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Turns out my front chain rings are biopace, after I thought they had disappeared!

I've got a problem with the chain rubbing the front deraileur at a certain point every rotation. I only get this in the highest gear on the inner ring and it happens from about the 4th gear when on the outer ring. getting gradually worse when changing up to the highest gear.

Not sure if it's a trim or height problem or both :S

when in the small inner ring height should not be a problem.

Front deraileur

But it could be height issue when on the highest gears on the outer ring.

Setup

  • Dura ace deraileur front and back
  • 105 cranks and chain ring
  • unknown back sprocket set

  • If it's only rubbing once a revolution then the ring or crank is bent. If it's rubbing twice a revolution you need to adjust your derailer. Generally the largest ring is not Biopace. – Daniel R Hicks May 2 '12 at 10:17
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    Can't make out much on the video. (And I see that your large ring IS Biopace.) – Daniel R Hicks May 2 '12 at 11:24
  • Of course, nothing is ever perfect, so there may be some very slight wobble as the crank is turned (and some compression of the bearings when the crank is "mashed" with your foot), but in theory a chain ring is supposed to be perfectly symmetrical, so if you get rubbing only once per revolution (or rubbing is significantly worse on one half vs the other) then that indicates that something is NOT symmetrical. Ie, bent. If the rubbing is reasonably equal twice during a full revolution then the problem is the derailer adjustment. – Daniel R Hicks May 2 '12 at 22:31
  • Given this question is quite old - can you give an update on how it went? – Criggie Oct 5 at 8:03
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Keep in mind that there are 5 adjustments on a front derailer:

  1. Low limit
  2. High limit
  3. Cable adjustment -- selects where the derailer stops for intermediate cogs
  4. Height above crank
  5. Angle on seat tube

For subtle chain rub problems sometimes adjusting the angle (very slightly!) is the best approach.

But remember that if the rub is only once a revolution it suggests that the ring, arm, or crank is bent.

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I'm working on fitting a biopace big and middle chainring to a roadbike triple at the moment. Chain rub on the front derailleur is always a problem to be worked through, and having a non-round chainring only adds to that.

In your case it looks like the horizontal position is correct, but some minor tweaking of cable tension and limit screws may solve the rub.

In terms of a vertical position, the large chainring teeth must just clear the bottom of the cage by 1~2 millimetres. With non-round chainrings, this has to be at the highest points on the chainring, giving correspondingly more gap at other points.

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