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So what's the deal folks? I'm seeing a lot of off-handed comments about rim upgrades being a strain on overall power. I'm not a physicist, let alone a mechanic, so just curious what others have to say. Are wider diameter wheels too hard to turn? Do they add so much extra weight over stock that it makes a noticeable difference on performance? Also, I've seen in SCC that some makers are putting out "Lite" rims... anyone comment on the pros/cons of this?

 
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The less unsprung weight the engine has to turn the more efficent it will be at putting more power to the ground, and acceleration will also improve as well as braking. That is why some are putting forged wheels (which are generally lighter) on their cars. Or just moving up one size over stock with a cast aluminum wheel (most common construction) so the gain in weight will not be as great. I really don't see any cons of putting lighter weight wheels on a car other than the cost of the wheel itself. After owning a Civic, I know it does'nt have a ton of power so generally a owner is more concerned about looks than performance, with a few members being exceptions.

 
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you must also take into consideration the moment of interia changes dramatically when you upgrade the wheel size. a car wheel can be (physically) generalized as a thin walled hallow cylinder. the equation for the moment of interia of a thin-walled hollow cylinder is MR^2 where M is the mass in kilograms and R is the radius in meters.

take for example a 15kg 17" wheel (radius is .2159 meters). using the formula the moment of interia is calculated to be about .70 kg*m^2.

now take a 15" wheel (radius .1905meters) that weighs two kilos more than the 17". using the formula ones gets .62 kg*m^2!!!!--meaning that the 15" wheel that weighs 2kilos more (4.4lbs) actually has a smaller moment of inertia!!!!!!

this means that the 17" wheel that is 4.4 lbs LIGHTER than the 15" takes more torque (twisting force) to rotate than the 15" wheel that is 4.4lbs HEAVIER than the 17"!!!! now there's something to think about.

remember i didn't take the tire on the wheel to consideration. generally a 17" low-pro tire is heaver than a 15" tire--most of the weight savings on a larger rim are negated by the heavier tire that the new rim requires.

-jro

 
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You need to search this... this has been gone over countless times. To match the weight of a 15" wheel off a 2000 Si including the tire, you need a 17" rim that weighs roughly 14 lbs. The Si rim/tire come out about 35 lbs. And a 17" tire is around 21-22 lbs. So be prepaired to fork over some cash for rims that light. Inertia isn't something that can be exactly calculated because of the rim weight distribution, but figure to match the inertia of a 15" with a 17", you're gonna need something that doesn't exist.