[PVC]

Originally Posted by CanAutM3

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

Those look even worse . All 19 or all 20 would have been better IMO, but I guess BMW can't satisfy everyone. It's just a bit sad that their changes this cycle are too radical.

[heavyD^2]

Originally Posted by CanAutM3

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.

I know I shouldn't have come back but facts are facts. I have selected two Michelin Pilot Sport 4S tires (275/30-19 and 275-35-19. If you look at the specs you will see the 30 series tire has 0.6" more tread width than the 35 series of the same tire. This is what they mean by gaining a larger contact patch by going to a lower profile tire.

https://www.tirerack.com/tires/tires...omCompare1=yes

Half an inch is a fair bit of extra contact area so you can see why tire profiles keep getting lower and lower.

[CanAutM3]

Originally Posted by heavyD^2

I know I shouldn't have come back but facts are facts. I have selected two Michelin Pilot Sport 4S tires (275/30-19 and 275-35-19. If you look at the specs you will see the 30 series tire has 0.6" more tread width than the 35 series of the same tire. This is what they mean by gaining a larger contact patch by going to a lower profile tire.

https://www.tirerack.com/tires/tires...omCompare1=yes

Half an inch is a fair bit of extra contact area so you can see why tire profiles keep getting lower and lower.

You indeed “selected” only one example, which proves nothing. Further, the tires you selected don't even have the same rolling diameter.

See below a counter example. Comparing the 265/40R18, 265/35R19 and 265/30R20 PSS where not one of the five 265/35R19 offering has a wider tread than the two 265/40R18 offerings and with the 265/30R20 having the narrowest tread of all. Picking such examples does not prove that higher profiles have larger treads, but it does prove what I said earlier: that the tread width is defined by the tire's construction and is not directly related to sidewall profile. Period. Yes, facts are facts...

[brad850csi]

CanAutM3

I remember your comments and comparisons with various PSS and PS4S threads such as the below ones and definitely respect your tyre knowledge!

https://f80.bimmerpost.com/forums/sh....php?t=1089535
https://f80.bimmerpost.com/forums/sh....php?t=1730589
https://f80.bimmerpost.com/forums/sh....php?t=1350040

Those pages will give some people more things to argue about





What I wanted to say was that I find it interesting that as the G8x cars have had their weight distribution become less balanced and get closer to an Audi weight distribution with an increase of about 1.5% heavier at the front compared to F80. From this I believe that is why the front tyres are now only 10mm narrower than the rear whereas they were previously 20mm.

Of course there is an overall weight increase as well and I'm sure that the wider front tyres are there to provide more grip as it is now needed due to the extra weight. I wonder if the staggered diameter allowed them to meet the compromise they were looking for between grip, ride quality and so on. This article touches on it slightly for the 992. https://www.motorauthority.com/news/...r-the-2020-911


My only complaint about the different sizes is that with 20mm difference in width the previous sidewall height difference was about 7-8% whether the wheels were the same size with the rear sidewall being a bit bigger on normal cars or with the staggered diameter setup the difference was still about 8%, just the other way around as the rear sidewall was of course smaller due to the larger wheel.

Now that sidewall height difference is 12.5% larger at the front which is the problem here - it will be much more noticeable and can look a bit weird.

The other manufacturers doing staggered diameters front and rear are making the rear tyres a lot taller for their outer diameter which means that the sidewall size is much closer from front to rear, ie the C8 is 6%, the 992 is 6% and I'm sure there are other similar examples out there - as mentioned in the first post.

I'm sure someone out there will be running 265/35r19 and 295/30r20 on their GTS or CS and I bet that will look really good but I'm not sure if that is fixable easily on the G8x, 275/35r19 and 305/30r20 would fix the looks issue but I'm not sure if the stability control will understand that difference. The stock rear wheels are 10.5" wide so will support a 305/30r20 if there is enough room for the tyre.

[lemetier]

Originally Posted by CanAutM3

You just picked a single example, which proves nothing. Further, the tires you selected don't even have the same rolling diameter.

See below a counter example. Comparing the 265/40R18, 265/35R19 and 265/30R20 PSS where not one of the five 265/35R19 offering has a wider tread than the two 265/40R18 offerings and with the 265/30R20 having the narrowest tread of all. Picking such examples does not prove that higher profiles have larger treads, but it does prove what I said earlier: that the [COLOR="Blue"]tread width is defined by the tire's construction[/COLOR]. Period. Yes, facts are facts...

