[CanAutM3]

Originally Posted by heavyD^2

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.

You still have a lot to learn young padawan.

[CanAutM3]

Originally Posted by mkoesel

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.

As having started my career as a test engineer, I do believe a lot in the learnings from testing . But testing scenarios always start with established engineering principles .

[solstice]

Originally Posted by jmg

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.

In this I will trust that Porsche know what they are talking about. I have great respect for the knowledge here from members as for example yourself and CanAutM3 and manufacturers aren’t always truthful but in this case I’m sticking with the explanation that the reason to not run the same wheel and tire size in the rear as what is selected to for the front is a larger contact patch without having to increase width.

That’s enough arguing for me on the reason, something I’m not that interested in. I have no intent to mess with it and I really only interested in the result. Some of the best car companies decided to do it on their cars with some track intent, I’m not about to argue against them when I don’t really care

[CanAutM3]

Originally Posted by solstice

In this I will trust that Porsche know what they are talking about. I have great respect for the knowledge here from members as for example yourself and CanAutM3 and manufacturers aren’t always truthful but in this case I’m sticking with the explanation that the reason to not run the same wheel and tire size in the rear as what is selected to for the front is a larger contact patch without having to increase width.

That’s enough arguing for me on the reason, something I’m not that interested in. I have no intent to mess with it and I really only interested in the result. Some of the best car companies decided to do it on their cars with some track intent, I’m not about to argue against them when I don’t really care

For sure the Porsche engineers are not wrong, but I think you are having a too narrow view here by directly transposing what you read about the 992 to G8X. The staggered wheel diameter on the 911 is a consequence of wanting a larger diameter tire to improve REAR grip. On the G8X, they have NOT increased the tire diameter. I know for a fact that the staggered wheel diameter on the M4cs is to increase FRONT grip, so I can only assume it is the same for the G8X.

[solstice]

Originally Posted by CanAutM3

For sure the Porsche engineers are not wrong, but I think you are having a too narrow view here by directly transposing what you read about the 992 to G8X. The staggered wheel diameter on the 911 is a consequence of wanting a larger diameter tire to improve REAR grip. On the G8X, they have NOT increased the tire diameter. I know for a fact that the staggered wheel diameter on the M4cs is to increase FRONT grip, so I can only assume it is the same for the G8X.

Do you mean that it’s not the actual tire wall or tire size of the front that increases front grip but the relation between rear and front tire walls? Because if it’s mainly the larger front tire wall of a 19” or 18” front tire you don’t need to upsize the rear.

And if so, how does the relation to the rear tire wall increase front end grip?

My comments here was solely for the rear upsize. I agree that a smaller front wheel with larger tire wall has positive impact on the front axle as for grip and steering. Never argued that.

[mkoesel]

Originally Posted by CanAutM3

As having started my career as a test engineer, I do believe a lot in the learnings from testing . But testing scenarios always start with established engineering principles .

No doubt. Certainly they go into it with knowledge and experience as a guide.

I guess my point is that once you have a sufficiently complex set of variables, you can't necessarily argue absolutes because things have a way of yielding surprising results when they combine in the real world. So, it may very well be that until recently, BMW simply used the same (or virtually so - as close as they could using common tire widths, inside diameters, and aspect ratios) sidewall height front and back because they had no reason to expect appreciably better results from using different sizes. But, if they came to learn from simulation or experimentation that traditional wisdom wasn't necessarily true in all cases, then certainly they'd want to apply those findings to product design, even if they could not immediately explain the results.

[jmg]

Originally Posted by solstice

In this I will trust that Porsche know what they are talking about. I have great respect for the knowledge here from members as for example yourself and CanAutM3 and manufacturers aren't always truthful but in this case I'm sticking with the explanation that the reason to not run the same wheel and tire size in the rear as what is selected to for the front is a larger contact patch without having to increase width.

That's enough arguing for me on the reason, something I'm not that interested in. I have no intent to mess with it and I really only interested in the result. Some of the best car companies decided to do it on their cars with some track intent, I'm not about to argue against them when I don't really care

I don't know what the context of this porsche quote is. Do you have a link to it? An article? A video?

[solstice]

Originally Posted by jmg

I don't know what the context of this porsche quote is. Do you have a link to it? An article? A video?

