I thought that was the main point of hydro-forming, to put extra material where it’s needed but keeping the tubes thinner where it’s not.
Well based on conversations with an engineer in the bike industry, it seems that the point of hydroforming is to make shapes you wouldn’t otherwise be able to make. No one has yet come up with a problem that that solves but the industry likes to say that it’s for things like standover clearance or fork clearance when really it’s mostly about aesthetics and a little bit about packaging.
But also note that there are two issues here; one is the process of making different shaped tubes the other is making tubes that are antyhing other than straight. I’m not sure at what point a tube stops being optimal in strength terms if it has bulges in it, although I still can’t see why we need the bulges in the first place; there are plenty of great bikes that don’t use any kind of shaping, like the Cotic Rocket or any Nicolai you care to memtion.
Butting a tube, which is what you’re describing, is something the tubing industry has been doing for years and it certainly doesn’t need hydroforming to do that. Double and even triple butted tubes have been used in bike construction since I don’t know when; Reynolds was doing it with their 531 steel in the 50s I think. I even wonder if you can butt a hydroformed tube; can you?
Besides, the basic engineering premise is that the strongest tube between two points is a straight one, so the minute you make it anything other than straight, you have to make it heavier than it otherwise would be.
To quote one design engineer in relation to the UK’s most popular 140mm bike, i.e. the one he designed: “when we made that top tube kinked, we had to make that tube twice as heavy to make it just as strong and the only reason we had to make it kinked is because that’s what the marketing department told us the customer was demanding”.