Here's the design. Sorry it's not in CAD! That's one technology I've never really got into. Also, I haven't drawn all 13 planks that go across, but I think you can get the idea. And sorry that I wrote on it while I was working out what lengths of wood to get.
The problem with a lot of the commercially available futon sofa beds is that they don't seem to realise that the sofa is likely to have its back to the wall, and when you turn it into a bed, you just want to pull it away from the wall into the space in the middle of the room, and not have to faff around moving the base in and out. This is not so difficult, but maybe I've just thought about this more than everyone else!
Even if the sofa does not have its back to the wall, the chances are that there will be a space in front of it which could be turned into a space to sleep. If not, then they don't really want a sofa bed, they want a futon cupboard. Perhaps a traditional Japanese futon cupboard. These come free with every traditional Japanese house.
Also, in terms of design criteria, it's going to be in the children's room, so I want something that a seven-year-old can turn into a bed, but that won't collapse if a ten-year-old and two of his mates are jumping on it. This is more of a challenge, but I think doable!
There are wheels on the floor-ward end of the seat. When the seat is lifted up, these should make contact with the ground, making it very easy to pull the seat forward, turning the A frame into a flat bed. It will be very difficult to do this when someone is sitting on the sofa, since the wheels are going to be pushed away from the ground by their weight, so I hope the wheels will only come into action when they are needed for conversion.
I'm hoping the weight of the frame, and the angle of the middle section as it meets the ground, will keep the back robust. If not, a latch can easily be retrofitted to lock the frame in the sofa position.
Putting the bed back into sofa position may be a two-handed job. Although the wheels are going to go back quite happily, the other part of the bed may need lifting up a little before it starts moving. We'll have to see how it works when it is made!
The materials I've used, by the way, are 2 by 4 (89 x 38mm) spruce for the frame with 89 x 15mm planks on top.
