I found a bunch of new reference for this bike's gas tank, it turns out it's a metal gas canister with plastic fairing over it. This is another failed attempt, I'd say. I think I was trying to do two things at once, understand the form in detail, but also how to model this thing in a sensible way. I'm going to keep revisiting this until I get it. Maybe not tomorrow, but I won't consider myself good at modo unless I nail this. Are really tight automotive forms the hardest thing in 3D, period, or what?
This attempt was using ZBrush to get a rough massout, retopo it in modo, and tweak into shape. So overall a failure with bad surface. I did however get an insight for creating the curves> using 'align all' on a series of verts to get a straight line, then using a linear falloff set to repeat at end, create an even 3D curve. The only problem is, that it's an even curve, and not an accelerating one.
I think the answer is going to be using a spline cages for the main mass, create an even, clean polymesh, and work from there. We'll see. I might even write it up like my hand tutorial. It's very easy to save iterations in layers photoshop style.