Originally Posted by JetBlack5OC
I bet it isn't much at all. Hence Nissan put 20's on the GT-R. They must not be big on the whole inertia thing. And if you put 20's on that are 3-4 lbs per/wheel lighter than the stock 19's. It would have to offset some/most of the negative affect.
The GT-R is a REALLY big car, 20s on that car are very much like 18s or 19s on an M3. There is a reason you fight I EVERYWHERE on a performance car. If you don't understand the effect of inertia you really need to grab a Physics 101 text book. I assure you Nissan engineers understand the importance of I.
As briefly as possble rotating bodies obey an equation that is a perfect equivalent of Newtons law, force = mass x acceleration. Less mass, same force = larger acceleration. For rotating bodies torque = moment of inertia x angular acceleration. Again low I is absolutely fundamental to making objects easy to "spin up" (i.e. have a high angular acceleration for a give applied torque).
All rotating masses have to be accelerated linearly along with the translational movement of the car, this takes force and power. As well rotating objects have to not only be accelerated they have to be spun up, this takes even more torque and power. Surely you need to fight weight as well, both are key. Large I values in your wheels not only affect acceleration but also braking and turning, all negatively. In other places that contain rotating mechanical parts like an engine, axles, driveshafts, etc. it is always a balance to have low weight, low I and acceptable stiffness and strength. For these components low mass and I also produce much less bearing loads.
There is a similar reason to be greatly concerned with unsprung mass - that of the suspension and everything that moves along with the suspension. High unsprung mass is even worse than high sprung mass (say like the car body or a passenger).
Lastly (and again, and again...) these effects are NOT huge. We are probably talking tenths of seconds in most acceleration metrics and maybe a couple feet in braking metrics will separate 18s, 19s and 20s. Very rough educated guesses here. However, do recall that saving X lb in a rotating component is MUCH more significant than saving the same X lb from a non-rotating component.