I am not trying to put a wider tire on my i3.
I am looking for a wheel+tire combination that is the same bolt pattern, tire width, and tire outside diameter as the original setup.
It must weigh less, and have a lower moment of inertia about the axis of rotation than my current 19in setup.
Because the wheels are pretty unique, and BMW does not offer 17 or 18in rims, I think they may not exist.
Vehicle dynamics is a complicated subject.
It tends to take sophisticated simulation to get the results you want in a car.
I am not going to ever be allowed to see BMWs discussion, modeling, or experimentation about 18 vs. 19in rims.
In this forum we can go back and forth about how a wheel+tire combination affects dynamics (and every statement we make should be qualified with "generally" or "usually" unless we talk about specific rims and tires).
I am glad that we agree that a smaller diameter rim with more tire sidewall usually provides a more comfortable ride.
But I disagree that this is the "only advantage".
Another advantage would be that the tire and wheel would be more likely to survive an impact with a pothole.
Given the same tire width, and tire outside diameter, and the same style of forged rim by the same manufacturer, in general:
larger rims with less sidewall are heavier.
larger rims with less sidewall have a higher moment of inertia about the axis of rotation. This is true despite the fact that smaller rims with more sidewall need more material in the tire sidewall to achieve the needed sidewall stiffness.
There are detailed discussions of this on sports car and racing forums. You can read many stories about miata drivers tracking their car on larger rims and low profile tires, only to be disappointed with a higher lap time.
Even formula 1 has considered bigger rims, but "it's not an attractive direction performance-wise":
(the pic in this article is not a Formula 1 wheel, this is a Formula 1 wheel http://jalopnik.com/5922489/the-wheels- ... rmula-one/
I think the 18in rim and tire that I want does not exist.
In the case of an i3, large diameter wheels were an engineering choice to allow the use of narrow tires with less aerodynamic drag and rolling resistance. The total contact patch area is independent of tire size and depends only on inflation pressure and the weight supported by the tire, so narrow tires don't reduce the contact patch area. To have the necessary contact patch area to support the weight of an i3, a smaller circumference tire would need to flex more to create the contact patch which would increase its rolling resistance. Narrow tires also allow the wheel wells to be narrower to maximize interior space.
The only advantage of a smaller diameter wheel that I can think of would be a higher aspect ratio tire to keep the circumference the same which would provide a more comfortable ride, especially compared with the 20" tire.