Yebbut, you've already conceded (I think?) that titanium is strong and light, so why is that a bad thing for bikes, but a good thing for aircraft (the latter point you've said isn't a sensible comparison, but I don't follow the argument why)
OK, titanium is difficult to work with, and may not be readily available in the tubing you'd ideally choose, but neither of these points make the material itself unsuitable.
Prof. I didn't concede that titanium is strong or light. I simply place it right between the other two frame metals in terms of strength and density (weight). Strong is subjective as is light.
Who said aircraft are built from titanium? Not me. In fact, I doubt any aircraft is made from titanium. These things, just like cars or bicycles, are generally made from a variety of materials. Aluminium, carbon fibre, steel and indeed titanium can be found in most of them. I, apparently have some titanium bolts in my Campagnolo brake calipers. It doesn't mean that my bike is made from titanium. Same for the fictitious aeroplane. The argument of why titanium is good for a hypothetical airoplane and thus good for a bicycle is drawing non-existent parallels. We can't argue like that in engineering. By analogy, I can say the Spruce Goose was built from birch, therefore it should be good enough for a bicycle. The Spruce Goose was indeed one of the largest aircraft ever built. Or, I could say that I like ice cream and because you're also a male homo sapiens, you also like ice cream. That is a fallacy and not a sensible comparison. Someone threw that one at me but for me to now go and build an argument listing all the components that are suitable for aircraft use and then try and project them onto bicycles is futile. He will nitpick and build a strawman quicker than I can say "grumpy." That's why I rather said it is not a valid argument and invite the proponent of that argument then break the case down into detail, which he is clearly not going to do.
As for your last point: if any material is more difficult to work with than steel/alu/carbon, is more expensive, is less reliable and tubing in the right sizes are not available (they mostly are for certain style bikes) and it cannot conform to the demands of modern frame styling, I like to think the material is unsuitable. Beryllium for instance, can also be used to build frames. However, it needs to be manufactured in an environment where no-one must come into contact with fumes given off when it is heated or even just machined. That makes it an inappropriate material for the job. We can get the same job done better, using different materials.
I hate using this one because it smacks of call to authority, but if ti was so great why aren't the pro's using it? I'll redeem myself for asking it because I think I have answered that question adequately.
Prof, have you done the design-a-pen mind experiment yet? What is your chosen material?
