I don’t quite understand how these things are joined. In two crashes recently I have ended up with twisted bars, and in the second case I have no idea why I crashed. Now this is probably a stupid question deserving of a response telling me how badly I ride. But is there any possibility whatsoever that the steerer tube might rotate inside the crown? I’d like to be quite sure before trying to roost the gnar (or whatever it is I’m meant to do) on that bike again.

I’d go with the steerer twisting inside the stem as being the most likely issue.

It is probably theoretically possible but I wouldn’t want to be in a crash so severe that it cause it to happen. It is far far more likely that the stem rotated on the steerer tube than anything else.

I’d go with the steerer twisting inside the stem as being the most likely issue.

That.

It’s not impossible, don’t envy you trying to eliminate it as a cause, as it will often end up in a nasty crash (as you’re potentially found!). I’d very slightly mark the steerer where the stem clamps, so you can see if it’s the stem or steerer twisting.

Yes, I’m planning to mark it, and yes I think the stem rotating is more likely though it was already done up bloody tight after the first incident :/ Hopefully the stem is rotating because of the crash, rather than me crashing because of the stem rotating.

It seems very unlikely. I’d prefer the simplest explanation:

I have no idea why I crashed

Sometimes that just happens.

Are steerers not keyed into the crown?

Fwiw i used to have a Sunline stem that seemed to be a reasonable fit on the steerer but in some circumstances in would move. When I swapped it, I realised that not much of the stem was actually in contact with the steerer apart from the areas where it was clamped

If you’re going to mark it use something like paint rather than centre punching or scoring it.

I had a steerer tube twist in the crown of a set of maunitou xverts.
Going down the DH course at Strathyre, turn the bars to corner, bike ploughs straight on.

done up bloody tight after the first incident

not always the best idea, in a crash something has to give, stem rotating on steerer is preferable to bars snapping, stem snapping, etc.

Are steerers not keyed into the crown?

I would think they are if they’re a separate assembly. Seems an obvious thing to do. Whatever bonds steerer to crown would be susceptible to twisting rather than pulling.