Rob_E

This is the theory, and it might actually work that way. An object in motion tends to stay in motion, so if you've got a higher mass spinning, it could theoretically spin longer. But also remember that nothing comes for free when it comes to energy. Smaller wheels usually mean smaller mass, and the less mass you have to push forward, the less energy you have to spend to do it. So basically, you spend more energy spinning up your bigger wheel because it's heavier, and you may "bank" some of that energy because you have now expended more energy, but in the long run, you will expend more energy to push more weight down the road, no matter the wheel size.

So ultimately, to my mind, momentum and rotational mass are non-starters. There is no question that a heavier wheel requires more energy to move it, and if a bigger wheel is a heavier wheel, the question is, "What do I get in exchange for my bigger wheel?" Momentum is nice, but it's still a net loss, energy-wise. What you get is smoother rolling. The angle between the tire and the road/surface is smaller with a larger wheel. It makes any irregularities less jarring. Currently my bikes are either 700 wheels or 20" wheels. My speed varies very little between the two sizes. The amount of cussing I do when I hit a pothole varies, though.

I think that's why mountain bikes and all terrain bikes are moving away from 26 inch tires. Partly I suspect it's marketing, but theoretically a larger wheel will handle rough surfaces better. But the sacrifice, as you point out, is that a larger wheel doesn't accelerate as well, which is a bigger problem when the terrain forces you to have uneven speed. I think that's why 29" has not taken over off-road bikes, and the newer trend seems to go towards the 650b, which is only slighter bigger than 26".