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Popeye
07-25-2012, 04:38 PM
Ok, so I was wonderin'......
I'm trying to learn a little theory here, so I'm gonna spitball some numbers. Say a car came stock with 17" wheels that weighed like 22lbs a piece. If someone wanted to upgrade to an 18" wheel, but wanted to stay close to the same performance, what would they need to look at when purchasing their new set of wheels? I'm sure, with a little searching, finding an 18" wheel close to the same weight as the oem 17" wheel wouldn't be terribly hard. Let's say I found an 18" wheel that weighed as much, or maybe like a pound less. Would I still have the same performance? In MY head, as long as I keep it around the same weight, or even lighter, the motor uses the same amount of force to move the same amount of weight. But does the wheel diameter affect that at all?
Now this isnt taking into account purchasing good tires. That's not what I am making this thread about. I know that getting the best tire that meets your need is best, etc. I am just wondering about the weight of the wheel and how "upgrading" to larger diameters, or even wider widths, affect performance.
What brought this all up for me is I happened to catch something online that talked about the new Subaru BRZ, and how they did a comparison with the OEM wheels and tires, then upgraded JUST the tires to Dunlop Star Specs, then upgraded to 18" wheels with the same exact tires. They lost a half of a second with the 18" wheel and Dunlops, but gained like a second on just the upgraded tires. BUT, they never discussed the wheels. All they stated was that they used an 18" wheel with the Dunlops. If they would have kept the 18" wheel at the same weight, or lighter weight, could they have kept their lap times close?
Is any of this info different if it's a FWD or a RWD car?

powdbyrice
07-25-2012, 10:48 PM
rotational inertia. more weight farther from the axis requires more effort to spin.

eccramer
07-25-2012, 11:43 PM
The big difference here is inertia... it takes force to get the wheel moving (or, once the wheel is moving, to slow it down).

Inertia mass times the radius squared. Boiled down, that means that given the same weight, as you move that weight further out from the center of the wheel, inertia is increasing on an exponential scale. Here is an example that is to some extent pulled out of my ass:

We'll base this off the BRZ... stock wheels appear to be 21lbs and lets not forget the tires which weigh an additional 20lbs. (to be truly accurate you'd need to count all the rotational mass here, such as the brake rotors and the wheel hub and rotating portion of the bearing, but I'm ignoring all of this here). This is a total of 41lbs or roughly 18.5kg.

Biggest oversimplification of all... Where is this mass? How far is it from the center of the wheel? (what is the radius for the equation above)
Spokes of the wheel are pretty light, wheel hub is generally fairly small--most of the weight of the rim is concentrated at the outer edge. Likewise, the sidewalls of the tire are lighter than the tread surface.
Wheel diameter for the BRZ is 17" and the overall tire diameter is 24.6" (.43 and .62 meters respectively) Lets *assume* that, on average, the weight of the wheel and tire assembly is somewhere out in the tire outside the rim, we'll call this circle 20", making the radius 10" or roughly .25m.
Put all this together and you have a rotational inertia of 1.16kg-m^2 and a equivalent static mass of 31.3kg or about 69 pounds. (that is to say, 41 pounds of wheels is equivalent to having 69 pounds of crap in your trunk)

Now lets say we move up to 18" wheels. We've done some good shopping and found a wheel and tire combination that is the same weight and overall circumference but the rim has gotten a little larger. Instead of a 10" radius, lets say that moving the mass out from the center of the wheel effectively puts this mass out at 10.5".
Do the math again and you come to rotational inertia of 1.35kg-m^2 and an equivalent static mass of 33.5kg or about 74 pounds.

Without adding any actual weight to the car (remember we held it constant), moving the weight further from the center of the wheel increased rotational inertia and thereby also increased equivalent static mass by 5 pounds per wheel. Essentially, this is like adding 20 pounds (4 wheels x 5 pounds) of dead weight to your car. This negatively affects acceleration, braking and handling.

Of course there are tradeoffs... a wider tire will provide more contact patch (though it will be heavier) and the added grip may more than make up for the equivalent static mass that you've added.

Its late and I've got to work in the morning but I can probably expand on that tomorrow if its not clear.

lemonhead228
07-25-2012, 11:44 PM
rotational inertia. more weight farther from the axis requires more effort to spin.

