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

Tappington · Nov 3, 2009, 05:08 PM

I have been looking into wheels lately. On a post, from a while back, Eric from AMS had weighed the Ralliart stock wheels (the GTS pizza cutters) at 48lbs each. I do not know if that is with the tire on the wheel but let’s pretend it is without the tire (if anyone can confirm the stock wheel weight without a tire that would be pimpin)

I will be keeping the wheels 18 inches in all examples.

Again, let us assume all the wheels are 48 lbs, which would give us a 192lb total wheel weight. Now if the heavy stock wheels are replaced with 20lb wheels that would be a loss of 112lbs to the weight of the car, which is about the weight of a nice mamacita in the car with me.

Lurking on Tire Rack, I see the weight of performance wheels are around 17 to 24 lbs.

Thinking about wheel weight, I remembered my physics and angular momentum/inertia theory. I started asking myself: "Will removing a 112 pound mamacita from my sprung weight have the same effect as removing 112lbs from my wheel weight?"

It should not, I think. Because torque is the force times distance, losing that assumed 28 lbs per wheel should have a greater effect on engine torque and HP than the Normal force exerted on the wheels from a 112lb mamacita. The force needed to move 48lbs one foot is going to be much greater than the force needed to move 20lbs in one foot, when dealing with angular momentum and inertia.

You may be saying to yourself, "Don't be daft Tappington, the relationship between wheel weight and weight in the car seat is linear, and henceforth the gains will be the same." Well I say to you the torque formulas require multiplication, so a small change in weight should create a large gain in torque at various RPM's.

I realize how much calculus I would have to attempt to do in order to see numbers…but I have a better idea.

Does anyone have Dyno run numbers with stock wheels and then lighter aftermarket wheels immediately after? I would like to see if wheel weight makes as much a difference as I think it does.

Anyone have anything to add or contest?

Ladogaboy · Nov 3, 2009, 05:36 PM

First, the stock rim AND tire weigh just over 48 pounds. Mine came to 48.6 lbs. I have not measured the weight of the rim, but I would estimate that it weighs between 24-26 lbs.

Second, you are correct in that reducing unsprung weight has a greater impact than reducing sprung weight. I did not personally do the math on this, but I remember reading that the reduction was on a scale of 4:1. In other words, reducing one pound of unsprung weight would be like reducing four pounds of sprung weight. If someone can either confirm or deny those numbers with actual, authoritative findings, I would greatly appreciate that.

Third, while I do think it is beneficial to keep track of unsprung weight reduction, I do not think that we should spend THAT much time focusing on it. That being said, there are quite a few options for reducing unsprung weight, but these are my top three:

1. Wheels. You should be able to reduce between 5-10 lbs per wheel fairly easily.
2. Lug nuts. You can save more weight than you might think just with lug nuts; maybe as much as two pounds total.
3. Brakes. You could, possibly save more weight than the #1 and #2 options combined, but due to the cost, this upgrade is not worth it unless you are actually looking for the improved braking. Also, many of the bigger brake kits might not save that much weight even with vented rotors and aluminum calipers.

Tappington · Nov 3, 2009, 05:46 PM

You are correct. The wheel/tire was 48lbs. Their rim and tire choice were six pounds lighter.

So unless I want to drive around without tires... I better get some light tires. Despite my wrong weight ratio 6 lbs would be a ton at 4:1

I have seen that 4:1 number around the internet but I can't prove or disprove it.

Ladogaboy · Nov 3, 2009, 05:49 PM

Also, I just noticed that you were more interested in the difference in power output. Well, as far as I know, weight reduction should not have that significant of an impact on a dyno pull. It could be one reason why mustang dynos read a bit lower than dynopacks, but I could be wrong about that.

Tappington · Nov 3, 2009, 05:51 PM

As a kid, I remember an occasion I tried to pull my fat brother while he sat in a wagon. All I know is... when he was not in the wagon it was much easier to pull.

Ladogaboy · Nov 3, 2009, 05:56 PM

Quote:

Originally Posted by Tappington

As a kid, I remember an occasion I tried to pull my fat brother while he sat in a wagon. All I know is... when he was not in the wagon it was much easier to pull.

True.

But what I am saying is that when a car is on the dyno, it is not actually being moved. The dyno is absorbing the energy of the car and measuring its output. I could have four 300 lb men sitting in my car as it is dynoed, and its power output should be the same. All that would change is the power-to-weight ratio.

Tappington · Nov 3, 2009, 06:00 PM

I see. So then the weight would matter on the street but not on a Dyno, and that's is why Dyno's can construe real life performance benchmarks?

Fourdoor · Nov 3, 2009, 08:16 PM

For power to the ground measurements, the further the weight is from the center of rotation, the more it matters! If you have studied the math, you realize this already... torque is measured in lb./ft of force.... the number of lb.'s of force as measure exactly 1 foot from the center of rotation.

