mzeffex 

So I'm about to order the xt rear derailleur for my bike, but couple questions before that.

A. Do I get medium (sg) or long (sgs) cage? I think I have a long cage now. I'm running 3x7 drivetrain.

B. What is the difference between the XT and XT shadow, and which should I go for?

Retro Grouch 

You will want the long cage version for your 3 X 7 drivetrain. There's really no penalty for having the longer cage.

As near as I can tell, the Shadow is the newest redesign. It's supposed to have some technical refinements, but I'd get it just for the better, more modern look.

mzeffex 

Alright great. Thanks.

I_bRAD

The shadow is the low profile design with a more direct housing routing. If you need a barrel adjuster at the derailleur end, you'll need to get an older model. I have 3 generations of XT derailleurs installed at the moment, all are working flawlessly

mzeffex 

Oh. Ok. thanks.

Mondoman

It'll help to list what type of bike you've got, what type of riding you plan to do with it, and the gear sizes of your rear cassette. The longer the cage, the bigger the range of gear sizes it can handle (since the gears with fewer teeth take up less of the chain, the derailleur takes up the extra chain length so that the chain doesn't go slack).

Assuming you have a mountain bike (sounds like it from the 3x7 gearing), SGS has been the standard for most mountain bikes for some years now, so I'd go with that. If you ever go for a "Megarange" cassette with a large range like 11t to 34t in the rear, you'll need it, and there's not really a practical drawback. (the arm on the derailleur, being longer, will hang down lower, slightly increasing the chances of it hitting a rock or log or something in really rough terrain)

Regarding the "Shadow" version, it seems to be designed for an extreme low profile to minimize even more the chances of it hitting something on rough terrain (see the ad copy here: https://www.treefortbikes.com/498_333...erailleur.html ).

Given that your current bike is 3x7, it sounds like it's a lower-end bike. Personally, I'd probably go with a Deore RD at 1/4 to 1/3rd the price of an XT RD for such a setup. You can always upgrade to an XT later if the Deore fails on you. Here's a Deore for $22: https://www.cambriabike.com/shopexd.a...%20DEORE%20RD&

mzeffex 

I have a specialized. Sort of. It is not lower end, its old. And barely stock and as you can see it is going to be less so. I already have deore. Thanks.

Mondoman

Thanks for the clarification - have fun with the upgrade! You might also think about upgrading to 8 speed in the rear while you're at it -- 11t to 3Xt is less jerky with 8 steps than 7, and 8spd cassettes are quite cheap these days.

mzeffex 

I would but I would need a new wheel.

CCrew

Actually, if it's the Hardrock in your sig it's both lower end *and* old. Hardrock is about the bottom of the Specialized MTB line.

I'd Deore it like others have said.

mzeffex 

Ok. /has deore
/has fox fork
/has avid juicy 7
/has carbon handlebar
/has dt swiss wheels

I wouldn't really consider it lower end.

Mondoman

Worth is all in the eye of the beholder; let's leave the p***ing contests to pre-school.