OK. I have been thinking about this a bit and will finally throw in my
. Ben's right - I am a scientist, but I am a biologist, so my physics and math is probably as rusty as many of yours. But, I can bring some scientific thinking to this, so that is what I will try to do here. IAlso, I am working this out as I write, so corrections are welcome. It also means that this is an adventure to me, since as I start this, I don't know what the result will be!
Let us keep things simple for the moment with some assumptions. A scientist starts by simplifying things and then adding back any complicating issues. I will take at face value the statements that a pound on the wheels is equivalent to 5 pounds on the car (since I don't know) and that the moment of inertia increases exponentially with diameter (if you think of pi r
2, that makes sense). Let's pretend that the 19" wheels weigh the same 26 pounds as the 18" wheels.
Also, if you remember back to algebra, you can scratch out things in common in a comparison (2x/4x = 2/4 = 1/2). Since we are comparing moments of inertia for 2 wheels, I am going to make the bold assumption we can scratch out all the rest of the moment of inertia equation and use the part that changes, the radius of the wheel.
First, you will be moving weight out, but only by 1 inch, not two. The 19" is the diameter of the wheel rim. Not knowing the equations, but assuming it has to do with r
2 by rim weight, let's look at the percent change for 1 pound (1 pound keeps it simple). For the 18" wheel, r=9, so 9
2*1 = 81 (I am not assigning units because I am treating this as an index, not something real since I don't know how to calculate that). For a 19" wheel, that is 9.5
2*1 = 90.25.
That's an 11% difference between 81 and 90.25.
Second, you will not be moving the weight of the tire out because of the lower profile keeping the total tire diameter the same (in reality, considering you are trimming some from the inside of the tire due to the increased wheel diameter, the tire actually might weigh a little less, but let's say they weigh the same to keep it simple). Not all of that 26 pounds is at the rim - I am guessing it is 50% rim, 50% hub and spoke. So, you are moving 13 pounds out half an inch from the center of the wheel. So 13 lbs * 0.11 (the 11% difference) = 1.43 lbs change in effective weight.
Now, let's say Ben has it right that a pound on the wheel = 5 on the car, and that my figuring is correct that our wheel's effective weight. So 1.43 lbs * 4 wheels * 5 (the Ben factor) = 28.6 lbs. So, if this is anywhere near correct, there is about a 30 lb effective difference car weight.
A stock GXP weighs about 3,000 lbs. The 30 lb effective difference in weight is about 1%. Ignoring gearing, various frictions and resistance, and probably a bunch of other stuff, I go back to the formula for acceleration - a=F/m (acceleration = force divided by mass). So, acceleration is linear (negatively) with mass and we have about a 1% change in effective mass. So, we should see acceleration times decrease (again being simplistic) by 1% which for a stock GXP takes us from 5.5 seconds 0-60 to 5.55 seconds 0-60. I would care about that if I am trying to win a drag race but seriously doubt I would notice that on the street.
All of this assumes the wheels are of equal weight. But, they are looking at lighter wheels, so there should be that much less of an impact, if in fact they are not making things better over stock.
The other thing, as U-238 points out, is unsprung weight. That would not be affected by wheel diameter. Since they are going lighter, this is also positive.
So, unless you are seriously racing (where every ounce is meaningful), I doubt these wheels will have any impact other than on the looks of the car.
