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Finished a ride at Afan on Saturday and as i reached the bridge to cross the river on my way back to the Drop Off for a welsh rarebit I sat down to hear an almightily crack and thank god im not a big chat as stood up straight away.
Is it something that happens to most riders? The seat post is 3 years old and i am not a heavy rider.
Is it one of the single bolt designs? Like:
[img] 
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In which case yes. I've done it a couple of times, and had others on rides with me do the same. I personally think it's a design flaw; any pressure/impact that isn't directly above the bolt will be levering the bolt into tension and while they can go years without failing, eventually they will, and the bigger / heavier the leverage, the sooner that'll be.
I try not to use single bolt designs now, but one bike still has one on and I carry a spare bolt just in case. And seeing as you can buy spares for pennies if you know where to go, I change the bolt every couple of months or so anyway just in case. I still shudder at the feel of the shaft of the seatpost on my inner thigh as my saddle suddenly folded backwards at D2D.
I think this is one of those failures (like a snapped mech-hanger) that can be completely prevented by carrying a spare at all times. 🙂
It is a very common problem for me, I have snapped single bolt clamps and two bolt clamps probably about 5 times. Positioning the saddle correctly helps.
It is strange as no one else in our riding group has the same problem, must be my fat ass 😉
I can see that 2-bolt ones (eg: Thomson) have the same issue but I'm slightly comforted that if one of those goes, at least the second is holding the saddle vaguely over the top of the large metal spike, making an impact with my scrotum or femoral artery slightly less likely.
I've seen single bolt ones go quite a few times - most of them on red Spesh Rockhoppers from a few years back.
Its happened often enough that I carry a large M8 bolt in my guiding kit for such eventualities.
It is very much like the one above in the pic. The guys at Skyline cycles where able to help out with a £2.99 bolt to fix the problem but best get a bolt to carry in the bag.
I've snapped a couple. First time, before I carried a spare resulte din 10 mile cycle home standing up which was a bit tiring. ;o)
I had a problem with a particular brand where the bolt had been machined (excessively) to save weight. The clamp didn't hold the post with the suggested torque, so i upped the ani a little (only 2NM) and bang it went. The same thing happened with the replacement. Just guess the combination of post, frame and clamp weren't compatible. I have replaced the bolt with a Ti one now and whacked it right up!
It is in fact the only part of a bicycle I have ever actually broken.
Broke one last week but through overtightening. Reckon it was on the way out bugger to get replaced as FSA didn't seem to be the standard size.
race face posts use a better system (imo) that removes the possibility of this happening. Used to happen to me enough that I bought one on purely this point.
I've snapped a single bolt design before but looking at the bolt afterwards it appeared to be made of aluminium which doesn't strike me as a good choice of material, I would have thought that steel (even over titanium) would be the material of choice.
I've had no problems with my dual-bolt Thomson in several years of use.