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Old 12-16-2007, 11:03 AM   #5 (permalink)
MetroMPG
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 55.97 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)

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Thank you for questioning this. It forced me to do a bit more digging for a credible source on the subject --- and put this bit of fluff to rest: "taking off 1 lb of rotating mass is the same as removing anywhere from 7 to 16 lbs elsewhere"

First - agreed on the handling benefits of unsprung weight.

On the issue of rotating mass...

Quote:
Dropping 10-15lbs of my vehicle weigh via lighter wheels is equivalent to dropping 10-15lbs off my ass.
In fact, you'd have to drop 20-30 lbs off your ass to equal it. The relationship is more like 1:2.

Here's a credible source giving an overview of the 1:2 relationship, in response to this question:

Quote:
I just bought a set of nice chrome wheels with low profile tires for my car. Since these 4 wheels are 40 pounds heavier than the old ones, I removed 40 pounds of weight from the body of the car to compensate. My acceleration times and braking distances have increased dramatically. Why? (source)
And a perhaps less credible source adding to the above... with formulae!

Quote:
As has been said here, and as I've heard elsewhere, lighter wheels can make a noticable difference in acceleration. Takes less work to get them rolling.

It does take less work: the kinetic energy in a circle rolling about its circumference is mv2. This is exactly twice the energy of a non-rotating mass moving at the same velocity, ½mv2.

So removing 5kg from your tires (if you could somehow remove the mass entirely from the part of the tire which is actually in contact in the road; removing it from the interior of the tire has less effect), would be the same as removing 10kg from the non-rotation portion of your car. (source)
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