Rims and tires - opinions

[HeavyH20]

So, I am looking at some 20 inch rims. I want to keep the stock look to a degree, just with 20 inch rims and tires. So, weigh in on the choice.

I am looking at these rims and tires:

20 inch x 8.5 Voxx Misano - these were the closest in style to the stock rims

275/40/20 Ventus ZR rated or 255/45/20.

The alternate tire is the Kumho

 

[Killer]

you do realize 20" rims will SLOW DOWN your CX7 and further reduce gas mileage?

Just letting you know...to each their own.

 

[HeavyH20]

I assume you are referring to unsprung weight and rolling resistance? If so it is a negligible change. The tire circumference is within 2%, so acceleration is almost identical (a little better, actually). Overall tire width is within 2%, so wheel well clearances are also fine. There is simply more tread on the ground due to the shorter side wall. It introduces no impact to performance or mileage.

 

[Killer]

No..it is a tad more involved than that...

Google "Rolling Mass and Inertia". I see nothing about weight of the wheel and tire on your chart, which is the BIGGEST consideration.
There is also a STICKY THREAD that may help you in the Mazda 6 Tire and Wheel Section.

Not trying to bust on your decision..just want you to have all the info.

 

[HeavyH20]

Thanks for the info. It corroborates what I suspected, already. I used to drag race alot, and I keenly aware of the effect of unsprung weight (if you want good times ). I also am aware of the handling change introduced by wider tires, if you are not careful. You need to calculate out the lbs load on the contact patch. Bigger patches can yield more stable turn ins, but, since they do not load properly, grip is actually reduced. I used to race my 300Z twin turbo at a circuit track, as well.

On many of those articles, they are poorly founded. The plus sizes they used are not true matches. Many times, they get a larger rim that weighs more and a tire that is 2% larger in circumference. So, of course performance changes since you are affecting gear ratios. Simple premise, actually.

The 20 inch rims I am looking at are a tad lighter than the the stock rims, so the unsprung weight is not affected negatively. In fact, will likely improve marginally. The tire weight is also similar. The advantage is the rolls. The 275-40-20 will be a tad faster on acceleration than the stock tire. As for the track, the relatively tall 60 is a little too mushy on turn ins. For that, you really want 50 or lower. So, handling should be more predictable. As for the rolling resistance, you can add a few more PSI and change the contact patch to be a little shorter and wider versus stock, but maintain the same patch area to weight ratio.

So, I think it will be a simple swap with no peformance impact. If anything, it will improve a little.

No decision, yet. Just figuring out the best route

 

[Int3grity]

good information Heavy
rim looks nice although I want chrome.not a fan of alloy

 

[HeavyH20]

They have a chrome version, as well. $60 more each.

 

[spike blue]

they look awesome!! nice choice though!!

 

[Killer]

seems you have made up your mind...good luck. But as I said, Inertia and rolling mass, torque....which you seem to leave out of all of your figures(boom02)

I posted this in another thread, but for any non-believers.. here are the physics behind this.

The torque equasion refers to how much force you can apply due to your current leverage (a wrench on a nut, and the length of the wrench is linearly important to how much force you can put on the nut), Don't confuse this leverage as the same concept as loss of torque due to mass being on the outer edge.. torque is spinning that edge (the edge is not what is generating torque).... it's a very different equasion when you are talking leverage radius for a force applied to spin an Axis point (what the engine is doing), VS something being spun around an Axis at a radius causing a loss in Intertia.

what we are talking about here is the loss in Inertia due to radius while taking into account an object's Mass. You need to use the "moment of inertia" equasion for a hoop or solid cyllinder rotating about it's diameter. In this equasion Inertia=1/2Mass*Radius^2 (radius squared).. this is what causes the loss in torque due to mass being pushed farther out. The radius of the wheel is twice as important as 1/2 the mass, which is why the same weight wheel if it's larger will take more torque to generate the inertia. Using this equasion you can calculate how much lighter a wheel would HAVE to be to maintain the same inertia with the same torque applied at the hub.

lets say I have a 20lb 17inch wheel, and I want an 18" wheel that won't cost me torque (and lets say the weight distribution along the radius of those 2 wheels is the same.. although it won't be exact, this is still ROUGHLY close).... so 1/2N*18"^2 = 10*17"^2
N=17.83

So to maintain the same amount of torque on the road, I need a 17.83lb 18" rim to replace my 20lb 17inchers (or my 26.6lb 15" wheels)

now, you start talking something like 12lb 14" Motegi RT5s? VS an 18" rim?
You would need 7.25lb 18s to match torque on the road.... LOL

 

[HeavyH20]

Never left that out. Physics 101. That's part of the unsprung weight calculation. The stock wheels are heavier than the new ones. Yes, torque from a larger weight diameter will matter, but, tire weight is also changing. There is an additional item to consider, is tire growth at speed. When a tire spins at high speed, it gets taller. A 60 will have more growth than a 40 which maintains its true aspect better. Belt designs help, but do not eliminate this altogether. So, if the new rims are about 10 lbs lighter, we are golden.

Also, even if we lose a little, what does it matter? On a 3900 lb car we are looking at 100 lb difference to get another .1 secs in the quarter mile. Not much of a big deal. A single bad shift will cost you much, much more.

 

[SuperStretch18]

Just for my reference; how much do the stock rims weigh anyway?

 

[HeavyH20]

Well, I have not weighed them directly. But, they are supposed to be right around 25 lbs, each. The Goodyear tires are about 34 lbs each., so we are close to 60 lbs per corner. That is my goal weight (or less) for any rim/tire replacement.

 

[1Sleepy93]

Damn, that's a heavy combo. (boom06)

 

[SuperStretch18]

Hey Heavy, just wanted to touch base and see if you ever wound up getting the VOXX rims. If so, I am curious to see what bigger mesh wheels look like on the 7...

 

[HeavyH20]

Actually, I put off the purchase a bit. I invested in another endeavor for the time being. I will probably revisit next spring since winter is just around the corner, anyway.

 

[Killer]

well probably a good thing.