I dropped off my new wheels (centerline rpm 18") today to get my new tires (yokohama parada spec 2) mounted.

my stock wheels w/ 235/40-17 kumho 712's weigh in at ~ 52lbs.

the new wheels and 225/40-18's weigh in at ~ 39lbs.

so the weight savings is around 13lbs. per corner, 52 lbs. total


but, the 18" package ends up being +1" outside diameter, so i'll lose a wee bit of gearing advantage.

so anyway, what do you think my gains (if any) will be? EricL already said it could be a wash because of the increased O.D.

i find that difficult to believe

 

i have no clue, if you were lookin for gains why didnt u go w/ 17's?? but 18's do look alot sweeter

 

you answered your own question.

it took all of my self control not to get 19's. 17's were out of the question.

 

Mattg,
Should stayed with 17s BTW, PICS!!!!!

 

What was the RPM number? That is better than the OD number as I've seen too much variance there.

If you are an inch off, I still think you will see a better 60' 330' and 1/8mile possible, time wise. Your trap speed may go up.

With mine I dropped almost 10lbs per set up and saw the 1/4 time drop to 14.65, about 3 tenths, but my trap speed dropped to 93.5 MPH.

Also, the tire compound will have an effect too.

So get out there an let us know how it goes.

BTW - wish me luck, I'm taking on an SC430 at the track this Friday.

Ruf

 

That's a lot of weight you lost there bro!!!!
I think you are gonna notice a difference. The OD will take some away, and EricL is a brain surgeon and a rocket scientist, so what he says probably will hold true!!!

Lets hope that less weight out weighs more OD!!!!!

Keep us posted!!!

 

225/40/18 has overall diameter less than stock and has improving the gearing ratio!

 

hey mattg,
i have those exact wheels.....theyre pretty nice and grip alot better than the stock.....so u should see better times.....but most of all...DONT THEY LOOK FRIKIN SWEET??????? I think theyre badass....people comment on how mean they look to me all the time......good luck

 

i mean tires...oops......=)

 

thanks guys.

i'll post pics as soon as they're on the car, and time slips soon to follow

 

yes, the tires look sweet.

 

those tires have more revs per mile, so you'll gain some in gearing. not really, but you won't lose any at least.

ericl may have been talking about the distribution of the weight, most likely your new set up will have more weight on the outer most part of the wheel. That's what really matters when reducing unsprung weight. I'm not positive about this, but if you were to draw a circle connecting all the lugnuts, everything inside the circle is not considered rotational weight.

 

19" look the best on this car, especially as low as your ride is. New 17" are just a waste of time and money. You like to run hard so 18" is the rbi on looks and centerline is a homerun on performance.








....sorry for the baseball analogy

 

You will have to let me kow how you like the tires... I have the piradas same size and will move to something new due to the small diameter and often bing bam ding ding on da rims.... First you will notice the flex with hard corning or "swooping" corners. It takes a lillte getting used to. Also look at the sidewalls and see where the tire flexes too: scarry..... and the scarry bit is enought to make me not want to corner as hard as I do. I do think I would be a gonner if I were to push that tire a little further and pop the bead... mess up da kool rims too.....
Any hoo let me know I'll keep an ear open Ohh yea speed ohhh will bee off too

 

Here we go again....

3 x 52 lbs ~= 160 of equivalent static weight loss (round #s)

Figure car + driver at 3500 lbs (pick you own figure).

(3500) / (3500 - 160) = 3500 / 3340 = 1.047 (or about 1.05 factor that could be multplied by HP) for speeds up to 80MPH (or so).

[NO DIAMETER CORRECTION FOR THE 18s YET]


The 235/40-17s run about 4.2% fast relative to the stock 215/50-17s. (1.042 lower effective overall gearing and more effective torque.)

So, the new correction factor gives you a power-to-weight boost of 1.05. (figure high with tire wear)

The effective torque increase provided by the 235/40-17's smaller diameter is close to 1.042. (or 1.05 with wear)

1.042 is pretty close to 1.047, but this is comparing against the stock 215/50-17.

