Car's Performance on bigger wheels

[CrowsFeast]

Originally Posted by Grady

Rim diameter has nothing to do with it.

That's not entirely true. If you somehow had 2 wheels that weighed exactly the same but one was 17" and the other 18" the 18" wheel would still have a higher moment of inertia; which is what you're actually working against in terms of acceleration. The reason for that is the barrel of the wheel has a significant amount of the weight of the entire wheel in it and it's been moved outward. When you move a mass away from the central axis it's inertia goes up (d^2).

This is also the reason that wider rims would slow you down as they would add weight around the outer rim.

Depending on the sensitivity of your butt-dyno you might be able to notice it or you might not.

[Grady]

Originally Posted by CrowsFeast

That's not entirely true. If you somehow had 2 wheels that weighed exactly the same but one was 17" and the other 18" the 18" wheel would still have a higher moment of inertia; which is what you're actually working against in terms of acceleration. The reason for that is the barrel of the wheel has a significant amount of the weight of the entire wheel in it and it's been moved outward. When you move a mass away from the central axis it's inertia goes up (d^2).

This is also the reason that wider rims would slow you down as they would add weight around the outer rim.

Depending on the sensitivity of your butt-dyno you might be able to notice it or you might not.

Your answer agreed that rim diameter has nothing to do with it? It is about weight and yes where its center of mass is located for the wheel/tire combination.

[CrowsFeast]

Originally Posted by Grady

Your answer agreed that rim diameter has nothing to do with it? It is about weight and yes where its center of mass is located for the wheel/tire combination.

No it explained why diameter DOES make a difference. When you go up in diameter it moves more weight outward which DOES make a difference.

In fact it's not even so much about weight so much as it's about moment of inertia. Weight is only one component of the moment of inertia. It's possible to have a heavier wheel with a lower moment of inertia.

[Yoshoobaroo]

Originally Posted by Grady

Your answer agreed that rim diameter has nothing to do with it? It is about weight and yes where its center of mass is located for the wheel/tire combination.

God damn your reading comprehension is poor:

Originally Posted by CrowsFeast

- 2 wheels that weighed exactly the same but one was 17" and the other 18", the 18" wheel would still have a higher moment of inertia; which is what you're actually working against in terms of acceleration.

Center of mass has nothing to do with it, that's going to be on the axis of rotation no matter what. Moment of inertia cares only about 2 things:
1. Mass (constant in this example)
2. Distance of that mass from the location you're measuring MOE around, in this case the axis of the wheel hub.

[Seiryuu]

Originally Posted by why?

I wish this idiotic massive wheel craze never happened. 14x5.5 Volk ce28n weigh about 6 pounds each. That would be awesome on this car, but because of the silliness it would never ever happen.

No kidding. Not sure if you are referring to cars in general but it is appalling to see economy cars like the Mazda3 comes in 19 (NINTEEN) inch wheels for no good reasons! Even the Corolla comes in 18.

Originally Posted by ichitaka05

Like Grady stated, wheel size or weight doesn’t matter too much. It’s all about tires & brakes balanced with the car setup.

As for the weight, I wouldn’t worry too much... unless you’re racing your car and need to shave every oz. I’ve seen FR-S that had 3 piece wheels and it was AT and ran faster lap around the track AND autox than 80% of ppl that was there. So at first, don’t worry too much about the weight of the wheels. As long as each wheel doesn’t weight 35lbs and stay around 20’s should be good enough.

As for tS and TRD model running 18’s vs other running 17’s. There are few reason for it. First is marketing. Second is performance. There’s nothing wrong with running 18’s. Having 18’s doesn’t mean it loses performance. If you look at Drift King’s GT86, he’s running 18x8.5 wheels w 235/40 tires. Iirc he ran 1min 11secs around the Tsukuba stock vs his GT86 ran 1min 7secs. Of course that’s a lot to do with tires and suspension setup, but the point is, it varies. You gotta find the right setup for you. I’m currently on 3rd wheels (soon 4th) and still figuring out what’s the ideal setup for my driving style.

