end shifter cannibalized from a time trial bike's aero bars, possibly in friction mode. I haven't decided yet whether I'll like the carbon fiber frame I'm building up -- I won't know whether it fits until I ride it awhile, since it appears to be a 58cm, right on the verge of being too big for me. So rather than commit to buying pricey brifters I'm gonna hodge-podge it together for now.

I'm expecting it to be finicky in friction mode with 10 cogs. I've been running my 7-speed Ironman with friction shifters for a couple of weeks and it's a chore to shift smoothly on chipseal and rough pavement. Some of my favorite rural routes are so torn up it's like riding on bricks or cobbles, and I've blown a lot of shifts with a 7-speed setup and downtube friction shifters.

BTW, Sheldon Brown's site has quickie compatibility charts for Dura Ace components. I dunno if he has another page for all Shimano, Suntour, etc., but it's worth browsing his site to find out.

It gets complicated as a number of items determine compatibility.
cog spacing, cable pull of shifter, and ratio of RD movement per length of cable pull. Then you throw in throw distance of RD & shifter, max cog size, and chain wrap capacity of RD.
Then you can throw in a number of different manufacturers and their variations and "special designs" as well.

Their are some "general rules" with lots of exceptions and workarounds.

Science Behind the Magic | Drivetrain Compatibility ? Art's SLO Cyclery

Assuming the same cable pull (indexed shifting) or friction shifting. I did run into a problem setting up a Campagnolo Mirage 9-speed derailleur on my wife's 5-sprocket cluster. Even with the lower limit adjuster fully screwed in, it couldn't limit the travel of the derailleur enough to keep it out of the spokes. I had to replace the limit adjuster with a longer bolt to make it work.

Aside from some serendipitous combinations, anyway.

Assuming it can sweep the entire cluster, yes. But the more sprockets you pile onto the rear hub, the more deft your friction shifting must be to reliably hit the gear you want.

Sometimes relevant is that lateral float is greater for derailleurs designed for index shifting than for earlier models. You will get a bit more precise shifting in friction mode if you have less lateral float/slop.