> Power needed to overcome rolling resistance increases somewhat linearly
> with speed. Power needed to overcome aero drag increases with the cube of
> the increase in speed. Aero drag is pretty much independent of load.
> Rolling resistance is greatly affected by load. Therefore the best
> compromise between drag and rolling resistance depends on the load and the
>Check the equation for Fd (drag force), its got a V^2 in it.
That's the equation for drag FORCE. If you place an object in a wind tunnel and double the velocity, 4 times the force is appled to that object. However, to move it you need POWER which is force x distance / time.
To move an object thru the air, at twice the speed, (Sub sonic of course) you apply 4 times the force, twice as far in the same time. 4x2/1=8. I've been reading about aerodynamics for 40 years and I've always seen it spelled out this way.
>Can you explain this more. I don't understand what the distortion is.
2 factors here;
With a loaded narrow tire, the difference in rolling diameter at the edge of the contact patch is significantly smaller than at the center. A certain amount of this discrepancy is taken up in flexing the rubber tread. However if the load is high, (increasing the difference in rolling diameters) or the tread is thin, slippage occurs. This is the only component of rolling resistance that I would consider "Friction" You can even hear it. Have you ever noticed that large smooth tires are much quieter than skinny ones? Anyway, the part that's slipping is supporting weight but not gripping well.
Remember doing tests in high school physics proving that friction is independant of area. And that static friction is greater that sliding friction. Race car tire are huge for 2 reasons. One is to better dissapate heat, but the one that concerns us is that they want smaller G loadings per square inch of contact patch, to get less distortion, so that more of the cars weight is supported by rubber getting static instead of sliding friction. In a turn, G loads simply distort a smaller tire more as force per unit area is greater. The more the tire is squirming, the less of it is planted firmly on the pavement and getting maximum grip.
Mark E. Stonich, Minnesota Human Powered Vehicle Assn
Thursday, 29 January 2009