Psyc

I searched around and couldnt find an answer, but I want to build up a frame with double fsa tt chainrings and was wondering if i could go with a short cage rear derailleur or do i need to purchase a long cage?

thanks for any help

larue

You only need a long cage for a triple. Long cages have greater pulleys needed to accommodate a triple crankset.

Psyc

oh ok, so it doesn't have anything to do with the teeth on the chainrings?

slagjumper

The only factor is the max difference between your largest front and smallest rear gear. It makes no difference if you have a double or tripple.
For example--
I need a long cage with a 50/34 - 11-34. Check out the "capacity" number for the rear derailleur.
50 - 11 = 39, so a max capacity of 34 would not be enough. Check out the Shimano website, and look at the deraileur specs. Get the shortest cage that will work.

https://bike.shimano.com/catalog/cycl...=1153103396082

SGS = Long cage
Rear Derailleur
RD-M751-SGS
Pully Cage SGS
---
Speeds 9
Max Front Difference 22T
Total Capacity 45T <<<<<<<<<<<<<<<<
Max. Rear Sprocket 34T
Min. Rear Sprocket 11T

https://bike.shimano.com/catalog/cycl...=1153103407566

Rear Derailleur
RD-M751-GS
Pully Cage GS
---
Speeds 9
Max Front Difference 22T
Total Capacity 33T <<<<<<<<<<<<<<<<
Max. Rear Sprocket 34T
Min. Rear Sprocket 11T

Retro Grouch 

Well, you're right but: The purpose of the derailleur cage is to take up the chain slack that's generated as you shift into different gear combinations. As a rule of thumb, a long cage rear derailleur isn't necessary with a double crankset because the smaller difference in chainring sizes (vs. a triple crankset) reduces the need for slack take up capacity.

Slagjumper's example is an exception to the rule of thumb because he is using both a wide ratio double crankset and a wide ratio cassette. That's a pretty uncommon combination.

Psyc

thanks, that was exactly what i was looking for!

Psyc

it seems like the math isnt working right for road bikes..?

Pully Cage SS

Speeds 10

Max. Front Difference 16T

Total Capacity 29T

Max. Rear Sprocket 27T

Min. Rear Sprocket 11T

standard front large chainring is..
50-53T

with 11 or 12 t rear, that is only

38 in the best case scenario, but the parts are part of the same group?

Psyc

after a few more days of research, i finally came across this page
https://www.stanford.edu/~dru/triplecheat.html

and it says...

Both front and rear derailleurs have a listed capacity. For the front derailleur this is the difference between the number of teeth on the largest chainring and the smallest chainring, and is difficult to exceed by more than 2 teeth. For example, many "Racing Triple" front derailleurs have a listed capacity of 22t, which happens to be just enough for the popular 30-42-52 setup (52-30=22, ain't arithmetic grand!)

Rear derailleurs usually have both a largest cog (which is generally hard to exceed) and a capacity listed. What's capacity anyway? It's:

(biggest chainring - smallest chainring) + (biggest sprocket - smallest sprocket)

just in case anyone else runs into this problem later