Rotating weight savings claim...

[Safety Guy]

I can't recall the car forum in which I read this, but someone claimed that reducing one pound of rotating weight (as from the wheel or tire) results in a weight reduction enhancement the equivalent of four times the weight if it were the body of the car (sprung weight).

In other words, if I go from my Aspire wheels (13 lbs. each) to Civic VX wheels (9 lbs. each), that's saving four pounds per corner (assuming the same weight tires here) of rotational weight, for a total weight savings of 16 pounds. This "formula" posits that that rotational weight reduction would be like reducing the weight of the car proper by 4 X 16 pounds = 64 pounds.

Does anyone know if this idea is valid, at least mathmatically on paper? I know that if so, the real world results will often vary a bit, but I would like to see where this comes from and if the source is credible and has some accurate science to it.

All I do know is that reducing the weight of the wheels and tires is a very good, perhaps "best" place to reduce weight, and one of the worst places to add weight.

Thanks,

Karl

[eurotiva]

I know 1 ounce of wheel weights at 60 mph on a 15 inch wheel equal like 16 lbs. Learned that when I worked at a tire shop.

[Safety Guy]

Eurotiva wrote:

"I know 1 ounce of wheel weights at 60 mph on a 15 inch wheel equal like 16 lbs. Learned that when I worked at a tire shop."

Hmmm. Well, that would be static weight of the lead vs. the "g force" weight at speed. Any part of the wheel or tire at rotational speed will weigh "more" due to the centrifugal force acting on it. Perhaps then part of this equation would be speed dependant? But then, at any point in the rotation, the weight may be pushing down on the road, or up to the sky. Or front or back, for that matter. So I don't think this will really affect what I'm talking about. Especially since, at speed, the tire/wheel combo will have energy saved as a flywheel would.

I think the main weight savings "equation" is concerned mainly with acceleration and to a certain extent, deceleration, particularly of the rotating mass. The engine has to rotate the tire/wheel mass while also moving it forward as part of the car.

I don't have the physics/mathmatical background to be able to explain this very well.

Karl

[JPT]

Here's a good article on the subject.

http://www.modified.com/tech/modp-0906-performance-wheel-test/index.html

From what I've read (that article and others), Diameter is a big factor too. The more weight located further from the center, the more power you'll need to get the wheel moving.

[Safety Guy]

Thanks for that article, FF!

The weight difference between all those tires was a total of 3.5 lbs., on much larger diameter wheels. They dealt with relatively small differences in their tests, too.

I don't think there was any indication of weighing each of the twenty tires they used. I believe there could be some variations which should have been controlled for, but I have no idea how much weight difference between all the same type and size tires there could be. A few grams or over an ounce? My guess is any differences over an ounce may be significant enough for those tests. I do know that brand new guns of the same type can vary in their velocities, but that's comparing apples to mangos.

Compared with that article, and given our smallish wheel sizes, and differences in weight between widely different brands and weights of wheels, the four pound difference between 13 X 4.5" Aspire alloys and the 13 X 5.0" Civic VX alloys is pretty large.

Same with 14 X 6.0" Miata hollow spokes at 10.7 pounds and say, another brand factory alloy at maybe 15 pounds.

Comparing different sizes of wheels, especially different diameters, would further complicate things, though some comparisons would probably be valid. Compare Miata hollow spokes with my 13 X 5.5" VW alloys at 15 pounds each!

Lots to think about, but has anyone seen that formula I spoke of?

Karl

[muscle_Car1]

http://www.audiworld.com/tech/wheel13.shtml

http://premium.nascar.com/2001/TECH/09/13/powell_circletrack/index.html

[Safety Guy]

Thanks Kemp!

That first guy seems to know what he's doing, but I can't do the math so I can't check him. He says that the weight difference is more like one to two or actually, one to 1.7. However, I'm wondering if he's considering this at the lower speeds of "city driving," where constant acceleration and deceleration would be the norm.

The second article is interesting. Obviously applies mainly to RWD cars and internals of engine and rear end.

Thanks!

Karl

[CharlieZ]

Without getting into the science of it. Empirical knowledge gained from much reading of auto books and car mags since 1950, shows that the ratio of sprung to unsprung weight is an important parameter effecting the ride and general control-ability of a vehicle. For one, jounce and rebound requires less robust control by the dampers. Inboard brakes incorporated in many designs are an effort to reduce unsprung weight. (Brake cooling then becomes a factor though.) All interesting stuff worth looking into for the more serious minded gear-head..

[blkfordsedan]

F=ma

[muscle_Car1]

Sure it boils down to that but we are talking rotational forces as well.
Its just like saying all of our fuel sources came from the sun. True but wind power is different is both acquisition and application than say fossil fuels. Both can move an object but there still are variables.

Such as:
velocity is the rate of change of position
acceleration is the change in velocity over time

Since both are constantly changing when in motion they do have an effect on the total energies supplied into the equation, which in our case is the difference in the torque needed to rotate said mass around

[CharlieZ]

<snip>to rotate said mass around

The ominous "Centrifugal" Force, a dreaded and feared division of the BATFE, will assure compliance when one foolishly attempts to mass around. :-)

[JPT]

the ominous "centrifugal" force, a dreaded and feared division of the batfe, will assure compliance when one foolishly attempts to mass around. :-)

lol!!

[tecnopolis]

Does anyone know the weight of the Festiva 12" Factory Alloy Wheel?

[Kraig]

From any performance perspective...less weight is better.

[tecnopolis]

Agreed. I just wanted to know what the 12" alloys weigh -vs- the 12" steel rims. Without Tires.