[Thomas]

Thank you, Steve! That are important comments (more from others?). When discussing with a local tube bender I will ask him about the tube thicknesses. I simply have no feeling for it. I agree, the 135 degree bend will probably need some support (horizontal tubes between them?). The rack could get two additional supports at the mudguard right below the rack centre. There are two bolds which could do the job (see picture). Yes, the cautious approach is always the silver bullet, especially for this case. I don't want to loose my girl, indeed!
Again thank you and cheers, Thomas

[Greybeard]

Hello, Tomas.
I tend to work to the old engineering/ship-building adage that if something looks right, then it probably is right. I think that using 12mm tube as you suggest would not be strong enough to take a pillion safely. The upright sections with the 135 degree bend, are in my opinion, likely to be a weak point. I would go for a 15-16mm tubing with an additional vertical section at the midpoint in addition to mountings onto the mudguard itself.
I may of course be tending towards the cautious approach, but it's not worth putting a pillion at risk. There's a large weight difference (say that quietly!) between a full picnic basket and the Missus 
A pillion seat would of course be raised by the height of the rack too, which in itself might cause balance issues.

Steve

[Thomas]

Hi all! I think about building a rear rack out of steelfor my 1937 Model 9 to place a sprung pillion seat on it. Attached is a respective CAD. The tube diameters will be 12mm (not 10mm). It shows that the major support comes from the two single points on both sides although an additional bar will go to the mudguard so that tilt along driving direction is avoided. If that's fine I could change between pillion and transportation by simply connecting and disconnecting the pillion seat, not the whole mess altogether. I wonder if a rack out of 12mm tubes can manage a typical pillion load if the machine has no rear suspension. Is there a high risk that the rack breaks?
Cheers, Thomas