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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.

We get lots of inquiries where people send a photo of the damage found on their bike and ask us for a repair quote.

 

Mostly it isn't possible to do this because the majority of structural damage is often not visible on the surface, but within the carbon "laminate". As discussed in previous articles, carbon composite is made up of many thin layers and the common failure mode is for these layers to separate, we call this delamination.

This is why we use Ultrasound scans to determine the extent of the structural damage that requires repair, in the same way the aerospace industry does.

Ultrasound is the method of choice for finding delamination in carbon composite due to it's sensitivity and accuracy. With our methods we can find critical damage which could cause a catastrophic failure that would otherwise be unseen until it's too late.

There have been many cases where a frame comes in with what looks like a small mark on the top tube or even no visible mark at all. When the Ultrasound scan is done however we find large areas of tube to be delaminated, sometimes this renders the frame beyond the point of a safe repair and it is recommended to replace the frame.

 

Photos can still be useful for quoting on the paint repair of the item and based on experience if the part should be even considered for an assessment, however photos do not tell much of a story on what the significance of the structural damage may be.

 

Ride safe.


 

I often see and hear advertisements from repairers saying that after their repair the bike is "as good as new" or "even better than new". This may be the case for the paint, however the structure is a bit different.

These sort of comments just highlight a lack of understanding on composite materials. The fundamental principal of advanced composites is having a strong well aligned fibre carrying the load. Once this fibre is broken or cut, it will never be the same.

The way loads flow through the structure will be different and with an optimised laminate there is not much room for error as the margin of safety is quite low. This is evident in the way frame weights have been getting lower and lower. As the design loads are better understood the factor of safety tends to go down.

So what does all this mean for a repair?

A well executed repair should meet the original parts properties allowable specification. That is the part should be able to meet acceptable load criteria.

It is also very important to match the stiffness of the original part to avoid stress concentrations adjacent to the repair, thus it is important to match the materials as close as possible and infact not be "much stronger" than original.

Because there is an area of discontinuous fibre it will never be exactly the same because the repair is dependant on a good bond interface to the existing material. This is why resin selection for repair is critical as it needs to be an adhesive as well as a resin.

So theoretically a repair will never be as good as new, however it should be within the acceptable design limits of the structure allowing further use of the part.

How does this relate to bike frames?

The good news is that typical bike frames are made up of sections joined together, often in a similar way a proper repair is done. The frames are made of many smaller pieces of fibre joined together so the discontinuous fibre theory is less relevant, depending on the location within the frame. Frames are nowhere near the fully optimised continuous fibre theoretical ideal due to production limitations.

In almost all cases with a well executed repair on a bicycle frame you will not be able to notice any of these small changes in the way the load flows through the part. The bike will ride the same as before and the final weight after repair should also be close to the original.

 

The other popular myth is "undetectable repairs", well, they are detectable to us as we have specialist Non Destructive Inspection methods such as Ultrasound and our other techniques. With these methods we can find defects that the manufacturer doesn't know exist. These methods are required for composites used in aerospace, particularly repair so as to provide quality assurance that the repair is within the specification. These methods provide safety for flying in an aircraft and with our process when"low flying" on your bike!

 

For further information read the article below:

Black Art or Engineering article here.

Why Use Ultrasound article here