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I've been using a 30.9mm SDG carbon fibre I-beam seat post on an alloy Giant Anthem with a Hope quick release seat clamp for the last 12 months. The post is fitted to the minimum insertion mark but there have been no problems whatsoever. When having a dropper post put on the Anthem I transferred the post and saddle onto a carbon Giant XTC hardtail with a bolted seat clamp. Again put in to the minimum insertion mark. This was tightened with a torque wrench to Giant's recommended figure of 5Nm. Towards the end of the second ride on the bike something felt wrong and I found the post had sheared through 75% of its circumference (basically the sides and rear), almost as if it had been sawn through. I hate to think what might have happened if I had hit a large bump whilst seated! I have returned the post to SDG's distributor and await their answer.
Carbon is the only type of seatpost I've never broken. But I'd never use one with a QR.
almost as if it had been sawn through
I had the same thing happen with an Easton EC70, looked just like it had been cut through just above the clamp.
It appeared to have failed under compression.
LBS/Easton changed it under warranty 🙂
That 5Nm torque recommendation is the one that Giant recommend but what about SDG?
I don't know if they still do, but Specialized used to have quite specific seatclamp requirements for their carbon posts (to do with the placement and design of the clamp "gap". I don't suppose SDG have anything to say in their "instructions"?
Just as an aside - lots of folk use the I-Beam seatpost as it gives a lot of fore/aft adjustment. A saddle set way back on a long seatpost is always going to be stressing the material somewhat.
Sounds like its suffered from being overtightened at some point, this splits some fibres and causes a crack, it slowly works its way round the post. You'd notice as it gets more and more flexy. Id not worry about total failure when riding, it will just flex loads before snapping off. Noticed mine (EC70) had gone mid ride so we took it out and tried to snap the bottom bit off so we could shove upper bit in so id still have a seat, it took bloody ages! Mine had been overtightened, it took hardly any force but you could see the indentations in the post, lasted 9 month before it went.