[heavyD^2]

Originally Posted by CanAutM3

OK, explain the physics to me.

Ha ha ha. I can see where this is going. Why would I go through that when there's a wealth of information on the internet supporting this? You can disagree adamantly but then I would simply lump you in with the flat earth crowd and move on.

[CanAutM3]

Originally Posted by heavyD^2

Ha ha ha. I can see where this is going. Why would I go through that when there's a wealth of information on the internet supporting this? You can disagree adamantly but then I would simply lump you in with the flat earth group and move on.

Because it's urban legend. You can't believe everything you read on the internetz. Regarding flat earth crowd, when pot meets kettle...

A tire's contact patch size is defined by the tire's structure, tread mold and inflation pressure. A lower profile tire does not automatically translate to more rubber on the road.

[heavyD^2]

Originally Posted by CanAutM3

Because it's urban legend.

A tire's contact patch size is defined by the tire's structure, tread mold and inflation pressure. A lower profile tire does not automatically translate to more rubber on the road.

LOL and I'm out.

[solstice]

Originally Posted by CanAutM3

You'll have to explain that one to me: how does a larger wheel diameter for a given tire diameter puts more rubber on the road ?

Swedish translation error in my head. Back home wheel is translated to “hjul” rim + tire. I.e. over here I should have said tire diameter not wheel diameter, not the first time this causes confusion for me

I’ve not calculated the exact resulting tire diameter between front and rear in all cars we mention here, I guess I trust Porsche here when they stated the larger contact patch without adding width and resulting added stability of the rear as their reason for staggering them...they usually don’t do things just because fashion.

[Bimmr7]

Originally Posted by Transfer

Yes I did and I agree with that. Like I said in my post so then why a 20" rear?

As opposed to what? If the staggered set up (according to BMW and Porsche) is the ideal set up, and the front diameter is 19 inches the 20 would be the preferred diameter of the rear. Likewise the M-Performance set up is 20/21. I

[CanAutM3]

Originally Posted by solstice

Swedish translation error in my head. Back home wheel is translated to “hjul” rim + tire. I.e. over here I should have said tire diameter not wheel diameter, not the first time this causes confusion for me

I’ve not calculated the exact resulting tire diameter between front and rear in all cars we mention here, I guess I trust Porsche here when they stated the larger contact patch without adding width and resulting added stability of the rear as their reason for staggering them...they usually don’t do things just because fashion.

I believe the staggered diameter rim is mainly to have different tire sidewall height front and rear. The larger front sidewall improving front grip to reduce understeer and the shorter rear sidewall providing more stability from improved response in transitions.

[solstice]

Originally Posted by CanAutM3

I believe the staggered diameter rim is mainly to have different tire sidewall height front and rear. The larger front sidewall improving front grip to reduce understeer and the shorter rear sidewall providing more stability from improved response in transitions.

That could be it but the Porsche guy specifically said that the reason on the 992 was the larger contact patch on the rear tires that can be achieved without adding more width.

To be honest I’m not all that interested in the exact science behind this I’m more interested in the result. I’m getting impatient for quality reviews

[heavyD^2]

Originally Posted by solstice

That could be it but the Porsche guy specifically said that the reason on the 992 was the larger contact patch on the rear tires that can be achieved without adding more width.

Ha ha ha. Don't poke the bear as some people will argue nonsensically against this.

[mkoesel]

Originally Posted by CanAutM3

I believe the staggered diameter rim is mainly to have different tire sidewall height front and rear. The larger front sidewall improving front grip to reduce understeer and the shorter rear sidewall providing more stability from improved response in transitions.

That could very well be.

In any case, it seems at least plausible to me that the car simply performed best with the different diameter wheels, so that's what they went with. That is to say, irrespective of the actual science behind the engineering, sometimes you test several different configurations and go with what works best.

[CanAutM3]

Originally Posted by solstice

That could be it but the Porsche guy specifically said that the reason on the 992 was the larger contact patch on the rear tires that can be achieved without adding more width.

To be honest I’m not all that interested in the exact science behind this I’m more interested in the result. I’m getting impatient for quality reviews

In the 992 does have significantly larger diameter tires in the rear. But that can be achieved without having a larger diameter rim. The different rim sizes are used to alter sidewall height.

[CanAutM3]

Originally Posted by heavyD^2

Ha ha ha. Don't poke the bear as some people will argue nonsensically against this.

Blissfully ignorant.

[Transfer]

Originally Posted by solstice

From the link above:

"Advantages of larger rear wheels in an RWD car?

Having a larger diameter rear wheel increases the amount of rubber that contacts the road along the axis of the vehicle which increases your acceleration potential. When you accelerate, much of your vehicle's weight shifts to the rear. By comparison, the front wheels don't need as large a contact patch to take care of steering and braking."

In summary more rubber to the road.

I did read that and disagree with it. Wheel diameter means nothing to this because if the tires are the same diameter then contact patch is not increased. The Lambos might have larger diameter rear tires but even if they do, a 1" taller tire won't add much to the contact patch. The tire width counts substantially more.

G8x has equal front/rear tire diameters so that is irrelevant.

[solstice]

Originally Posted by CanAutM3

In the 992 does have significantly larger diameter tires in the rear. But that can be achieved without having a larger diameter rim. The different rim sizes are used to alter sidewall height.

