Question about Torque and HP gain: Wheel Weight versus Sprung Weight

[Ladogaboy]

Originally Posted by FJF

FWIW, we reduced the rotational mass of our IX by ~40lbs via lighter tires, wheels, and rotors. The front and rear rotors, alone, dropped ~20lbs. Though we didn't dyno each change individually, our IX with minimal power mods pulls the same numbers as IXs with full bolt-ons, cams, ported parts, etc on the same dyno, blah, blah, blah. Em, from STM, feels rather strongly that it's due to decreased rotational mass.

More importantly is the change in the feel of the car. The car feels lighter, a lot lighter. It adds to the driver's confidence to a degree that's difficult to express without putting one behind the wheel.

Is that on a mustang dyno or a hub dyno?

Either way, in real world terms, I guess there would be an increase in power. The engine is not putting out more, but less power is lost. Ultimately, the driver is experiencing the power after loss, so a 20% (just throwing a number out) decrease in drivetrain power loss, would mean an increase in whp and the driver would feel that increase.

[FJF]

Originally Posted by Ladogaboy

Is that on a mustang dyno or a hub dyno?

STM's Mustang.

Either way, in real world terms, I guess there would be an increase in power. The engine is not putting out more, but less power is lost. Ultimately, the driver is experiencing the power after loss, so a 20% (just throwing a number out) decrease in drivetrain power loss, would mean an increase in whp and the driver would feel that increase.

I haven't come across a single, practical drawback to reducing rotational mass, besides the cost. All aspects of performance and drivability are improved. One does have to prioritize. For example, I just got new tires. Instead of opting for Dunlop Star Specs, the current high-performance favorite that tip the scales at ~27lbs per corner, I chose Hankook Evos - ~23.5lbs each. There are even lighter tires, but I wasn't comfortable with the compromises. I think you see where I'm going....

Edit: There was a mention of lug nuts earlier. I weighted the stock Evo lugs and the Muteki Super Tuner lug nuts that I run on RPF1s. The difference was a couple of grams short of a pound (total) for all 4 corners. Too, as they sit the closest to the hub, their impact is small compared to the same reduction at the tire.

[Xenozx]

Originally Posted by Tappington

I agree with that. The wheel power gets affected so the gains would be from the wheels spinning easier. It is like a person lifting weights. If I am trying to lift 80 lbs and then reduce the weight to 20, my overall body power potential doesn't change, I can just do more work with a lighter load [work being force over distance]

I also agree guessing is very complex due to the weight distribution of the wheels. There is no way to know for sure without testing sets of wheels.

+ 1 for the weight lifting comment, that is exactly how it works

[Ladogaboy]

Originally Posted by FJF

Edit: There was a mention of lug nuts earlier. I weighted the stock Evo lugs and the Muteki Super Tuner lug nuts that I run on RPF1s. The difference was a couple of grams short of a pound (total) for all 4 corners. Too, as they sit the closest to the hub, their impact is small compared to the same reduction at the tire.

Yeah, the one pound difference sounds about right. BTW, good choice on rims!

[Tappington]

Okay all, so this leads us to the question: Should we explore 17" rims for our Ralliarts or stay with the 18" rims?

That 1 inch reduction will make a big difference in foot-lbs for the wheel.

[Ladogaboy]

I'm currently running 17s, but that is not necessarily for weight reduction. The thing to remember is that the tire still weighs a lot, and most of that mass is on the outer edge. The other thing to consider is that the higher the tire profile, the more sidewall flex you will have to deal with.

Again, in my case, I am running 17s for cost purposes and the fact that I'm not tracking/autoxing my car right now. If anything, most of my non-daily driving involves dirt and gravel roads, so having a higher profile and more rubber to protect the rim makes sense. I will, eventually go with a lighter-and-wider-than-stock 18, but I'm probably going to wait until next summer for that.

[Fourdoor]

Originally Posted by Tappington

Okay all, so this leads us to the question: Should we explore 17" rims for our Ralliarts or stay with the 18" rims?

That 1 inch reduction will make a big difference in foot-lbs for the wheel.

Ahhh, but to maintain the same overall diameter do you gain or loose weight going to a smaller rim? You lose 1" of metal from the spoke area on one side of a wheel, and a small amount of metal taken out of the rim area of the wheel to make up for the smaller diameter (no change in hub, or rim width) and gain 1" of sidewall on both the inner and outer side of the tire. The rim will be lighter, but the tire will be heavier... but how much on each? So all other things being equal (same wheel style and brand, same tire, just different size) is this setup lighter or heavier? If you go with a 17" setup that is wider than stock, it will probably be heavier than stock... but for the same width I don't know!

