I agree with you. I think that a Ti bike can be more than stiff enough for a big rider and I certainly never said, nor meant to imply, that only small riders should consider Ti. My only point, which I seem to not be so good at making
, is that if a big rider is asking what would be the ultimate best choice for them that for many the answer would be steel. And again, not all Ti is created equal just as all steel isn't created equal........... meaning you can make appropriate or inappropriate bikes from either and it's not a matter of one material being better than the other.
I think that there is an optimum design and material for every rider. The key word(s) being 'optimum' and not 'one and only'. If someone owns a Ti bike, and loves it, that kicks butt and I never said that you shouldn't love it. And at the same time if a big guy were to come to me and ask for my professional opinion, based on my over 20 years of designing and building with both Ti and steel, I would lean toward steel for him.
There is one place I disagree with you and that is your thoughts on the tubes that transfer power from your feet to the rear wheel. It seems overly simple to some but the reality of the situation is that while the down tube does give a feeling of stability to the BB is does nothing to provide efficient transfer of power from your feet, through the crank, into the chain and then into the rear wheel. The only tubes that have a mechanical advantage and are in a position to transfer the force from the BB to the rear wheel are the c-stays. In fact you can remove the down tube altogether and still have a very stiff BB and great energy transfer from the BB to the rear wheel (i.e. the Slingshot bikes). The down tube, along with the top tube, provides torsional rigidity to the front of the bike which will make it steer and handle as it should but contributes little to the efficiency of the rear end of the bike and power transfer from your feet to the rear wheel.
Lastly, I think it's very important to not lump all similar materials in with one another. You have real world experience showing that your Ti bike is better for you than your steel bike and I would never argue with your findings or conclusion. I would however question if either of these materials were designed and used in the optimum way for you, your weight and how you will use the bike. Meaning that, as a big rider, a custom builder would pick specific tubes for you and your needs and I'll bet that both your Ti and steel bikes could be improved upon if you had a good custom builder design and build the bikes for you.......... and I think that if the designs were both optimized for you, your size, and how you ride that you would find that the steel bike would have a slight advantage over the Ti bike. The admittedly difficult thing in this is the fact that since they are custom built bikes you can't test ride them nor will most riders go as far as having two bikes built with differing materials so that they can compare them. This leaves the rider needing to ask the pro builder what they think would be best for them and then taking a small leap of faith and trusting them. If you are going to have a new bike built to replace your Soma I would suggest that you go to a builder like Tom Kellogg who offers both top shelf Ti and steel bikes and ask him what he feels would be best for you based on your needs. Only then can you be as sure as you can be that you will have the best thing for you.
All the best,
Dave