Bicycle Mechanics - Shimano 6-speed and 7-speed compatibility?

I'd like to put a 105 derailler, originally designed for 6-speed indexing in the late 80's, on my dad's bike that has a 7-speed RSX drivetrain. Do Shimano 6-speed deraillers have the same cable pull as 7-speed (and thus be compatible with 7-speed shifters and cassette)? I've not been able to find any solid information on this yet.

Well if you try it out and report back, you can contribute to our knowledge.

I have the impression that while 6 speeds were fairly common, 6-speed _indexed_ was relatively uncommon, hence the lack of concrete info.

sydney probably would've known... :(

I have an early eighties Suntour Cyclone II derailleur; which is a 6 speed design. I used it with upto 8 speed shifters as that is the derailleur's functional limit. I am sure that a Shimano 105 will go at least to 8 speed and possibly 9 speed.

Yes, that should work fine. But, the derailler might not have enough throw to get all 7 gears.

I have an old 6-speed 105 rear derailler that I used on a Shimano 8-speed drivetrain. It shifted fine, indexing, for 7 of the 8 gears.

I'm pretty sure the cable pull and gear spacing for Shimano 6, 7, and 8 speeds is the same.

I'd like to put a 105 derailler, originally designed for 6-speed indexing in the late 80's, on my dad's bike that has a 7-speed RSX drivetrain. Do Shimano 6-speed deraillers have the same cable pull as 7-speed (and thus be compatible with 7-speed shifters and cassette)? I've not been able to find any solid information on this yet.

It will work fine. Derailers don't care how many speeds.

See also my article on this topic: http://sheldonbrown.com/speeds

Sheldon "Go For It" Brown
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It will work fine. Derailers don't care how many speeds.
See also my article on this topic: http://sheldonbrown.com/speeds
Thanks Sheldon, good to know. My worry wasn't that a derailler designed for a 6-speed drivetrain was speed-specific, but rather that it was cable-pull specific - like pre-8-speed Dura Ace, or using Campy deraillers with Shimano shifters, etc.
I'd looked over your article before, and it says nothing about 6-speed shifters, only 7, 8, 9, and 10. Could you note 6-speed compatibility along with 7-10 deraillers in the article? It might help a few other strange souls who might use the old stuff :)

It will work fine. Derailers don't care how many speeds.


Except for Positrons of course... ;)

Thanks Sheldon, good to know. My worry wasn't that a derailler designed for a 6-speed drivetrain was speed-specific, but rather that it was cable-pull specific - like pre-8-speed Dura Ace, or using Campy deraillers with Shimano shifters, etc.
I'd looked over your article before, and it says nothing about 6-speed shifters, only 7, 8, 9, and 10. Could you note 6-speed compatibility along with 7-10 deraillers in the article? It might help a few other strange souls who might use the old stuff :)

Done.

Sheldon "Glad To Oblige" Brown
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Thanks Sheldon. Great page. One other point which may be helpful on it. There's another difference between "road" and "mountain" deraillers, and that's the angle at which the cage swings along the cogs. The thing that allows an XT derailler to be low enough to not bump the bottom of a 34t cog, when a 105 GS would bump into the cog. It may not be worth mentioning this difference, because the only thing it affects is max cassette tooth capacity, but it is a technical diff. between "road" and "mountain" deraillers in the Shimano family. (Of course, a "mountain" derailler will shift fine with a straight-block "road" cassette, even though the derailler pully ends up further away from the cogs as you move to lower gears than would be the case with a "road" derailler.)

I don't know if I will work perfectly, but I suspect it will work. When I have encountered real world qureries about cross compatibility, generally the answer is "TRY IT". That's what a respected mechanic at a local bikeshop has told me. I still use 6 speed and I'm in the same position of gradually moving to 7 speed. I know I'm way behind the time but all three of my road bikes have 126mm rear triangles. I don't think the spacing is any different between Shimano 6 speed cassettes and freewheels compared to the 7 speed cassettes and freewheels. So the derailleur pull should be the same. Try It and post back here your real world empirical result...O.K.?

AFAIK, rear deraillers don't care about indexing; that's the shifter's job.
They do care about chain size, but chain size is the same for 5-8 cogs.
Sheldon Brown has lots of good info on this: http://sheldonbrown.com/

AFAIK, rear deraillers don't care about indexing; that's the shifter's job.
They do care about chain size, but chain size is the same for 5-8 cogs.
Sheldon Brown has lots of good info on this: http://sheldonbrown.com/
Did you notice that Sheldon already replied to this post?
And also, I've detailed above that the rear derailler does matter for indexing, because the amount of cable pull.