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I recently changed the crankset of my Cannondale Synapse 5 from the stock FSA Gosssamer crankset to a Rotor 3D24 crankset with power2max power meter. This was a bit of an iffy modification to make - the Cannondale frame has a BB30A bottom bracket with a 73mm shell width for 30mm spindles. However, the Rotor cranks are 24mm. I took the bike to my LBS and we eventually got everything to mount using some Wheels Manufacturing shims (BB30-UNIV).

However, there is a problem now - the front derailleur now rubs against the crank arm as the crank passes over the derailleur, causing a very annoying clicking noise. I can stop the rubbing by tightening the high limit screw in all the way, but now the derailleur itself rubs against the chain, and doesn't have that 1-2mm clearance that is recommended.

I have a bunch of spacers that came with the Wheels Manufacturing shims, and I've taken the crank off and installed some on the drive side. This has the effect of pushing both the crank arm and the chainrings out - so the clearance between the chainring and the crank arm is still constant and small.

Is using these spacers the right thing to be doing? I'm wondering if this is a chain line issue?

I am also considering getting http://kogelbearings.mysimplestore.com/products/cannondale-bb30a-24-for-rotor-cranks but I don't really see how that will solve my problem - it seems to be the distance between the crank arm and the chainring itself...

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    You could always grind away a small bit on the back side of the arm, but that would create a weak spot. Or you could possibly grind away the high spot on the derailer. Otherwise, is change the arm or change the derailer.
    – Daniel R Hicks
    Commented Dec 19, 2015 at 13:27
  • I believe other brands also have different adapters, and the spacing may be a bit different with other brands. If you are going to modify something though, I'd go with the derailleur, not the crank.
    – Batman
    Commented Dec 19, 2015 at 16:47
  • Can anyone suggest derailleurs with a narrower cage? I currently have some Shimano 105 derailleurs.
    – ocharles
    Commented Dec 19, 2015 at 17:17
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    (Note that twisting the derailer on the seat tube may give you a hair more clearance..)
    – Daniel R Hicks
    Commented Dec 19, 2015 at 19:46
  • @DanielRHicks that only works for a band on presumably - I have a braze on.
    – ocharles
    Commented Dec 19, 2015 at 23:18

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If you already adjusted the front derailleur, there is not much to do. As you wrote yourself, the distance between crank arm and chainring is too small for your derailleur. Ask Rotor about compatible derailleurs.

The main reason to have derailleur not touch is not noise. The derailleur scratches the crank, which creates a starting point for cracks.

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  • I was wondering if it would be possible to remove the power meter spider from the crank & chainrings and then adding a spacer there to push the crank arm out further, do you think that is worth trying? I haven't been able to do it yet as I need a vice to remove the power meter from the crank with the proprietary Rotor tool...
    – ocharles
    Commented Dec 19, 2015 at 17:18
  • Possibly. It's hard to say from pictures on the internet if there is enough contact surface between the crank and spider, you'll have to see yourself.
    – ojs
    Commented Dec 20, 2015 at 21:18
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Kind of a cheap-shot answer, but you could revert back to the original. That would certainly cure your noise problem.

I'd be concerned on the follow-on effects of upgrades, where one simple change creates an avalanche of changes and new parts. Perhaps your choice of power meter needed more research?

Is it too late to return the power meter and find a better-fitting one?

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  • I think it is too late to return the power meter, but the meter was bought with the intent of mainly going on a new bike in the future - and this seemed the most compatible. I would have been hard pressed to do more research though - I googled, posted here, and emailed power2max twice. I just got unlucky, I think :(
    – ocharles
    Commented Dec 20, 2015 at 7:33
  • The reason I opted for a pedal-based powermeter. With the easy switch between bikes as a bonus.
    – Carel
    Commented Dec 20, 2015 at 9:16
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You can try modifying the derailleur so it fits into the narrower space. Without seeing it, I have no idea if that's practical or not. If not, there's nothing you can do besides replacing the crank and/or derailleur.

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