Press Fit Bottom Bracket : No thanks

[Tim Hall]

The older design carbon bikes did have a threaded BB but that was achieved by gluing in a threaded aluminium sleeve inside the carbon BB shell. This has huge problems with galvanic corrosion. Water enters the frame and a reaction between the carbon and aluminium causes the aluminium to corrode to the point where there's huge jagged gashes in it. Eventually the BB just breaks out. Other problems are that the alu insert comes loose and creaks like hell, just like a pressfit BB, so they may as well bypass the alu shell and go directly to the creaking PF BB.

A galvanic reaction between the carbon (or resin) and aluminium sounds a bit odd. Got any more info on it?

 

[Yellow Saddle]

presumably they'd all last longer if there was a grease nipple on the BB.

A galvanic reaction between the carbon (or resin) and aluminium sounds a bit odd. Got any more info on it?

Yes I have. I used to fix quite a bit of out-of-warrantee Cervelo frames with that problem and somewhere in my archives I have some nice photos of the problem. I'll look for them. It was so severe that the sleeve would break off inside after being eroded to foil thickness. The warrantee period on the frames was such that second owners could not claim. I suppose galvanic implies a bi-metal reaction, which it definitely isn't.

 

[Tim Hall]

Yes I have. I used to fix quite a bit of out-of-warrantee Cervelo frames with that problem and somewhere in my archives I have some nice photos of the problem. I'll look for them. It was so severe that the sleeve would break off inside after being eroded to foil thickness. The warrantee period on the frames was such that second owners could not claim. I suppose galvanic implies a bi-metal reaction, which it definitely isn't.

Yes, it was the galvanic bit I was having problems with. Sounds more like an unholy mixture of water, salt and other road chemistry getting in there under capilliary action, sitting unseen and eating the aluminium away.

PS, warranty not warrantee. The double "e" is for guarantee (pet hate of mine).

 

[Yellow Saddle]

A galvanic reaction between the carbon (or resin) and aluminium sounds a bit odd. Got any more info on it?

I found this one. It was an interesting case in that the seatpost was an alu tube wrapped in carbon. The corrosion happened between the carbon wrap and alu. I had to remove it by sawing off the post and then sawing vertically through the aluminium bit. I can't remember what bike it was in, whether it was a carbon or alu frame, but the problem was between the carbon and alu in the post itself.

Edit: I have changed my mind about his seatpost. This problem, I now remember was pure oxidation, because the post had swelled to the point where it could not be removed and the white is a dead give-away.

The Cervelo problem was pure galvanic. It just had the look. Metal that had disappeared, holes and jagged edges. The insert on those BBs is probably 3 or 4mm thick, so the corrosion required to eat right through that was pretty severe.

 

[Profpointy]

Yes, it was the galvanic bit I was having problems with. Sounds more like an unholy mixture of water, salt and other road chemistry getting in there under capilliary action, sitting unseen and eating the aluminium away.

PS, warranty not warrantee. The double "e" is for guarantee (pet hate of mine).

I was about to pooh-pooh galvanic, then remembered that carbon, including carbon fibre, is a conductor of electricity, so the galavanic mechanism quite possibly does operate - and the aluminium will naturally be sacrificed protecting the less reactive carbon
Caveat- above is half remembered theory rather than expertise .

 

[Yellow Saddle]

Yes, it was the galvanic bit I was having problems with. Sounds more like an unholy mixture of water, salt and other road chemistry getting in there under capilliary action, sitting unseen and eating the aluminium away.

PS, warranty not warrantee. The double "e" is for guarantee (pet hate of mine).

No salt where this happened. Just water and whatever washes off the roads. In the Cervelo case all the water had to come in via the seatpost since the BB was not drained.

I guarantee you that I will not warrantee anything ever again.

 

[Steve Malkin]

Galvanic corrosion between carbon fibre and aluminium is a well known effect.
There's an article here which discusses it for those interested:
http://www.corrosionpedia.com/2/1556/corrosion/galvanic-corrosion-of-metals-connected-to-carbon-fiber-reinforced-polymers

 

[ushills]

Galvanic corrosion between carbon fibre and aluminium is a well known effect.
There's an article here which discusses it for those interested:
http://www.corrosionpedia.com/2/1556/corrosion/galvanic-corrosion-of-metals-connected-to-carbon-fiber-reinforced-polymers

Very interesting and something my engineering mind had not considered.

Looks like I will be sticking to Alu or Ti for my next frame.