From my perspective (bicycle frame building), the biggest problem with Ti other than $$$ is the hassle and cost of back purging. Coming up with proper fittings to run argon behind the welds would be a pain for the average builder and wouldn't be worth it.
Regarding cro-mo steel, I know I have read on here about post-weld processing of it. That might be the case in the automotive/race car world but I can guarantee that millions of bikes worldwide have been made with cromoly tubing and none of them have gone through any sort of heat treatment, annealing, etc. Just FWIW.
I'll second what Bbox said:
Quote:
All other things being equal, stiffness is a linear function of Young's modulus (called "E"). The E value of Aluminum is 10 million (no units), E for titanium is 20 million and E for steel is 30 million. NOW HERE'S THE INTERESTING BIT: Titanium weighs twice as much as aluminum and steel weighs three times as much as aluminum. Their E values are in exactly the same ratios as their weight!!!
So what's the bottom line? In a properly designed chassis, the weight for a specific stiffness will WEIGH THE SAME NO MATTER WHAT THE METAL USED. That is why most spaceframe chassis, like the Super 7, were made from carbon steels, not chrome/moly, titanium or aluminum.
Again, my perspective is from bicycle frame design but this is precisely why an aluminum bike frame does not weigh 1/3 of that of a
properly engineered steel frame. The one advantage that aluminum (and perhaps Ti) has over steel is when you need
something that has to hold
something else...for instance a bracket that holds a bearing and has to be welded to some tube. Sometimes you just need a certain amount of real estate to hold the bearing AND have enough area to weld to something else.