Bicycle Mechanics - XTR RD m952............

i have an RDM952 and on the cage is says mega 9. does this mean that this is an SGS derailleur that can be used with a 34-tooth cog?

ed rader

Is it obviously the long cage version? I assume the "mega" means it will clear a 34T cog.

M952 came in two models: -GS and -SGS. You should be able to tell from the exploded drawings whether you have the longer arm. http://www.paul-lange.de/produkte/shimano/support/explosionszeichungen_archiv/RD/RD_M952_99.pdf

M952 came in two models: -GS and -SGS. You should be able to tell from the exploded drawings whether you have the longer arm. http://www.paul-lange.de/produkte/shimano/support/explosionszeichungen_archiv/RD/RD_M952_99.pdf


what i'm wondering is if hillrider is correct: does the mega nine necessarily mean this is a long cage. it sure seems like i've seen longer cages and i know GS derailleurs are often called long cage.

i can't really tell from the drawing.

ed rader

"Mega 9" simply refers to 9-speed. It's confusing because in the lower model freewheels, "mega" means a cassette with a larger cog capacity, like 34T. Both GS & SGS Shimano mountain bike 9-speed RDs, will be labeled "Mega 9". But, I guess since all "Mega 9" Shimano MTB RDs can handle a 34T cog, the "mega" still makes sense.

However, the cage length has nothing to do with the maximum cog size. That is determined by the upper parallelogram, which is different between the MTB and road lineups. Most Shimano MTB rear derailleurs (both GS & SGS) go up to 34T, whereas most road RDs (both SS & GS) go up to 27T.

The cage length determines the total capacity of the system. Essentially, how much slack the rear derailleur can take up between the big/big and small/small combos, without leading to the chain rubbing itself under the RD in the latter combo.

For a general, Shimano "party line" guide to rear derailleur compatibility, see post #114:
http://bikeforums.net/showthread.php?t=316561&page=5

"Mega 9" simply refers to 9-speed. It's confusing because in the lower model freewheels, "mega" means a cassette with a larger cog capacity, like 34T. Both GS & SGS Shimano mountain bike 9-speed RDs, will be labeled "Mega 9". But, I guess since all "Mega 9" Shimano MTB RDs can handle a 34T cog, the "mega" still makes sense.

However, the cage length has nothing to do with the maximum cog size. That is determined by the upper parallelogram, which is different between the MTB and road lineups. Most Shimano MTB rear derailleurs (both GS & SGS) go up to 34T, whereas most road RDs (both SS & GS) go up to 27T.

The cage length determines the total capacity of the system. Essentially, how much slack the rear derailleur can take up between the big/big and small/small combos, without leading to the chain rubbing itself under the RD in the latter combo.

For a general, Shimano "party line" guide to rear derailleur compatibility, see post #114:
http://bikeforums.net/showthread.php?t=316561&page=5

according to harris cyclery you need an SGS RD for a 34-tooth cog. what i'm trying to determine is if my RD is SGS. is there a way i can tell?

thanx,

ed rader

You can usually just tell by the length between the two pulleys. I measured my Deore SGS RD, it's about 90mm (edit, that's center to center). Don't have a MTB GS RD in front of me, because they're generally used on MTBs with single or double chainring cranks, which aren't as common.

Did this RD come stock on your bike? For mountain bikes, SGS is used on bikes with a triple chainring (the majority of MTBs), whereas a GS can be used with a single or double crank (but so can an SGS).

You can usually just tell by the length between the two pulleys. I measured my Deore SGS RD, it's about 90mm (edit, that's center to center). Don't have a MTB GS RD in front of me, because they're generally used on MTBs with single or double chainring cranks, which aren't as common.

Did this RD come stock on your bike? For mountain bikes, SGS is used on bikes with a triple chainring (the majority of MTBs), whereas a GS can be used with a single or double crank (but so can an SGS).


thanks for your help. no i built the bike. it's a 1 x 9 litespeed pisgah. many XTR equipped bikes use a shorter derailleur like the GS and some even use short cage.

i would think that GS would be more common then SGS for off the rack XTR.

hey i'll go measure and see what i come up with :D.

edit: i'm getting like 87mm c-c.

ed rader

i would think that GS would be more common then SGS for off the rack XTR.

Again, determined more by number of chainrings, so I'd bet SGS is more common for any grade of model since triple chainrings are just more common for mountain bikes.

Just measured a little more closely, and (edit) 87 or 88mm looks right. So, looks like you've got yourself an SGS. It will work on a 1x9 system just fine. Actually, the MTB SGS RD is the most versatile and will work on most any bike, it's just unnecessary on many.

.........you need an SGS RD for a 34-tooth cog.

No, this isn't the case. All of the Shimano SGS or GS rear derailleurs in the mountain bike line will work with a 34t cog. The only difference between SGS and GS mountain bike derailleurs is the length of the cage, which affects how much chain slack the derailleur will take up, not the largest cog compatibility-

No, this isn't the case. All of the Shimano SGS or GS rear derailleurs in the mountain bike line will work with a 34t cog. The only difference between SGS and GS mountain bike derailleurs is the length of the cage, which affects how much chain slack the derailleur will take up, not the largest cog compatibility-


thanx. this

http://sheldonbrown.com/harris/derailers-rear.html

explains just that and 87mm is a SGS derailleur so i have what i need. thanx everyone.

ed rader

according to harris cyclery you need an SGS RD for a 34-tooth cog.

