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I bought a new Shimano 3-ring crankset for my aged MTB (5-speed at the rear) and updated the BB to the recommended length (9.5mm longer than I previously had with 2-ring cranks)for the new cranks. Now the chain rubs on the outer side of the rear derailleur cage when shifted to the large sprocket and the middle ring. Does this suggest the chainline of the new cranks is too big?

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    It suggests that the rear derailer needs to be adjusted. – Daniel R Hicks Jul 19 '15 at 18:15
  • Is there any particular adjustment likely to fix this (high/low limit, cable tension)? – tim Jul 19 '15 at 19:00
  • Increase cable tension and move the "high limit" further out. I had this issue on a bike once. My derailleur barely had enough range to cover the new crankset. If yours is a severe case, you may need a new derailleur. – BSO rider Jul 19 '15 at 20:43
  • Thanks to Daniel and BSO rider. I will try re-adjusting the derailleurs (front & back). To avoid exceeding the derailleur range, I am intending to use only the two larger rings (42/34 - the original rings on the bike were 48/40) - using all 3 would definitely exceed the range as it has an SS cage (trial & error proved this - either the chain was far too loose on small ring/small sprocket or too tight on large ring/large sprocket). – tim Jul 19 '15 at 20:51
  • I finally got to look at the bike yesterday. No amount of adjustment made any difference and I noticed the chain was rubbing on all sprockets. Fortunately, I had another rear derailleur handy and compared it with the one on the bike - the bottom of the cage (and the lower jockey wheel) had got bent. I switched the derailleur out and now all is good... thanks for all the advice. – tim Aug 2 '15 at 15:00

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