My bike is trying to kill me. Twice in two weeks I've been thrown over the handlebars. The first excursion was puzzling, the second simply painful.
It started with a relatively low-speed crash as I came off the bike track in Hobart after a morning spin. One moment I was bobbing along out of the saddle, the next I was on my arse. At the time I put it down to a mudguard stay popping out at an inopportune moment and lodging in the front wheel, although there was no marks on either to indicate they'd jammed together. Nonetheless, I fixed the stay and carried on assuming all was well.
Then yesterday on my way home from my regular Sunday ride, it happened again. I rose from the saddle in a low gear on a slight rise and bang, down I went, taking the impact on my right knee again. This time the damage was a lot more - a big open wound on my knee with lots of dirt and gravel. I picked up the bike and noticed the chain had snapped, although I suspect this isn't what caused the accident. Puzzled and not a little sore I walked the bike the three kilometres back to the car, coasting down the hills where I could.
Once I cleaned up my knee I had a closer look at the bike. Not a mark on it. I thought it might be a mudguard problem: a sudden flexing of the wheel under load stopping the back wheel. But that didn't seem likely. I consulted my brains trust on Facebook who came up with a bunch of possibilities, the most likely of which seems to be chainsuck, a jamming of the chain which happens when the chain fails to disengage from the back of the chainrings properly. It's more common with worn chainrings apparently. Two things confirm this diagnosis; a slight hooking of the chainrings indicating they're a bit worn - hence likely to catch - and a few telltale chips in the paint on the chainstay.
I'll be replacing the worn chainring and the chain, and likely the cassette as well. The two incidents have spooked me a bit, but I guess it will make me a bit more focussed on regular maintenance, something I might have become a bit slack with of late.