Short or Long cage der.
 

I did a search to try and find out what I need, but the discussion on derailleurs was way over my head. Can anyone tell me which kind I need for a Surly Cross Check with 52-42-30 ring and 12-27 cog? The previous derailleur was a Deore LX. Thanks in advance.

You will need a long cage.

What, may i ask, is the difference between a short and a long cage?

The difference is the amount of chain slack they will take up. As you shift gears different sprocket combinations require the chain to be differing lengths. This difference in chain length requirement is made up by the spring loaded derailleur arm. Long arm derailleurs will wrap up more chain slack than short arm derailleurs.

The example given was 52 and 30 front chainrings = 22 teeth difference. and 12/27 cassette = 15 teeth difference. Together you ideally need 37 chain links of wrap up capacity. Shimano short arm derailleurs are rated to wrap up 29 links of chain and long arm derailleurs are rated at 37 links of chain.

However, I'm running an XT long-cage rear derailer with an excess of 49-50 links.
I can't shift onto the smallest five rear sprockets when using the granny, but that's the only problem. And it's not even a problem, since those combinations are duplicated using better combinations.

As long as you understand the limitations, a derailleur with insufficient wrap capacity is perfectly fine and it's done all the time. I'm running Shimano 105 9-speed with a triple crank but have substituted a 26T granny for the 30T. My rear derailleur, theoretically, can't handle the range. It does fine but I have to avoid the 26 x 13 to 15 combinations. No loss.

yep, that's what you need!

While the lack of tension makes it easy to drop the chain, a poor tension yields a lousy response of the drivetrain that begins to waver. Usually the last 3-4T of the rear derailleur capacity makes riding lousy. However, everyone is free to make his choices.

I've never dropped the chain - honestly!

I simply never shift into the slack combos.

What would be cool though, is a telescopic arm/cage, with capacity for a significantly longer range than the larger models offered today. If I'd been into metalworking and welding, I'd have modified one myself. The spring seems plenty strong enough to handle a longer moment arm.

okay, heres a related question. Heres my drivetrain specs as of now.

Cassett:SRAM PG-850 - 12-25T - 8-spd
chain: Shimano/KMC HG-50 Narrow
crankset: TruVativ Touro Triple - 52/42/30T

I have a Sora RD (long) and am going to put on a
Ultegra RD (short) do I need to remove links? If so, what is the formula used to figure how many links need to be removed.

I was toying with the idea of extending a cage and have seen, actually, one extended through welding. In the end, however, I have built a tensioner on the return side, see:

http://www.bikeforums.net/attachment...achmentid=2742

I now ride with a 49T fully tensioned range. I have only used a saw, file and a drill. With your experience, you might produce something slicker.

Hey, that's not a bad idea at all! But how do you cope with the changing chain line?

And, out of curiosity, since we seem to have the same "excess" chain length, what is your front and rear setup?
Mine is a 22-32-48 in front and an 11-34 in the rear.

The tensioner wheel can slide laterally on its axle, covering the full chainline range. However, in winter I noticed that the axle may get gunked up, with the wheel capable of getting stuck in one lateral position. In practice, that turned out not to be a big deal, but it annoyed me. So there is still some tuning up going on.

I ride with a setup like this for nearly a full year now. As my confidence in the practicality of the arrangement grew, I have been upgrading the quality of the setup's elements. Right now, I should be close to the end.

Mine is 20-34-46 with 11-34. The small granny is especially useful for hill climbing in uncleared snow.

20T granny? Not with Shimano, right?

TA Zephyr. For that reason, however, the smallest middle ring I can have is 34T. What's yours?

The middle one? A 32...

No. What brand/BCD is your crankset?

Ah... :D
Shimano, 104/64 BCD.

If you are going to run a rear derailleur with insufficient chain slack take up, it's better to run a full length chain - long enough to safely cover the big/big combination. If you do that, your derailleur will handle the chain slack down to your 30/18 combination. Do you really envision yourself using a smaller rear cog than 18 with your granny chainring?

To size your chain for the big/big combination, wrap the chain around the two biggest sprickets bypassing the rear derailleur. Add 2 links (one inch) of chain. The answer is the same regardless of derailleur cage length.