It's not a myth at all. A wheel with more mass far from the center of rotation, i.e. the rims and tire, does take more energy to accelerate to a given speed as you concede. By keeping its momentum better, you seem to imply that the rotationg energy of the heavier wheel can somehow be used to overcome the energy losses encountered, mostly the energy consumed driving the bike and rider against air resistance, and a much smaller energy consumption by friction at the tire contact patch.
That's an interesting theory, and I'd be interested to hear a real expert analysis of that. I suppose an empirical experiment to test it would be to accelerate two wheels with the same overall mass and diameter, but different mass distribution, to the same RPM, then allow both to roll along the same smooth surface in an enclosed building and see if one rolls farther than the other.
But even if that is true, in actual cycling it would only apply to a time trial. That is, if you are right, the time trialist with the heavier rims would have to use more energy to get up to top speed, but then need to use less enery to sustain that speed. The problem is most cycling on the road involves the use of brakes. So the extra energy of a heavier rims just means more energy dissipated at the brake pads each time one brakes to a lower speed. I still think an ounce on the rims is worth a pound on the frame. Schwinn seems to have believed that too. My filet brazed 1973 Schwinn Sports Tourer probably weighs close to 30 lbs, as Schwinn had a lifetime waranty on frames, and therefore tended to overbuild them. But the wheels are top quality stuff, Weinmann alloy rims on Normandy Competition hubs, keeping the weight low where it really counted - although actually they could have used heavier hubs, since those are close to the axle.
Regards,
Jerry Moos Big Spring, Texas, USA
John Wood <braxton72@gmail.com> wrote:
On Thu, Jul 3, 2008 at 4:26 AM, Neil Foddering
wrote:
>
> I think that someone may have touched on this, but a phrase that sticks i
> n my mind is "an ounce on the rim is worth a pound on the frame",
Sorry to say that this is yet another myth that has been debunked in recent years. I'm sure there are others on the list that can explain it better than I can, but weight is weight regardless of where it's located. A heavier wheel will indeed take a bit more energy to accelerate, but will keep it's momentum better. For climbing, a lighter will help, only because it reduces overall bike weight. An ounce on the rim is worth an ounce on the frame. This has been much discussed on other forums.
--
John Wood
Washington Island, Wisconsin, USA