Here you go, one of them. I recall reading two at the time from different Porsche reps.

https://www.motorauthority.com/news/...r-the-2020-911

In relation to this thread:
August Achleitner, vice president of the 911 and 718 product lines

2) Enlarge the diameter of the rear wheels.

Calculations and simulations showed that enlarging the diameter of the rear wheels would improve dynamics even though it would also add weight. The 2020 Porsche 911 S and 4S now ride on 21-inch wheels at the rear and 20s up front, though the tire sizes haven't changed at 305/30 rear and 245/35 up front. While this influenced all of the surrounding areas of the suspension and the packaging of the car, Porsche felt it was worth the effort because it would make the handling more neutral and controllable. The company had experience with the practice as the 918 Spider and GT2 and GT3 cars already use staggered wheel sizes, and there are no better handling Porsches than those. Handling becomes more neutral because the bigger rear wheels provide greater rear end stability and grip without having to go with a wider tire.


Here’s the other one and it’s actually Achleitner again but a different outlet:

https://www.roadandtrack.com/car-sho...specs-details/

So what's the effect of the increase in wheel size and track width.

I'll let Achleitner explain:

- When you widen the track in the front and in the rear...you can transmit rolling forces a little bit more with just the spring and damper, and then you can soften the stabilizers. When you soften the stabilizers, you get better traction out of the car, [which] makes the car faster.

-We have a bigger rear wheel, and that's the first time in the history of the 911 Carrera, that the rear wheel is a diameter bigger than the front one...From this, we get a better, bigger [contact patch] not only by width, but also by length. And with the enlargement of the tire, we were also able to reduce the tire pressure a little bit.


So it’s interesting that they actually soften the impact of the tire wall by reducing tire pressure and by that further emphasizing traction and grip over stiffness and response in transitions. Not one word on front axle advantages, staggering is all about the rears for Porsche.


Main take away from this should not be who is right or wrong of us but for BMW to step up and provide similar insight about the G8X as Porsche does in these two articles. That’s currently sorely missing leaving the friends here argueing about it.

[CanAutM3]

Originally Posted by solstice

Do you mean that it’s not the actual tire wall or tire size of the front that increases front grip but the relation between rear and front tire walls? Because if it’s mainly the larger front tire wall of a 19” or 18” front tire you don’t need to upsize the rear.

And if so, how does the relation to the rear tire wall increase front end grip?

My comments here was solely for the rear upsize. I agree that a smaller front wheel with larger tire wall has positive impact on the front axle as for grip and steering. Never argued that.

Yes, front to rear relationship.

Balancing over and understeer is a question of matching front and rear grip. To reduce understeer, one needs to increase front-end grip relative to the rear, hence the staggered rim diameter setup. Be it 18"/19", 19"/20" or 20"/21", the purpose is to have more front end grip relative to the rear but by always keeping the rear with a sharper response through a lower profile than the front to obtain the desired overall handling characteristic.

[CanAutM3]

Originally Posted by solstice

Here you go, one of them. I recall reading two at the time from different Porsche reps.

https://www.motorauthority.com/news/...r-the-2020-911

In relation to this thread:
August Achleitner, vice president of the 911 and 718 product lines

2) Enlarge the diameter of the rear wheels.

Calculations and simulations showed that enlarging the diameter of the rear wheels would improve dynamics even though it would also add weight. The 2020 Porsche 911 S and 4S now ride on 21-inch wheels at the rear and 20s up front, though the tire sizes haven't changed at 305/30 rear and 245/35 up front. While this influenced all of the surrounding areas of the suspension and the packaging of the car, Porsche felt it was worth the effort because it would make the handling more neutral and controllable. The company had experience with the practice as the 918 Spider and GT2 and GT3 cars already use staggered wheel sizes, and there are no better handling Porsches than those. Handling becomes more neutral because the bigger rear wheels provide greater rear end stability and grip without having to go with a wider tire.


Here’s the other one and it’s actually Achleitner again but a different outlet:

https://www.roadandtrack.com/car-sho...specs-details/

So what's the effect of the increase in wheel size and track width.

I'll let Achleitner explain:

- When you widen the track in the front and in the rear...you can transmit rolling forces a little bit more with just the spring and damper, and then you can soften the stabilizers. When you soften the stabilizers, you get better traction out of the car, [which] makes the car faster.

