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Choosing Medium or Long cage derailer for cannondale scalpel
I have destroyed my XTR rear derailleur on 2012 Cannondale Scalpel and I don't know if I need to replace with XTR medium cage or long cage. The re cog is 11-36 and two ring at the front. Help!!
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26er or 29er? I'm assuming 29er with a Twoby crank. Gimme a minute while I go check mine.
29er
Bugger: I was hoping it would have GS or SGS printed on it somewhere. So we'll have to work it out.
This link says GS has a 35T capacity. http://www.wiggle.com.au/shimano-rd-m986-xtr-10-...
Assuming 38-24 crank and 11-36 cassette, that gives 38-24+36-11=39T capacity, so it would need to be the SGS model that you buy.
I tried looking on websites and couldn't work it out.
My understanding, which I'm open to being corrected if wrong, is:
Rear cassette:
# of teeth on largest rear sprocket - # of teeth on smallest
+ Front Chainring:
# of teeth on largest front chaining - number of teeth on smallest front chaining
Eg 34-11 = 23
40-30 = 10
23+10= 33
If running a single chainring upfront the second value is 0.
You got it
Can't you tell by looking at the destroyed one? Or have you binned it already? Should be pretty obvious just by looking at it.
It works like this.
A double ring requires a mid cage rd, that's a GS.
A triple ring requires a long cage rd, that's an SGS.
It works according to what range of difference you have across both cassette and chainring.
GS tops out at 35T difference, as worked out above. Most 2x10's exceed that. 38/24 with 11-36 works out at 39t. My 39-26/11-36 comes to a 13 tooth difference at the front and 25 at the rear for a 38T total. I too need a SGS.
If you are still using a 9-speed rear end your rule of thumb would probably work, though.
I have medium cage XT + on both my bikes. One is 40/28 11/36, the other is 38/24 11/36 both get the full range easily in each ring. I only ever go up to about 5th in granny ring and don't usually go below 3rd in big ring but can if I need to.
I prefer the medium cage because that extra cm of clearance makes me think it is less likely to catch on things (in reality its probably no different).
The ratings for teeth are conservative. The generally used rule of thumb is simply 2x is medium cage, 3x is long cage.
More often, people end up with long cage derraileurs when they don't need it because that's what was in stock when they needed it yesterday. If you have a big difference at the front and a medium, you just have to pay attention to chain length . D
Agree with Pharmaboy, Shimano's capacities are on the conservative side. I have a 39T range and run GS or Medium cage. However... on a full suspension bike you also need to take into account the effect of chain growth due to rear suspension travel - which can vary a lot depending on the design of suspension and the stroke. So, nobody can really tell you unless they have the same bike with the same setup.