I continue to think that a modern titanium frameset made from oversized tubes is more than stiff enough for a big rider who is not racing.

Based on my own experience, I cannot agree with the idea that lighter riders only should consider titanium.

I consider my Soma to be a well made steel bike, Tange Prestige is a very good quality heat treated steel. Yet the bike simply does not accelerate or climb as well as the ti bike. I could not get the steel bike up to higher speeds in the 25 to 35 mph range unless I was on a slope. The titanium bike, on the other hand, is much better at climbing and will sprint to 33 mph without assistance from wind or gravity. The two bikes could not be more different in this respect.

Some of my century rides feature 15 to 22% grades and the titanium bike allows me to make the most of every pedal stroke. The steel bike will also climb the grades, but with more effort and less speed. On a 200k, ride the small differences do add up.

I somewhat disagree that the chainstays provide all the power to the wheels. The chain provides the power, and the frameset may flex, which will reduces some of that power. All that is required is that the BB and rear axle be able to hold the bearings in place without unwanted flex. Its not especially important if the bearings are held rigid by the chainstays, seattube or down tube. All that matters is that the bearing housing not flex in a way that absorbs power.

I have seen 75hp chain driven systems in industrial application where the motor and the material handling systems were set on two different structures. As long as both structures are rigid, the only power loss is the 2% mechanical loss in the chain itself.

I'm planning on replacing the Soma to get a stiffer frame that can also be used for light touring. I'll use steel or ti. Based on experience, ti has all the advantages.