Grassroots Motorsports Magazine ran a test on the effects of wheel inertia on a short autocross course and 0 to 60 acceleration.
Granted they used a low powered Maita for the testing.
The whole article is interesting, but I clipped the acceleration details .. here


Are Lighter Wheels Really Better?

https://grassrootsmotorsports.com/ar...really-better/





Masitaly Wheels
size: 15x7 in.
backspace: 57/8 in.
spacer: 1/2 in.
total weight per corner: 42 lbs., 4.6 oz.



Volk TE37 Wheels
size: 15x7 in.
backspace: 53/8 in.
spacer: none
total weight per corner: 30 lbs., 14.6 oz

This test showed clear differences both on the kart track and at the strip. Times were substantially worse with the heavy wheels, which slowed the car by 0.31 second on the average lap and 0.46 second on even the best lap. Even though Alan’s final run on the heavy wheels was substantially quicker, it still was 0.16 second behind his best time on the light wheels.

0-60 times:

Volk TE37 = 8.61 sec

Volk w/ballast = 8.75 sec

Masitaly = 9.04

Our zero-to-60 launch was our big test of angular inertia, and it showed substantial results. We ran three tests here, the third with the lightweight wheels and an additional 42 pounds of ballast in the trunk. This would illustrate the difference made by the fact that the heavy wheels aren’t just weighing down the car, they’re increasing the amount of energy required to spin them.

That test was fruitful, too. Though the ballast slowed the car’s zero-to-60 time by 0.14 second compared to the Miata equipped with simply the light wheels, putting that weight on the hubs slowed the car by an additional 0.29 second.

 

Not just low-powered, but also really light. It's a 92 Miata that has a curb weight around 2200lbs, stripped down for track duty. So it probably weighs 1800lbs, half the weight of a new Corvette. And it had 116hp, less than 25% of the power of a C8 and even less than half of an L98. And they removed 43lbs of rotating mass! So yeah, I'll grant that in a fairly extreme example such as that, the difference in both acceleration and track time would be noticeable.

 

Interestingly, Engine Master JUST released a new vid covering....light flywheels! They ran a ~500hp SBC with they typical flywheel and dyno coupler, which they claimed, weighed 44 lbs, along with a 10 lb harmonic damper. THEN, they ran an little aluminum hob up front (not a damper) that weighed basically nothing, and a ~4" diameter coupler on the rear...no flywheel. I don't know how they got the engine started. Anyway, with a 300 RPM/second acceleration rate for the pull, they gained 12.7 lb/ft and 15.8 hp by going to the light components....





With the 600 RPM/second acceleration rate for the pull, they from heavy to light, they gained 17 tq, 23.1hp....see for yourself:

 

In another interesting comparison, my friend has an '88 fox 5.0. Sweet car. He has done underdrives, GT40 intake, 1.7 rockers, P heads, LT headers/exhaust, TB, MAF swap, 11lb aluminum flywheel...basic "5.0 starter kit". His car dyno'd 280RWHP.
My '92 is stock and did 278 RWHP. Which engine makes more power? Steady state HP, mine does. Compared to his car, my power is getting "eaten up" on an inertia dyno with:
18" wheels (he has 16's)
ZF6 trans and gears (he has a T5)
35 lb flywheel (he has 11 lb)
Stock accessory drive to his UD set up.

Now for the weird part (weird to me anyway): My car KILLS his at the drag track. We met a 'Vegas years ago and I went 13.5x's/103ish to his low 14'.4x's/95. We even swapped drivers and I couldn't drive his car into the 13's. The drag track is where those light components should shine, and his 280RWHP should out do my 278. Odd.

Interesting stuff to ponder/scrutinize etc.



.

 

The car I am talking about removed ~64lbs in total on the wheel/tire swap and another ~20lbs with the carbon fiber driveshaft.

Also, a tenth isn't all that huge in the grand scheme of things but in drag and roll racing it equates to a car length or more at 125+mph. We mainly roll race so going from getting edged out by a fender or half a car length to dragging that *** by 2+ car lengths is a pretty damn big difference. Forget the cost, it is impressive for a wheel/tire/driveshaft swap to make that big of a difference. At least I think so anyway.

 

Let's look at the inertia of a wheel and what weight of the wheel is worth compared to weight on the car.
One extreme is a wheel with all weight exactly in the center. That wheel would not have any inertia and 1 kg on the wheel would be worth 1 kg on the car.

