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Can someone please explain why Shimano has two types of XT deraileurs available? "GS" and "SGS". Seems to be related to your gearing choices, but I am still not sure exactly.......

thanks--
 

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Here's the short of it: The longer the cage, the more "capacity" the derailleur has to take up excess (slack) chain.

A long cage will take up slack all the way down to your small-small combo (22x11) and hold some amount of tension.

A medium cage will handle any gear combination from your big and middle rings (i.e. 44x11 or 32x11) but only has enough capacity to get you half way through your cassette while in the granny ring (i.e. 22x20 or so).

If you're never going to run in the small-small, no problem. If you accidentally do, your chain will probably drop due to lack of tension.

Benefit of shorter cage is greater tension across the range & reduced chain slap, plus increased obstruction and spoke clearance.
 

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To add to the earlier post:

the length of the cage does influence the degree of slack the mech can take up.

a typical mtb cassete would be 11-34 or 11-34. Couple this with triple chain rings and you have a wide range of gears to cover. Hence the need for a long cage to take up the slack at the extremes.

a typical road cassette would be 11-23 or 12-25. Couple this with only a double chain ring and you have a smaller range of gears to cover. Hence you can get away with a short cage.

Shimano's website http://www.shimano-europe.com/cycling provides stats of their rear mech's a the range they cover (double or triple chainring - capacity etc)
 

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Tippster said:
To add to the earlier post:

the length of the cage does influence the degree of slack the mech can take up.

a typical mtb cassete would be 11-34 or 11-34. Couple this with triple chain rings and you have a wide range of gears to cover. Hence the need for a long cage to take up the slack at the extremes.

a typical road cassette would be 11-23 or 12-25. Couple this with only a double chain ring and you have a smaller range of gears to cover. Hence you can get away with a short cage.

Shimano's website http://www.shimano-europe.com/cycling provides stats of their rear mech's a the range they cover (double or triple chainring - capacity etc)
I should probably be posting this over on Road Bike Review but, since we are on the subject... does this apply to road bikes as well? I am in the process of building up a road bike with a triple ring and wasn't sure if I needed a long cage or a short cage RD? It sounds like a long cage? Is this correct?
 

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Shimano do specify on their road mechs - Double / Triple.

They also specify capacity in teeth. Most have a 27 capacity. meaning you couldn't run a mtb 11-34 cassette.

I would always recommend checking with the specs on teh shimano website.
 

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Mantis, Paramount, Campy
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Not so true

Road triples will work fine with Shimano's SS cage which is the shortest cage and not even mentioned in the original post. MTBs in the early and mid 90's were speced with SS cage derailleurs frequently with 12-28 read spreads and 24-48 front spreads with no issues.

I am running a MTB with 12-26 rear and 24-36-48 front with a Campy Record rear derailleur from 1992. This derailleur had the shortest cage made durring the indexing era. Works excellent!
 
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