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Old 12-01-2013, 08:39 PM   #40
Doc
Dances With Mustangs
 
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Drives: 2010 1SS/RS MT
Join Date: May 2009
Location: USA
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Quote:
Originally Posted by Number 3 View Post
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.
If you read my posts in detail and took the time to examine my project thread you should be able to see I'm not saying just replacing the wheels is the magic bullet to gaining 2-3 mpg. There is no magic bullet. It's a combination of several things; wheels and tires being part of that equation.

I don't know how cars are EPA certified; are they tested in a lab by a machine or out on the road with real drivers? I know that people push the gas pedal as far as they feel is necessary to get the feeling they're "moving" at a rate they feel is acceptable. That "feeling" depends on acceleration response and how quickly they reach that "feeling". The farther they push the gas pedal to get going, the worse the gas mileage. If you take the same driver and put them in a car with more "pep" so that they get to that "feeling" quicker and only have to push the gas pedal half as far or even less, then they are using less gas to get going and will get better mileage as a result. How far they push the gas pedal depends on their perception of how the car is responding to their throttle input. If they feel the car is sluggish, they're pushing the pedal down pretty far to get it going. If it feels like they barely push it at all (like mine; I can push the pedal no more than an inch at the most and in a couple of seconds I'm at the speed limit on the street) then they're using less fuel; especially for around-town stop-and-go driving. Freeway driving is very different; there it's rolling and wind resistance you have to overcome more than anything else. Unsprung reduction would have little effect on that unless the freeway traffic is heavy and you're having to speed up and slow down a lot.

It would have to be a total approach; drivetrain from the flywheel through the clutch/trans, driveshaft, differential and axles, as well as unsprung rotational in the wheels, including using low-rolling-resistance tires and lighter brake rotors. Any of that mass you can reduce means less power is required to get moving which means less gas is used. Just changing the wheels isn't enough; you need the total package, but that's something GM can certainly do.

The reason why manufacturers haven't so far I'm guessing is because they're also focused on making the parts as cheaply as possible, so the idea of putting what they would probably perceive as racing grade engineered parts in a mass-manufactured street vehicle would seem cost prohibitive and be nixed by the bean counters.

Times have changed though. With new materials and manufacturing techniques what was once highly custom engineered is now reasonably possible with computerized manufacturing. And again, they need to recalibrate their focus on targeting to the performance perception of the customer. When a customer does a test drive it's not the numbers that impresses them, it's how the car makes them feel when they drive it. Aim for that. A car could literally have less horsepower than a competitor's car, yet feel better when driving because of how it responds to the driver. Aim for the perception because to the customer, perception is reality.
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