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which size rear derailleur do I need please?

daffyavfcdaffyavfc Posts: 237
edited October 2010 in Workshop

I've got a 2009 GT GTR series 4 road bike and the rear mech got damaged in the car!!
I'm going to get a 105 mech as the one on it at present is a Sora shimano.

It's 16 speed and has a 50/39 chainset and an 8 speed rear cassette 12-25.

What size cage rear mech do I need for this set up please ?



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