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