-We have a bigger rear wheel, and that's the first time in the history of the 911 Carrera, that the rear wheel is a diameter bigger than the front one...From this, we get a better, bigger [contact patch] not only by width, but also by length. And with the enlargement of the tire, we were also able to reduce the tire pressure a little bit.


So it’s interesting that they actually soften the impact of the tire wall by reducing tire pressure and by that further emphasizing traction and grip over stiffness and response in transitions. Not one word on front axle advantages, staggering is all about the rears for Porsche.


Main take away from this should not be who is right or wrong of us but for BMW to step up and provide similar insight about the G8X as Porsche does in these two articles. That’s currently sorely missing leaving the friends here argueing about it.

From what I read in the texts you posted above, they are perfectly aligned with what I've been saying all along in this thread .

My comprehension from the Porsche text above is that their reference to "wheel" is the tire/rim combination as you had earlier interpreted. A larger tire diameter does result in a longer contact patch. But a larger rim diameter for a given tire diameter DOES NOT translate to a longer contact patch (with all else being equal). As I've stated before: while Porsche have indeed increased the tire diameter stagger with the 992, BMW have not done so for the G8X (they have actually reduced it compared to the F8X).

I fully agree with you, technical insight is definitely lacking with this G8X launch.

[CanAutM3]

Originally Posted by mkoesel

No doubt. Certainly they go into it with knowledge and experience as a guide.

I guess my point is that once you have a sufficiently complex set of variables, you can't necessarily argue absolutes because things have a way of yielding surprising results when they combine in the real world. So, it may very well be that until recently, BMW simply used the same (or virtually so - as close as they could using common tire widths, inside diameters, and aspect ratios) sidewall height front and back because they had no reason to expect appreciably better results from using different sizes. But, if they came to learn from simulation or experimentation that traditional wisdom wasn't necessarily true in all cases, then certainly they'd want to apply those findings to product design, even if they could not immediately explain the results.

I'd argue that they tried the different wheel stagger because there was something specific they wanted achieve in terms of handling. They tried it because the theory
said it would help in achieving the desired result. I doubt they tried it just for the sake of trying it.

Many things have indeed changed over the years and through the different M3/4 generations, such as tire and damper technology that have evolved significantly just to state those as examples. But also weight distribution. The G8X carries more weight on its front axle than previous generations. That also needed to be addressed to achieve the desired handling characteristics.

[solstice]

Originally Posted by CanAutM3

I'd argue that they tried the different wheel stagger because there was something specific they wanted achieve in terms of handling. They tried it because the theory
said it would help in achieving the desired result. I doubt they tried it just for the sake of trying it.

Many things have indeed changed over the years and through the different M3/4 generations, such as tire and damper technology that have evolved significantly just to state those as examples. But also weight distribution. The G8X carries more weight on its front axle than previous generations. That also needed to be addressed to achieve the desired handling characteristics.

Agreed that science say it could be helpful but I don’t think it’s the first option you reach for. As the Porsche guy eludes to its a rather involved change and I bet they tested it out together with other options if not now then with the GTS and/or CS.

[CanAutM3]

Originally Posted by solstice

Agreed that science say it could be helpful but I don’t think it’s the first option you reach for. As the Porsche guy eludes to its a rather involved change and I bet they tested it out together with other options if not now then with the GTS and/or CS.

Again: it was "very involved" for Porsche because they significantly changed the rear TIRE diameter by 1", which impacts wheel well clearances, ride height and chassis rake. On the F8X CS/GTS and the G8X, they only changed the RIM diameter without significantly altering the tire diameter, so no required changes to the body in BMW's case.

[jmg]

Originally Posted by solstice

- When you widen the track in the front and in the rear...you can transmit rolling forces a little bit more with just the spring and damper, and then you can soften the stabilizers. When you soften the stabilizers, you get better traction out of the car, [which] makes the car faster.

-We have a bigger rear wheel, and that's the first time in the history of the 911 Carrera, that the rear wheel is a diameter bigger than the front one...From this, we get a better, bigger contact patch not only by width, but also by length. And with the enlargement of the tire, we were also able to reduce the tire pressure a little bit.

Originally Posted by solstice

So it’s interesting that they actually soften the impact of the tire wall by reducing tire pressure and by that further emphasizing traction and grip over stiffness and response in transitions. Not one word on front axle advantages, staggering is all about the rears for Porsche.

By lowering the pressure you get more tire deflection, which is in line with what I said earlier: more deflection, more grip.

Originally Posted by jmg

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(this is wrong, I should have said tire sidewall flex) 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.

However, I stand by my original statement with a caveat:

Originally Posted by jmg

The increased contact patch from increasing diameter is minimal.

However combine the small (minimal) amount of increased contact patch due to an increase in diameter with the ability to run a lower pressure with less sidewall flex, and you get the benefits stated by Achleitner. Engineers smarter than I have probably done the math and determined the increased weight is worth the gains.

Think of it this way: take a tennis ball and set it on the ground. Take a bowling ball and set it on the ground. What is the difference in contact patch? Minimal. Only when to take into account material compression or in this case tire deflection does the contact patch change significantly.

Originally Posted by solstice

Main take away from this should not be who is right or wrong of us but for BMW to step up and provide similar insight about the G8X as Porsche does in these two articles. That’s currently sorely missing leaving the friends here argueing about it.

Just to be clear, I was not trying to draw a line between right and wrong, with one argument definitely being on the right side of the line. I was merely giving my "take" on the subject. As CanAutM3 also said, it is not wheel diameter alone that increases contact patch, it is the balance of tire material, pressure, sidewall hight. Then you have to consider the differences between the front and rear not only in size, but how it changes the center of gravity, the rake, even which tires give more or less traction... all these factors contribute to how the car handles. My issues was the general statement that an increased diameter (alone) increases contact patch.

I also have an M3 CS which has a staggered 19 front 20 rear setup from factory, so obviously I can't argue against the fact that they did it for a specific reason.

[solstice]

Originally Posted by CanAutM3

Again: it was "very involved" for Porsche because they significantly changed the rear TIRE diameter by 1", which impacts wheel well clearances, ride height and chassis rake. On the F8X CS/GTS and the G8X, they only changed the RIM diameter without significantly altering the tire diameter, so no required changes to the body in BMW's case.

I’m not going get my self dragged into more detail again of something I only remotely interested in. The GTS and CS decision to do the staggering levels a lot of question marks though taken the balance of the base cars and grip distribution if increased front end grip vs rear end is the goal with that type of staggering.

[CanAutM3]

Originally Posted by solstice

I’m not going get my self dragged into more detail again of something I only remotely interested in. The GTS and CS decision to do the staggering levels a lot of question marks though taken the balance of the base cars and grip distribution if increased front end grip vs rear end is the goal with that type of staggering.

Can't speak for the GTS as I have not driven one, but from my own experience the CS definitely has a more neutral balance (less understeer) than the base and competition versions. Tires are part of the equation while the softer front sway bar is another.

This is demonstrated by the M4cs achieving the highest recorded skidpad figure by a significant margin for any F8X they tested despite being shod with PSS tires in C&D testing.

And BTW, you are dragging yourself into this, you even started it .

[CanAutM3]

Originally Posted by jmg

By lowering the pressure you get more tire deflection, which is in line with what I said earlier: more deflection, more grip.



However, I stand by my original statement with a caveat:


However combine the small (minimal) amount of increased contact patch due to an increase in diameter with the ability to run a lower pressure with less sidewall flex, and you get the benefits stated by Achleitner. Engineers smarter than I have probably done the math and determined the increased weight is worth the gains.

Think of it this way: take a tennis ball and set it on the ground. Take a bowling ball and set it on the ground. What is the difference in contact patch? Minimal. Only when to take into account material compression or in this case tire deflection does the contact patch change significantly.



Just to be clear, I was not trying to draw a line between right and wrong, with one argument definitely being on the right side of the line. I was merely giving my "take" on the subject. As CanAutM3 also said, it is not wheel diameter alone that increases contact patch, it is the balance of tire material, pressure, sidewall hight. Then you have to consider the differences between the front and rear not only in size, but how it changes the center of gravity, the rake, even which tires give more or less traction... all these factors contribute to how the car handles. My issues was the general statement that an increased diameter (alone) increases contact patch.

I also have an M3 CS which has a staggered 19 front 20 rear setup from factory, so obviously I can't argue against the fact that they did it for a specific reason.