TLDR: Drive and figure things out.

You're right. If Tsuchiya can run run that setup, then guess there is no need to worry about performance differences.

For anyone who wants to watch the video referenced.

Originally Posted by soundman98

check the wheel fitment stickies-- there are a number of 17's that fit with the PP calipers yet. by no means are you condemned to 18's.

Ah, I didn't mean to sound like I was limited to 18. But from narrowing down to the design I like, and then checking upon sizes... I realized I had to look deeper (and wider) because what some specs said fits for BRZ, they mean the base model and doesn't clear the BBK. So at the minimum, for some I have to look at .5" wider. Otherwise, for some reason, some wheels don't even come in 17".

[LancePower]

If you have 17in vs 18in wheel but you're running tires that give you the same rolling diameter for both, then generally which is better?

Does the extra "inch" of wheel hurt you more than the extra "inch" of tire profile?

[Yoshoobaroo]

Originally Posted by LancePower

If you have 17in vs 18in wheel but you're running tires that give you the same rolling diameter for both, then generally which is better?

Does the extra "inch" of wheel hurt you more than the extra "inch" of tire profile?

Good question! The smaller wheel is better in the end. The extra "inch" of tire is in the sidewall, which is much lighter than the thread area. for most tires the weight difference will be negligible. I just checked on tire rack, 2 tires with ~ the same outside diameter (25.0" and 25.1"), both Michelin pilot sport 4S:

225/40/18: 21lbs
225/45/17: 21lbs

[Yoshoobaroo]

Originally Posted by Seiryuu

You're right. If Tsuchiya can run run that setup, then guess there is no need to worry about performance differences.

He's running Pilot Sport 4S tires. Outside of the US they are not available in 17", in fact, we only got them in the US very recently. Michelin might bring the 17s to the rest of the world too, but when setting that car up they just weren't available. That's probably the reason he's not running 17s.

[LancePower]

Originally Posted by Yoshoobaroo

Good question!

Once in a blue moon....

or should I say once in a global pandemic.

[Grady]

Originally Posted by CrowsFeast

No it explained why diameter DOES make a difference. When you go up in diameter it moves more weight outward which DOES make a difference.

I am just saying the center of mass is not located at the rim. If you go from a heavy 17 to a light 18. the tire/wheel combo center of mass may not move out.

[Grady]

Originally Posted by Yoshoobaroo

God damn your reading comprehension is poor:

Really?

[Ohio Enthusiast]

Originally Posted by Grady

I am just saying the center of mass is not located at the rim. If you go from a heavy 17 to a light 18. the tire/wheel combo center of mass may not move out.

Just a nitpick - center of mass of a wheel/tire is always at the very center (unless it's unbalanced). What you are interested is the concentration of mass further from the center. Ideally you'd want the mass as close to the center as possible; sadly this is not how wheels and tires work - most of the mass has to be closer to the edge (barrel of the wheel/tread of the tire).

[Yoshoobaroo]

Originally Posted by Ohio Enthusiast

Just a nitpick - center of mass of a wheel/tire is always at the very center (unless it's unbalanced). What you are interested is the concentration of mass further from the center. Ideally you'd want the mass as close to the center as possible; sadly this is not how wheels and tires work - most of the mass has to be closer to the edge (barrel of the wheel/tread of the tire).

Not really a nitpick, as you pointed out it’s a fundamental difference. 3 different people have already explained the difference between COM and MOE, we’re not really getting anywhere here.

[Yoshoobaroo]

Originally Posted by Grady

Really?

I apologize, I let my frustration get to me. That was out of line. You use MOI and center of mass interchangeably, even though they are very different concepts.

Center of mass is just the geometric point at which the wheel and tire can be represented as a point mass. You can pick it up at this point and it won’t rotate or wobble off your finger.

Moment of intertia is a quantity that shows you how much torque (or ‘angular force’) you need to change the rotation of an object. If your MOI is 20% higher, you need 20% more power to turn that wheel in a certain direction.

Diameter does not affect the location of the center of mass, but it does affect the magnitude of the MOI.