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Recently we have had a number of bikes come in with problems with the bottom bracket shell insert.


The problems have been with the aluminium insert, either disbonding from the carbon with a crack around the insert, or voids in the adhesive joint, again leading to a crack at the insert. The failures typically occur due to a combination of a flaw and a less than ideal design, or user error in assembly or dis-assembly.

The bikes were all big name brands.

A couple of these came to us after being "repaired" by another repairer, in both cases the repair didn't last the first ride before the crack reappeared. The "repair" was not successful for many reasons including them not having the ability to find the problem in the first place. Putting glue on the surface and painting over a crack does not actually make a structural repair.

The other thing which showed the lack of specific knowledge about bikes was that the threaded bottom bracket shell had not been faced after painting. Threaded BB shells must be faced to ensure proper operation of the bearings, so the previous "repair" cost the rider money and watts!!

Another bike was scrapped because it was not safely repairable. In this case it had a aluminium BB30 shell. It had previously been fitted with a BB30 to BSA threaded adapter which is pressed and bonded in place. The owner then had a bike shop remove the adapter after which the damage appeared. I suspect that the adapter was not removed properly, probably by hitting it with a hammer, which caused the fracture of the bonded joint.

Bonding aluminium to carbon is not an easy thing to do properly, special treatments and materials are required to even have a chance of the parts holding. Other issues such as galvanic reactions and thermal mis-match also contribute to problems along with poor design. See our other tech article on metal bonding for more information.

So the rules are:

  • Don't hit a sub 1kg carbon frame, (or any frame really) with a hammer.
  • If using a threaded BB shell ensure that the shell is faced and the threads are clean.
  • If seeking a repair ensure that the repairer can actually find the damage.
  • Painting the bike does not make a structural repair.
  • If in doubt seek professional advice.