I searched and could not find the answer to this so sorry if its common knowledge or I just missed it.

Why is wheel weight so important? I see people bragging about 18 pound wheels vs. 22 pound wheels. To me 4 pounds per wheel is nothing...16 pound weight loss total for a lot more money. So is there another reason wheel weight is so important?

 

A small part of it is the unsprung weight, as unsprung weight makes it harder for the suspension to follow the ups and downs of the road. This is usually more of an issue in rallying and rallycross than road-racing and autocross.

The larger part is the rotational inertia. This makes it harder for the wheel and tire to be accelerated or slowed down. Now it's not just the total mass that matters, but also where the mass is located. This is the one downside of having huge brakes, such as on the Evo X; it forces you to run a large wheel (to clear the brakes) which puts a large part of the wheel's mass far from the center. The rule of thumb is that mass out near the circumference is four times as important (in a bad way) than total mass because of the effect on rotational inertia.

 

thanks!

so the 4 pounds really does make a big difference in accelerating and slowing the wheels? does this affect 0-60 time?

 

As I failed to explain clearly enough, it's not that simple. If the extra 4# is out near the tire, then yes. If the extra 4# is all near the hub, then no. Odds are, a large part of the extra weight is pretty far out, so probably yes.

Of course, what someone means by "really make a difference" is quite subjective. Do you find that having a (relatively small) passenger in the car noticeably lowers performance 0-60? If not, then you won't notice the effects of 4# extra per wheel.

 

how many thenths of a sec does a lightweight wheel shave off from the 1/4 mile?

 

thanks for clearing it up!

 

I was once told 1 lb of unsprung weight is like 5 lbs of sprung weight.
Also 10 lbs of sprung weight is like losing 1 HP.
For 16 lbs of wheels you're looking at 80 lbs of sprung weight or an 8HP "hit".

While these numbers are far from accurate, it gives you a good idea of why it matters. 8HP is maybe a 10th of a second in the quarter, but it's something.

 

First, it does make a difference (as others have stated above).

Second, I wouldn't scoff at 4 lbs of weight savings (unsprung or not). As you said, 4 lbs times 4 corners is a total of 16 lbs. Cutting that much weight off of these cars is very difficult, expensive, and often comes with draw backs. Outside of the "easy" weight cutting, such as battery and cat back exhaust, double-digit weight cuts are hard to come by AND expensive. Most likely, more expensive than rims.