The other extreme is a wheel with all weight on the thread maximizing inertia. Assuming that the wheel is not slipping the speed of the thread around the center would be the same as the speed of the car. The rotational energy would be the same as the energy of motion forward. In this case 1 kg on the wheel is worth 2 kg on the car.

From this it is clear that removing 1 kg from a wheel would be the same as removing between 1 kg and 2 kg from the car. Normally 1 kg on the wheel is worth about 1.6 to 1.7 kg on the car.


Spoiler from the video:
Inertia 0.016 Kg m2
Mass 1.63 Kg
Radius 0.126 m

By dividing the inertia by the square of the radius we get the equivalent weight to accelerate from inertia.
0.016 / ( 0.126*0,126 ) = 1.01 kg.

So in this example the wheel weight of 1.63 kg is worth 2.64 kg on the car.

2.631 / 1.630 = 1.62 times

 

Oops, I'm an idiot. In all my calculations previously in this thread, I used the circumference of the wheel/tire/flywheel/driveshaft to calculate inertia. I should have been using the radial center of mass. So I seriously overstated the inertia of all the objects in my examples.

 

No, cost is always a consideration.
You discuss costs in many of your posts. I mean we all do, but you are the one saying "forget the cost" .
Here you are laying out your costs
Talking about the cost of your gages
Talking about the cost of a 6.0 Swap
Here talking about the cost to put your car together
Here you are talking about spending less and beating more expensive cars.

So now that we have established you do care about cost, I am sure I can find more posts in necessary.....

That Carbon Drive Shaft... $1250 best I could find.
That VMS Mustang Drag Pack... Sold Out, but shows as $965 for the set

Can't look at the price of tires as you didn't specify, but the math is $2215 plus tires for .1sec.... Forget the cost? I guess you would have to.
I know this is why high end race cars start to get very expensive and I am very aware of diminishing returns. Once you get to a certain stage, especially in certain classes, you have to spend deep to further improve in areas you are allowed to, but I am not impressed by gaining a fender or .1sec if you spent north of $2250. Based off the math, you are claiming trap speeds of 125mph which puts a 10-speed Mustang in the 575hp range up from 460HP. So obviously some bolt ons, I would just wonder which.

Seems a .1 or more could be had cheaper by:
Rear gear change
Pulley, Injector, and Tune (if a SC was put on)
Does it have an intake change?
What about a throttle Body Change? I am sure the additional consumption of 100hp would like a bit more air.
Does it have headers and High flow cats?
If we are willing to go big and skinnies, can we remove the rear seat, passenger seat, spare and jack and other non essentials
I guess without a full bolt on list I could keep making suggestions, but you see my point. I would think all those would be cheaper and net greater gains. Doesn't sound like that car has a tapped out engine.

Is it an improvement, yes. Does the science work, yes. What I don't agree with is choosing this mod path without a specific restriction keeping you from doing other things, like class rules.

 

Well let's just call the driveshaft what it is: bling that costs a lot and adds zero performance to the car. It just doesn't do anything measurable. The skinny front-runners could cut the ET 1/10th, but due at least as much to drag and friction as to any weight/inertia reduction. Those aren't cost-effective if you go the new/fancy route like KyleF mentioned, especially when you consider that they can't be your daily-use front rolling stock. Back in the days of Fox Mustangs, we used to get aluminum space-saver wheels and put skinny cheap tires on them to get the same result. That was cost effective. Also consider a more typical use case: someone is buying a second set of wheels and tires to use for track or autocross events. My personal C4 example was buying a set of four 18x11 wheels and 315/30/18 tires. The tires are sort of a given, so you won't select them based on mass. But I could have bought:

1. Piece-of-****, blingy wheels made of Chinesium that weighed 37lbs for $200ea, or
2. Cost-effective flow-formed, rotary forged one-piece wheels that weighed 25lbs for $360ea (think Forgestar), or
3. Super high-end forged three-piece wheels that weighed 23lbs for $1500ea (think Forgeline).

I chose option #2. The extra $640 I spent over the craptastic boat anchors not only saved me 48lbs of rotating mass, it also got me a truer wheel with a better finish. The extra mass not only got me a little better performance, but also kept my back much happier when I changed wheels (which I did a lot). That was easily worth the extra money. OTOH, spending another $4560 to drop 8lb additional mass just didn't make any sense. That change would not have made any measurable performance difference (keep in mind that the radial center of mass with these wheels is probably only around 6-7"), and that's a crap ton of money. So with rolling stock, there are levels involved and with mass removal in general there is always the law of diminishing returns to consider.

