[ajb]

I don't think that's true of the mechanism I saw, which I believe was quite common, but it's 20 years since I looked at the mechanism.

Ok I've found a video with one in: https://m.youtube.com/shorts/zLuJYlHBSkU

The counter weight is always taking the full weight of the door, there's just a linkage which rotates the door on the way up. So, not the same as the spring ones, which follow a curved channel.

This makes me less keen on them as in theory it's going to fall when it wears out, albeit with only 50% of the weight. Unless there's another failsafe.

[ajb]

Thinking about it, the failsafe should be that if one side fails first, it gets stuck against the track. Although I don't think the one in the vid would, because the track points inward