Saddle clamp bolts
It’s a regrettably common occurrence for heavy riders. Single bolt designs are most vulnerable. The bolt is often only a medium strength grade (e.g. ‘8.8’) and electro-plated, which process tends to reduce the resistance of steel to fatigue. Your photo nicely illustrates that. The smooth surface is the zone of gradual, progressive crack growth, culminating in a few more obvious crack fronts, further apart, at the edge of the final catastrophic tearing fracture.
Considering your experience, it would be prudent to replace the pretty shiny bolts in your seatposts with plain black high-tensile (12.9 grade) bolts – that will go rusty but nevertheless remain strong – of identical thread, length and head size, bought from an industrial fastener supplier. There is sure to be one on a trading estate somewhere near you.
When failures such as this are brought to the attention of those importing the product (it is rarely possible to discover the actual manufacturer), one is invariably told that it’s never happened to that product before. And since products change cosmetically all of the time, that might even be true! But we know this happens and continues to happen, with seatposts of all but the finest quality.
People very rarely get seriously injured this way, fortunately. Until someone does, and is able to prove that it wasn’t the bike hitting the road that broke the saddle off, and that they didn’t clamp the saddle beyond the limiting measurements on its rails (both tricky), we shall have to go on living with this risk – or upgrade the bolts.
This was first published in the October / November 2014 edition of Cycle magazine.