That may be true for some cast wheels, but you are not considering that all wheels are not constructed the same. For example my 17x8 aluminum wheels use a cast center disk welded to a thin spun rim (see below picture). The center of the wheel is quite thick for strength, but the outer rim is almost the same thickness as a steel rim. Even though the volume and radius of the outer rim increased some, the large density decrease going from steel to aluminum resulted in a net decrease in both mass and rotational moment of inertia. The moment of inertia of the center cast section did increase, but the outer portion of the rim decreased drastically resulting in a ~20% reduction in the overall inertia value compared to a 15x8 steel wheel.

Here's mine when I had 235/50ZR17's Michelin PS2's on it, which had a 26.6" diameter. I run a considerable amount of "rake" in the suspension to optimize the handling characteristics of both the front and rear suspensions. That's why the front is so much lower than the rear.



Here is an earlier picture when I had 255/50R17 Nitto 450's (27.0" dia) on it and the rear end was set a little lower, before I figured out that raising the rear end resulted in less bumpsteer. I preferred the way the car looked with it sitting lower, but I gave up some aesthetics for better handling.

 

Correct

Mine are a 2 piece rim, a spun Aluminium barrel and a billet aluminum CNC cut center section, then welded in to correct back spacing for 275’s or larger

 

I understand that, but you are comparing apples to oranges. If you consider a single wheel style alone like the Torq Thrust II which is an affordable and popular choice among us C3 owners; the extra weight and the way that extra weight goes to outside is always going to counter performance a bit as you you go up in size.

 

I never said steel wheels. My 81 has aluminum wheels.
I was simply comparing un-sprung weight from 15" to 18".

 

I do know this:

If after winning the lottery, and I get that Ferrari or Lambo, I won't be taking it down to the tire store for a fresh set of 15"Goodrich T/A Radials.

 

I'm not sure what you mean there, the stock alloys are 19.2 lbs which are still a bit lighter than 18x9 Torq Thrust II wheels at 20.6 lbs. But if you compare APPLES TO APPLES, the 15x8 Torq Thrust II is only 16.5 lbs, which adds up significantly considering each wheel, and how the weight moves to the outside of the assembly.

I really don't how to make it any clearer and I am getting tired (and I'm sure you guys are too) of repeating the same thing.

 

Of course you wouldn't, because a new Farrari or Lambo is not a classic Vette, and 15 inch Radial T/As would not look right in the first place, for one of the same reasons some of us don't want to put new Farrari wheels/tires on our C3s. This discussion is not just about performance, its about weighing the pros and cons of sacrificing the classic look too.

 

Quick shopping expedition on TireRack gave me this data for tires in the 235/245 and 27” diameter range. Wheels are American Racing. I took 1/2” off the outside of these tires and wheels, just because that felt right.

17” wheel - 39.7 lb-ft^2 / 44 lbs
Rubber tread - 13 X 13 X 27 = 31.7 lb-ft^2
Aluminum rim - 8 X 8 X 18 = 8 lb-ft^2

15” wheel - 40.85 lb-ft^2 / 46.6 lbs
Rubber tread - 13 X 13 X 30# = 35.2 lb-ft^2
Aluminum rim - 7 X 7 X 16.6# = 5.65 lb-ft^2

15” wheel - 43.375 lb-ft^2 / 54 lbs
Rubber tread - 13 X 13 X 30# = 35.2 lb-ft^2
Steel rim - 7 X 7 X 24# = 8.16 lb-ft^2

Message here is pay a lot more attention to the tires than the wheels

 

Yes. A size 9 shoe of the same style weighs less than a size 12.

I also thought it was about running shoes vs. dress shoes.

As Ignatz pointed out, it's about the package, not the construction.

My 81 has 200HP, and I consider it a GT car. Stock wheels are fine.

If I dropped in a 400HP beast, I would be compelled to go modern.
Goodrocks are not capable of controlling higher HP.

 

Whoops, those numbers did seem kind of big when I started thinking about horsepower. Should be inches not feet squared in my math - need to fix that. The proportions should be right though, just need to divide by 144 to get the units i used. I was going to do a horsepower calc to see what it took to spin four tires up and noticed the error.

 

The wheel and tire isn't the problem here.

It's all about the idea that the Corvette pictured retains that 4X4 ride height look.

In this shot my car rides on a 17" tire and wheel that retains just about exactly the same circumference as the 15" they replaced......the car has been lowered in a big way with ease though as it rides on Transverse Front and Rear Composite material springs. Long gone ore the coils and steel rear leaf.


For comparison same lowered car with it's older wheels......different look but the exact same amount of filling the wheel openings.

 

After my first foray into looking at some numbers, and after thinking about this for a bit, I decided that the crux of the matter relative to performance and not looks is: "how much horsepower do the wheels alone take relative to the power required to accelerate or decelerate the car?". A couple of useful constants here to help things along.

60 MPH = 88 ft/s
1HP = 17696 lb ft^2/s^3 (pounds-weight - feet-squared/seconds-cubed)

Assume a 3500 pound car that accelerates to 88 fps in 6 seconds
The 17” wheels in my first post have a moment of inertia of 39.7 lb-ft^2 and a weight of 44 lbs.

The car itself actually weighs 3500-(4x44) pounds or 3324 pounds

The wheels have both kinetic energy (1/2 x M x v^2), and rotational energy (1/2 x I x omega^2), where ‘omega’ is the rotational speed of the wheel in radians per second. A 27” tire has a circumference of about 7’, so it rolls about 12.5 times per second at 88 ft/s. Times 2 pi gives about 79 radians/sec.