Yup what he said.. just like comparing a 15' to 17' rim but they weight the same.

Popeye
07-26-2012, 03:53 AM
Nah, it makes total sense! I appreciate the in-depth explination.
So to keep the same performance, it seems like there's no way other than to add power. Well, you could always spend as much money on wheels as you did the car and get some killer "racing" wheels. I'm not doing THAT crap! It seems like buying a wheel that is just one size bigger, and 5lbs less, would be damn impossible.
BUT, correct me if I'm wrong, but if it's only 20lbs, could one not just put a carbon biber hood and maybe a carbon fiber trunk and probably save a little MORE than 20lbs? Im thinking that saving weight from the front and the back would kind of even out the weight savings keeping the car fairly balanced.
This also applies to cars like our EP's. Sure, 15" wheels/tires perform better, but 17" LOOK better.
So, if someone upgraded to the 18" wheels, put a decent tire on the wheels, and added the carbon fiber goodies, you would be pretty close to the OEM performance?

los
07-26-2012, 06:28 AM
so then if you were to go smaller wheel inertia is less so wheels spin faster ? does that = faster car ? sorry if im coming off as dumb lol

b_ron
07-26-2012, 07:07 AM
so then if you were to go smaller wheel inertia is less so wheels spin faster ? does that = faster car ? sorry if im coming off as dumb lol

Well as long as your have the same diameter tires on them (that aren't significantly heavy), it should still spin the same but with less torque/force.

You can do some physics to estimate the difference between smaller diameter wheels to weight ratio. (lighter 17's compared to heavier 16's)

eccramer
07-26-2012, 07:39 AM
Nah, it makes total sense! I appreciate the in-depth explination.
So to keep the same performance, it seems like there's no way other than to add power. Well, you could always spend as much money on wheels as you did the car and get some killer "racing" wheels. I'm not doing THAT crap! It seems like buying a wheel that is just one size bigger, and 5lbs less, would be damn impossible.
BUT, correct me if I'm wrong, but if it's only 20lbs, could one not just put a carbon biber hood and maybe a carbon fiber trunk and probably save a little MORE than 20lbs? Im thinking that saving weight from the front and the back would kind of even out the weight savings keeping the car fairly balanced.
This also applies to cars like our EP's. Sure, 15" wheels/tires perform better, but 17" LOOK better.
So, if someone upgraded to the 18" wheels, put a decent tire on the wheels, and added the carbon fiber goodies, you would be pretty close to the OEM performance?

There are a million other different variables that affect the performance of the car, but generally yes, if you were to remove an equivalent amount of weight from the car you might make up that performance gap.

As far as upsizing and saving the weight in the wheels goes, you wouldn't quite need to shave 5 pounds off the weight of the wheel. That 5 pounds is equivalent static weight. To match the old tire and wheel combination you'd need to decrease the weight by 5 / (74/41) or 2.77 pounds (weight difference divided by the ratio of effective static weight to actual weight)

dj addicted
07-26-2012, 10:30 AM
I also think there has to be some sort of medium. There are a lot of 18" wheels that weight significantly less than 22lbs. Volks 18x9.5 can weight into the 19 lbs range. Plus the fact if you keep the same tire diameter, the weight of the tire would probably be a bit less due to less actual rubber. Even with intertia factored in, it would probably be a better performing combination.

Blah1219
07-26-2012, 10:40 AM
Someone TEACH me something!

That was too easy.

Zzyzx
07-26-2012, 01:22 PM
So, if someone upgraded to the 18" wheels, put a decent tire on the wheels, and added the carbon fiber goodies, you would be pretty close to the OEM performance?

not really.

Grassroots motorsports did a profing of that a few years ago. They found that rotational interia (wheel weight) has about a 70% greater influence then static weight on a cars acceleration and stopping.

so, for a really simplified example

(stealing this from a post of mine from years ago)


I=MR^2

Where
I = INertia
M= Mass
R= Radius

So... IF the Stock 16" wheel weighs 16LBS, that means it has about 4096N of Inertia...

compare that to an 18" wheel that also weights 16LBS.... which using the same formula has about 5184N of Inertia...

for that 18" wheel to have about the same Inertia of the 16, it would need to weigh about 12LBs.

and given the 70% rule... you'd have to remove 70% more weight on to of that as far as static weight goes to achieve the same results.


