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Buying a new rear derailleur.

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la569 Offline
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Posts: 2
Joined: Aug 2014
Post: #1
Buying a new rear derailleur.
Hi all, I'm a bit new to the maintenance of a bike so would really appreciate any help. I have a road bike that needs a new rear derailleur fitted.

My current rear derailleur just says shimano on it but my gears are shimano sora triple deck. I've looked online for a replacement shimano sora derailleur which are all 9 speed but mine is an 8 speed. Will this cause a problem?

Also, I've seen that you need to know the total tooth capacity. Can you just confirm how you work this out?

Thanks for any help!
Aug 29, 2014 07:37 AM
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nfmisso Offline
Veteran Member

San Jose, CA
Posts: 2,316
Joined: Jul 2010
Post: #2
RE: Buying a new rear derailleur.
Any 8 or 9 speed Shimano or Microshift RD with adequate capacity will work fine.

Total tooth capacity:
A = number of teeth in your big chain ring; for example 54
B = number of teeth in your smallest chain ring; for example 26
C = number of teeth in your largest cog of your cassette(or freewheel); for example 34
D = number of teeth in your smallest cog of your cassette(or freewheel); for example 11

Total TakeUp = (A+C) - (B+D)
in the example = (54+34) - (26+11) = 51 teeth
The example above is our Trek T50 tandem, we never use 26/11 combo, and would not go past 26/21;
= (54+34) - (26+21) = 41 teeth
so the RD only needs a take up capacity of 41 teeth.

The other number of teeth you see with RD's is the largest cog of the cassette or freewheel that it will work with. Most road RD will handle 28 teeth, some a bit more. MTB and Hybrid RD are usually rated for at least 34T.

Nigel
Aug 29, 2014 07:55 AM
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la569 Offline
New Member


Posts: 2
Joined: Aug 2014
Post: #3
RE: Buying a new rear derailleur.
Thanks for the help!

I'm also fitting new gear shifters but can't afford to replace like for like so was going to get some shifters that just fit onto the handlebars like on a hybrid or MTB. Do you think there'd be any problems with doing this? At the moment I'm looking at shimano Altus shifters and a shimano Sora rear derailleur.

Any help appreciated!
Sep 2, 2014 07:31 AM
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nfmisso Offline
Veteran Member

San Jose, CA
Posts: 2,316
Joined: Jul 2010
Post: #4
RE: Buying a new rear derailleur.
Road FD have a different cable pull ratio than MTB/Hybrid FD. Indexed shifters need to match. There are flat bar road bike index shifters that match road FD.

You can find 8 speed Shimano drop bar shifters on Amazon.

Nigel
Sep 3, 2014 06:10 AM
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