bugly64

I have already purchased 10spd 105 cranks, cassette, and Ultegra brake/shifters, but I don't know which derailleur to buy. The long or the short one. What's the difference? What's preferable for racing and part time commuting?

jhota

depends on your cassette.

nitropowered

it more depends on if you have a double or triple. For most applications, you use a short cage for a double

Aeroplane

If you are going single ring you can probably go with a short-cage. The only way to know for sure is to look up how many teeth of difference your derailleur can take up, and figure out what the maximum difference your setup has. For instance, if you are running a 36-44 and a 12-28 cassette (random numbers), your minimum is 48 (36 + 12) and your max is 72 (44+28), so you will need a derailleur that can take up 24t (72-48) of slack.

Andy_K 

Just to make explicit what I don't see explicitly above, you want the shortest cage derailleur you can get away with. If I'm not mistaken this is because a longer cage derailleur gives you a bit more chain slack in many gear combinations and the shorter cage derailleur deliver crisper shifting.

To give an example, I've got a 50-39-30 front crank (triple, because like you I also use it for commuting). I'm looking to upgrade to an Ultegra rear derailleur. I've got a 12-27 cassette. Going to the Shimano web site, I get the following data:

Ultegra GS
Maximum Sprocket = 27T
Minimum Sprocket = 11T
Maximum Front Difference = 22T
Total Capacity = 37T

Ultegra SS
Maximum Sprocket = 27T
Minimum Sprocket = 11T
Maximum Front Difference = 16T
Total Capacity = 29T

Looking at this, I can't get the SS (short cage) derailleur because I have too much front difference (20) and require too much total capacity (35). So I'd have to get the GS (medium cage). But when I use this bike for cross racing, I never get onto the big chainring, so if I wanted to, I could take it off and set up the front derailleur to keep the chain on my 30-39 rings. Then I'd have a front difference of 9 and a total capacity of 34, and I could happily use the short cage derailleur.

Notice also, that if I had a nine speed setup and wanted to run an 11-32 cassette, I'd need a MTB derailleur regardless of capacity because I'd be exceeding the max sprocket size for road derailleurs.

Also, you can also calculate the total capacity as the front difference (50-30) + the rear difference (27-12). It's mathematically the same, but when you're thinking about things like reconfiguring cranks and cassettes, it's sometimes easier to juggle.

flargle

It's not just about chain slack, though, but also whether there's interference between the jockey pulley and cassette.
https://www.sheldonbrown.com/gloss_ca-g.html#capacity

Since the mfgrs publish max cog and max tooth difference, might as well just follow their numbers.

Sheldon says there's no real reason to use a short-cage: "SS cage derailers don't work any better than GS models, just a couple of grams lighter."

Short answer: Medium cage up to 28t, large cage ("mountain" or "touring") for anything bigger.

Aeroplane

I hate to disagree with Sheldon, but in my experience short cages are a LOT less likely to pick up a stick, and we all know how a stick in the derailleur can ruin a ride.

flargle

Fair enough.

I can't say it's been a problem for me. Also, I think the amount of chain slack would play into the equation, too. Shortening the chain as much as possible is a good idea anyway, for cross especially.