pstock

This is something I have been wondering about lately. I know there is a lot of chatter and many posts on drivetrain cross- compatability - what works well, reasonably or Not At All - with what across differentmakes and models.
and I understand that Indexing and pull distances for the various shifters is obviously different. and that the chains and cassettes (sprocket and spacer widths) for different setups (8S vs 9S vs 10S etc.) are different (to accomodate different pull ratios/distances etc.)

but I am a bit stumped as to how various RDs - 8S, 9S, 10S even 6 or 7S - behave differently, mechanically,
so if I had say 9S shifters, would a 6S RD move a different distance laterally - across a cassette - with each Click than say an 8S or 9S derailleur would move? is there some design element built into an RD that dictates how far it moves.?
Or, are they "dumb" and all move the same distance?

Bill Kapaun

ASSuming Shimano, with rare exception, 6-9 speed indexed RDER's are the same.
The shifters determine how far to move the cable. The RDER is just a mindless slave.
The greater the number of cogs, the closer they are spaced from each other and thus require less cable pull per shift.

KenCarlson

In addition to having the right number of clicks, a shifter's job is to pull the shifter cable by a certain amount. That amount varies depending on whether it is 6-speed, 7-speed, etc.

For a given amount of cable pulling a derailleur, the derailleur moves sideways a certain amount. The ratio of cable pull to sideways movement is commonly called the pull ratio of the derailleur.

shifter pull X the pull ratio = amount of sideways travel of the derailleur. That sideways motion of the derailleur needs to be very close to the spacing of the cogs.

Although that multiplication needs to come out very close, the two individual numbers that contribute to the final result can vary somewhat as long as their product turns out close. As Bill points out, Shimano for a long time made derailleurs with a fixed pull ratio of 1.7, and just adjusted the amount of pull in the shifter to make the final travel match the cog spacing. That went out the widow starting around 10-speed systems based on the practicality of moving ever smaller amounts of cable within the shifter. So we end up with derailleurs having different pull ratios depending on what "speed" system we are talking about.

Learn more here:
Science Behind the Magic | Drivetrain Compatibility ? Art's SLO Cyclery

-Ken

icemilkcoffee 

The 8, 9 and 10 speed cassettes have the same total width. They only differ in the cog thickness and between-cog spacing. The 7 speed cassette/freewheel on the other hand, is something like 4mm thinner than the 8,9,10 speed one. The 6 speed freewheel is again thinner than the 7 speed. 5 Speed thinner than 6 speed. Will a Shimano 8 speed RD work with a 10 speed cassette? Most likely. Will a Shimano RD designed to sweep across 5 and 6 speed freewheels, work with a wider 10 speed cassette? There are no guarantees because you are now pushing it outside of its design envelope.

Bill Kapaun

Then why does a 10 speed cassette require an additional 1.8mm spacer?
The FH bodies are the same length, but the cassettes differ slightly.

icemilkcoffee 

That's only when you are running a 10 speed cassette on an 11 speed hub. The 11 speed hub/cassette is wider than the 8,9,10 speed one.

Atlas Shrugged

As mentioned earlier, there are three variables which must align.

1 Cassette cog spacing
2 Cable Pull
3 Pull Ratio

All this info is available on the inter webs regarding cross compatibility.

Bill Kapaun

WRONG!
A 1.8mm spacer was required before 11 speed came along.

cxwrench

Only if you have a Mavic freehub.

dabac

I used a Shimano 600 Arabesque on a 10-speed Shimano cassette, Campy 10 shifter and a shiftmate. Upshifts were a little slow, but indexing and reach was fine.

rccardr 

With the exception of Dura Ace 740X, ALL indexing Shimano road 6-7-8-9-10 speed (and Shimano MTB 6-7-8-9 speed) rear derailleurs are cross compatible.
So to use an extreme example, a 10 speed shifter will correctly interface with a 6 speed RD to shift a 10 speed cassette.

It’s not completely ideal with regard to the cage width vs. chain width, but it works and the shifting quality is very acceptable.