I find I'm able to stretch using a hammer, but I've never mastered shrinking. But I dropped on a low-cost shrinker / stretcher pair which, as you say, are very useful. In fact I think one of the first parts I made for my project was that section you show there.
Imo one of the hardest panels to make is an inner wheelhouse. For my bmw 2500 I need to make my own. I started by folding the Z-like shape into a straight strip. Then I continued with my stretcher to make the arch. Because the arch also bends inwards, I used the shrinker to make the strip follow that shape too. It’s a lot of walking back and forth between the workbench and the body. But I’m pretty happy with how it fits.
That's nice, I have a similar need for a part for my current project. The only problem I have is that where you have that lowest bend onto the lip, I have another straight section followed by a 90-degree bend to the lip. I was wondering whether I'd get away with the fact that I can't stretch it, though it looks like you've got away with that on the centre of the three sections of your profile there. Mine is more like this in profile, and if I can, I'd really like to make it in one piece.
I’m not native in English, so I hope my answer is helpful. I’m not sure I got your question right.
To make the arch work, I started with making the folds. It left me with the problem that I had to stretch the metal of the top strip (upper side in your picture) quite far to make it actually arch. I solved this as follows. I used 1,5 mm instead of the normal 1 mm to give me more tolerance. Regarding the stretching I stretched the top part evenly and then I stretched a 1 cm rim an other time.
As you may see, the part I was stretching is also at a bit of an angle relative to the bottom that welds to the outer panel. It gave me no serious issue as long as I checked and undid any torque that happend.
I really had to take care that the piece would not torque. That distorted the shape of the arc.
Then I started the bit that is on the bottom of your drawing. In small steps I shrunk the metal. It gave me the ‘inward’ arc lower down. This proces was rather slow, but in the end it did fit.
The only thing I can imagine that might happen is that the lowest bit (that welds to the outer panel) wants to bend upwards a bit as you make the arch. You may need to shrink it first a bit to make it 90 degrees again. But my 3d thinking was a tad rusty when I cooked this up, so it maybe will simply work.
I had a go at this yesterday. As you said, I made the folds first, then started stretching the upper section to force the arch shape. It sort-of worked, but there seems to be a limit on how much of a curve I can get, and I might need to try a different stretcher. I am getting quite a bit of distortion, and as you also said, I needed to shrink the lower section to keep that straight - this is no problem, as I'd need to do that anyway because it's not really straight, it curves inwards towards the bottom of the arch profile - where it hits the sill is further in to the centre of the car than the top of the arch.
I will have another go today. I wonder whether I can help things along by hammering the top edge of the centre vertical section to try to get some stretch there. I also wonder whether it might work better if I do only the bottom fold, then stretch the top part, then when it's close to the arch shape that I need, make the second fold and adjust it to keep the curve.
For speed, it might be easier for me to just make it in sections and weld it in place, though I can make larger sections than I did last time I tried this - that time I was making about 100mm at a time. It worked well, but if I could do one piece (or maybe three pieces) for the whole arch, that would be better.
Droopsnoot. I guess there’s a natural limit to the amount you can stretch. On my first try I had to make the strip that I stretch smaller. Perhaps that helps. With regard to the distortion, I managed to control it by shrinking the other end,
I have gone back to this now, and made it in more than one section. I've also found that I'm very lucky - in about 80% of the length of the arch, I don't actually need the top part of the three planes, only the right-angle section, and of course that's much easier to work with. For the small sections front and back, I'll just make the "top" bit separately. I'd probably have had to do most of that anyway, as it's a different profile there as the outer panel comes inwards to join the rest of the car.