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Compact Gearing - usable gears?

ForumNewbieForumNewbie Posts: 1,664
edited November 2011 in Workshop
I thought that with a Compact chainset (opposed to a triple) you would be able to shift up and down near enough all 10 sprockets at the back, whether on the 34 or 50 ring at the front without chain stretch or rub on insides of the front mech - especially with all Ultegra components.

However I noticed that on the 34 front ring the chain rubs the front mech when on the smallest gear at the back (the 28 sprocket) but when you go up the gears at the back it rubs the other side of the front mech when in the 3rd highest sprocket. So even if I managed to adjust it so it doesn't rub the left side of the front mech when on the 28 sprocket, it will probably then rub the other side when in the 4th highest gear at the back when still on the 34 front ring.

To summarise, when on the 34 front chain ring, due to the compact's front mech being narrower than a triple's front mech, it seems that there is only the width to accommodate 6 usable gears (i.e. without chain rub) rather than 9 or 10 as I would have expected. Is that normal?

Posts

  • There is a trim function for the front mech, its kind of like half a click and only moves the mech a couple of mm.
    Dolan Preffisio
    2010 Cube Agree SL
  • John.TJohn.T Posts: 3,698
    Correctly set up and using the trim function you should be able to avoid any chain rub on the mech. You will however probably get rub on the inside of the big ring when on the small ring and the smaller sprockets (usually only the first 2). These gears are best avoided and are available on the big ring. 34/13 and 50/19 are near enough the same ratio. Put your rings and cassette into this calculator and you can see what you actually have.
    http://sheldonbrown.com/gears/
  • cyberknightcyberknight Posts: 1,238
    I assume you have got the high/low limiters on your front mech set correctly?
    FCN 3/5/9
  • dennisndennisn Posts: 10,585
    John.T wrote:
    Correctly set up and using the trim function you should be able to avoid any chain rub on the mech. You will however probably get rub on the inside of the big ring when on the small ring and the smaller sprockets (usually only the first 2). These gears are best avoided and are available on the big ring. 34/13 and 50/19 are near enough the same ratio. Put your rings and cassette into this calculator and you can see what you actually have.
    http://sheldonbrown.com/gears/

    +1
  • cougiecougie Posts: 22,512
    Avoid the extreme angles on the chain.

    If you work out your ratios -- you'll see they'll be very similar to usable ratios anyway.
  • ForumNewbieForumNewbie Posts: 1,664
    Thanks for the replies. I wouldn't normally want to drop down to the bottom 2 gears when on the big ring, but I'd expect to be able to move up to at least the 3rd top gear when on the 34t front ring without rubbing against the front mech. But maybe I have to accept that I should only use the first 7 sprockets when on the 34t front ring and the last 7 sprockets when on the 50t ring?

    I have seen old Sheldon's gear page many times recently and looked at the ratios (how sad am I getting?). Surprisingly I haven't been able to find anywhere on the internet that actually says what gears make up the 11-28 sprocket. I'm guessing it's probably 28,24,21,19,17,15,14,13,12,11 - is that right?
  • As mentioned above, if your kit is properly set up and you're familiar with trimming then you will be able to use all ten gears on both front rings. Generally its best to avoid the extremes but its by no means critical.

    jon
  • Surprisingly I haven't been able to find anywhere on the internet that actually says what gears make up the 11-28 sprocket. I'm guessing it's probably 28,24,21,19,17,15,14,13,12,11 - is that right?

    Yeah thats right for the Ultegra 11-28.
  • amaferangaamaferanga Posts: 6,789
    Thanks for the replies. I wouldn't normally want to drop down to the bottom 2 gears when on the big ring, but I'd expect to be able to move up to at least the 3rd top gear when on the 34t front ring without rubbing against the front mech.

    Get it set up properly. The chain shouldn't be rubbing on the front mech when you're on the largest 8 or 9 sprockets (at least). If you set it up yourself then maybe get someone who knows what they're doing to do it for you?
    More problems but still living....
  • ForumNewbieForumNewbie Posts: 1,664
    amaferanga wrote:
    Thanks for the replies. I wouldn't normally want to drop down to the bottom 2 gears when on the big ring, but I'd expect to be able to move up to at least the 3rd top gear when on the 34t front ring without rubbing against the front mech.

    Get it set up properly. The chain shouldn't be rubbing on the front mech when you're on the largest 8 or 9 sprockets (at least). If you set it up yourself then maybe get someone who knows what they're doing to do it for you?
    I've found that to set the front mech so that the chain doesn't rub it when in the lowest gear (34x28), because the compact has a narrow front mech cage compared to a triple, it will naturally start to rub a bit once I move up to the higher gears on when on the 34t front ring, say (34x14 or 34x13) . However I've now learned that if I do want to stay in these gears without rubbing the mech I can use trimming, as someone mentioned earlier in the thread, i.e. move the mech a bit, without going up to the big ring.
  • John.TJohn.T Posts: 3,698
    amaferanga wrote:
    Thanks for the replies. I wouldn't normally want to drop down to the bottom 2 gears when on the big ring, but I'd expect to be able to move up to at least the 3rd top gear when on the 34t front ring without rubbing against the front mech.

    Get it set up properly. The chain shouldn't be rubbing on the front mech when you're on the largest 8 or 9 sprockets (at least). If you set it up yourself then maybe get someone who knows what they're doing to do it for you?
    I've found that to set the front mech so that the chain doesn't rub it when in the lowest gear (34x28), because the compact has a narrow front mech cage compared to a triple, it will naturally start to rub a bit once I move up to the higher gears on when on the 34t front ring, say (34x14 or 34x13) . However I've now learned that if I do want to stay in these gears without rubbing the mech I can use trimming, as someone mentioned earlier in the thread, i.e. move the mech a bit, without going up to the big ring.
    You could always do as I have for several years. Use the wrong front mech. :wink: IE 8sp on 9 or 9sp on 10. Or as I now have on both my TT and winter bikes 8sp on 10. (This because I am too tight to buy new ones.) They all work fine.
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