A flywheel that weighs 20 lb. and is 2 feet in diameter with the weight concentrated at the hub is much easier to spin up to 10,000 RPM than a flywheel that weighs 20 lb.'s that is 2 feet in diameter with the weight concentrated at the rim. So for power to the ground lug nuts don't make much difference.

For handling, all unsprung weight matters the same amount. Any mass removed means less work load on the springs and shocks and an improvement in handling. This is where lug nut weight reduction pays off.

Light weight tires are out there, but there is a price to pay. Lighter construction results in a less stiff side wall, and less stiff tread blocks. Drag radials are lighter than road racing tires, but you wouldn't want to take the drag radials to the road course for the little bit of power to the ground gained

Keith

PS: The 4:1 is a huge simplification. If you are running 20's the rotating weight reduction has more effect than if you are running 17's due to the rim of the wheel on the 17's being closer to the center of rotation... or does it? I don't know, because the larger amount of rubber in the 17" tire (of the same over all diameter) may actually weigh more than a tire made for a 20" rim... beats the crap out of me

ambystom01 · Nov 3, 2009, 09:51 PM

Reducing the weight of the car, regardless of how you do it, shouldn't actually increase HP or torque. It has no effect on the engine output at all. However, it can give the illusion of gained power by reducing the overall weight and by reducing the weight the engine has to spin up to speed. I've heard that wheel weight can have an effect on dyno numbers but it depends on the type of dyno (hub dynos would obviously not be effected) and it doesn't represent a true increase in power.
Fourdoor, that's a good point regarding the tires, I've never thought of that before.

Piper747 · Nov 3, 2009, 09:56 PM

Quote:

Originally Posted by Fourdoor

A flywheel that weighs 20 lb. and is 2 feet in diameter with the weight concentrated at the hub is much easier to spin up to 10,000 RPM than a flywheel that weighs 20 lb.'s that is 2 feet in diameter with the weight concentrated at the rim. So for power to the ground lug nuts don't make much difference.

Very true. The change in moment of inertia is not linear. It is a squared relationship.

Quote:

Originally Posted by Fourdoor

PS: The 4:1 is a huge simplification. If you are running 20's the rotating weight reduction has more effect than if you are running 17's due to the rim of the wheel on the 17's being closer to the center of rotation... or does it? I don't know, because the larger amount of rubber in the 17" tire (of the same over all diameter) may actually weigh more than a tire made for a 20" rim... beats the crap out of me

That is a good question. Its too late to think on that one.

Fourdoor · Nov 3, 2009, 10:12 PM

Quote:

Originally Posted by ambystom01

I've heard that wheel weight can have an effect on dyno numbers but it depends on the type of dyno (hub dynos would obviously not be effected) and it doesn't represent a true increase in power.
before.

There is zero gain in flywheel HP from a change in wheel weight. No mater what you change, the same amount of fuel is burned with the same amount of air, making the same amount of Horsepower. But the amount of wheel HP is affected by wheel weight. The less torque that is wasted spinning up a heavy set of wheels, the more of that torque is available to actually accelerate the car.

There is zero "true" increase in power, but there is a definite "effective" increase in power.

Keith

Ladogaboy · Nov 4, 2009, 11:06 AM

Quote:

Originally Posted by Fourdoor

There is zero gain in flywheel HP from a change in wheel weight. No mater what you change, the same amount of fuel is burned with the same amount of air, making the same amount of Horsepower. But the amount of wheel HP is affected by wheel weight. The less torque that is wasted spinning up a heavy set of wheels, the more of that torque is available to actually accelerate the car.

There is zero "true" increase in power, but there is a definite "effective" increase in power.

Keith

And that gets us back to the question of mustang dynos versus hub dynos. Mustangs do generally read lower power output, but I wonder if there is programming to compensate for the added rotational inertia of the wheels and tires.

ambystom01 · Nov 4, 2009, 11:10 AM

I wouldn't call it an increase in power. The main reason for that is it can confuse people who just skim threads, there are people who are still convinced that a lightweight flywheel increases the HP of the engine. It's more of a redirection of power.

Tappington · Nov 4, 2009, 11:17 AM

Quote:

Originally Posted by Fourdoor

There is zero gain in flywheel HP from a change in wheel weight. No mater what you change, the same amount of fuel is burned with the same amount of air, making the same amount of Horsepower. But the amount of wheel HP is affected by wheel weight. The less torque that is wasted spinning up a heavy set of wheels, the more of that torque is available to actually accelerate the car.

There is zero "true" increase in power, but there is a definite "effective" increase in power.

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.

FJF · Nov 4, 2009, 11:19 AM

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.