Rear Mech

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uclown2002

Guru
Location
Harrogate
Looking to replace my shimano 105 rear derailleur, but see they do a short cage (SS) and a medium cage (GS). Which is more suitable for use with a 50/34 compact and 12-25 cassette?? What does total capacity refer to below? Guessing above 12-25 cassette has total capacity of 37T?

  • short cage
  • Maximum front difference 16T, total capacity 33T
  • Weight: 221g
  • Medium cage
  • Maximum front difference 22T, total capacity 39T
  • Weight: 232g
Do hangars typically come with a new rear mech or are they considered part of the bike?
 
Looking to replace my shimano 105 rear derailleur, but see they do a short cage (SS) and a medium cage (GS). Which is more suitable for use with a 50/34 compact and 12-25 cassette?? What does total capacity refer to below?

  • short cage
  • Maximum front difference 16T, total capacity 33T
  • Weight: 221g
  • Medium cage
  • Maximum front difference 22T, total capacity 39T
  • Weight: 232g
Do hangars typically come with a new rear mech or are they considered part of the bike?
A short is best unless you want to fit a rear cassette bigger than 28t (you are not, you have a 12-25)
The hangar does not come with the mech bike, if you need one you'll have to order one separately.
 
OP
OP
uclown2002

uclown2002

Guru
Location
Harrogate
A short is best unless you want to fit a rear cassette bigger than 28t (you are not, you have a 12-25)
The hangar does not come with the mech bike, if you need one you'll have to order one separately.

Thanks for that. What does 'total capacity' in the description refer to?

Are hangars typically generic that fit most/all road bikes and mechs?
 

TheDoctor

Europe Endless
Moderator
Location
The TerrorVortex
Total capacity refers to the amount of slack the rear mech will take up - the length of spare chain, if you like. It's the difference between big-to-big and little-to-little.
In your case, you've got a 50/34 chainset, so there's 16 teeth worth of chain between big and little rings.
And a 12-25 cassette has 13 teeth worth of chain - that makes 29T in total.
Basically - you need the medium cage for a triple, and short for a double or compact. Cassette size is irrelevant.
And if your hanger is OK there's no need to replace it. If it's busted, you'll need to get one that matches your frame. Which is a whole new can o' worms, as there's lots of different mech hangers...
 
Thanks for that. What does 'total capacity' in the description refer to?

Are hangars typically generic that fit most/all road bikes and mechs?
Lifted from Sheldon:
For example, if your rear sprockets run from 14 to 26 teeth, the rear difference is 12. If your chainwheels are 36-52, the front difference is 16. Adding the figures for front and rear, we get a total difference of 28 teeth.
There's many different types a lbs should be able to sort you out.
 
OP
OP
uclown2002

uclown2002

Guru
Location
Harrogate
Total capacity refers to the amount of slack the rear mech will take up - the length of spare chain, if you like. It's the difference between big-to-big and little-to-little.
In your case, you've got a 50/34 chainset, so there's 16 teeth worth of chain between big and little rings.
And a 12-25 cassette has 13 teeth worth of chain - that makes 29T in total.
Basically - you need the medium cage for a triple, and short for a double or compact. Cassette size is irrelevant.
And if your hanger is OK there's no need to replace it. If it's busted, you'll need to get one that matches your frame. Which is a whole new can o' worms, as there's lots of different mech hangers...


Excellent thank you.

I had a huge off last week and I'm likely to change a few components, including rear mech. I'm told the hanger is bent as well, so thought about just replacing it with the other bits.
 
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