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.

I often hear the words "It's just a bike!", meaning that the level of care required can be less than that for other items.

This phrase has always set off alarm bells for me, as the people who say it obviously don't ride or have a passion for cycling.

It's not a plane they say, why do you need to do Ultrasound scans and other methods that the aerospace industry use?

Well for me the answer is simple, I ride, and I would like to continue to ride for many more years to come, without the avoidable risk of my bike failing.

The technology used in bikes these days has far more in common with aerospace than other modes of transport, utilising advanced composites and alloys, fly by wire and data acquisition. In fact bicycles and aircraft have always been linked.

The Wright brothers were bicycle engineers before they did their epic first flight over 100 years ago. There has always been a connection between cycling and flying.

Fig 1. Photo of the Wright Brothers Workshop.

Torque wrenches have replaced shifting spanners and hammers, these bikes cannot be treated like farm tractors, they are performance machines.

As such they need to be cared for to maintain performance and ensure safety, just like an aircraft. Boeing or Airbus are not going to replace their Ultrasound scanners with someone who just looks at the part and says "it should be ok", and neither should you.

Riding a damaged bike may not have the consequences for as many people if it fails, but it will have a major consequence on you the rider and maybe also on family and friends around you. At best a failure reduces the raw pleasure of getting out to ride your bike, which is what it is all about. 

Bikes are not planes, that is true, but they have much in common and that is the reason to use the best technology and methods available.

It has been suggested by some people that Ultrasound scans aren't useful because they don't find cracks and cannot differentiate between fillers used during manufacture and real defects.

It has even been suggested that because bikes are not aircraft, you don't need the same level of quality or care!

As a person with over 25 years Non Destructive Inspection experience that has held CASA (Civil Aviation Safety Authority), certification as well as the equivalent FAA and a range of military approvals, I can answer these questions accurately.

Cracks that are perpendicular to the laminate plane are dificult to find with Ultrasound, however in carbon composite there is usually delamination associated with a crack. Delamination is easily found as it is a planar defect which Ultrasound is the proven method for detection. Cracks on the surface are also visible, therefore can easily be found by visual inspection, unlike internal delamination which cannot be found visually..

Using a combination of methods instead of being limited to only one method is the best approach for detecting, identifying and sizing damage and defects.

Fillers are sometimes used, it was common to find fillers on many older frames however the latest frames tend to use very little as the moulding methods have improved. Fillers also add weight to the bike which is undesirable so they avoid it's use where possible.

We know this because we section cut frames to see how they are built and use these as Ultrasound reference standards.

Ultrasonically, the velocity of sound of the fillers used is a little different to resin and carbon, which also varies due to fibre volume, type and resin content. The key ingredient using Ultrasound is experience and the difference is quite obvious between fillers and rejectable indications. The Ultrasound display may show a small interface which the inexperienced may see as a defect, however a back wall signal (the internal face of the part) will still exist. With a delamination the back wall signal will be scattered or may even disappear, so the experienced technician can tell the difference between fillers, voids, porosity, foreign object and delamination. To pass the CASA level exams you have to be able to do this.

Of course another big advantage of having Ultrasound scan capability is that we can ensure that our repairs (or other peoples repairs) do not have porosity or voids and are laminated correctly. So apart from finding all the damage, we can also provide the same level of confidence that the repair is sound as well. Without the scan you just don't know how good the repair is internally.

The level of care in aerospace has always been at a higher level than other fields such as automotive, boating etc. Be thankful that when you step on board an aircraft, they use Ultrasound scans to avoid catastrophic failure, not only on carbon parts but metal parts as well.

I don't know about you, but when I am flying (pun intended) down a big hill on my bike, I like to know that the highest level of care has been applied to avoid a component failure causing a crash with potential serious consequences.

I apply the same standards to your bike and that is why we are the "Gold Standard" in carbon bike assessment and repair.

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.