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how to increase derailleur capacity

Old 08-02-21, 09:23 AM
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rbrides
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how to increase derailleur capacity

My bike's rear derailleur is Shimano Ultegra R8000, short cage, with 30T max cog. I would like to install an 11-34 cassette for better climbing. How can I increase the derailleur capacity? Can I simply install a medium or long cage, with a longer chain? Or, is there more to it than that?
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Old 08-02-21, 09:45 AM
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I've never done it but I think you can swap plates. Part #'s are:
Y3E998080 Outer plate and fixing bolt
Y3E916000 Inner plate
Good luck finding them, your shop will most likely have to order them from Shimano.
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Old 08-02-21, 10:33 AM
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Originally Posted by rbrides
My bike's rear derailleur is Shimano Ultegra R8000, short cage, with 30T max cog. I would like to install an 11-34 cassette for better climbing. How can I increase the derailleur capacity? Can I simply install a medium or long cage, with a longer chain? Or, is there more to it than that?
Yes, installing a longer cage derailer is an option. A medium cage derailer will handle a 34 tooth cog. About $100 for the derailer and extra for the chain.

A less expensive alternative that might work, is the Wolf Tooth RoadLink. I used one to increase the capacity of an old XTR derailer so that I could use a 36 tooth low. I could probably put in a 40 but I’m already at a silly low gear. It’s only $21. It might or might not work but it’s a bit cheaper than the other route.

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Old 08-02-21, 10:53 AM
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Originally Posted by cyccommute
Yes, installing a longer cage derailer is an option. A medium cage derailer will handle a 34 tooth cog. About $100 for the derailer and extra for the chain.

A less expensive alternative that might work, is the Wolf Tooth RoadLink. I used one to increase the capacity of an old XTR derailer so that I could use a 36 tooth low. I could probably put in a 40 but I’m already at a silly low gear. It’s only $21. It might or might not work but it’s a bit cheaper than the other route.

I had looked at Wolftooth but they explicitly state the road link does not increase derailleur capacity and that you should not exceed the derailleur's capacity.
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Old 08-02-21, 11:16 AM
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Originally Posted by rbrides
My bike's rear derailleur is Shimano Ultegra R8000, short cage, with 30T max cog. I would like to install an 11-34 cassette for better climbing. How can I increase the derailleur capacity? Can I simply install a medium or long cage, with a longer chain? Or, is there more to it than that?
From what I have read and seen online there are two differences between a Shimano short cage (SS) derailleur and medium cage (GS) derailleur: (1) cage length (obvious); and (2) parallelogram shape difference (less obvious), such that the derailleur travels a different path from the first to the eleventh positions. Difference #1 is mainly to take up the additional chain length necessitated by a larger range cassette, whereas difference #2 is mainly to accommodate the greater difference in diameter between the smallest and the largest cogs in a larger range cassette.

By swapping only the short cage for a medium cage, you are only accounting for the first difference, so it may or may not work with a 11-34, the nominally largest cassette that a complete GS derailleur would accommodate.
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Old 08-02-21, 11:17 AM
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Originally Posted by rbrides
Can I simply install a medium or long cage, with a longer chain?
Yeah it's pretty much that simple. I did it to my tandem recently. The longer chain is only because you are (probably) also increasing the total wrap.
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Old 08-02-21, 11:37 AM
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Originally Posted by rbrides
I had looked at Wolftooth but they explicitly state the road link does not increase derailleur capacity and that you should not exceed the derailleur's capacity.
I think you are misunderstanding the difference between derailer capacity and maximum cassette size. The RoadLink increase the maximum cassette size by making moving the derailer down and away for the cassette. The maximum capacity of the Ultegra 8000 SS rear derailer is 35 teeth. You haven’t said what the front chain rings which figures into the calculation. The cassette difference in an 11-30 cassette is 19. The cassette difference in an 11-34 is 23. Add the difference to the difference between your chainrings to get the capacity.

One thing to remember is that all those capacities listed by Shimano aren’t hard and fast numbers. That derailer in the picture is supposed to have a capacity of only 43. I’m running a 48/36/20 crank with an 11-36 cassette. The capacity of that is 53. I recently reduced the crank to a 44/36/20 with the same cassette. That capacity is 49…still above the supposed maximum. Yes, the chain can get a bit slack if I’m trying to run in the highest 3 cogs on the back with the 20 in the front, but I would never run that combination anyway. The whole system works very, very well.

By the way, I’m also way outside the specs of how the front derailer is supposed to work as well. It’s supposed to have a 20 tooth capacity. The 48/20 crank has a difference of 28 teeth and the 44/20 has a difference of 24 teeth. According to Shimano, that shouldn’t work. I can tell you that it works flawlessly as 1200 miles in Wisconsin and Michigan, can attest. And that’s not the first time I’ve used that gearing…not by a very long shot.

Finally, as a fellow researcher was fond of saying, “you can always talk yourself out of an experiment”. Try it. If it doesn’t work, you are out $20. If it works, you saved $80 to $100.
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Old 08-02-21, 11:45 AM
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Originally Posted by cyccommute
Yes, installing a longer cage derailer is an option. A medium cage derailer will handle a 34 tooth cog. About $100 for the derailer and extra for the chain.

A less expensive alternative that might work, is the Wolf Tooth RoadLink. I used one to increase the capacity of an old XTR derailer so that I could use a 36 tooth low. I could probably put in a 40 but I’m already at a silly low gear. It’s only $21. It might or might not work but it’s a bit cheaper than the other route.

The Wolftooth will certainly increase the max sprocket size capacity, by simply lowering the RD to provide more clearance. However, it won't increase the chain wrap capacity, which may or may not be an issue with a larger (ie, wider-range) cassette. If the OP is currently using an 11/30 cassette and wants to go with an 11/34, his RD's cHain wrap capacity will have to increase by 4 teeth. Depending on his chainring setup and the capacity of his current RD, the Wolftooth might be all he needs to achieve this. Before buying anything, calculate the chain wrap of the proposed new drivetrain and determine if the current RD's chain wrap capacity can handle it (with some allowance for RDs generally exceeding official specs). If you're in the ballpark re chain wrap capacity, the Wolftooth alone will likely do the trick.

Last edited by Litespud; 08-02-21 at 11:49 AM.
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Old 08-02-21, 11:56 AM
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Originally Posted by SoSmellyAir
From what I have read and seen online there are two differences between a Shimano short cage (SS) derailleur and medium cage (GS) derailleur: (1) cage length (obvious); and (2) parallelogram shape difference (less obvious), such that the derailleur travels a different path from the first to the eleventh positions. Difference #1 is mainly to take up the additional chain length necessitated by a larger range cassette, whereas difference #2 is mainly to accommodate the greater difference in diameter between the smallest and the largest cogs in a larger range cassette.

By swapping only the short cage for a medium cage, you are only accounting for the first difference, so it may or may not work with a 11-34, the nominally largest cassette that a complete GS derailleur would accommodate.
Thanks for additional intel. I already have an 11-34 Ultegra cassette so I've decided to buy the medium cage GS derailleur. I'm about due for a new chain anyway.
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Old 08-02-21, 04:56 PM
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11-36

I went one more.
50/34 and 11-36 using FD-R8050-GS derailleur

I even tried to go 46/30, but the rings I purchased placed the chainline outside the adjustment range of the Di2 FD.
SRAM makes the PG1170 in 11-36 (way over Shimano's spec'ed capacity BTW)

If you do choose to go over the capacity number, be sure to get the chain length correct.
I used Park Tools Large/Large +2 method. It's a great youtube video.

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