Thanks, Randy, guess I won't have to find my Engineering Dynamics book after all. To elaborate, the inertia is a product of the weight (actually the mass) and the distance from the axis of rotation, and inertia determines the ease with which the wheel can accelerate or decelerate (brake). Since the hub is very close to the axis of rotation, its weight is almost irrelevant. Since the rims and tires are at the greatest distance from the axis, a small decrease in their mass leads to a large decrease in inertia, so their weight is all-important. Also, since the weight of the bike and rider acts thru the axle, it isn't important in acceleration either. Total weight does play a small role in riding at a steady pace, as it does affect rolling friction of the tires somewhat. It matters a lot in climbing, since neglecting friction and wind resistance, the energy required to lift the bike and rider to the top of the climb is the product of the total weight and the vertical distance climbed. This is why track sprinters, or road sprinters for that matter, are usually heavily muscled types, since they have more power output, and their weight is no handicap, as the weight of the rims and tires and the power applied mostly determines acceleration. In climbing, however, it is the ratio of power to weight that matters, so a 130 lb rider only has to have 2/3 of the power output of a 200 lb rider to make it to the summit first. Never thought polar moment could explain why Cipollini thrashes Pantani in the sprint, but Pantani destroys him on the l'Alpe d'Huez did you?
Regards,
Jerry Moos
RMAugust@aol.com wrote:
> << Why doesn't someone offer some proof that weight matters all that much to
> begin with and that rim and tire weight matters more. All I see are
> unsubstantiated claims. I say until some one can prove different rotating
> weight doesn't matter more, and that weight in general doesn't matter all
> that much.
> >>
> Given two wheels of equal weight, one with a greater proportion of its weight
> in its rim and tire will have a greater polar moment of inertia which
> therefore will accelerate at a slower rate of speed. A good example of this
> is an ice skater rotating with arms extended (high polar moment) and then
> moving the arms in very close to the body (lower polar moment). The result is
> that the speed of rotation increases dramatically.
>
> Wind resistance plays a small role in this but mainly it's the reduction of
> the polar moment. In the case of wheels, reducing the polar moment makes a
> bike feel more fleet and in competition can give one an actual speed
> advantage off the line. That's the proof of why reducing rim and tire weight
> matters more than reducing weight in general.
>
> As to reducing weight in general, I think it's generally known that, given no
> other variables, a lighter bike will be more efficient to move owing to fact
> that less calories are required to fuel it.
>
> Randy
> Corral De Tierra, Ca.