It can with different tire profiles but again he did state the larger contact patch as an explanation to why they had staggered wheel sizes.

They wanted more contact patch and likely tried all options as wider wheels, more profile at the same wheel size and larger wheels and the best overall results were achieved with larger wheels.

I don’t think the main target was a lower profile rear tire instead it was a larger contact patch. The sidewall was likely just tuned as a secondary parameter.

For the G8X we haven’t gotten a detailed explanation to why it ended up were it did. Your guess is as good as any. I think mkoesel is close to the truth that this is more an empirical result of testing than a formula based decision.

[CanAutM3]

Originally Posted by BBoyM3

I found this on the Lamborghini of Palm Beach website. It discusses the rear wheel diameter:
https://www.lamborghinipalmbeach.com...o%20the%20rear.



The UK product manager explains the front wheel diameter for the M3/M4 at 2:43:

Funny tidbit, the "product manager" says the ZF8 gear ratios "are very specific to these cars (the G8X)". That's The G8X shares the same gear ratios as all other 8HP75/8HHP50 ZF8 applications .

Just shows how you can't rely on everything you hear, even if it seems from reliable sources...

[Transfer]

I'm still gonna stick with they did it because it looks cool and has no ill effect on the car. Plus they can ramble on about improved turn in up front (presumably compared to a 20" front wheel). The G8x design is clearly about brash looks and 19" front/rear doesn't fit that theme.

[solstice]

Originally Posted by CanAutM3

Funny tidbit, the "product manager" says the ZF8 gear ratios "are very specific to these cars (the G8X)". That's The G8X shares the same gear ratios as all other 8HP75/8HHP50 ZF8 applications .

Just shows how you can't rely on everything you hear, even if it seems from reliable sources...

Well, I did check when I drove the 992. It really has staggered wheels

[CanAutM3]

Originally Posted by solstice

It can with different tire profiles but again he did state the larger contact patch as an explanation to why they had staggered wheel sizes.

They wanted more contact patch and likely tried all options as wider wheels, more profile at the same wheel size and larger wheels and the best overall results were achieved with larger wheels.

I don’t think the main target was a lower profile rear tire instead it was a larger contact patch. The sidewall was likely just tuned as a secondary parameter.

For the G8X we haven’t gotten a detailed explanation to why it ended up were it did. Your guess is as good as any. I think mkoesel is close to the truth that this is more an empirical result of testing than a formula based decision.

They did increase the tire diameter stagger significantly on the 992 (5.2%) vs the 991 (1.4%). They learned that from the 991.2 GT2/3RS . OK, so in the case of the 992, lets say you want a larger rear diameter tire but do not want to increase tire sidewall too much relative to the front to maintain response balance, how would you do it ? Of note, the G8X has less than 1% tire diameter stagger.

While we do not know the specifics of the G8X tire engineering, I did read up a lot about my M4cs, which also has staggered diameter rims.

[CanAutM3]

Originally Posted by solstice

Well, I did check when I drove the 992. It really has staggered wheels

Your own eyes is definitely the most reliable source

[heavyD^2]

Originally Posted by CanAutM3

Blissfully ignorant.

Dude it was not my intention to make you feel stupid. I just answered your question accurately and here we are.

[CanAutM3]

Originally Posted by heavyD^2

Dude it was not my intention to make you feel stupid. I just answered your question accurately and here we are.

False...

[heavyD^2]

Originally Posted by CanAutM3

False...

True: You asked a pretty obvious question (in automobile circles) and didn't like the obvious answer even after another poster has continuously posted Porsche did this specifically for the larger contact patch. Just give it up as this is childish.

[jmg]

Originally Posted by solstice

It can with different tire profiles but again he did state the larger contact patch as an explanation to why they had staggered wheel sizes.

They wanted more contact patch and likely tried all options as wider wheels, more profile at the same wheel size and larger wheels and the best overall results were achieved with larger wheels.

I don’t think the main target was a lower profile rear tire instead it was a larger contact patch. The sidewall was likely just tuned as a secondary parameter.

For the G8X we haven’t gotten a detailed explanation to why it ended up were it did. Your guess is as good as any. I think mkoesel is close to the truth that this is more an empirical result of testing than a formula based decision.

I haven't watched the video and I am no expert, but this is my take on it:

The increased contact patch from increasing diameter is minimal. Tire width is what actually effectively increases contact patch (and in turn the wheel width to accommodate the tire). But with an increased tire and wheel width, comes an increase in rubber, which increases tire deflection on load at a given tire pressure. This gives more grip but sacrifices responsiveness as CanAutM3 pointed out. To help balance this out, we can use a bigger diameter wheel to decrease the tire sidewall which will decrease deflection but maintain the contact patch. So no, WHEEL diameters do not increase contact patch, but they can allow for a wider tire without excessive tire sidewall.

Additionally, having a larger overall diameter wheel/tire combo changes the overall driving ratio of the car. A larger tire means a higher potential speed. I don't know if the G80's rolling diameter will be more than the fronts because I am too lazy to look up the tire sizes, but that would be one potential reason for using a larger rear wheel.

These arguments about the increased weight of larger rear wheels are kind of ironic given BMW increased the weight of the G80 by increasing its size, which isn't as correctable as new rims.