105 Rear mech capacity

proto
proto Posts: 1,483
edited September 2010 in Workshop
I've not got too much experience of Shimano drive train, and I'm putting together a winter bike for my daughter to use. Anyone know if a 105 rear mech, short cage, will be able to cope with a 50/34 and 12/27 combination. I'm pretty sure the spec says 29T max, and I'm looking for 31T.

Reckon it'll work?

Comments

  • Pokerface
    Pokerface Posts: 7,960
    Conflicting statements there.

    It will cope with a 50/34 and 12/27 no problem. A 31 on the back would require a long cage.

    Or did you mean a 31T on the front? 31 on the front with a 27 on the back would be fine.
  • proto
    proto Posts: 1,483
    Sorry, I didn't make that clear, I meant TOTAL difference, front and rear together.

    Shimano quote 29T total. I want 31 total (50/34 = 16 + 12/27 = 15)

    It shouldn't work, but I'm asking if, just maybe, it does!

    Anyone?

    PS Sorry Pokerface, read your post again, and you say it will work. Thanks, I'll give it a go.
  • John.T
    John.T Posts: 3,698
    I think the OP means 29 and 31 max teeth difference.
    In any case 50/34 with 12/27 works fine. I have it on 3 bikes with no trouble what so ever. Just run the chain as long as possible as it changes better. The little tab on the rear mech cage should be just clear of the chain when in small / small gear.
    My Fleet.
  • Pokerface
    Pokerface Posts: 7,960
    edited September 2010
    Edit - see post below
  • proto
    proto Posts: 1,483
    Pokerface, I don't think you are right.

    There are are two critical limits for rear mechs

    1) the biggest rear cog that the mech can accommodate

    2) the total capacity of the rear mech, f+r combined (the ability of the mech to 'soak up' changes in chain length.

    My question was really refferring to 2) above. How much 'wrap' can the 105 take up.
  • Pokerface
    Pokerface Posts: 7,960
    edited September 2010
    EDIT: You are correct. The specs for a 105 do indeed specify no more than a 27T rear cassette and no more than a 29T difference in total (by your math).

    It's different for Ultegra. (Goes up to 28T cassette and 33T total)
  • Pokerface
    Pokerface Posts: 7,960
    John.T wrote:
    I think the OP means 29 and 31 max teeth difference.
    In any case 50/34 with 12/27 works fine. I have it on 3 bikes with no trouble what so ever. Just run the chain as long as possible as it changes better. The little tab on the rear mech cage should be just clear of the chain when in small / small gear.

    What mechs are you running? In doing a bit of looking up - it seems the 105 mechs don't have as much give as say, the Ultegra ones. If you are running 105s - then the OP is safe with his choice.
  • Slater
    Slater Posts: 80
    proto wrote:
    I've not got too much experience of Shimano drive train, and I'm putting together a winter bike for my daughter to use. Anyone know if a 105 rear mech, short cage, will be able to cope with a 50/34 and 12/27 combination. I'm pretty sure the spec says 29T max, and I'm looking for 31T.

    Reckon it'll work?

    I have the 105 short cage on my 50/34 12/27 and it works fine.
    _____________________________________________
    https://giffgaff.com/orders/affiliate/jslaterb
  • proto
    proto Posts: 1,483
    Thanks all, it would sem that I'll be okay, but Pokerfaces comments about Ultegra having more capacity reminded me I could pinch the Ultegra mech off her TT bike (put the 105 on instead). Belt and braces.

    She's at Bath University and has been complaining about some of the hills around there. Bath must surely be the worst cycling city in Britain. She's said that she doesn't fancy cycling up Bathwick Hill (and there are some much steeper, apparently) with a bag full of books every morning . Kids today, eh :roll:
  • rhext
    rhext Posts: 1,639
    Even if it struggles a bit, the first gear combinations you'd lose are ones you'd probably not want to use anyway (eg 34 front to 12 back).....
  • John.T
    John.T Posts: 3,698
    Pokerface wrote:
    John.T wrote:
    I think the OP means 29 and 31 max teeth difference.
    In any case 50/34 with 12/27 works fine. I have it on 3 bikes with no trouble what so ever. Just run the chain as long as possible as it changes better. The little tab on the rear mech cage should be just clear of the chain when in small / small gear.

    What mechs are you running? In doing a bit of looking up - it seems the 105 mechs don't have as much give as say, the Ultegra ones. If you are running 105s - then the OP is safe with his choice.
    I am running 9sp Ultegra on the winter bike and 10sp (6600) and Ultegra SL on the others. All the Shimano mechs with exposed cables will take this. Shimano are just being careful. The new 7900 and 6700 are more tolerent but I have not tried them to comment on how far they can be pushed..
    My Fleet.

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