(Yes I am aware that the equation is for a thin walled hoop & that I mixed imperial and metric units, Its an old post and I don't really feel like correcting it)

Popeye
07-26-2012, 01:49 PM
Old or not, it's still good info. I was wondering about the whole "should I get a cheaper heavier wheel and change it back to the oem size when I wanna have fun" issue. Would it be worth it to save the money and just get what you think looks good and just use the lighter oem wheels on "track day?" Probably. I just think that a lot of people out there arent going to do that. So trying to find that good balance of good performance and being able to get a wild hair up your ass and driving out to where you wanna have fun, have your fun, and then drive home. I live only an hour or so from The Dragon. It would suck to have to swap wheels and torque everything down just to drive an hour to hit a few curves, then drive home and swap them back out again, you know? It would be a whole other story if I wanted to drive to Road Atlanta (which Im an hour and 15 minutes from in the opposite direction as The Dragon) and spend a good portion of the day there.

Zzyzx
07-26-2012, 01:56 PM
Most of us "old timers" that got in to competition ended up with two sets of wheels. But I understand the whole seeking balance thing.

So let me put it this way... If you were chasing a clock I'd say having a set of small light weight wheels would be paramount to getting the most out of the car; most events are timed down to the thousands of a second. But your are not, so the relatively small amount of time you lose by going with your larger "fancy" wheels is going to be more or less irrelevant. Its a question of are you really going to notice a few tenths of a second per turn out on the road? Probably not. (I'd really be impressed if you could)

black05ep
07-26-2012, 04:20 PM
Skunk2 did an article on this when they were building the black ep. Those te37 18s weighed the same as the stock ep15". But when it comes to handling. It would make sense to have a bigger wheel that weighed the same right? I tryed to read all that but equations and math shit f with my head. Ill see if i can find the article.

Popeye
07-26-2012, 04:23 PM
That's pretty much what I thought, Z. But I dont want to dog dog my car, regardless.

b_ron
07-27-2012, 07:19 AM
Skunk2 did an article on this when they were building the black ep. Those te37 18s weighed the same as the stock ep15". But when it comes to handling. It would make sense to have a bigger wheel that weighed the same right? I tryed to read all that but equations and math shit f with my head. Ill see if i can find the article.

Well also depends on the width of the wheel/tire. More contact with the road theoretically has more handling?

Zzyzx
07-27-2012, 08:45 AM
Skunk2 did an article on this when they were building the black ep. Those te37 18s weighed the same as the stock ep15". But when it comes to handling. It would make sense to have a bigger wheel that weighed the same right? I tryed to read all that but equations and math shit f with my head. Ill see if i can find the article.

all other things being equal for you to go from a 16 to an 18 inch wheel and still maintain the same moment of inertia that 18 inch wheel would need to be ~25% lighter then the 16 inch wheel.

the saying in the racing world is that you should run the smallest diameter wheel that will clear your calipers that is as wide as possible with out rubbing on the uprights and is as light as you can afford.

The benefits you are going to see with larger wheels then OEM are going to be focused more around street tires and their limitations. The sidewalls on most street tires (All season & low end summer tires) are relatively soft. which means slower response from your steering inputs and kind of a mushy feel overall. With larger wheels, you end up swapping out soft side walls with ridged metals... This is a non issue with track day inspired summer tires & competition tires which come with reinforced sidewalls.

as far as how much tire is on the ground... well that is true, however you can get wide 16's just as easily as wide 18's and really that's more a question of tire choice. wheel width will influence it a bit, but its mostly tire size that's going to determine your contact patch size.

hyu
07-27-2012, 12:08 PM
Sometimes the wider you go the less of a selection in tires you can get. I wanted to go 225 but they have limited tires/grip rather than 205. So a 205 theoretically has a greater selection of better compounds than 225

poeticfinesse21
08-30-2012, 12:28 AM
I like threads like this, it's like a splash of cold water on a hot summer day :thumbu: