LSOHOLIC

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..

AU N EGL

add your body wt into the equations. 2975 plus your wt = X

X is the number you use, not 2975 which seams light anyway

An other factor is where is that wheel wt? on the rim or near the hub?

ie 18" vs 19" rims and 18" vs 19" tires ? The farther the wt is away from the center of rotation the energy it takes.

There are a few formulas to calculate this as well.

and as you mention the stopping distance changes too.

a fairly good "approximation" is for each 100 lbs of wt removed, is approx 1/10 sec quicker in a quarter or 10 hp more.

Next your tire choices are extremely different in grip or traction

Drag race tires want to have tall side walls for better performance and grip. 16" rear rims may be a better choice with DR tires.
That taller sidewall will flex on launch, but provide grip. vs a short side wall tire will or may cause tire spin.
HTH a bit

drivinhard

the whole "X" rotating lbs is equal to "X" static lbs is a tough one to nail down, because where that rotating weight is located is more important than the weight itself. Ie, a 50 lb driveshaft is not going to have the same affect as putting a 50 lb heavier tire on the car, since the tire is 26" in diameter and much farther from the axle centerline, vs the driveshaft that's just a few inches in dia and rotating very close to the centerline.

and the moment of inertia gains are felt more in deeper gears where mechanically, the driveline has the ability to rev quicker. I've got a clutch that's 38 lbs lighter than the stock, and has a very small dia, and it's gains are less and less as you bang through the gears and move to taller gears. 1st gear feels and by 4th or 5th, it's just ho hum. with the taller gears and associated slower rpm sweeps, you lose that inertia advantage.

unsprung weight shows up big time in handling, especially on rough surfaces or if you are jumping curbs at the track, etc. the shocks work so much better if they are trying to control less weight.

VetteDrmr

Why don't you consider some 305/315s on your existing C5Z rear wheels? That's what I run on the track (autocross and DEs, not DR).

Have a good one,
Mike

Joe_Planet

Going to c6z wheels is a waste also you dont want a 345 rear timre if you go stock cause it
Throws off the height ratio. Either go a tad wider on stock rims or get stickier rubber toyo r888, hoosier, etc

hamdog

Leave the wheels alone and focus on the tires. Run the Hoosier R6 275/40 17 (23 lbs) and 315/30 18 (27 pounds). The traction will blow your mind!

Everett Ogilvie

Good topic and the subject of reducing unsprung and rotational mass is very important to me. My car came with carbon fiber Dymag wheels and I also bought a set of the 3 piece OZ wheels, and the head engineer told me the performance difference (acceleration) between my car with the Dymags vs. my car with the aluminum OZ wheels is the same as adding 40HP. I don't know how accurate that number is but there is no doubt I can feel the seat of the pants difference in the car when swapping wheels back and forth. With that said I stopped using the Dymags on the track and ended up buying a THIRD set of wheels for the track. They are not as light as the Dymags (but they are stronger), and they are lighter (and wider) than the OZ wheels. Additionally the street wheels for this car were 18" front and 20" rear, the OZ wheels are 18/19, and the new track wheels are 18/18. You can see the trend - smaller diameter wheels, which gave me more tire choices and reduced the diameter where the outer wheel weight is carried.

Right now I am in the process of installing Gary's new T1 brake setup front and rear. I have one main reason for doing this (b/c my current brakes already seemed up to the task of stopping this lightweight car) - reducing rotational effects. My current brakes are 14" front and rear, and Gary's new AP rotors are 12.8 front and rear. I am going with smaller rotors! I won't get into the advantages of the new AP rotor b/c there is a thread dedicated to that, but suffice it to say that I will still have plently of braking power (maybe more b/c the AP calipers and rotors are race quality) and a rough calculation is that rotational effects will be reduced by up to 30% (there are some spreadsheets out there which attempt to calculate performance changes by changing wheel/tire sizes but I have not had time to play with them much).

I guess my point is there are mods and there are smart mods! Hopefully a few of my choices are the latter.

mousecatcher

The question for rotating mass is about inertia -- so you have to consider the moment and not just the weight. Holding a 5# barbell at your shoulder and holding it with arm extended is a HUGE difference.

So for wheels you have to consider not just the absolute weight, but the diameter and the weight placement. That's why 3 piece wheels, which tend to be heavier than 1 piece, can actually be better from a performance standpoint -- because the weight is more centralized.

Now with unsprung weight you also have to consider the ability of the shocks to control it, so there's another level of complication.

It's extremely difficult to equate unsprung weight to sprung weight by any rule of thumb due to this high variability.

But we can use one good rule of thumb: use the smallest wheels that the brakes fit under.

SIK02SS

a larger tire adds more wight too..

LSOHOLIC

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 ..

Slalom4me

In addition to points already made about inertial mass, consider
speed of rotation.

A 10 lb change at the wheels is different than 10 lbs at the
driveshaft and different again from 10 lbs at the engine's
rotating assembly.

Changing wheels & tires led to a reduction of 20 lbs, which
was noticable in improved SOP acceleration/braking and ride control.

On a different vehicle, increasing wheel & tire weight diminished SOP
acceleration and ride control. Braking was likely affected too, but
this didn't come though in SOP terms.

I disagree.

A number of considerations prevents a direct correlation - but,
in general terms, my vote is that the relationship is reversed. A
few dynamic lbs gained or lost is equivalent to many static lbs.
Reducing flywheel weight by 10 lbs has greater impact on
acceleration than taking a 10 lb jack out of the car.

.

AU N EGL

44lbs of rotating mass is equal x 6.5 lbs or 286 lbs of static weight

Jason

Great topic. Scary OP.

Slalom4me

This relationship might occur under specific circumstances.

But as a general catch-all formula - I disagree.

LSOHOLIC

Jason, please elaborate on the statement "scary OP" please.

AU N EGL

Each 1 lb or rotating wt is ~ 6.5 lbs of static wt

LSOHOLIC

yes, my original post states that. I took the low average of 5 lbs so I would not insight a riot. You know, some people don't believe that heavier wheels cost(hp) you more weight than just their static weight. But yes I agree that 6.5 could be a legit figure. In my original post I claimed 220lbs or something like that.

drivinhard

It's not a linear number though. It's going to show up more in 1st gear, less in 2nd, less in 3rd, less in 4th, etc. This is just the angular acceleration part of it though, the unsprung weight factor is always going to be there for ride quality/handling.

For drag racing, it will show up as an advantage in the first 2 gears the most.

AU N EGL

and under braking.

mousecatcher

He obviously meant, ONE # of rotating mass = 4-6 # of static weight.

Which is far far too simplified, and also he left out the part about it being unsprung rotating mass vs a sprung mass.

About the only thing you can really say is that adding unsprung weight is worse than adding sprung weight. Always optimize your unsprung weight first. Run the smallest brakes (and therefore wheels and tires) possible.