Increasing a tire's diameter has more impact than one can believe in terms of contact patch size. A larger diameter tire will generate a larger contact patch for the same radial deflection. There are quite a few good white paper articles on the topic. Further, increasing the contact patch length instead of the width also has benefits on longitudinal traction for acceleration and braking.

But this is all moot for the G8X discussion, because they did not increase tire diameter to a significant extent.

[bm323]

Does anyone have a photo of the recommended tire specs and pressure on the metal tag below the B pillar? The one I have seen on the G80 does not have any 19/20 inch listed. It included 19/19 and 19/21 but did not include 19/20. I don't recall the details of the specs.

[AndroidRobot]

Originally Posted by Ilyam5

If you look at the contact patch geometry - width is the most contributing factor. Diameter of the tire and thus growing it lengthwise is miniscule.
275/40/18 tire is within single percent of 275/30/22 but weight gains are huge

and narrower 255/30/22 will have less rubber on the road and less stability and grip vs 275/40/18

I think you underestimate the marketing and sales pressure of the bigger looking wheels. Small wheels do not sell. Look at what people do to their cars first - drop them and buy huge wheels to fit the wheel gap tp achieve cartoonish stance. It has nothing to do with performance and most of the time deteriorates it for the looks.

there's also a lot more sidewall flex with with that extra 1.3" of sidewall on the 18" wheels not to mention those aren't even close to the same rolling diameter at all. sidewall flex makes steering feel less sharp, basically your whole car is moving a few cm outside of the normal range under cornering before returning to normal. That's not a great feeling at all. sure you can have stiffer springs or adjust other things but that only does so much. While low profile tires aren't great for the ride, they are better for dynamics to a degree.

there's a lot more to the picture that we should consider. Of course we cns find a good balance between the two for the best daily set up

[PVC]

Originally Posted by Ilyam5

If you look at the contact patch geometry - width is the most contributing factor. Diameter of the tire and thus growing it lengthwise is miniscule.
275/40/18 tire is within single percent of 275/30/22 but weight gains are huge

and narrower 255/30/22 will have less rubber on the road and less stability and grip vs 275/40/18

I think you underestimate the marketing and sales pressure of the bigger looking wheels. Small wheels do not sell. Look at what people do to their cars first - drop them and buy huge wheels to fit the wheel gap tp achieve cartoonish stance. It has nothing to do with performance and most of the time deteriorates it for the looks.

I agree 100%. I even remember hearing a sales guy on the interwebz several years ago complaining about the competition and how BMW was failing to listen to its customers asking for bigger wheels. If you wanna track your car, you're probably better off going with 18's.

When I was getting my F80 with 20's, the sales guy was warning me about cracked wheels because there is not as much rubber around the wheels to protect them against impact. Bigger wheels don't improve performance, the ride is harsher, and they're more likely to crack...they look f**** great though!

Back to the topic, the difference in size between front and rear wheels is fairly noticeable. I pointed this out in another thread a few weeks ago. I am pretty sure I'm being too picky, but they would have been better off sacrificing performance if they were picking the 20's to begin with. Now, they've kind of half assed it. It doesn't look that great, the ride is as harsh and the performance is probably not as good as it could have been. They should have done what they did with the F8*

[solstice]

Originally Posted by CanAutM3

And BTW, you are dragging yourself into this, you even started it .

I know, and I keep on doing it

[CanAutM3]

Originally Posted by PVC

I agree 100%. I even remember hearing a sales guy on the interwebz several years ago complaining about the competition and how BMW was failing to listen to its customers asking for bigger wheels. If you wanna track your car, you're probably better off going with 18's.

When I was getting my F80 with 20's, the sales guy was warning me about cracked wheels because there is not as much rubber around the wheels to protect them against impact. Bigger wheels don't improve performance, the ride is harsher, and they're more likely to crack...they look f**** great though!

Back to the topic, the difference in size between front and rear wheels is fairly noticeable. I pointed this out in another thread a few weeks ago. I am pretty sure I'm being too picky, but they would have been better off sacrificing performance if they were picking the 20's to begin with. Now, they've kind of half assed it. It doesn't look that great, the ride is as harsh and the performance is probably not as good as it could have been. They should have done what they did with the F8*

There's always the base 18"/19" wheel option