# Bicycle Mechanics - how long of cage for rear derailleur

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sprocket47
03-11-11, 08:40 PM
I'm building up a new 29er with 3x10 gearing. How do I know if I need a long cage or medium cage. Assuming I need long for 3x10, would I use medium if I choose to go with 2x10 gears?

jasonrobo02
03-11-11, 08:44 PM
You need to figure out your required chain wrap and compare that to the chain wrap capacity of the derailleur(s) you are interested in. To identify your required chain wrap you follow this formula: (a-b)+(c-d).
Where:
a = number of teeth on largest chainring
b = number of teeth on smallest chainring
c = number of teeth on largest cog
d = number of teeth on smallest cog

jasonrobo02
03-11-11, 08:46 PM
Here's an example for my roadbike that I ride:
It's a 2x10 setup with a 50/34 crankset and a 11-26 cassette.
a= 50
b= 34
c= 26
d= 11

So my required chainwrap is: (50-34)+(26-11) = 31

prathmann
03-11-11, 08:58 PM
Yes, the formula given above is correct. Note that the number of chainrings and cogs doesn't matter, just the differences between the largest and smallest. If you're at all close to the borderline in cage size I'd suggest you pick the larger option. That way it'll still work even if you later make a change in your chainrings or cassette.

sprocket47
03-12-11, 12:34 AM
You need to figure out your required chain wrap and compare that to the chain wrap capacity of the derailleur(s) you are interested in. To identify your required chain wrap you follow this formula: (a-b)+(c-d).
Where:
a = number of teeth on largest chainring
b = number of teeth on smallest chainring
c = number of teeth on largest cog
d = number of teeth on smallest cog

36 for my 3x10 option
38 for my 2x10 option

What are the limits for the cage lengths long and medium?

prathmann
03-12-11, 07:26 AM
What are the limits for the cage lengths long and medium?

Depends on the specifications of the specific derailleur you're considering. What you're looking for is the specified "capacity." But typically getting a capacity of 36 or more teeth will require a long cage.

Another important spec. for a RD is the largest cog size; i.e. the size of the biggest cog in the cassette you plan to use.

jasonrobo02
03-12-11, 09:57 AM
36 for my 3x10 option
38 for my 2x10 option

What are the limits for the cage lengths long and medium?

You need to go to the derailleur manufacturer's website and look at the specs for the specific derailleurs that you are interested in. You may also find that spec on certain retailer's websites, but it's usually quicker to just go to the manufacturer's site.

Also, as Prathmann noted, you need to also check the max cog specification and compare that to your target derailleur. For example, if you were to build up a 1x10 drivetrain with an 11x34 rear cassette, a short cage roadbike derailleur would have enough chainwrap capacity, but would fail on the max cog size. Thus you couldn't use it on your bike build.

strock
03-13-11, 08:00 AM
36 for my 3x10 option
38 for my 2x10 option

What are the limits for the cage lengths long and medium?

Top-tier Shimano and SRAM long-cage MTB rear derailleurs generally have a total chain wrap capacity of 45T. Shimano's XT medium-cage RD has a capacity of 33T, I believe. I suspect you will need a long cage for either your 2x10 or your 3x10 option, but a check of manufacturers' websites will tell you for sure. I would say that, if in doubt, go with the long cage. Even if you don't need the extra wrap capacity, it will work just fine. Some people will tell you a long-cage RD will shift a little less quickly than a medium (or short) cage. That might be true, but in my experience, the difference isn't worth worrying about. As others have mentioned, the long cage will also give you flexibility if you later decide you need to expand your gear range. Finally, if you're riding a 29er, you shouldn't have to worry much about ground clearance with a long cage.