You can't know that you need an SGS (vs. GS) without knowing what your front gearing is (and to a lesser extent what your cassette range is). As JT pointed out, the cage length has nothing to do with the max sprocket size.

Now, going up from, say, a 28T to a 34T increases the overall range by 6T and may necessitate going from a GS to an SGS. But you don't have that problem.

For a 1x9, either one will do.

edit: I see you're set. Enjoy!

You can't know that you need an SGS (vs. GS) without knowing what your front gearing is (and to a lesser extent what your cassette range is). As JT pointed out, the cage length has nothing to do with the max sprocket size.

Now, going up from, say, a 28T to a 34T increases the overall range by 6T and may necessitate going from a GS to an SGS. But you don't have that problem.

For a 1x9, either one will do.

edit: I see you're set. Enjoy!


i need to add at least a link to my chain to use a 32-tooth cog in the back. it looks like to me that i need the longest cage possible to keep tension on the chain....but i could be wrong......

i may also try a 34-tooth cog and a 10-speed shifter. this would give mne a 10-speed bike that is a far cry to the schwinn varsity i was riding as a teen :D.

thanx for all the help!

ed rader

i may also try a 34-tooth cog and a 10-speed shifter. this would give mne a 10-speed bike that is a far cry to the schwinn varsity i was riding as a teen :D.


You'll need a 10-speed cassette to go with a 10-speed shifter and Shimano doesn't make a 10-speed mountain cassette. I believe IRD does.

it looks like to me that i need the longest cage possible to keep tension on the chain....but i could be wrong......

There are two primary "capacities" a rear derailleur has: chain wrap capacity, and large cog capacity. They are two separate issues.

A derailleur's large cog capacity is simply the largest cog that can be used without the derailleur's upper pulley bumping into the teeth on the cog. Derailleur bodies have different "geometries" that determine this, and there can even be different results from one frame to another, using the same derailleur. Generally, a max. large cog of around 30t can be used with a Shimano "road" derailleur (even though Shimano's "official" large cog capacity for these derailleurs is 27t), and at least a 34t can be used with a Shimano "mountain" derailleur.

The chain wrap capacity is related to the cage length. There's a simple formula for figuring the maximum chainwrap requirement for a drivetrain: largest chainring teeth minus smallest chainring plus largest cog minus smallest cog. Example (on a bike with a 44/32/22 crankset with an 11 x 34 cassette): 44-22+34-11=45. An SGS mountain bike derailleur would be required because of the large max. chainrwrap requirement. Check out the chain wrap capacities of GS and SGS XTR rear derailleurs here (I think Shimano refers to it as "total capacity"):
http://bike.shimano.com/catalog/cycle/products/group.jsp?FOLDER%3C%3Efolder_id=2534374302053217&ASSORTMENT%3C%3East_id=1408474395181785&bmUID=1202779692379

For your 1 x 9 drivetrain (or 1 x 10), you could certainly use a GS mountain bike derailleur. You'd be covered regarding the 32t or 34t large cog you plan to use, and you would have a relatively small max. chainwrap requirement since you're only using one chainring (subtract the smallest cog from the largest cog and that's your max. chainwrap requirement for your single chainring setup). Since it sounds like you have an SGS derailleur, no worries there either, it will work fine. You just won't be utilizing anywhere near the amount of chainwrap capacity the SGS derailleur has-

You'll need a 10-speed cassette to go with a 10-speed shifter and Shimano doesn't make a 10-speed mountain cassette. I believe IRD does.


i'd use a SRAM or shimano 10-speed and remove one cog and add a 9-speed 34-tooth.

ed rader

There are two primary "capacities" a rear derailleur has: chain wrap capacity, and large cog capacity. They are two separate issues.

A derailleur's large cog capacity is simply the largest cog that can be used without the derailleur's upper pulley bumping into the teeth on the cog. Derailleur bodies have different "geometries" that determine this, and there can even be different results from one frame to another, using the same derailleur. Generally, a max. large cog of around 30t can be used with a Shimano "road" derailleur (even though Shimano's "official" large cog capacity for these derailleurs is 27t), and at least a 34t can be used with a Shimano "mountain" derailleur.

The chain wrap capacity is related to the cage length. There's a simple formula for figuring the maximum chainwrap requirement for a drivetrain: largest chainring teeth minus smallest chainring plus largest cog minus smallest cog. Example (on a bike with a 44/32/22 crankset with an 11 x 34 cassette): 44-22+34-11=45. An SGS mountain bike derailleur would be required because of the large max. chainrwrap requirement. Check out the chain wrap capacities of GS and SGS XTR rear derailleurs here (I think Shimano refers to it as "total capacity"):
http://bike.shimano.com/catalog/cycle/products/group.jsp?FOLDER%3C%3Efolder_id=2534374302053217&ASSORTMENT%3C%3East_id=1408474395181785&bmUID=1202779692379

For your 1 x 9 drivetrain (or 1 x 10), you could certainly use a GS mountain bike derailleur. You'd be covered regarding the 32t or 34t large cog you plan to use, and you would have a relatively small max. chainwrap requirement since you're only using one chainring (subtract the smallest cog from the largest cog and that's your max. chainwrap requirement for your single chainring setup). Since it sounds like you have an SGS derailleur, no worries there either, it will work fine. You just won't be utilizing anywhere near the amount of chainwrap capacity the SGS derailleur has-

thanx -- i think i'm all set :D.

ed rader