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105 Rear Mech Oddity

SlowingDownSlowingDown Posts: 129
edited May 2009 in Workshop
Help please guys and girls.

I swapped my 9 speed 105 shifters and cassette from my old road bike to my Tricross Sport (as one of the Sora Shifters was knackered). Before that the Tricross was running an 8 speed cassette and chain on a compact drivetrain. Out of laziness I left the old chain on the Tricross and as expected the gear changes were terrible, so I chucked on the chain from the road bike. I'm presuming this is a 9-speed chain but since it was put on recently by a LBS that I no longer have any confidence in it could be anything. Shifting was perfect and I happily tucked the bike away in the garage while I played with my new bike.

Looked at the Tricross later and realised that in high gears the bike looks fine but in low gear the rear derailleur isn't hanging as expected but is pulled almost horizontal towards the front of the bike. Have I done something wrong, could this be a cage length issue (road bike was double, Tricross is compact), chain length problem? As I say, shifting is perfect but obviously something isn't right.

Thanks!
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Ribble Sportive Racing: FCN 2
Tricross Sport: FCN 4
Cannondale R600: FCN 3
Scrapheap Rescued SS, in bits: FCN 9

Helmet wearer
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Posts

  • alfabluealfablue Posts: 8,497
    I think it may be a chain length problem - the Shimano method for chain length is that the jockey wheels of the rear mech should be vertically aligned when the chain is in the small sprocket and large chain ring - put it on that, if the mech points forward the chain is too short. When at the correct length it should be okay when on the large sprockets, though it will point forward.

    okay:

    chain-OK1.gif

    Too short:

    chain-short1.gif
  • SlowingDownSlowingDown Posts: 129
    Interesting, thanks. It is closer to the 'Okay' picture to be honest but I've never seen such a difference in the rear mech angle on any of my bikes. Would a 10-speed chain be worth a try if it really bothers me?
    ===============================
    Ribble Sportive Racing: FCN 2
    Tricross Sport: FCN 4
    Cannondale R600: FCN 3
    Scrapheap Rescued SS, in bits: FCN 9

    Helmet wearer
    ===============================
  • alfabluealfablue Posts: 8,497
    A 10 speed chain isn't going to affect chain length - 10 speed chains are narrower to cope with narrower spacing between the sprockets. The mech should be fine for either compact or double. Try the big ring, small sprocket test to see if the jockey wheels are vertical, this will quickly tell you if the chain is too long or too short (with chain on big ring and small sprocket, bottom jockey wheel forward of vertical = too short; bottom jockey wheel rearward of vertical = too long).

    I suspect it was too long on your road bike and is just right on the tricross, hence the mech points further forward than you are used to. Remember that chain length will be influenced by the length of the chainstay, which is likely to be longer on the Tricross, as well as the gears used. The change from double to compact should mean that the chain is too long, but the chainstay length may have counteracted that (and some).

    Do the check I suggested.
  • SlowingDownSlowingDown Posts: 129
    Thanks alfablue - I did the test you describe yesterday and the jockey wheels were vertical. So if the jockey wheels go close to horizontal, with no noticeable bend in the chain at each jockey wheel, when on larger sprockets I just need a longer 9-speed chain?
    ===============================
    Ribble Sportive Racing: FCN 2
    Tricross Sport: FCN 4
    Cannondale R600: FCN 3
    Scrapheap Rescued SS, in bits: FCN 9

    Helmet wearer
    ===============================
  • alfabluealfablue Posts: 8,497
    Yes, a new chain will have more links than you need. Assess required size as I have suggested, use a chain splitter. I can recommend KMC chains, they have a quick link. Merlin Cycles have them at good prices.
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