A friend brought me an old frame that he found on the roadside on bulky waste day (is that a thing outside of Austria and Germany?). The rear wheel was gone, the front one bent beyond repair, but the rest of the bike was in quite good condition. From the parts on it (a complete Shimano 105, 2x7 groupset) I guess it's from the early 90ies.
The interesting feature on it, one that I have never seen before, are these "bulges" on the main tubes (top tube, down tube, and seat tube).
The feature is visible on the second photo, and in the third image I have made a crude sketch of the cross-section of those tubes.
Does anyone know WHY they are there? Is there some engineering idea behind that? Is it just an optical feature?
PS: I thought that maybe it was some kind of identifying feature of the manufacturer, but I can not find any information about a bicycle manufacturer "Oberriet" (see photo nr. 4) at all. This just made me even more curious.