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[Zeph]

Originally Posted by Socom

Curious if anyone has thought about the rotational mass. Do larger wheels add more rotational mass even if they weigh less than those which are slightly smaller/heavier?

Yes because you are spreading the weight out further, even if they are light... remember the outer diameter of a wheel has a large percentage of the weight because you have that 8-10 inch "dish" that the tire sits on/in. A true track setup would probably use 18's on this car (17's even if you could clear the brakes properly, which I am not sure of on a 335i for example).

[F32Fleet]

Originally Posted by Zeph

Yes because you are spreading the weight out further, even if they are light... remember the outer diameter of a wheel has a large percentage of the weight because you have that 8-10 inch "dish" that the tire sits on/in. A true track setup would probably use 18's on this car (17's even if you could clear the brakes properly, which I am not sure of on a 335i for example).

Which would then possibly lead one to conclude that just because your new larger rims weigh less doesn't mean they don't hurt the driving dynamics of the car?

[TSM330i]

Of course if you go with larger diameter wheel/tire combo, there is a negative affect on dynamics. I'm not sure if 1/2" or 3/4" will affect it as much since the balance of the weight (where it's located in the radius) isn't much different.

Wikipedia explained it pretty well:

The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. For example, consider two discs, A and B, made of the same material and of equal mass. Disc A is larger in diameter but thinner than B. It requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed farther from its axis of rotation: mass that is farther out from that axis must, for a given angular velocity, move more quickly than mass closer in. In this case, disc A has a larger moment of inertia than disc B.


Divers minimizing their moments of inertia in order to increase their rates of rotation.The moment of inertia of an object can change if its shape changes. An ice figure skater who begins a spin with arms outstretched provides a striking example. By pulling in her arms, she reduces her moment of inertia, causing her to spin faster (by the conservation of angular momentum).

The moment of inertia has two forms, a scalar form I (used when the axis of rotation is known) and a more general tensor form that does not require knowing the axis of rotation. The scalar moment of inertia I (often called simply the "moment of inertia") allows a succinct analysis of many simple problems in rotational dynamics, such as objects rolling down inclines and the behavior of pulleys. For instance, while a block of any shape will slide down a frictionless decline at the same rate, rolling objects may descend at different rates, depending on their moments of inertia. A hoop will descend more slowly than a solid disk of equal mass and radius because more of its mass is located far from the axis of rotation, and thus needs to move faster if the hoop rolls at the same angular velocity. However, for (more complicated) problems in which the axis of rotation can change, the scalar treatment is inadequate, and the tensor treatment must be used (although shortcuts are possible in special situations). Examples requiring such a treatment include gyroscopes, tops, and even satellites, all objects whose alignment can change.

The moment of inertia can also be called the mass moment of inertia (especially by mechanical engineers) to avoid confusion with the second moment of area, which is sometimes called the moment of inertia (especially by structural engineers) and denoted by the same symbol I. The easiest way to differentiate these quantities is through their units. In addition, the moment of inertia should not be confused with the polar moment of inertia, which is a measure of an object's ability to resist torsion (twisting).

[stressdoc]

This is why I covet Volks and BBS FI -- I want wheels <17.5 lbs. Lighter dampers, springs, M3 control arms, rotors, non-rfts = 100lbs. reduction in unsprung weight. Braille battery, sunroof delete, BMW Perf seats, exhaust, ... = 4 door Lotus killer.

I am really excited about some of the new efforts at BMW R&D to reduce weight. The Z2 supersport is going to be sooo much fun!!!!

[HRC]

Love my Ce28n's. 19's but lighter and stronger...

Danny

[chaz58]

Yes, you need to look at the wheel and tire package.

Interestingly I save 80lbs with my winter wheels. 10lbs per wheel, 10lbs per tire. Those OEM wheels are heavy, the RTFs are heavy, and the Dunlops are about the heaviest in there size even without RTF!.

Biggest difference I notice is the rear tires stay planted - they don't hop as much over bumps (besides, the car is faster with sub zero intake are anyway).

Originally Posted by 200Duece

+1

you need to take the weight of the total wheel and tire combo and see what the difference in weight is. the non runflats may save you a lb or two

[Marcus-SanDiego]

Originally Posted by chaz58

Biggest difference I notice is the rear tires stay planted - they don't hop as much over bumps (besides, the car is faster with sub zero intake are anyway).

Which makes perfect sense. The lighter the wheels and tires the quicker they rebound. +1