I had my helmet camera running, but it doesn't give a great picture in the dark. Basically, I went up onto the pavement, then I felt the saddle lurch underneath me. I'm amused by my running commentary on the video, since I didn't swear, I just said "Woah, woah, that's not good!" I came to a halt, and hopped off the bike to take a look. The saddle stayed in place when I got off, but as soon as I put my hand on it the whole post came off the bike. I then heard a tinkling noise and saw a short metal pole lying on the ground. At this point, I wasn't quite sure what had happened. It shouldn't be possible to pull the seat post all the way out of the bike (because the bung on the end is a bit wider than the hole), and I initially wondered whether the short fragment was from my pedals or bottom bracket.
I put the short pole into my front pannier, then put the seat post back into the hole. I wouldn't normally ride the bike in this condition, but I needed to catch the last train. So, I put the saddle a lot lower than usual: I felt a bit like the teenagers you see riding around on mountain bikes who can put their feet flat on the ground with their knees bent.
I made it to the station ok, and folded the bike up. However, when I picked it up it promptly unfolded. The seat pillar normally holds the folded assembly together, but it's now too short to achieve that! So, I had to carry the bike so that it leant against my leg, in order to keep it together.
So, I now need a replacement seatpost. Brompton list the range on their website. My seat pillar is telescopic (since I'm too tall for the standard one), but the top stem (the small bit that telescopes out) is ok; it's just the main stem that broke. So, I either have to replace the entire thing (QSPTA) or just the main stem (QSPTS). The main stem would be cheaper (e.g. £25.50 from SJS Cycles), and my top stem is titanium which they don't make any more (just steel or aluminium) so it might be stronger than a replacement part. On the other hand, if I order it online then I won't get it until at least Tuesday. If I go to Evans then they only stock the complete assembly, but I should be able to get it right away, and the extra cost (£48.50) probably evens out with postage and extra travel costs. I'll phone around in the morning to see what my options are.
My main concern is that I don't want this to happen again. The original seat pillar has lasted me for 8600 km (over 5000 miles), so one option would be routine replacement. I didn't notice any signs of damage before this happened, but maybe I just didn't know what to look for. Anyway, if you cycle then be aware that this can happen.
Here's the video from my helmet camera:
You can't really see much of the bike, but you can see that there's a steady stream of traffic on the road next to me.
I took some clearer photos when I got home. I turned the bike upside-down, and balanced the broken part of the seat pillar in its correct position:
Here's a close-up of the break:
Here's the entire length of the seatpost:
I assume that the break happened roughly where the pillar emerges from the frame, so the pillar was effectively acting as a long lever with my weight on the far end. However, the bike is supposed to be able to handle more than my weight (up to 110 kg).