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Is this true or a myth ?

JimmyKJimmyK Posts: 712
edited May 2008 in Workshop
My trek 1000 is double front ring and 8 speed 12-28 at the rear. I kept finding if I was on my large ring at the front and my 28 cog at the back and then changed gear so i would be on the smaller ring at the front and 28 cog at the back..............my chain falls off :shock:

I phoned the LBS and was told my bike was perfectly normal and the gear change I described should be carried out ( at least ) on the next cog up on the rear cassette ( I think its a 25 tooth cog :?: )

Is this correct ? guy in LBS said its to do with the way the chain crosses over downwards onto the inner front ring whilst the chain is on lowest cog at the rear ( 28 tooth cog ).

hope ive described this clearly enough, anyway.............truth or myth ?

Jimmy

Posts

  • Tom ButcherTom Butcher Posts: 7,137
    It's certainly more likely fall off if you change gear like that - if it happened every change then you could probably set the bike up a bit better.

    it's a hard life if you don't weaken.
  • JimmyKJimmyK Posts: 712
    It's certainly more likely fall off if you change gear like that - if it happened every change then you could probably set the bike up a bit better.


    no, its only on the largest 28 tooth cog at the back, unless the physics behind it is to do with the sudden decrease in tension on the chain ?

    Jimmy
  • Tom ButcherTom Butcher Posts: 7,137
    Sorry I meant if when you have it on the 28 and change to the small ring at the front - that would make it more likely to drop the chain but if it did it without fail in that situation I'd suspect that there were other contributing factors.

    it's a hard life if you don't weaken.
  • John.TJohn.T Posts: 3,698
    Tom is right. It should not do this but this change is the one most likely to do so as the chain is pulling that way. You need to adjust the low limit screw on the front mech. First make sure the outer plate of the mech is paralel to the big chainring then put the bike in the small ring and the 28 cog. Adjust the stop screw until the chain is just rubbing on the inside of the cage when riding (ie- with the chain loaded) then back the screw off until you have about 1mm clearance. This should work OK. You may have to trim it a little more or less but this is where it should be.
  • nick hansonnick hanson Posts: 1,655
    Best bet is to use your gears more sensibly,& get it on the little ring a bit sooner :wink:
    so many cols,so little time!
  • meanwhilemeanwhile Posts: 392
    The rule is: try to keep the chain straight. A change forced into an extreme diagonal won't be flowing smoothly and will be subject to forces trying to throw it.
  • John.TJohn.T Posts: 3,698
    Nick and Meanwhile are both right but it should be able to do this change if set up right. You should be able to use all gears but to avoid the extreme cross over as much as possible. You often see Pros using big/big combinations rather than a double change onto the small ring and the smaller end of the cassette.
  • y tiny tin Posts: 102
    I'm guessing (and without turning this into a campag / shimano debate) that you're using shimano cos it'd be easier to trim the front mech with campag to prevent chain from derailing. Either way it'd be better to shift to a smaller rear cog before changing the front mech.
    >^..^<
  • JWSurreyJWSurrey Posts: 1,173
    Although as pointed out, technically riding crossed-over is not recommended, I do it all the time, especially if cresting a hill or getting over a bump. It should be entirely possible if set up right.
    Given that you have an 8 speed rear mech. it may be worth checking the derailleur tension, and maybe just giving it a damn good de-gunk, as it may be a bit tired and sluggish at taking up the slack, as pointed out above.
    An old gritty worn chain will not help matters.

    Just for the record, 2007 Shimano 105 and 2007 Campag. Veloce both have smart-trim on the front double configuration - in fact, I forgot the Shimano had it until last weekend!
  • John.TJohn.T Posts: 3,698
    It is nothing to do with trim. When you change to the small ring the mech goes to the low stop screw. That is what needs setting. This is not about how you should use the gears, just that they should work correctly at all times. If the cage does not throw the chain too far it will never drop beyond the ring. The cage will not let it. I have 3 bikes on 50/34 rings which are more prone than standard ones to over throwing. Non of them have ever done it. Just set it up as the maker says and it will work correctly.
  • maddog 2maddog 2 Posts: 8,114
    it's a miff
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • meanwhilemeanwhile Posts: 392
    John.T wrote:
    Nick and Meanwhile are both right but it should be able to do this change if set up right. You should be able to use all gears but to avoid the extreme cross over as much as possible. You often see Pros using big/big combinations rather than a double change onto the small ring and the smaller end of the cassette.

    The one caveat I'd have about "set up right" is that it might require a looser chain, which could transmit power less efficiently.
  • JimmyKJimmyK Posts: 712
    ive NEVER ever adjusted a front deraillure, its a shimano , can anybody point me to so simple a child could do it step by step instructions please.

    thanks for your help guys

    Jimmy
  • SDPSDP Posts: 665
    get a deda dog fang to stop it jumping off inner ring .....as per pro bikes ! :wink:
  • JWSurreyJWSurrey Posts: 1,173
    There are two screws mounted on the front derailleur - One adjusts the end-stop point for the upshift, one adjusts the end stop point for the downshift.
    If it's falling off the small ring, then you can drop it down on to the small ring, then play with the screw that affects this particular setting.

    Either check for a Shimano manual, or the Park Tool website is very useful.
    Sheldonbrown.com also has a lot of good instructions, though I'm not sure if there is anything Shimano specific there.
    Alternatively, do what I do - which is to peruse the derailleur, and if you are mechanically minded it should be pretty obvious which screw is hitting an end stop at this point....
    You then simply have to turn the screw a little to move the inner stop back towards the outer stop.

    The catch is getting it right, so it doesn't rub when the chain is at either end of the rear cassette, so when you're done you should go for a test ride/try out the extremities of travel.
    Bear in mind that you will probably need to make use of the trim function to stop rubbing at both ends of the cassette.... I certainly have to on my Shimano 2006/7 and Campag 2006/7 setups.
  • John.TJohn.T Posts: 3,698
    Surely the chain tension that transmits power is provided by you pushing on the pedals, not by any adjustments. The tension in the lower chain is governed by the springs that Mr Shimano provides and that is non adjustable.
  • magliacelestemagliaceleste Posts: 748
    Chains just shouldn't fall off if the bike is set-up right. Crossed over or not. The chain does have to be the correct length though.

    It's pretty detailed article but here you go:

    http://www.parktool.com/repair/readhowto.asp?id=75
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