Wednesday, June 04, 2008

3TTT seatpost failure

3TTT seatpost failure_0432
Originally uploaded by gtveloce
Doe sit look like a strange angle? It's a lightweight 3TTT seatpost that fitted into the slightly smaller than 'usual' diameter seat tube of my cabon-framed Look KG76. You can see that it has had fluting milled out and painted black. It and the Look frame (with Mavic ensemble) were sold new to me by Steve Hogg in 1990, so it was about 17 years old when it finally snapped, although it wasn't used for maybe 5 or those years and was never my only bike.

Did it look like it was about to fail? Nope, not at all. I looked and looked again several times at this very component and saw nothing untoward. Did it make any sounds or strange sensations? Well that's harder to answer... maybe. For perhaps 3 or 4 years the bike made a strange creak that seemed to come from the saddle, and the "fix" was to lower the saddle slightly. However I swapped saddles somewhat later and raised the seat height without inducing the creak. So I thought it was indeed saddle-related. Nevertheless it made me nervous and I feared that the carbon frame (with bonded aluminium lugs) was getting "flexy". So I examined it very carefully and could find no cracks or even a creak.

However 50 metres before it broke I felt that the saddle had swung very slightly to one side. Not an especially unusal event as the post can't be over-tightened without cracking the carbon tube. So I slowed to stop but had to get around a corner first... and it was in that corner (at about 25kmh) that 2 things happened. I leaned the bike at a small angle and I put weight on the saddle. (Bear in mind that 1km earlier I had descended at 50kmh and took a hard left hander - seated - at speed without drama.) Anyway, I think now that the crack was invisibly growing from the fluting and gradually circumnavigating the seat post. Only the merest connection remained when I entered that final corner, and I probably rose out of the saddle slightly and knocked the post out of alignment. When I sat down again the crack no longer aligned and simply collapsed under my weight. It took me by surprise and I ended up on the deck.

So the lesson is to expect the unexpected, look at everything for fatigue and don't push aluminium to destruction.. replace it early. And yes, I still use the KG76 today and the frame itself is fine. Touch wood (or carbon).

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