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Old 12-01-2013, 01:34 PM   #35
Number 3
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Drives: '19 XT4 2.0T & '22 VW Atlas 2.0T
Join Date: Dec 2008
Location: Illinois
Posts: 12,141
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Originally Posted by Doc View Post
Are you absolutely sure about that? I never said it was easy; it wasn't easy for me but I was only modifying my one car. It would be "easier" to some degree for GM because they can actually make the technology they need; I had to wait and research like crazy to put together what I did. But based on my actual experience I can say it would be worth GM's time and money to do more research in this area. Yes it will be expensive initially, but what isn't in order to meet cafe standards?? The problem with pouring millions into certifying one engine is it's only good for that one engine; they have to do it for every new or modified engine if I understand it correctly. The tech they would develop for reducing unsprung I believe would actually be cheaper in the long run as it can be used across many vehicle product lines and can be carried over year to year. And there wouldn't be expensive certification tests involved.

I don't know how they go about getting an epa certified rating but I firmly believe it would be worthwhile to create one "mission modified" test car that's only had a reduction in unsprung to see what the actual results are. My car is modified for the track with appropriate cam and everything and I STILL get pretty decent gas mileage; especially on the freeway. If I was just modifying for street/daily driver I have no doubt I could get better mpg out of a stock SS without engine mods and it would be a noticeable improvement in performance. And that's just me modding an already manufactured car. If I had the backing and resources of GM behind me I'm absolutely positive I could improve the performance and mpg WITHOUT modifying the already-certified engine.

It would be an investment on GM's part but a worthwhile one that would pay off for years and years.
You are correct that the responsiveness of a car with less rotating unsprung mass will be better. The rotational moment of inertia of a larger and/or heavier wheel tire combination will take more to spin. So from a real world standpoint you would be absolutely correct.

If you look at the Cruze ECO, you will see that it comes with what I believe are lower weight aluminum wheels. But this is only a part of the equation.

So I am agreeing that you are correct in your assumption. Reducing the rotational inertia of the wheel/tire/rotor will make a difference. Just not as big as you are suggesting.

All I am suggesting is that if it were simply a matter of reducing rotational mass you would see a much different set of wheels and tires on cars today.

GM and every other OEM would pay dearly for the 2 to 3 MPG you are suggesting is possible and if all it took was 12 or 13" aluminum wheels...............you'd see every car come standard with those. You wouldn't see some base models coming with steel wheels and hubcaps either.

Just look at the Camaro. Do you think GM would love to have the same or higher EPA highway number as the Mustang???? YES in as big a font as I can come up with. I know the effort that went into getting 30. And if all they had to do was put the aluminum wheels on as standard to get even 1 MPG do you think they would have done that? Again, YES in as big a font as the last yes. I believe the aluminum wheels save something in the neighborhood of 30 to 50 pounds, half of your suggestion.

So I am pretty sure that rotating unsprung mass won't get the MPG you are thinking. For highway numbers it truly is Aero, Tire Friction and Powertrain. Mass of the vehicle highly impacts the tire friction. For City, it's less Aero and more Mass. Much more to the art and science of it and there is a bunch that can impact "real world" FE as well compared to the EPA cycles.
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