Date: Fri, 11 Sep 2009 19:43:37 -0400
Reply-To: Kim Brennan <kimbrennan@MAC.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Kim Brennan <kimbrennan@MAC.COM>
Subject: Re: Rolling resistance
In-Reply-To: <20090911215859.700A01E853F@tc2.main.nc.us>
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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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