Originally Posted by Carusoswi
Also, someone correct me if I'm wrong. I thought softness and flex in a tire would tend to increase its rolling resistance. Some here are stating just the opposite. I would think that solid steel tires would have less rolling resistance than rubber. Wood tires also would have lower rolling resistance. Obviously, these substances would be heavy and very uncomfortable, and probably give you terrible traction. But they would have lower rolling resistance.
Rolling resistance comes from a phenomena in wheel materials called hysteresis. To make things simple, the way a tire works is this: when a section of tire comes in contact with the road, its surface, and the shape of the tire/wheel underneath, is deformed and compressed. That takes energy. When the section of tire leaves the road, the tire/wheel recovers its initial shape and releases the energy stored in the deformation process by "pushing the road down" so to speak. Since you have a leading section of tire coming in contact with the road and the trailing section leaving the road at all time, the forces cancel each other and the wheel is seen as rolling with very little resistance.
Now what causes a tire to drag is this: when a tire is forcibly deformed, it has no choice, it deforms. But when it regains its shape, there's a sort of inertia due to the friction in the rubber material, and due to the density of the rubber, that makes the tire regain its shape with a slight delay, so that the tire doesn't "push the road down" immediately as the compression force ceases, effectively wasting part of the energy that was initially put into the deformation.
That's why the most efficient tires have walls as thin as possible: air has virtually no hysteresis, so by reducing the amount of rubber separating the atmosphere and the compressed air in the tire, you decrease the overall drag of the tire. That's also why airless tires, despite being very hard, drag like pigs (they waste up to 30% of the pedaling energy). But of course, there's a balance between too thin a tire (that blows at the slightest thorn) and an airless tire that steal all your energy. There are also rubber compounds that produce more or less hysteresis, so all tires aren't born equal for a given shape.
And of course, another, lesser source of drag is the heating of the rubber due to constant deformation, which is also why a high pressure tire performs better because it deforms less. As for solid steel wheels, yes, they're virtually perfect as far as drag is concerned, but you wouldn't ride 100 yars with them without falling down