 

Exactly, if you are already changing components doing a cost benefit analysis to help with the decision is spot on. This example is the definition of diminishing returns. The dollars spent between your Options is significant but the benefits are reducing at each step. It would stand to reason that Option #2 would be the best as you save that $4560 and can invest it elsewhere that would return greater performance. The only exception to this is if you have class rules that you have met and the only other place to gain an advantage is to spend more on the wheels than the guy in the lane next to you.

 

I really don't even know how to respond here other than to say..... I am not defending my acquaintance's expenditures, I am just giving you the results and his reasons for the purchases. The only thing I take issue with here is that you keep calling it a .1s improvement on ET. It is likely a .2-.3s improvement in total and at a cost of about $3-3.5k. That being said, I have already agreed with you that the costs are outrageous. All I am doing is telling you a story, you seem to agree with the science but you are getting mad at the story teller for the actions of the story's subject. All I was saying with my "forget the cost" is to get you to focus on the science as that is the subject of my posts and I believe the subject of the OP's post. I am not suggesting that anyone with a C4 should try and invest a single dime into this type of stuff. I know it is expensive and I also know that with the mediocre power that most C4 Corvettes make, these types of high dollar mods would be cost prohibitive and also not produce as good of results as they will for higher powered cars.

If I have offended you in any way, I apologize. You win.

 

This is why:

You have not, and I apologize if my postings came across in any offending way as well. I was just trying to make the point that cost is an important factor, not the only one, but usually significant. I did think you were defending it, but I was not mad. Internet postings are bad for getting communication crossed up.

 

My prior daily, GM originally used a hollow aluminum driveshaft (roughly 4500lb truck) for the first 2 years of production for "better" fuel economy. They later swapped to the steel driveshaft I had. There was a substantial difference in weight. (probably about 15lbs between the two) I clocked the exact same MPGs (within tenths I'm talking, which falls within STD deviation) So with that I would agree, not much to be gained there in a light driveshaft.... GM would probably too. If it were worth something they would've left it IMO. The aluminum one transmitted more harmonics from the driveline too. I've been told that can happen, but I'm leaning more toward something else was different or going on that caused that.

 

I was saying the front skinnies equate to .1s alone. 12whp and total weight reduction account for the other .1-2s. He will just have to find out at the track to be certain but has ran it against other cars out at the rolls and he and I are both certain it is quite a bit faster.

A thing to add about the driveshaft is that maybe his 3whp gain is coming from removing the two piece drive shaft that was also turning a carrier bearing and now it is a one piece. Who knows.

I do have another buddy with almost the same exact car minus having long tubes. He was running a best of 11.40s at 123, swapped to a one piece aluminum driveshaft and a similar VMS wheels/tire combo and started running high 11.10s and 11.20s at 124-125. Both of these guys are on a mission to clip a high 10 with just bolt-ons, E85 tunes and weight reduction with their gen 3 Coyotes. Quite a few people have done it and they, like these guys, are spending a lot of dough doing it.

 

Z28s and GTAs could be had with aluminum driveshafts. As we mentioned above, its more about acceleration than shear MPGs. It takes more mass off the entire vehicle to improve MPG that mass off the rotating components to improve acceleration. I hope that was just one part of multiple steps to reduce mass if MPG was the goal. At highway speeds, improved aero would be most beneficial.

 

Yea... its hard to improve the aero on a ******* brick. Like I said truck. They tried everything. I never noticed an acceleration difference either. Again though... 4500lbs with only ~300hp.

 

Butt Dyno for the win!

I don't think any of us would notice a few tenths in acceleration. To really evaluate that would take some real measurements.

Don't have to tell me about a brick, I moved from my pig Challenger Scat Pack to a Jeep Wrangler for a DD. Want to talk about a brick.

 

My favorite part was after they abandoned the aluminum driveshaft they also went from the spare being an aluminum wheel to a steel wheel. Still no difference lol.

I bet your wrangler did better than 10 in town and 16 highway.

I agree. Testing needed. Aluminum is inherently safer... if it breaks it'll deform vs taking out anything in its way.

 

Eh, I primarily drive highway to work and a combined average of around 18 on winter gas and 19 on summer gas. I suppose too, air density may play a roll.

I think when it comes to mass, its better to talk in % reduction rather than actual mass. 40lbs off a 5000lb truck is all but meaningless. 40lbs off a 1800lb race car is going to show up in track times.

Pee on a match and put it out, pee on a bonfire

 

Depending on how much I drink that night I could give that bonfire a run.