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My 20 inch folder has a single chain ring of 56T and a 11-28T cassette, 9 speed Shimano sora 9 speed Rear Derailleur short cage. I want to change the cassette to 12-36T for some hill routes. Based on my understanding the Derailleur capacity needed is biggest chain ring - smallest chain ring + (biggest cog - smallest cog) = 56 - 56 + (36 - 12) = 24.

I am not sure the capacity of the short cage Sora 9 speed, because different source gave different info. And they all assume a double or triple crank set which is not the case here. So will my RD support a 12-36T cassette?

  • I cheated by putting a triple crank on my folder. Its about 15 gear inches at the bottom now. Downside, theres no way to mount a derailleur so changing front gear is done by hand. – Criggie Jul 27 at 21:45
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Not really. A current short cage Sora derailleur (RD-3500-SS) has the total capacity (i.e., 37 > 24), but does not support a maximum sprocket over 32. While the Sora short cage derailleur should be able it can take up the chain slack, the top jockey wheel may not properly clear the 36 tooth sprocket.

Also see: How to calculate the capacity of a rear derailleur

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I've done this. You need to fit a small nut on the end of the longer B screw because the change in angle means the screw misses the frame. the nut essentially widens the screw.

  • Yay for actual experience! When theory contradicts evidence, it's not the evidence that's at fault :) – Móż Jun 22 '16 at 2:59
  • You should be aware that when you're pushing derailleurs that far, the hanger can play a part in the largest cog you can fit. Longer hangers give more clearance. – alex Jun 22 '16 at 3:36
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Generally, with a 36T, you want a long cage (Shimano SGS), such as a Deore rear derailleur.

While the capacity calculation you've done says OK, the max sprocket for a short cage road will be in the 20's or low 30's (in this case, its listed as 32t).

Thus, I'd buy a long cage RD (or at least medium cage).

  • The only question is whether a long cage would fit with a 20 inch wheel. It might hang really close to the ground in certain gears. – Kibbee Apr 13 '15 at 20:07
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    That's true. A front chainring swap may be better in this case. – Batman Apr 13 '15 at 20:27
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    Actually in this case the cage size isn't important at all. A "short cage" Deore would work fine. What you need is the length of the parallelgram and available B adjustment to allow clearance on the large cog. There is no difference in these measurements between short/long cage versions of different derailleurs. – Fred the Magic Wonder Dog Apr 13 '15 at 21:47
  • Swapping the 56T chainring for a 46T will give the same low gear approx. and save some weight! – Carel Apr 14 '15 at 7:38
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While using a Sora with a 36t cog may not be "approved" by Shimano, it's at least worth trying. The real problem is getting clearance on the large cog, not the capacity of the derailleur.

This clearance is generally adjusted using the B screw derailleur adjustment.

http://sheldonbrown.com/derailer-adjustment.html#btension

If you are not able to use current derailleur with it's maximum B screw setting, what you can try is getting a longer version of that B screw and putting the derailluer even farther out. As a temporary hack to see if this is worthwhile, you can put a small spacer in between the end of the B screw and the spot where it rests on the derailluer hanger cage.

This will at least let you know if it is possible.

I am reasonably sure that if the derailleur can work with a 32t cog, you can probably tweak it enough to work with a 36t cog. If you absolutely can't make this work, then get a "short cage" Deore MTB 9 speed derailleur. It has a long enough parallelogram and sufficent B screw adjustment to work with a 36t cog.

You don't need the long cage version of a derailleur. You just need one with a sufficiently long parallelogram.

  • Won't adding an extra long B screw also affect shifting in the lower gears? That said, like you suggest it could be worth giving it a shot before purchasing another derailleur. – Rider_X Apr 13 '15 at 22:12
  • In my experience the B adjustment just doesn't matter all that much with the smaller gears. – Fred the Magic Wonder Dog Apr 14 '15 at 20:40
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According to this page, the tooth capacity of the short cage version is 31 teeth. But let's look at a road bike with a relatively small gear range. you're likely to find. Filling in your formula, we end up with

52-39 + (23 - 12) = 24

Even the smallest cage derailleur would most likely cover that range. 52-39 is a standard road double crankset, and although there are smaller cassettes, they are quite uncommon. I would say that just about any derailleur could cover a 24 tooth range.

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    You also need to consider the maximum supported cog size in addition to the total capacity. – Rider_X Apr 13 '15 at 18:50
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I can confirm this works. Tested exactly this setup today. Small-Large/Large-Small won't work that great.

B-Screw is approximately half way in.

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    Welcome to Stack Exchange - can you please expand your answer with edit ? Would be helpful to include all the details of your results. What kind of folding bike? Exactly what crankset ? Do note that OP has a single chainring whereas you're talking about large/small like your bike has at least a double chainring, so there are already some variations between your situation and OP's. Are you sure you have the same rear mech - a Sora short cage 9 speed? How long are your chain stays? Or also a useful metric, how many links are in your chain total? – Criggie Jul 27 at 21:48

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