I would love for someone to actually get some wheel and tire combo's and actually check

Keith

[Ladogaboy]

I know that I only saved a few pounds (1-2 per corner) going to the EVO VIII Enkeis. But I also went from a 215 to a 235 tire.

[Tappington]

Originally Posted by Fourdoor

Ahhh, but to maintain the same overall diameter do you gain or loose weight going to a smaller rim? You lose 1" of metal from the spoke area on one side of a wheel, and a small amount of metal taken out of the rim area of the wheel to make up for the smaller diameter (no change in hub, or rim width) and gain 1" of sidewall on both the inner and outer side of the tire. The rim will be lighter, but the tire will be heavier... but how much on each? So all other things being equal (same wheel style and brand, same tire, just different size) is this setup lighter or heavier? If you go with a 17" setup that is wider than stock, it will probably be heavier than stock... but for the same width I don't know!

I would love for someone to actually get some wheel and tire combo's and actually check

Keith

So will the same volume of tire be lighter or heavier than the metal? I am guessing rubber is not as dense as the wheels? So no?

[Ladogaboy]

Originally Posted by Tappington

So will the same volume of tire be lighter or heavier than the metal? I am guessing rubber is not as dense as the wheels? So no?

Well, I think it depends on a lot of factors. Is the wheel forged or cast? What tire are we talking about? As FJF mentioned, there can be a four to five pound difference in weight from tire to tire. And I've noticed that the weight between the same model tires does not differ that much from aspect ratio to aspect ratio and tire width to tire width. The bigger the size difference, the more the weight discrepancy, of course, but for tires that will fit on the same rim, there is not much weight discrepancy.

[Tappington]

Aye, need to test set-for-set.

For the time being I am looking for wheel on cost versus weight that will work with my somewhat aggressive DD style.

I was looking at these 18x8: http://www.tirerack.com/wheels/Wheel...All&sort=Brand

For the cost and weight, and the OZ racing brand, that's not too bad. Any comments on the quality of these?

Future conversation with girlfriend:

Her:"Oh you got new wheels on your car, how much were they"
Me: ::coughing loudly:: "Well they were..." ::coughing continues" I got a good deal on them... but whats important is that I love you!"
Her: "Awwwww!"
Me: ::whew::

[Ladogaboy]

The OZs are very good, and very light for their size. They are still a little expensive for my taste, but what can you do?

Well, one thing you can do if/when your GF asks how much they were, quote her the price for one wheel.

"Oh, the wheels cost $400... I know, I know. I splurged."

[Tappington]

Originally Posted by FJF

FWIW, we reduced the rotational mass of our IX by ~40lbs via lighter tires, wheels, and rotors. The front and rear rotors, alone, dropped ~20lbs. Though we didn't dyno each change individually, our IX with minimal power mods pulls the same numbers as IXs with full bolt-ons, cams, ported parts, etc on the same dyno, blah, blah, blah. Em, from STM, feels rather strongly that it's due to decreased rotational mass.

More importantly is the change in the feel of the car. The car feels lighter, a lot lighter. It adds to the driver's confidence to a degree that's difficult to express without putting one behind the wheel.

FJF, that is good to hear some dyno confirmation. So what was cheaper? All the bolt-ons and mods or the new rotors, wheels and tires? Considering all the time and money that can be put into engine mods, new rotors and wheels and tires would be much quicker to install and getting working correctly... and maybe cheaper?

[Ladogaboy]

Originally Posted by Tappington

FJF, that is good to hear some dyno confirmation. So what was cheaper? All the bolt-ons and mods or the new rotors, wheels and tires? Considering all the time and money that can be put into engine mods, new rotors and wheels and tires would be much quicker to install and getting working correctly... and maybe cheaper?

I think people generally agree that it is always cheaper and easier to make more power than it is to reduce weight. The only exception to that might be wheels/tires, but that has more to do with the ancillary benefits, which include better braking and handling.

[FJF]

Originally Posted by Ladogaboy

I think people generally agree that it is always cheaper and easier to make more power than it is to reduce weight. The only exception to that might be wheels/tires, but that has more to do with the ancillary benefits, which include better braking and handling.

Agreed. Making power, alone, is definitely less expensive. To add another benefit of reduced rotational mass, the car's overall performance is improved while making life easier for its drivetrain. It's the difference between making power and freeing-up power. I hope that makes sense.