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Date:         Fri, 11 Sep 2009 20:41:33 -0400
Reply-To:     Edward Maglott <emaglott@BUNCOMBE.MAIN.NC.US>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Edward Maglott <emaglott@BUNCOMBE.MAIN.NC.US>
Subject:      Re: Rolling resistance
Comments: To: Kim Brennan <kimbrennan@mac.com>
In-Reply-To:  <DDB28160-40E3-411F-A29E-896374D6BB45@mac.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

What you are describing about the mass and the distribution of the mass on the axis affects rotational inertia. (I just learned about that term from an off list tip and some recommended web surfing.) Inertia only affects the change in rate of motion. In the case of rotational inertia, accelerating or decelerating a spinning object. In the example of your flashlight, once you got it spinning, wouldn't it take the same amount of energy to keep it spinning at the same rate? The rotational inertia would be much higher spinning it lengthwise so it would take more energy to accelerate it, but the mass would be the same so to keep it going should be the same. (The flashlight is an extreme example, there would be much more friction with the air spinning it lengthwise.)

Edward

At 07:43 PM 9/11/2009, Kim Brennan wrote: >It is the rotating mass's position away from the center that uses more >energy. Let's give you a working example. Say a Maglight flashlight. >Which is easier for you to do? spin it on it's axial axis, or spin in >lengthwise? > >Back to Newton and inertia. Objects at rest tend to stay at rest. When >you spin an object (say a wheel), most of the mass of the wheel is in >its outer section. in order to move that outer section it takes a >certain amount of energy. How much energy, depends on how far you want >to move it. One rotation's worth of energy is proportional to the >distance traveled, which is the circumference of the wheel. > >Now increase the diameter of the wheel. One's rotation of worth of >energy is still proportional to the distance traveled, but the >circumference has increased. > >Most of the time when you go to a larger wheel, you are increasing the >weight of the wheel too (and tire). You loose a little bit of weight >on the tire, due to (usually) a lower profile on the tire, so that is >often a wash. > >Friction doesn't increase substantially (in this case it can be >treated as a constant). However, it prevents the wheel from spinning >forever, once we have given it energy to rotate. Friction is all over, >from the tire contacting the road, the tire sidewalls flexing, the >wheel bearings rolling, the grease the bearings are immeshed in, the >cv joint, the flexible boot on the cv, and on and on. > >Your last statement is correct...up to a point (i.e. if you have too >high a gear, the engine might not have enough power at low rpms, to >turn the tire.) It's one reason I really wish I had another gear above >4th for long distance interstate cruising. > > >On Sep 11, 2009, at 5:58 PM, Edward Maglott wrote: > >>I'm having trouble getting this. Are you saying is that it's the >>rotating mass and not the diameter that uses more energy? So if you >>had a larger diameter tire/wheel combination that had the same mass >>as a smaller one, there would be no change in the amount of energy >>required to spin it? Where is the friction that you refer to, and >>why is it increased with a larger diameter tire? It seems like the >>whole package from the tranny input shaft all the way to the tire >>determines how many engine revolutions equal 1 mile. If you have a >>smaller tire and higher top gear in the tranny to equal the same >>overall ratio, would you use less energy to go the same distance? >> >>Thanks, >>Edward >> >>At 01:59 AM 9/11/2009, Kim Brennan wrote: >>>I didn't phrase it particularly well. For best fuel economy you want >>>lightweight tires/wheel/rotating mass. The rest of the unsprung >>>weight >>>is irrelevant towards fuel economy (though yes, you are correct it >>>has >>>a lot to do with handling and ride.) >>> >>>Simply put, a low mass rotating body is much easier (i.e. uses a lot >>>less energy) to spin than a higher mass rotating body. Mass always >>>matters. If we lived in a world without friction, Newton's laws of >>>motion would rule supreme. But we live in a world with friction, so >>>what is in motion does not stay in motion. And to keep it in motion, >>>requires energy. This applies both to vehicles traveling at a steady >>>speed, as well as vehicles accelerating.


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