Agreed 100%. Unsprung weight matters more to me, so things like light weight forged wheels and 2 piece rotors are great to have from the factory on the Emira FE!
RAYS ENGINEERING stated for every 1kg (2.2lbs) of weight eliminated from unsprung weight is basically the same as eliminating 15kg (33lbs) of overall vehicle weight that your suspension has to manage...
So, let's just say forged wheels and 2 piece rotors are are something like ~4kg (8.8lbs) lighter. That's 16kg (35.27lbs) total (4 corners), right? So 16kg x 15kg = 240kg (or 529lbs) of reduced weight off suspension.
Because of the variables, there's really no simple number you can apply for something like this. Unsprung weight is simply those parts of the car that are not held up by the springs, which would be the tires, wheels, brakes, wheel hub, suspension members, shocks, etc.. Just reducing body weight can improve the handling and ride somewhat, but not dramatically unless you're removing large amounts of weight or building an all-out race car.
The real benefits are from reducing unsprung
rotating weight, such as tires, wheels, brake rotors. The less weight in those components, the less power (torque) it takes to get them spinning, and the less stopping power required to slow them down. It also makes it easier for the suspension to manage the vertical movement, so the response to surface changes are quicker.
The variables are the weight of the components, and where the weight is located. The farther from the rotating center the weight is, the more torque required to get it moving. This is why getting bigger wheels that are heavier than the old ones can make the car feel less responsive. Removing 4 lbs from a brake rotor isn't as dramatic an effect as removing 4 lbs from a tire, because the weight is closer to the rotating center, but it does have an effect. The thing is with wheels though, the weight is spread out across the entire wheel, and that can vary depending on the wheel style and design, so it makes it difficult to come up with a simple black and white number for gain or loss.
I've seen all kinds of numbers thrown around of the benefits. However Al Oppenheiser who was the chief engineer in charge of the Camaro group at GM, said when developing the Z/28 they had to remove 250 lbs of static weight from the car to gain 1 second in lap time. That's static weight; not unsprung rotating weight, so for those people who think spending thousands on carbon fiber body panels is going to make a big difference... not nearly as much as you might think. A lighter body helps in the corners on a road track, but on the street, you're not cornering hard over and over again like you are on the track. Unless you have gobs of money to spend and aren't worried about the cost, those thousands are better spent on removing unsprung rotating weight if the benefits you're looking for are better acceleration.
From my experience with the cars I've modified, if you can remove 10 lbs of unsprung rotating weight out of each wheel corner, that feels like the equivalent of twice the static weight. So that would be 20 lbs per wheel, times 4 wheels equals the effect of removing about 80 lbs of static weight from the car. This is on cars that weigh about 4,000 lbs to begin with (muscle cars). The lighter the car, the more dramatic the effect is going to be. On my Alfa Romeo which is about 400 lbs lighter, removing 10 lbs from each corner made a more noticeable difference than it did on my Camaro. If you have a 4,000 lb car and add an 80 lb child in the passenger seat, it's hard to tell the difference. If you have a 3,000 lb car and add 80 lbs, you'll notice it more. Same with removing weight. If you can remove 10 lbs of unsprung rotating weight out of each corner of a 3,200 lb Emira, you'll notice it.
Basically what you feel when you remove that kind of weight is the car feels lighter in its movement; accelerates easier, responds quicker to acceleration changes when it's moving, and slows down quicker. You notice it when pulling out to pass a slower vehicle, like a big rig on the highway. It doesn't take as long to reach the speed you need to get past the truck. That's an every day benefit you'll feel on the street. There's another side benefit; you use less gas. I gained 1 mpg in my Alfa (around town) after making those changes. Normally that's not that big of a deal, but with gas prices at the moment, that savings can add up.
