Shimano 105 rear derailleur upgrade

faceplant77
faceplant77 Posts: 25
edited March 2012 in Workshop
I have a specialized Secteur, and I want to upgrade the stock rear derailleur with a Shimano 105 5700. I'm not sure what cage length I require.

There are two options a short and a meduim.

I have three Rings at the front and 8 on the rear, the specs of the shimano states it will work with an 8 gear cassette, I just want to know if any one has tried this upgrade on a similar spec'd bike, and what type of cage length do I need.

Specs
Chainrings: 52 x 39 x 30T
Cassette: Shimano HG-50, 8-speed, 12-25t

Thanks

Comments

  • ddraver
    ddraver Posts: 26,810
    3 chainrings is the medium....
    We're in danger of confusing passion with incompetence
    - @ddraver
  • faceplant77
    faceplant77 Posts: 25
    On the specs of the Shimano derailleur it says;
    • Maximum front difference 22T, total capacity 39T

    what does this mean?
  • ddraver
    ddraver Posts: 26,810
    Honestly, I don't know...front difference seems like the gap between the biggest and the smallest ring....?
    We're in danger of confusing passion with incompetence
    - @ddraver
  • jameses
    jameses Posts: 653
    max front difference is the number of teeth on the big ring of the chainset minus that on the small, ie 52-30=22
    total capacity is the difference in chainring size on the front plus the range of the rear cassette (25-12=13). So total for you is 22+13=35.

    Short answer, you're riding a triple so you should get a medium cage rear mech.
  • keef66
    keef66 Posts: 13,123
    I have a 105 triple. Big ring is a 50t, otherwise identical to yours(but 10 speed). It has a medium cage rear mech
  • andy_wrx
    andy_wrx Posts: 3,396
    You need medium

    Short is for doubles or compacts

    The capacity it talks about is as JamesEs explains
    - the rough physics of it is that your chain needs to be long enough to cope with you being in biggest ring front vs biggest at rear i.e. 52 x 25
    - but if you were in the smallest ring front vs smallest at rear i.e. 30 x 12 then that chain is going to be pretty slack
    - which, apart from gear changing, is what the rear mech does, takes-up that slack and keeps the chain taut
    - you need a medium cage derailleur, with its longer cage/arm, to take-up that slack
    - whereas a short-cage one can only handle a 2-ring set-up, double or compact chainset, which has a smaller range between the extremes (say 53 x 25 vs. 39 x 12)
    - the medium cage one could be used with a double, but not the other way round
    - MTB set-ups, where you have 3 front rings and an even bigger range at the rear cassette, like 11-32 or something, need a long-case rear mech

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