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So how do you know which rear derailer to use?

Does it have to do with cassette?

Or does it depends on the bike (size and shape). If the bike came with a long cage then only a long cage will work?
 

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off the back
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yes, it depends on the cassette, and, though i'm not an expert, probably the greater use of compact chainrings will minimize that somewhat. .

if you use a cassette with bigger cogs in the back, such as the Campy 13-29, or a Shimano 12-27, you'll need a longer cage to allow for the greater chainwrap required and to keep the chain tensioned properly. all the manufacturers will specify what cage length is required for what cassette you use.

now, i'm just guessing here, as i haven't really looked into it much, but i would suspect, that if you were to use a compact chainset, such as a 34/50 combo instead of a 39/53, that you might be able to run a short cage derailleur with the 12-27 combo, as the fewer teeth on the front chainrings will allow you to meet the chainwrap figures specified. but i'd need confirmation from someone like kerry Irons on that matter.

i know Campy recomends at least a mid cage derailleur with their 13-29 cassette, but i also know that people claim you can use a short cage perfectly well, as long as you avoid the severe cross-chained positions(53-29 combo)
 

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NeoRetroGrouch
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rufus said:
yes, it depends on the cassette, and, though i'm not an expert, probably the greater use of compact chainrings will minimize that somewhat. .

if you use a cassette with bigger cogs in the back, such as the Campy 13-29, or a Shimano 12-27, you'll need a longer cage to allow for the greater chainwrap required and to keep the chain tensioned properly. all the manufacturers will specify what cage length is required for what cassette you use.

now, i'm just guessing here, as i haven't really looked into it much, but i would suspect, that if you were to use a compact chainset, such as a 34/50 combo instead of a 39/53, that you might be able to run a short cage derailleur with the 12-27 combo, as the fewer teeth on the front chainrings will allow you to meet the chainwrap figures specified. but i'd need confirmation from someone like kerry Irons on that matter.

i know Campy recomends at least a mid cage derailleur with their 13-29 cassette, but i also know that people claim you can use a short cage perfectly well, as long as you avoid the severe cross-chained positions(53-29 combo)
Exactly backwards!

The cage length is determined (mostly) by the chainrings, not the cassette. Assuming Shimano 9 or 10: Standard double= short cage. Triple= long cage. (If Campy, they throw in a super-short, called 'short', for use with a double and small cog set.)

A compact crankset has a larger difference is the size of the chainrings and may require a long cage.

To get the correct RD, (big ring - small ring) + (bigest cog - smallest cog). Check the RD spec for the 'capacity'. i.e 39/53 front with 12/23 rear is (53-39)+(27-12)= 29 teeth. A short cage Shimano RD has a capacity of 29 so it would be OK.

TF
 

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Campy has three sizes. Short is used for regular double cranks and cassettes, medium is used mostly for "pie plate" cassettes and long is used for triple cranks. The medium is also a good choice for compact doubles with a 34 tooth chain ring.

If you use one that is too short, your chain will hang loosely in some gear combinations because there isn't enough arm wrap to keep it tensioned. As mentioned above, using one that is too long won't hurt anything.
 

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The cage is there to take up the slack in the chain when you are using smaller cogs or chainrings. The slack is called "wrap". Derailleurs have a spec for how much wrap they can take up. For example standard Shimano road derailleurs are specd' to take 27t worth. To calculate wrap: (big ring + big cog) - (smallest ring + smallest cog)

Example, for my bike with a 50/34 compact with a 12-27 cluster: (50 + 27)-(34 + 12) = 31
 
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