To get the two energies converted to power we divide by 6 seconds and then the constant 17696 to convert to HP. You can check the units yourself if you want, I’ll leave them out of the expressions

The power required for the car per second is 1/2 x 3324 x 88^2 / (17696 x6 ) = 121 hp*
Kinetic power for each 17” wheel is 1/2 x 44 x 88^2 / (17696 x 6) = 1.6 hp
Rotational power for each 17” wheel is 1/2 x 39.7 x 79^2 /(17696 x 6) = 1.16 hp

So four wheels consume about 11 hp or less than 10% of the power required to accelerate the car itself. Given the small differences between the various combinations of tire and wheel sizes, I would say that matters very little. The steel rally are about 13hp. Obviously handling is a different situation with a lot more going on, but I was curious how much the wheel’s masses mattered. Not all that much.

Well that’s enough fooling around - I’m waiting from parts from Summit!

________________
* Note - just to check order of magnitude, I did find an online calculator that said I needed 240 hp at the flywheel and that was based on Edmunds sample data which would include lots of other real world things like air resistance, gearing, etc. Makes the wheels, which are a much simpler entity, even less of a concern

Finally.

Sorry man, that's not me - Don't take this wrong, you might try "The Pleasure of Finding Things Out" by Richard Feynman - a fun read

 

I agree with you, but to get a 17 tire the same diameter as the 15 inch tires, basically 27" diameter, you have to go with 50's. The 255/45/17 is 26.0 inch in diameter, a full inch less.....and that is noticeable. I can see it very easily without measuring it. My guess is your tires are 255/50/17 which are 27.0 inch diameter, 10 inch wide.....exactly same as 255/60/R15, and obviously they will fill the wheel wells exactly the same. I think that was your point.

In reality the cars wheel openings were designed in 68 to have a 27.5 inch diameter tire, so once you get to a 26 inch tire, like the 45's, you are 1-1/2 inch smaller in diameter than the wheel well was designed to be concentric with that tire,......and again, it is very noticeable to me.

Now, look at the selection of 255/50/R17 tires, and there are not many performance tires out there. I guess the tire to buy would be Nitto NT555 G2 tires, summer performance tires.

There are many more for 255/45/R17, but they are too small in diameter for me. You might be able to hide it by lowering the car, but the smaller the outside diameter tires, you start to see the tire is not concentric with the wheel opening.

See the attached link to a very useful Tire Comparison Visualizer, it really tells the story well.

https://tiresize.com/calculator/

 

Its difficult to see a difference other than thickness of the side wall profile. Both look like they should for me.








And when used for fun. 15" don't work at all.

 

My 17's look close to your #2 picture on how they fill the wheel wells:





Required a little work to get the ride stance but I do think in my opinion it is far superior than the 4X4 stance of the stock look, usually sagging in the rear, with balloon tires.

You car looks fantastic with the bigger rims and tires^^^^^^^^^^^^^...to me a real sports car.....

 

Can you tell us exactly what tires those are...i.e. size....are they 45 or 50 profiles?

 

I would bet they are 45's if 17 rims/40's if 18 inch rims....Mine are 255/45/17's in front and 255/50/17's in the rear, as a comparison....the difference is not huge, as long as you close the wheel gap up which I had to do

 

You are so smart! Problem is my reading level is only at AR 2.7 and even my kid’s Dr. Suess books are hard for me to comprehend, so I’m pretty sure I won’t be able to read that book.

No seriously, please forgive me for my smart a$$ sarcasm (I just couldn’t resist), and thank-you for doing your math work to help prove my point, even though I’m sure most people like me didn’t really understand it so it’s hard to trust it (no offense, you probably really are a super smart person).

Anyway, I never said that adding the extra weight and the fact the weight moves to outside of the assembly (when comparing same wheel makes) when going up in wheel size makes a huge difference, but it does make a difference that at least I am trying to be mindful of. I have built my Vette with weight reduction as a key goal to help improve performance.

In my real life experience (not mathematical theory), I noticed a significant decrease in acceleration just from going up from my 17 Inch to 18 inch combo; it was the same wheel style (AR TT2) but the tire and wheel combo added 5.7 lbs per wheel and the tire was about an inch taller because I didn’t want the thin rubber band look and wanted more ground clearance. I know tire height plays a big part in acceleration too, but the added weight/rotation resistance also made a difference.

Of course what good is having a lighter weight wheel/tire combo on acceleration if it doesn’t hold traction? This is a main reason I want to go back to 15s with street drag radials on light weight TT2 (other than wanting the classic look back); because they will hold traction better than any UHP street tire reguardless of the size, I just hope they don’t give up too much in terms of handling. I don’t plan on autocrossing but I do like pushing through curvy roads. I know the 15 inch Drag radials will improve my straight line acceleration, but if they take the fun out of driving through curves, I will most likely switch back to UHP street tires.

Going off topic a bit; but if you have the time, could you please do some equations to help prove how a car’s weight effects cornering performance? (Seriously, not being sarcastic this time). I know that a lighter car will cause less sidewall flex. I also know that a heavier car will force the tire to the ground and increase traction that way, but I’m also pretty sure that a heavier car will lose traction quicker when pushed.

 

Why did you decide to use 45 on front and 50 on the back, why would 50 not work all way around? Was it the tires available? Were you concerned about rub on front, because I know lots of folks worry about 255/60/R15 rubbing, but mine don't....so why would I not use 50's all around?