[TIME TO ADD IN THE 225/40-18 torque correction]

The 225/40-18 is 1.015 times smaller than the stock 215/50-17. So, an additional correction gets added to the new setup.

1.05 now gets compared against 1.047 (inertia) * 1.015 (effective torque) = 1.062

So, now its 1.062 (NEW SETUP) vs. 1.05 (OLD SETUP) (~1% difference?) [See caveats below]


The torque continues to help at higher speeds where V^2 * CD * frontal_area is an issue.

The lowered weight becomes irrelevant at higher speeds (inertial issues are "swamped-out" by the quadratic relationship of aero resistance.

Finally, the comparison presumes that the wheels are solid disks and can be compared on the basis of mass only. The smaller outer diameter of the 235/40-17 has a concentration of weight at the outside of the tire (the BELTS ARE HEAVY). Since they are moved inward, this is a factor that could influence the calcs. The new wheels are 18" and the inertia is based on radius squared (R^2).

I could go on and on (about the effects of radius on inertia) and so on. Even the change in rubber compounds will change traction (helping with 60-foot on one hand ... and possibly adding more drag depending on compound). All unknowns. Heck, even the tire footprint will be different (front-to-rear and side-to-side).

Previous 1/8-mile times to new 1/8-mile times should eliminate a good portion of the aero issues (this presumes 60-foot, temps, baro, and altitude are the same along with the rest of the car's configuration).

The previous 1/4-mile times to new 1/4-mile times in addition to the 1/8-mile times should help (along with 60-ft times). (Traction is also a consideration).


If the results aren't consist ant, erase the 1/8 and 1/4 mile comments -- it just becomes more guessing and you will find out soon enough.

 

First issue:

Yes, I did mention how the distance from the center impacts rotational inertia issues.

However, unsprung weight is a separate issue from rotational inertia. If you put 20 lbs of extra weight in the inner 1" of the wheel or even just made the lower suspension arm out of heavy material, you are increasing the unsprung weight. When you are talking about unsprung weight, you are not talking about rotational inertia.

UNSPRUNG WEIGHT != ROTATIONAL INERTIA (these issues seem to get mixed-up quite a bit.)

RE: Regarding inertial issues:

Even a drive shaft that is 3" in diameter will impact acceleration. Various RWD cars have lightweight aluminum, carbon fiber, and exotic materials for use to reduce this weight. When the weight is not moving WITH the vehicle -- as is the case with a car’s wheels, the gains in acceleration can be quite large (and higher that the 3x factor associated with lighter wheels).

RE: Regarding the weight towards the center of the wheels:

The weight towards the center of the wheel is "basically" very close to static weight. The distance from the very center of the wheel (radius squared) is what is important. The TOP of the tire (the topmost part of the tire directly above the tire-road contact point) is moving at twice the speed of the center of the wheel. So, the weight at the center of the wheel is at least as important as the static weight in the car. As you move outward, the mass becomes more important. There is no such rule that says that the weight inside the lug nuts is not considered rotational weight. From a theoretical perspective, any weight/mass that is not at R=0 (where R=radius), is contributing to the moment of inertia.

 

two things that work to my advantage,


1. the way the wheels are designed, most of the weight is concentrated in the hub/pcd area. the actual outer rim portion is thinner than a cast wheel.

2. the yokohamas weigh in 3-4 lbs. less than the kumho's.

Eric, what you said in the above post makes me think that spending $80 on superlight lug nuts may be a waste of money

 

you have the 225/35 series, i have the 225/40's.

what do you think about the take off traction? how are they in the rain?

your car must haul ass with that short of tire, have you taken it to the dragstrip?

 

RE #1: That was the first thing I noticed with the SSRs vs. the OEM 17x7 cast wheels. The outer flange was THICK. The forged wheel people are no dummies. The forging allows the outside to be just as strong, but much thinner

RE #2: Well that certainly will help. OTOH, it's the distribution of weight from inside to outside that will determine how that weight difference is leveraged.

