We often get bikes coming in where the metal inserts such as bottom bracket shells, dropouts and suspension pivots have come loose in the frame.

Sometimes, riders attempt to re-bond them themselves with adhesive from the local hardware, which fails again after a short time. We also get bikes in that other repairers have "fixed", which fail within a week.

So what is the trick, how do you get these inserts to stick?

The answer is surface preparation and using the correct adhesives.

Aluminium is difficult to bond to due to the oxide layer that forms on the surface. This layer forms a hard corrosion resistant barrier, however it does not bond very well.

To get a good bond the oxide layer needs to be fully stripped from the surface, this can be done in many ways.

Whilst working at DSTO (Defence Science Technology Organisation) there was lots of work done on this for bonding composite repairs to metal aircraft, F 111, Hercules etc. Sanding the aluminium is one way, but we did find that some types of sandpaper actually contaminate the surface and make it worse. Other methods include sandblasting and chemical treatments.

The chemical treatments provided the best results.

The correct adhesive selection is also important to match the properties and performance requirements. Getting the adhesive thickness correct is also important.

When we bond inserts here we use the same chemical treatments and adhesives used on aircraft repairs.

Of course some bonded inserts will fail regardless of how well it is bonded due to poor design of the join, thermal mismatch, corrosion etc. However if it is not bonded well in the first place it is just a matter of time till it fails.