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Seems very mysterious to me. The only time anything similar has happened to me was when track nuts weren't done up tight enough. The rear wheel would end up jammed against a chainstay. But if you are finding your rear wheel still centered then I would guess that it's a drivechain issue.
The only thing I can think of is that perhaps the chain managed to swing inboard, or was nudged by your foot, such that the tip of the chainring got caught under the edge of the chain link plate. Like halfway towards derailing the chain. This would lock up the drivechain instantaneously.
Did you feel the cranks lock up at the time?
Also can we have a closer side view of the front chainring?
Ndeipi-
Will grab a picture of the front chainring tomorrow, thanks both for the input! How do I check for stiff links? I could try running it free for a bit (reluctantly) but I can’t replicate the issue even if it’s fixed, and it’s occurred about once every two weeks or so with daily commutes to multiple places?
The cranks definitely did lock up momentarily, hence coming unclipped each time, but given it’s fixed presumably that would have happened regardless of whether it’s the wheel or the chainring locking up?
youClown
Sorry to ask a repeat question, but this is really bugging me because the problem reoccurred today and I feel it is only a matter of time before it happens when a car is just behind; opinions would be really welcome (or if this is the wrong thread to ask in, do let me know where I should go!)
The first incident was when I was commuting to work and all of a sudden heard a bang, like as if my wheel had gone into a pothole or my pedal had scraped the floor. But neither of those seem to be it because there were no significant potholes on the road and I was going straight.
My saddle had been turned 90deg sideways under me (like as if the bike had suddenly fishtailed while I was still seated, with the seatpost just slightly loose enough to move given sufficient force) but I thankfully remained on the bike and guided it to a stop. I found that my rear wheel was slightly mis-aligned and hence pressed into one side of my brake calipers, but when I shifted the caliper and spun the wheel to check, it spun freely and didn't seem out of true in relation to my chainstay.
Turned the saddle back round, got back on the bike and cycled the last 5 minutes to work completely uneventfully - and made the commute home absolutely fine like as if nothing was wrong, at the end of the day.
Happened again when I was going on a straight road, heard a bang/pop and fishtailed, came unclipped, rode my top tube for a while before braking the bike to a stop. Checked the rear end, no diagnosable issues.
Just today I was making a slow turn onto a road from a pavement, again heard a pop like as if my rear wheel had gotten jammed, and came off the saddle onto the top tube. Went into the wrong side of the lane but thankfully it was a quiet road so the oncoming car had plenty of time to see me and slow. Got back on the bike, checked the rear wheel was spinning freely, then made the rest of the commute just fine.
Pics of the rear end attached; it’s a fixed gear conversion so I have 0 idea what could have happened, because: