You may like this website:
http://www.analyticcycling.com/
It has lots of calculators for just this kind of thing and many others.
Dave Bohm Bohemian
----- Original Message ----- From: John Joseph Taglia To: Jerry & Liz Moos Cc: RMAugust@aol.com ; classicrendezvous@bikelist.org Sent: Sunday, April 22, 2001 3:19 PM Subject: Re: [CR]Re: Reducing the "Polar Moment of Inertia"
This is interesting, but fails to take into account the only acceleration that is material: the acceleration of bike and rider. Unitl I see an equation, I will continue to believe that weigh of bike is pretty much immmaterial, and that wheel matters no more than any other.
I guess I don't think it is too much to ask for proof via an equation. (And the skater analoqy is flawed as speed stays constant--larger radiusx slower rpm=smaller radius x higher rpm. So no net change in enery or speed.)
On Sat, 21 Apr 2001, Jerry & Liz Moos wrote:
> 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.
> >
> > _______________________________________________
> > Classicrendezvous mailing list
> > Classicrendezvous@bikelist.org
> > http://www.bikelist.org/
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