Short Cage Rear Derailleurs: HELP!
07-12-10, 01:35 PM
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Short Cage Rear Derailleurs: HELP!
Can someone explain when a short cage derailleur is appropriate?
I am building a frame up and have come across a few of short cage RD's for a much lower price than the regular/long cage versions. Specifically, I'm looking at SRAM Force/Rival RDs.
Not sure yet whether I will be running a triple or double up front (but most likely a double). Probably will end up running a 10 gear rear cassette but, unfortunately, I'm still uncertain since it wll depend on what kind of a deal I find on, amongst other things, shifters.
Thanks for any help you can offer!
I am building a frame up and have come across a few of short cage RD's for a much lower price than the regular/long cage versions. Specifically, I'm looking at SRAM Force/Rival RDs.
Not sure yet whether I will be running a triple or double up front (but most likely a double). Probably will end up running a 10 gear rear cassette but, unfortunately, I'm still uncertain since it wll depend on what kind of a deal I find on, amongst other things, shifters.
Thanks for any help you can offer!
07-12-10, 01:38 PM
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A long cage rear derailleur wraps more chain than a short cage. In other words the total number of teeth between the Big ring/ Big cog, and the Small ring/small cog combinations can be bigger.
So figure out the total tooth difference between your proposed Big/Big and Small/Small. Then compare that number to the capacity of your derailleur.
Also look at the maximum cog size it will shift to, and how that compares to the cassette you plan to use.
Typically a standard crankset (i.e with 53/39 rings) and rear cassette with no more than a 28 tooth bi cog is fine with a short cage derailleur.
If you go to a triple, or a very large rear cog, you may need a medium, or long cage derailleur.
So figure out the total tooth difference between your proposed Big/Big and Small/Small. Then compare that number to the capacity of your derailleur.
Also look at the maximum cog size it will shift to, and how that compares to the cassette you plan to use.
Typically a standard crankset (i.e with 53/39 rings) and rear cassette with no more than a 28 tooth bi cog is fine with a short cage derailleur.
If you go to a triple, or a very large rear cog, you may need a medium, or long cage derailleur.
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Last edited by merlinextraligh; 07-12-10 at 01:44 PM.
07-12-10, 01:41 PM
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Compare the RD max cog size and chain wrap capacity to what you want to run.
From Sheldon Brown:
From Sheldon Brown:
For rear derailers, the capacity relates to the amount of chain slack the derailer can take up, and is equal to the front range (22 in the example above) plus the rear range. Thus, if you have a 52/42/30 crank set, and a 12-28 (16 tooth difference) cluster, the total capacity required would theoretically be 38 teeth (22 front difference + 16 rear difference).
Manufacturers specify this fairly conservatively. They must do so, because they have to assume that some of their derailers will be sold to incompetent cyclists, who will abuse their drive trains by using the smallest chainwheel with the smaller rear sprockets.
Competent riders can considerably exceed the official rated capacity, since they will not misuse the granny ring by running it with the smaller rear sprockets, so it doesn't matter if the chain hangs slack in those gears.
Rear derailers are also commonly designed for a particular maximum size of rear sprocket. If you exceed this size by too much, the jockey pulley may rub against the sprocket when using the lowest gear.
Rated maximum rear sprocket size, however, is also commonly much lower than what actually works. For instance, Shimano's models designated as "road" derailers are generally listed for a "maximum" sprocket of 27 teeth...because 27 teeth is the largest size that they make in a designated "road" cassette. However, in almost all cases, these derailers, even the short-cage models, will handle rear sprockets as large as 30 teeth in practice. (This somewhat depends on the design of the frame's derailer hanger, so once in a while you will find a particular installation where you can't use a 30, but I've never seen one where a 28 wouldn't work.)
Manufacturers specify this fairly conservatively. They must do so, because they have to assume that some of their derailers will be sold to incompetent cyclists, who will abuse their drive trains by using the smallest chainwheel with the smaller rear sprockets.
Competent riders can considerably exceed the official rated capacity, since they will not misuse the granny ring by running it with the smaller rear sprockets, so it doesn't matter if the chain hangs slack in those gears.
Rear derailers are also commonly designed for a particular maximum size of rear sprocket. If you exceed this size by too much, the jockey pulley may rub against the sprocket when using the lowest gear.
Rated maximum rear sprocket size, however, is also commonly much lower than what actually works. For instance, Shimano's models designated as "road" derailers are generally listed for a "maximum" sprocket of 27 teeth...because 27 teeth is the largest size that they make in a designated "road" cassette. However, in almost all cases, these derailers, even the short-cage models, will handle rear sprockets as large as 30 teeth in practice. (This somewhat depends on the design of the frame's derailer hanger, so once in a while you will find a particular installation where you can't use a 30, but I've never seen one where a 28 wouldn't work.)
07-12-10, 02:30 PM
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If you are going with SRAM Rival or Force, I don't think they have a long cage SRAM RD...They don't offer the group in triple. Double and compact, but no triple. I could be wrong.
I am doing a SRAM Force upgrade from a Shimano triple.
If you are looking for a good deal on SRAM Rival Double Tap shifters I found a good deal at ediscountbikes.
I am doing a SRAM Force upgrade from a Shimano triple.
If you are looking for a good deal on SRAM Rival Double Tap shifters I found a good deal at ediscountbikes.
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