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Do short and longer cage mechs, of the same type, use different parallelogram geometry?
In my case, R8000 series Shimano Ultegra; I've got a R8050-SS but want lower overall gearing so wanted to replace the cage with that from a R8050-GS (the parts are available and detachable so theoretically it's possible).
Was discussing this in my LBS when the mechanic mentioned that the parallelogram geometry on the SS and GS may be different...
Firstly, does anyone know if this is true? Can't see anything specific in the Shimano documentation...
Secondly, even if it is, what impact will it actually have on shifting?
Thirdly, he said adding a longer rear cage is designed for increasing FRONT derailleur capacity, not REAR derailleur capacity, which confused me. Is he right?
A. Not true
B. None
C. And not right
The cage is purely to take up slack off the chain when the chain is longer cage needs to be.
The parallelogram is the same as it moves across the same space/cogs
vlad_the_invader
Thirdly, he said adding a longer rear cage is designed for increasing FRONT derailleur capacity, not REAR derailleur capacity, which confused me. Is he right?
Sort of - the rear mech has to take up all the slack, so capacity needs to account for (Biggest-smallest at front) + (biggest-smallest at back)
The parallelogram is the same as it moves across the same space/cogs
The mechanic was saying that the arc that some derailleurs moves through is/can be different if the derailleur is intended to be used with a close ratio cassette vs a wide ratio cassette, which sounds plausible but maybe he's thinking "marginal losses"!
Anyway, he said not all derailleurs are optimized like this...so I have no idea whether it's a problem for my derailleur or how to confirm one way or another, and I have no idea whether I'd even notice...
(For the record, I'm not a racer nor a particularly clumsy rider so not likely to make hurried gear changes under my full, awesome power 😄)
I've done this on R7000 105 and it worked fine. SS to GS cage.
I’ve done this on R7000 105 and it worked fine. SS to GS cage.
Interesting...
Did you put a wider range cassette on, or change chainrings (or both)?
Thirdly, he said adding a longer rear cage is designed for increasing FRONT derailleur capacity, not REAR derailleur capacity, which confused me. Is he right?
Front mechs have a capacity of up to 17 teeth, dictated by the geometry between the inner and outer cages - go bigger and you’ll have problems getting a clean, reliable shift. Different/modified rear mech won’t make a difference.
The top jockey wheel is placed slightly further away from the point at which the cage rotates on a larger capacity cage/mech. This means that when the chain is pulled tighter, the wheel moves further away from the sprockets so that it doesn't clash. I've swapped cages on a GRX & Force AXS mech recently & its not that hard.
Thirdly, he said adding a longer rear cage is designed for increasing FRONT derailleur capacity, not REAR derailleur capacity, which confused me. Is he right?
Sounds like he is talking about when road bikes came with double or triple chainsets. Back then the cage length on the rear mech was matched to the chainset. Short cage (SS) for double, medium cage (GS) for triple.
I’ve done this on R7000 105 and it worked fine. SS to GS cage.
Interesting…
Did you put a wider range cassette on, or change chainrings (or both)?
Neither - I had a crash and the derailleur cage took the brunt of the impact. I wanted to be able to use an 11-34 cassette so bought the GS parts and it worked fine. I've not needed a new cassette since then so still using the old one, and I've had no issues.