The topic is weight of wheels and how it relates to performance. I own a 04Z and I've tried really hard to keep the weight down on the Z with the performance up grades. The question is unsprung weight and the rolling mass/inertia/momentum which robs HP. Here's what I'm contemplating, switching my C5Z OEMs for C6Z OEMs. The research that I have done has revealed some flaws in my quest for more rubber under the rear(on a budget).
The facts:
C5Z Alcoas 17x9.5=19lbs
18x10.5=21lbs
C5Z tires Bridgestone Potenza RE760 265/40R17=26lbs(front tires)
Bridgestone Potenza RE760 295/30R18=29lbs(reartires)
Totals=C5Z fronts W&T=45lbs and rears W&T=50lbs per side

Now for the what I was considering
C6Z Speedlines 18x9.5=22lbs
19x12=26lbs
C6Z tires Kumho XS 275/35R18=28lbs(front tires)
Kumho XS 345/30R19=41lbs(rear tires)
Totals=C6Z fronts W&T=50lbs and rear W&T=67lbs per side

So the difference being; Going to C6Zs is an added 5lbs (x2)per side in the front and an added 17lbs in the rear (2x)per side. For a grand total of 44lbs to convert to C6Z OEM Speedlines. That seem ridiculous !!

So 44lbs of rotating mass is equal to 4-6lbs of static weight. We will take the average of 5lbs x the 44lbs gained = 220lbs. WOW. Thats not acceptable.
My car makes about 430 N/A and 600 on spray at the tire, and only weights 2975lbs.
So my question is will I feel this in the seat of my pants ?? The power loss to turn those big lugs is what I'm referring to. And does anyone have experiance with what I'm talking about or input to help ? I take it to the drag strip about once a month and road coure it about twice a year. But I'm not into hopping up the car just to turn around and slow it down with a bad choice of wheels. Not to mention taking a country mile to bring it to a stop.
I'm really looking for hard facts or peronal experiances. For example; if my car runs consistant 10.99's (N/A) could it potentially drop to 11.30's or something like that ?? By just changing the wheel and tire combo. The change is mainly for lack of traction on the street and the 19x12's look sick with the L5 fenders.
Again any input is much appreciated. Thanks..

 

Is your choice for street use? or track use?

If it is for the track, use stock 17" C5 wheels and 275/40/17 drag radials... about 40.5 lbs.

If it is for the street, I highly doubt you would notice the weight difference, no matter what combo you put on there.

 

The formula isn't exactly quite that extreme...many have tested it and the 'multiplied rotating mass' thing doesn't always add up to quite that much of a loss on the track in most cases. That 44 pounds might even equal no more than 100 pounds of static weight depending on power level and total vehicle mass etc.

 

I am trying to lighten my rotating mass by getting lighter wheels and tires. Need to dial in for the 11.50 Index.

 

Dude, you could throw some steel 18x15s on there and still qualify for the 11.50 on even the hottest summer day LOL.

 

Maybe these would work?

 

I've never understood the school of thought that some how the street shouldn't matter as much. A 10.90 car at the track is still a 10.90 car on the street. Period. So yeah, I will notice a difference, even on the street. I think ?? So really, I'm not wanting to give up anything anywhere(track or street). So you guys are saying yes, the car will be slower but I won't notice ?? I'm confused on the advice thats being given.

 

Of course it still matters and yes someone who is very 'at one' with their car will actually feel the difference the between wheels (for instance I can tell the difference between a full tank of fuel and 1/3 tank). But I think what Pete and I are trying to convey is that a 10.90 car on the track is rarely a 10.90 car on the street anyway (traction) so if it's only an 11.0/11.1 car on the street then it's still fast.
But I see your point as well, why not just be as fast as possible all of the time.

 

There are a lot of great points made here. Just to clarify, I have track wheels and tires. This set up is stricly for my street adventures . I'm just not wanting to slow the car down for the street. But needing more traction for the spray !! Hints the 345 with a sticky street tire. And please don't mention DR's because I enjoy the topend handling as well. But thanks again ..

 

I'll try to find it the equation for you's somewhere. Its approx 2.75 x weight of rotational weight up or down. So 10lbs off is like taking off 27.5lbs off the car.

 

The larger moment of inertia from both the larger diameter wheel and the heavier weight of the wheel/tire will help traction at the cost of slowing it down some. IMO not a bad sacrifice on a street tire when you're considering straight line acceleration only (different argument when you get into discussing the effects of unsprung weight on handling)

You have to remember that (if you're making the kind of power that you're implying) that your acceleration will be traction limited often, so you won't notice the difference in acceleration from some extra rotational inertia, because the tires will be slipping at lower speeds. You will lose some top end acceleration though once the tires hook up (but the tires should hook up at a lower MPH due to the extra inertia). Not that I am condoning the behavior, but if you're a 'highway roll' kind of guy and having issues with tire spin from highway speeds, the extra weight/inertia will help you immensely after the third honk.

I have no earthly idea what kind of math you're doing to come out with 220lbs, or the equations Fartpipe is talking about. To calculate the actual difference you'd need to find the moment of inertia of the wheel/tire assembly.

Rotational mass is not all equal. The effect of rotational mass is completely, 100% dependent upon its distance from the axis of rotation. 5lbs at the centroid of the wheel will show (theoretically...since its impossible to have it all at one exact point) no difference in acceleration from 5lbs on the chassis, versus 5lbs on the tread of the tire will have a huge effect.

 

myday121 "I have no earthly idea what kind of math you're doing to come out with 220lbs"
Let me explain. I was just going by what alot of the road racers use. Which is anywhere from 4 to 8 lbs per 1lb of unsprung weight. Depending on the location of the rotating mass. I understand the theory of the further away from center the more impact it has.
So let me break down the math I used again, I took the lower value of 5lbs per 1lb unsprung weight. So the rears were 17lbs more per side. Thats including the tires difference also. And the fronts were
5lbs more per side. Which is a grand total of 44lbs difference as compared to the C5Z wheels and tires. 44 x 5 = 220lbs.
So explain how this is confusing or wrong ??

 

Those equations are a very very very very very rough approximation. While they are not completely misleading, I wouldn't put a lot of weight into those values. 4-6lbs is not necessarily wrong and not a totally unrealistic estimate, but also definitely not right. It is also not a linear relationship, as it depends on the angular velocity (the effect will be greater at low speeds when the wheels are accelerating at a higher rate than at higher speeds when they are being accelerated at a lower rate)

Remember to take into consideration the benefits of the extra inertia at lower speeds to increase traction versus the loss at high RPM's -- depends on how much power you're making, and if you're trying to run say 0-75mph or 75-150mph. Sometimes losing a little power can make you faster. I'm fine with heavy street wheels for exactly that purpose.

I'm not trying to completely discredit what you're saying, I'm just saying be weary of any blanket approximations like that, because there are many, many, many more variables to take into consideration.