Product quality and durability are two issues that continue to be hotly debated in connection with carbon. Even 25 years after the first large scale production of carbon tube frames and seven years after the weight of the mass-produced frame fell below the threshold of one kilogram for the first time, it is impossible to give a general answer to such questions.
The reason lies in the material itself. As the correct name of carbon fibre-reinforced plastic implies, this is a composite material. Using a matrix, typically made of epoxy resin, carbon fibres in the form of fabric are processed in several layers to form a laminate. Only if a sufficient number of fibres are integrated into the respective directions of load and only if the matrix binds this construction cleanly together is the result a resilient structure.
In the case of bicycle parts, the layer stacking of a component is mainly done by hand. Robustness of shape is produced under pressure and heat as the resin hardens. The material - and thus its properties – comes into existence during the fabrication of each individual component. Only with meticulous work and careful control are the component properties of a production batch approximately the same.
In principle, production defects, delaminations, i.e. separation of individual fabric layers or cracks due to overload, can cause the component to no longer bear the load in operation and consequently fail. In the case of poor processing or weak fibre material, this can occur quite suddenly and even with serious impact, which is why the term "sudden death" has been adopted for such catastrophic failure.
What damages carbon?
Some years after the start of the second wave of the use of carbon in bicycle construction, it can be stated that sudden death is actually only occasionally the result of construction defects. Carefully produced carbon components, on which the designer has done a good job, are inconspicuous in normal operation and almost everlasting.
However, this is no reason to rest on one’s laurels, because expensive material does break time and again. Three damage mechanisms top the list of reasons for failure:
- Inexpert assembly
- Incorrect combination of components
- Careless handling by the end user
The first reason begins with the fact that screws are still regularly tightened by hand and therefore often with excessive force. Carbon assembly paste has not yet found its way onto every workbench, which can also mean that parts can only be fixed with high or excessive torque, causing pressure-sensitive carbon to fail.
Asymmetrical clamp zones, for example brake lever clamps, and sharp components are characteristics that the eye of a trained mechanic can recognise and which should not be used.
Many failures occur when bicycles fall over and collide with a sharp edge, or when the handlebar of a mountain-bike pivots and brake and gear lever hit the top tube; or during transportation if the carbon is inadequately protected against impact or the bike is transported on a rack. Claws commonly used on bicycle racks pincer delicate carbon frames in such a way that the material gives way and cracks.
What to do in the worst case
Basically, it is impossible to give overloaded carbon components the all-clear or carte blanche. Bicycles must be carefully checked after an overload has occurred. A visual inspection alone is not enough. Components made of carbon fibre-reinforced plastic do not bend, even after an overload; they take their original shape. In contrast to aluminium and steel, measuring the frame with a square ruler brings no usable result.
Some procedures such as X-ray and computer tomography are currently only theoretical and in no way applicable from an economically acceptable point of view.
Pulse thermography seems at first glance to be a viable and reasonably priced method. The colourful images generated show defects in laminate, resin accumulations, foam inserts, metal components, different paint layer thicknesses, labels, etc. Such thermographic images provide a great deal of information about many things, too many in fact. At present, no expert is able to say whether the positions that show up on the thermographic image are accident-related and whether or not they affect durability.
To make meaningful use of this procedure, it would be necessary to examine the component in question when new and store the images. Only a before and after comparison would provide reliable information on changes and therefore the current condition.
Stiffness tests are an adequate method to check the condition of carbon components. Some manufacturers have therefore switched over to testing products in this way after production. In case of variance the components concerned are removed from sale.
As a general rule, stiffness deteriorates when the composite disintegrates, is damaged or if the part is very fatigued. Here again, however, in the absence of a comparative value for an identical component in new condition, the measured value of a used or accident-damaged component says very little. The prerequisite, therefore, is a database of measured values.
Industrial and academic researchers are currently working on projects to alleviate potential risks and improve testing methods.
What to repair, and where?
Carbon is a material that can be repaired with basic materials, i.e. resin and fibre mats. As with a new bicycle, the verdict on whether repairs to small holes, cracks in the top tube due to being hit by the handlebar, or the signs of a chain suck will meet with success depends entirely on the expertise and experience of the repair technician. Layer-by-layer abrasion of damaged areas is technically good and visually clean. Stair-like abrasion allows the sound addition of new laminate layers until the original wall thickness is restored. In this way a durable composite is produced with no changes in stiffness.
When finished by an experienced paint shop, such repairs are no longer visible to the naked eye. A small number of companies active in motor racing have specialised in such repairs.
"To do list" for manufacturers and dealers
While it is durable in normal use, carbon responds with equal sensitivity to misuse and overload. Even though repair options have emerged:
- By no means should all components and types of bicycle be made from this material;
- The limitations of the fibre structures should be clearly highlighted with regard to weight, misuse, etc.;
- Workshop mechanics should be trained;
- Meaningful user manuals should be provided with each component and bicycle to ensure that consumers receive clear instructions on how to handle sensitive sports equipment.