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Chain length.....

white/blue flashwhite/blue flash Posts: 49
edited January 2013 in Workshop
Hi Guys and Girls,

Do I need to change my chain if I swap the rear casset from a 12-25 to a 11-28.??? :D:D

Posts

  • It will depend on many other factors such as your front chainring sizes and rear mech cage length, but generally if your chain length was spot on with the 12-25 installed you may find you need an extra link to accomodate the 3 extra teeth on the rear.

    However, unless the chain you've already got on there is almost new, your probably better off changing them both together from a mechanical/wear point of view. It would be a shame if you ran an old stretched chain on a nice new cassette.
  • Chain has only done a couple of hundred miles, so I should be ok with it....

    What do you think..???
  • maddog 2maddog 2 Posts: 8,114
    it depends how tight it was on big-big with the 12-25
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • Rear derailer was at 90 degrees to the chain stay..

    Should it be ok ???
  • sungodsungod Posts: 12,211
    to determine correct chain length, just follow the adjustment instructions for your specific make+model of rear mech

    they'll be available on the maker's website, for instance...

    http://techdocs.shimano.com
    my bike - faster than god's and twice as shiny
  • If your chain was the correct length before it will now be too short - no matter how much you don't want it to be true.

    You could use it and simply avoid crossing into the big-big combo (which you shouldn't really be in anyway from a mechanical point of view). but if you accidentally do it 50 miles from home don't be surprised if your chain snaps and the resulting roadside repair by taking out the broken link leaves you with a very 'spinney' low geared 50 miles back home.....
  • It will depend on many other factors such as your front chainring sizes and rear mech cage length

    Your rear mech length won't affect whether you need a longer chain (The cog sizes alone determine that). But it may be too short to deal with the slack in the longer chain that you do need while using smaller gears (Which is why MTBs have long cage derailleurs - due to the big differences in gear sizes from 11-34 on the rear and 22-42 on the front - that is a lot of chain slack between the two extremes of 11/22 and 34/42!).

    Also your front rings are a bit of a red herring - the only reason they would avoid you having to lengthen your chain is if you were changing your biggest ring for a smaller one at the same time to keep the max chain reqd the same as before - I am assuming you aren't?

    The easiest way to get the chain length is to loop is round your big/big gears (without it passing through the rear mech) then add 2 links to allow for the extra bends that passing through the mech requires). Any longer and you risk the chain falling off if you hit a rough patch of road while in the smaller gears. (Unless your rear mech has enough in it to take up the slack - in which case you are probably running a longer mech that you need to)
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