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Old 07-10-18, 04:20 AM
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PMK
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Location: Royal Palm Beach, Florida
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Bikes: 2006 Co-Motion Roadster (Flat Bars, Discs, Carbon Fork), Some 1/2 bikes and a couple of KTM's

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Originally Posted by twocicle View Post
Nothing on the market is perfect though.

I would like to see more modern carbon frame design with shaped tubing rather than using standard straight round tubes lashed
Recently, my opinion regarding the fitted carbon tubes followed by wrapped joints was changed. In the past, I always believed that the wrapped joint construction was a means to a carbon fibre frame that did not optimise the materials properties or exploit the weight reduction.

Recently, I was asked to evaluate, and possibly repair a high end Jamis road bike frame. There was no crash damage, yet the frame was showing distress on both sides of the top tube, down tube and head tube joint / area. Externally, these were minor blemishes, and tiny single cracks in the paint.

The owner, a long time friend as honest as god, never crashed the bike, is easy on his stuff, plus is average build of average strength for a weekend warrior / infrequent racer.

On account of Jamis rejecting warranty replacement, it was decided to see exactly what was going on with the frame, and if it was viable to repair. No doubt I could have added plies to save the frame, but with the bad experience with Jamis, it was decided to see truly why the frame was failing.

I cut a portion of the frame away, making cuts vertically along the headtube then sliced the top tube and down tubes in a mannner to remove the frames forward front side section.

As I do in evaluating damage to composite aircraft components, everything was inspected, measured and documented before ply removal.

With care, each ply was sanded away, in a method, that exposed each layer as it was revealed. Once the carbon layers were stepped until one ply remained, and with notes / photos taken during this process, it became obvious that the very minor visual indications on the frames exterior, was the fuse of impending timebomb failure.

For reasons unknown, either engineering improperly sized the reinforcing plies, the frames production facility cut the plies too small or possibly the person accomplishing the ply layup stacks positioned the plies incorrectly, whatever the reason, the length of the cracked epoxy with corresponding broken fibres was likely 10 times more damage than what was visible.

Sadly, both sides of the frame were failing in a similar manner and pattern.

Essentially, the stress path was not adequate to distribute the loads properly. With this, the carbon was ever so slightly being flexed over the edge of the next ply, causing interply stress cracks in a multitude of directions.

Even with the photos, a written evaluation report, and allowing Jamis to inspect the failure, they refused to warranty the frame.

The Jamis frame failure had me consider many things about the use of carbon fibre in bicycle construction. Long ago, I knew the material was not optimised for bicycle use whether in a molded shaped form or wrapped tube construction.

Seeing the Jamis, and other failures, it became obvious that the Calfee construction technique of wrapped tube joints that I so dislike, is in my opinion a very smart conservative design that has excellent distribution of stess into the tubes, and safety margins far superior to a molded frame. I do not care for the bulbous look, but truly consider this to be function over asthetics.

Last edited by PMK; 07-10-18 at 04:24 AM.
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