Jedwin

I'm in the process of building up a Surly Big Dummy frameset and am planning to use a Shimano XT M772 Shadow Rear Derailleur and a Shimano XT M770 9-Speed Cassette along with a Sram Chain.

My question is the derailleur length. I have the choice of a medium or long cage and was wondering if one has an advantage over the other on a cargo bike setup?

I'd also appreciate any comments about my choice of components.

jed

FBinNY 

My rule is to always use the shortest cage with sufficient capacity for the gear combination. There's only a marginal advantage, namely that the cage doesn't hang as low and is less likely to get damaged. There's also a weight savings but not enough to think about either way especially for a cargo bike.

But, before shelling out dough, take a moment to consider if you have enough gear range. If the shorter cage is at full capacity with the planned setup, you might opt for the longer cage, to leave yourself the option of expanding the gear range later on.

BTW- even though I prefer shorter cage RDs for their higher ground clearance, there's no meaningful drawback to longer cage RDs except their uncool factor on road bikes.

HillRider

Rear derailleur cage length is determined by the "total teeth" ( (big chainring - smallest chainring) + (biggest cog - smallest cog) ) you need to handle. For a 44/32/22 cranks set and a 11x32 cassette that number would be (44-22) + (32-11) = 43 teeth.

The manufacturer specifies the total teeth every derailleur model will handle so you just do the calculation for your planned gearing and see which model meets your requirements. That said, the longest available cage will work fine even if it's capacity isn't needed. The only downsides to a too-long cage are a very slight weight penalty and less ground clearance. There are those who claim a shorter cage shifts faster but I've never noticed the difference.

dsbrantjr

Modern derailleurs work uniformly well whether medium- or long-cage so there is not a lot of performance downside to going long-cage and it gives you the flexibility to possibly go to a larger chain wheel tooth difference (smaller granny gear) and/or larger spread on the cassette (larger big cog) without buying a new derailleur. It's not just chain take up of course; the front derailleur would need to handle the tooth difference and the rear derailleur the largest cog as well. Either alternative or both might be found useful on a cargo bike where a "stump-puller" low gear might save a long uphill walk.

Jedwin

The Shimano XT M772 medium cage has a max. of 33 teeth and I'm planning on running the 11-34 cassette in a 1x9 setup. The cage can handle the 11-34 but since I may want to add a FD later on, I'll go long cage for sure. Thanks for the help.

wrk101

Interesting, as I do just the opposite. Given a choice, I go long cage, as it gives me flexibility in the future if I start changing gearing. Now some of my RDs are short cage, they tend to be vintage parts that I wanted to use, so I accept the limitations.

+1 To below, I am less locked in my gearing, so I tend to change things up. If I was locked in, might as well get the shortest cage that does the job.

FBinNY 

We don't do the opposite at all, and I even mentioned the benefit of leaving room for future changes. I can go with the shortest practical cage because with 45+ years riding experience, I know what gearing I'll be using on any particular bike, and don't need to allow for future expansion.

It's kind of like buying jeans. A parent might buy slightly long jeans for a child to leave room for growth, but at some point we stop growing and can buy jeans that fit perfectly now.

HillRider

I'm at the same stage as you, I know with good certainty how I'm going to gear any specific bike so I can buy derailleurs the are as exact a match as I can get. However, at this point the OP's bike is still in it's early teens and needs some room for growth. It may not get any taller but the parent (owner) shouldn't bet on that. Too long jeans can be shortened. Too short ones have to be replaced.

FBinNY 

If you read my post #2 you'll see that I'm agnostic on cage length. I also tried to separate what I'd do for myself from what the OP might want to do for himself.

I don't try to convince people to go a certain way, or do as I do. I try to lay out the options, and things to think about then let folks decide for themselves. Also, RDs can be pushed beyond their limits (within reason) and in many cases I'd rather have a shorter cage, even if it meant that my granny couldn't be used with any sprocket smaller than 16t or so.

Retro Grouch 

That's what I think too.

This is one things that I'd do on a personal bike that I wouldn't do on a customer bike. On a customer bike I'd want the derailleur to take up the slack in the little/little. On a personal bike, I never use the granny in any but the largest 2 or 3 rear cogs, so I don't worry so much about slack take up.

Then again, if I were buying new parts, why not buy the derailleur that has the specifications to match the set up?

FBinNY 

Mechanically there's little difference in function between various cage lengths of the same derailleur, so there's no problem with more cage than necessary, other than weight (the RD + a few extra chain links). So the only argument for shorter cages would seem to be the cool factor issue.

But there are subtle benefits to shorter cages. First, most makers use the same cage spring regardless of cage length. That means a shorter cage will wind the chain slightly tighter, which means a quieter ride, and less chainstay slap (when coasting) on bumpy roads. There's also better ground clearance which can be very helpful off road, and lastly if you often have to pack the bike in a box or car trunk, wheels off, it's more compact with a smaller RD, and the cage unit is a bit tougher agains abuse.

So overall, I feel that it's best to use the shortest cage derailleur that'll serve. Of course, those who might be expanding the gear range in the future might want to provide some cushion, and opt bigger.

I also have another reason for my short cage bias. I'm a minimalist. I don't use anythng more than necessary for the job in general, so the shortest cage possible fits in with my overall temperament.

davidad

I don't like the fact that there is no der. adjustment on that model.