Thread: Prius 75-91mpg
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Old 04-28-2019, 08:36 PM   #27 (permalink)
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Originally Posted by Ecky View Post
Maybe I'm confused by the terms, but the math works out that the energy stored in a spinning tire is very slightly less in the tire with a larger diameter, all else being equal, if you want to be traveling at the same speed. It's counter-intuitive but the reduction in RPM is worth more (slightly) than bringing the mass closer to center and spinning it faster.

Angular momentum is likely the term I wanted.
I've mulled this over for a few days now (and gone back over my physics notes!). When you say, "all else being equal," are you including mass? The tires I used for my calculations weigh, according to Tire Rack, 13 lbs for the 185/55-R14 and 16 lbs for the 185/75-R14, which is quite a bit different. Considering the energy of a rolling tire/wheel, remember that rotation is not the only component we have to consider; rolling = rotation + translation, and Krolling = (1/2)mv^2 + (1/2)Iw^2. For that taller tire in my example, I is larger because of the larger radii (I is a function of the squares of the inside and outside radius for the "disk" of the tire tread), but its lower angular velocity, w, is not enough to counteract that increase and its overall angular momentum is larger. Plus, the higher mass means the kinetic energy of translation is higher as well, all of which results in a higher kinetic energy of the larger tire.

I might play around with this some more using other tires and their masses; this might be enough to split into a new thread.

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