Super light lug nuts -- well, if you got 'em, and they are the good ones that handle torque well, enjoy. In the overall scheme of things, a Big Mac (or two) before a 1/4-mile run will probably negate the impact of the lightweight lug nuts...

 

And if it were just as easy as applying some mathematics, then all the racing clubs wouldn't spend so much time testing out the various tires.

Anyway, EricL's approach to this can give you a rough idea and or help you narrow down your choices.

As we've said, gt out there and let us know how these puppies do.

Ruf

 

And this is why I stayed with 17" wheels, and worried more about the tire weight etc, than the wheel.

So while some may lose 10lbs, it's the percentage of weight that gets reduced further from the center of the hub that makes the most difference.

There were some wheels that I liked, but for the additional 2.8lbs I chose looks over the savings. While on the tires, I labored more over the 1.4lb of weight saving verses a tire that was $50 less $each).

Note my 60' times 2.186
and my 330' times 6.180

This was a Stock TLS - just wheels and tires

Ruf

 

what kinda tires goin on the centerlines matt g,

my bro jus got some centerlines for his truck, they are like 40% lighter than any other SUV rims.

 

the tires are yokohama parada spec 2, 225/40-18.

 

Go 16". Instant big brake kit.

 

well i got them installed and had the car aligned today.


i was worried i might have to raise it up a bit, since the wheels are +42 offset, but they fit, 3" drop and all. my front camber ended up being -1 degree, which is in spec. the rear is just under -2 degrees which isn't that bad.

i'll probably roll the rear fender lips just to be sure i wont rub w/ rear seat passengers. i could hear some minor rubbing from the right rear, but only for a second under wot acceleration.

when i drive the tires are actually tucking, especially in the rear

i haven't driven much yet but i did notice it barked the tires on the 1-2 shift under wot, that never happened before

i'll post a pic or two tomorrow.

 

If you don't break 14.0 I'ma come PUSH your car at launch to get you going quicker.

 

Mattg congrates on the wheels and the tires I bet your car looks sweeter than ever. I am curious to see if the increased rotational inertia will effect your 1/4 times. Keep me posted

 

Ahhh yess mattg you stand correctI am running a smaller tire.. As for the rain, they provide for much fun and lots of sliding. Another gripe is no gettin on the gas around corners... lol you go straight.. As for the strip, my luck has it that at the free night at the drag strip I was on the spare due to flat ie. nail which rendered the tire worthless due to low profile nature. I have a buddy with a GXT eclipse and everytime chase him down and take the win .... On dry ground with those tires one has to go easy with the clutch in second as well otherwise the tires will roast like duck but smells worse....

 

......and all this time I've been just looking for a nice set of chromies

 

get the bling and the zing.


NO FAT CHROME WHEEL!

 

and welcome to page three

 

solid in vtec

 

Hey Mattg,

Can you email me some larger pix of your car with the new Centerlines? I've narrowed it down to 19" Silver Centerline RPM's and 19" Ace Alloy Mantas. I actually like how the Mantas look a little better but I'm so tempted to save weight with the Centerlines. What does your butt-dyno say about the lighter wheels?


EMAIL: trohdy@yahoo.com

 

i'll have to take some hi res. pics.

butt dyno says i have some gains, i noticed it was easier to accelerate in 5th gear on the freeway. i also had it bark the tires on a wot 1-2 shift the other day, which never happened before.

 

mattg,
Could you please post a quick pic... I am looking into getting those wheels, keep finding dead links. Thanks!

p.s. Anyone know what Axis Se7en Mods weigh? I really like those too.

 

look in my sig.

i have more if you want.

 

hmm what do to....

lose my performanse w/ some bad ass lookin chromes

or go light weight??

matt,

do the polished rpm's almost shine as much as chrome when they are clean??

 

yes, they do.

a lot of people have confused them for chrome.

then i tell them they weigh 16 lbs.

 

yea, lemme know when u sell them!!!

 

Not exactly sure about the se7en mods, but the axis se7en wheels weigh in at about 25 lbs in 19x8. The RPM 19" are about 15 lbs I believe.