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Old 05-14-2009, 08:37 PM   #14 (permalink)
Bicycle Bob
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Join Date: May 2008
Location: N. Saskatchewan, CA
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Appliance White - '93 Geo Metro 4-Dr. Auto
Last 3: 42.35 mpg (US)

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[QUOTE=Big Dave;104375]Comments from the Grinch:

2. Empirical experience about tire diameter is at variance to the assertion that bigger tires are more efficient. Wheel-tire assemblies are big flywheels that the engine must accelerate along with the vehicle every time the vehicle speeds up. Rotational moment of inertia goes up with the square of diameter.

Aye, but wheel RPM for a given speed decreases with greater diameter. The tread moves at the same speed, there's just more of it.

3. The air resistance of bigger diameter tires is subject to debate. With lifted pickups, the air resistance goes up greatly. Maybe, as with the Aptera, sitting taller may get a very streamlined body out of what aeronautical engineers call “ground effect.” Ground effect is the high lift/high drag regime of a wing flying close to the ground. When the plane climbs out of ground effect the rate of climb decreases as lift is lost but air speed increases as drag drops away.

Aircraft in ground effect have less induced drag. The ground makes the wing more efficient; it does not affect form or skin drag substantially.

4. High diameter, narrow width “pizza cutter” wheel-tire assemblies have some definite non-efficiency problems. They make the vehicle more top-heavy, therefore for safety’s sake, the designer has to increase track width. “Pizza cutters” are murder on wheel bearings.

Or the designer can adjust the suspension and bearing sizes to suit. Given the wheel offsets that don't seem to demand bearing changes, and the more constant loading they induce, I'd not worry about that.
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