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I have a carbon Cannondale mountain bike with what appears to be a PF30 or PF30A bottom bracket: two 6806 bearings pressed into cups, which are pressed in turn into the frame.

With the bearings and cups still in the frame, the NDS bearing feels slightly gritty and the DS bearing feels very gritty with high initial friction. So I'm thinking I need to replace one or both.

Except when I pull the DS bearing (comes out with the cup), it feels fine. Maybe 90% of ideal, no obvious grit even under pressure. Press the cup and bearing back into frame, though, and then it feels terrible again.

I assuming this is a frame tolerance issue: it's squeezing the bearing too much. If so, then replacing the bearing won't get me anywhere.

Does this sound like the right conclusion? The frame is still under the factory warranty.

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What you're describing is common. One clue as to whether it's necessarily an issue depends on how a new bearing feels and lasts/performs. Press fits always take up some amount of internal clearance and so having some degree of what you describe on a worn bearing isn't necessarily a big deal. But if a new DS bearing behaves similarly, yes that's a problem.

To diagnose something like this, always take a bunch of measurements of the shell bore at different places to see if it's consistent or if the interference fit is greater than it should be. The PF30 standard is open and there's a technical paper for it out there somewhere that shows what these numbers should look like.

PF30 reamer/facers do exist and I daresay anyone selling cdale mtbs should have one. That should fix it better than new, although I don't know Cannondale's take on them on their carbon frames.

  • I didn't think to measure the bore when I had it in pieces, that was a good idea for the next go, thanks. I just installed new bearings. They're definitely smoother and easier to turn outside of the frame than inside, but they don't have the gritty feel of the old ones. Good enough! I skipped the Loctite this time and put it together with grease because it was such a bear to get the old bearings out of the cups. No creaks on a quick ride. – Alex Jun 8 '18 at 19:22

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