For what it's worth (and out of my realm), I was led to believe that the Rigid models had the rod on the upside and it worked better than the subsequent under-slung rod brakes...due to the way the cams activate the leading/trailing shoes.
I tried it on my Plunger Flacket (Flash-Rocket), but the way the suspension operates it rendered it insane, as I expect would happen with a rod operated Swing-arm brake. A cable operated brake *may* work better on the top... more*dunno*
Like you Dutch I really struggled that with a SLS type brake there is the “leading shoe should contact first theory” and diagrams on the net of this theory did not help me. I eventually figured it out (I think).
What bugged me most was reading that BSA racers commonly “reversed” brake levers to improve braking, which seems such a simple mod it makes you wonder why BSA did not get it right as standard!
This probably won’t make sense but I’ll give it a go!
it’s a fact that the cam contact point with each shoe happens such that one shoe contact point is further away from the spindle than the other. Ideally the setup of the brake lever should result in the leading shoe being contacted by the part of the cam that is nearest the spindle.
The reason for this is that the shoe with the cam contact point nearest the spindle moves outwards more than the other shoe, and you want the leading shoe to move more, and so contact first.
If the trailing shoe moves out further than the leading shoe then it will contact the drum first and potentially stop the cam moving, and hence stop the leading shoe even touching the drum.
It is generally understood that the leading shoe provides (say) 75% of the braking force, the trailing shoe only 25%, so if the trailing shoe contacts first then you are missing out on up to 75% of your braking power.
I did an experiment where (on a pretty poor 8” front brake) I built up the leading shoe flat cam bearing face by about 1mm with some weld, this meant the leading shoe HAD to contact the drum first even though the brake arm was the wrong way round on this particular brake set up. The result was a brake that really worked well. So my next job is to reverse the brake arm because I now know from my experiment that doing that will make the brake work better and stay working better, whereas with the experimental shoe mod the leading shoe will wear and the brake return to being rubbish.
My apologies if anyone reads this and it still does not help. I tried!