Originally Posted by
rpenmanparker
Funny thing is the term "rolling resistance" is defined as all losses of power transmission from the wheel to the road, not just frictional. So any skipping the tire does on a rough surface that wastes energy is also rolling resistance. It is counter-intuitive to me, but there it is. So if the harder tire can't grip the road as well, I would think that would be less rolling resistance, because it can't be losing energy to friction on the road if it isn't touching it. But I would be wrong since the energy that isn't transferred from the wheel to the road due to the poor tire contact also factors into the Crr. I can't say that effect can be seen between 80 and 110 psi, but somewhere it does factor in. You won't see it on a dynamometer, only on a real road surface, the worse the road, the more pronounced the effect.
All sounds right to me, but it may be generally hard to follow. But the question gets more interesting with this.
He is saying that we call it "rolling resistance", but some of the energy loss isn't due to the tire rolling at all! It's counter-intuitive because what we call it isn't fully descriptive of what it really is. rpenmanparker knows this but is trying to illustrate the problem.
The energy has to go somewhere (conservation of energy), and in this case it's mainly heat - flexing the tire heats it up, wind cools that off and the energy is gone. And whatever the frame and fork dissipates, but ultimately and mainly the tires.
There is also a momentum change to keep in mind. Otherwise we'd solve it all by using solid tires that won't deform but we know that's not right. I hit a bump and the front tire goes up. The vertical momentum is no longer helping me horizontally (conservation of momentum) so I slow down. I might get some of that back as long as I land on a slope angling me forward. But if my wheel lands on flat ground it will just bounce until it's all damped away.
With the tire deforming around bumps we don't get as big a momentum change, but we lose energy with the deformation. No way around it. In my view, the "right" pressure addresses a balance of these factors. That can't help but be utterly dependent on the materials of the tire, its construction, shape and size. In addition to load, speed and road surface.