The futon sofa bed is now working, and here are some reflections on the design. I can make some modifications and address some of the issues, but really I'd really like to make another one. To justify this I'd need to buy another futon, of course.
The front should probably be higher. I kept the angle to about 7 degrees, so the base is around 20 cm off the ground at the front. This height is great for kids, or at least would be if their upper legs were longer, since the chair part is quite long. They seem to sit on it quite happily, when it's not being used to accumulate crap.
Another issue is that the back is vertical, and the frame at the back of the chair is a right-angled triangle. When you sit back, this rocks a little and I think an isosceles triangle, more like an A-frame, would be more stable. As it is there is a chance of it banging back into the wall behind.
So geometrically speaking, I probably should have made the back a little taller and at more of an angle, the front a little higher, and the seat and middle seat-back sections a little shorter.
As I predicted, it is possible for a small child to open the sofa into a futon, but after pulling it out half way, it crashes down with a large amount of noise and power, and the health-and-safety part of me is worried about other children's fingers or toes getting in there. So some kind of damping would be a good idea. It may be possible to get more sophisticated hinges, but these are likely to work in the wrong way. I'd like something that will provide more resistance the faster it moves, but simple damping on the hinges is likely to be the opposite and resistance will decrease as it gets faster.
Also as predicted, putting the futon back into sofa position requires two hands, or a long arm, as the middle part needs lifting a little before it is pushed in.
It may be possible to work out a strap and pulley system that would both control the descent of the A-frame, and facilitate its return, but I think that's going to take at least another month, 12 envelope backs and ten sides of A4.
Also the spacing of the horizontal slats is a bit wide, and I think I need to increase it by three or six. There are four fixed slats, at the end of each of the movable parts, so they can only be increased in multiples of three. Currently there are 13 90 mm slats, with gaps of around 80 mm between each, which are big enough for little feet to push into when they are walking along the futon on top. Another three slats will bring the gaps down to about 45 mm, which should be small enough. Six extra slats will give a gap of just 25 mm, which may not be big enough to allow it to fold up properly.
Spruce, the material I used, seems quite stiff and pine may have been more springy.
Since I last looked, another kids' futon seems to have found its way between the frame and the futon on top, which has made the seat a little more comfortable. It has also created a home for another piece of loose soft furnishing that was floating around the house, which was the main job of the device. Perhaps it can attract even more bits of soft furnishing, and solve the height and leg-length problems too!
