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Old 07-18-2009, 01:32 PM   #1 (permalink)
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Wheel (rim) size and performance

I currently drive a 2002 chevrolet cavalier with the stock 14" steel rims and plastic 5 spole wheel covers. I was looking into getting the moon disc hubcaps to cut down on drag. I have access to a set of 15" factory steel wheels, from another cavalier and so was wondering if the larger rims (with the 15" moon disc hubcaps) would do more good becasue the tires are the same overall diameter so more of the outer area of the tire/wheel combo is actually covered by smooth metal and there would be less deflection because of smaller sidewalls, or would larger rims add additinal rotating mass where is should not be and negate any areodynamic advantage. Too expensive for A-B-A testing for me so I will take opinions as a good second opinion. thanks y'all

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Old 07-18-2009, 05:58 PM   #2 (permalink)
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I think you're right in that there might be a slight advantage in a slightly larger moon hub cap on a wheel/tire combination with same final outer diameter. However don't think the extra inch diameter would make that much difference.

You should weigh the two wheel/tire combinations and see if one is heavier. Tires are rotating mass and depending on who you read/believe, any additional mass will work against you 2x to 3x as much as if it were non-rotating mass. That's especially true for any driving where speed varies. Go with the setup that weighs less.

The "rotating mass" reduction thing is what originally drove performance enthusiasts to mag wheels and alloy wheels. Has nothing to do with bling - that came much later.

You can look up the weight of current available tires at tirerack.com if you need to. It's under "specs" for most tire make/models.
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Old 07-18-2009, 08:49 PM   #3 (permalink)
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Originally Posted by brucepick View Post
You should weigh the two wheel/tire combinations and see if one is heavier.
Just weighing won't give an accurate answer: what matters is the rotational inertia, which is mass * distance from axis. So if you had two wheel/tire sets that weighed the same, one with a heavy tire & light wheel, and one with a light tire & heavy wheel, the heavy wheel set would have less inertia.

I've wondered about this sometimes: would a big rim & skinny tire be more efficient?
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Old 07-20-2009, 08:42 AM   #4 (permalink)
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Larger diameter equals higher ratios equals better FE if loading is okay. Weight/inertia characteristics arnt too important at constant speeds
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Old 07-20-2009, 01:18 PM   #5 (permalink)
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But the question was which is better, assuming the same overall diameter...
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Old 07-20-2009, 06:36 PM   #6 (permalink)
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More weight distributed to the outside would increase rotational inertia, meaning it take more energy to accelerate. If the trade is between heavy tire on a light rim or a light tire on a heavy rim, go with the light tire on the heavy rim (assuming total weight is approximately the same).

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