You have to factor in Manufacturer specific dimensions of the reference sizes to do an accurate comparison. The larger Ø rear has to do with increasing the longitudinal contact patch (circumference) without impacting the necessary max load tire camber (loaded radius).

[CanAutM3]

Originally Posted by lemetier

You have to factor in Manufacturer specific dimensions of the reference sizes to do an accurate comparison.

Not sure how much of the thread you have read. My point here is simply to demonstrate that a lower profile tire does not automatically translate to wider tread, as was debated quite heavily earlier in the thread.

Originally Posted by lemetier

The larger Ø rear has to do with increasing the longitudinal contact patch (circumference) without impacting the necessary max load tire camber (loaded radius).

Curious here, when you say "larger Ø rear", are you talking tire diameter or rim diameter?

[Boss330]

Originally Posted by lemetier

You have to factor in Manufacturer specific dimensions of the reference sizes to do an accurate comparison. The larger Ø rear has to do with increasing the longitudinal contact patch (circumference) without impacting the necessary max load tire camber (loaded radius).

@CanAutM3 has already explained that point in a previous post on this thread.

There are quite a few misconceptions/misunderstandings in this thread. A few posters seem to confuse rim diameter and tyre diameter.

Porsche has a staggered setup on the 992 where both rim and tyre diameter is increased on the rear compared to the front. That obviously gives a larger contact area, not due to the increased rim diameter but due to the increased tyre diameter. Hence Porsche's reasoning of increased traction/grip on the rear with this setup.

BMW has, as CanAutM3 has pointed out multiple times, not done this. They have retained a tyre diameter of less than 1% difference between f/r, and only increased rear rim diameter. So no gain in tyre diameter or contact area from a 1" larger rear rim diameter in BMW's case.

Better turn in on the front axle and more stable rear due to a lower sidewall tyre. Best of both worlds!

This also lines up with Sport Auto's test drive of the G82 M4 on a track. It had a much better front end when it came to turn in than the F82 M4 Comp they drove back to back with on the same track on the same day.

[Boss330]

Originally Posted by jmg

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.

Have you calculated the contact area for the tennis ball and the bowling ball? (Bad example as the tennis ball has a textured surface while the bowling ball is smooth. Better to use similar surfaces when comparing/calculating).

How large was the difference in %

You might be surprised...

It's easy to disregard the difference in contact area because both are so small compared to the remaining area of the objects. But, even though both still have a very small contact area, the bowling ball will have a much larger area than the smaller ball.

A tennis ball is around 67mm (6,7cm) in diameter
A bowling ball is around 217mm (21,7cm) in diameter

Surface area of a sphere:
A = 4πr2

Surface area of the tennis ball: 141cm2
Surface area of the bowling ball: 1256cm2

For arguments sake, let's say that 0,01% of the surface of a sphere (ball) touches the ground.

Tennis ball area touching the ground: 0,0141cm2
Bowling ball area touching the ground: 0,1256cm2

Percentage increase in contact area: 890%

[Transfer]

Originally Posted by Boss330

Have you calculated the contact area for the tennis ball and the bowling ball? (Bad example as the tennis ball has a textured surface while the bowling ball is smooth. Better to use similar surfaces when comparing/calculating).

How large was the difference in %

You might be surprised...

It's easy to disregard the difference in contact area because both are so small compared to the remaining area of the objects. But, even though both still have a very small contact area, the bowling ball will have a much larger area than the smaller ball.

A tennis ball is around 67mm (6,7cm) in diameter
A bowling ball is around 217mm (21,7cm) in diameter

Surface area of a sphere:
A = 4πr2

Surface area of the tennis ball: 141cm2
Surface area of the bowling ball: 1256cm2

For arguments sake, let's say that 0,01% of the surface of a sphere (ball) touches the ground.

Tennis ball area touching the ground: 0,0141cm2
Bowling ball area touching the ground: 0,1256cm2

Percentage increase in contact area: 890%

Whoa whoa simmer down there sir. Maths proving points is not the way of the interwebz.

lol nice work.

[antzcrashing]

Originally Posted by nearwater4me

The looks have grown on me enough that I might consider one for my next car. The thing that I still find loathsome is the 18/19" or 19/20" wheel diameter. It sure was irritating on the F80 CS, and BMW decided to use the same setup on the new M3/4.

American sports cars like the Corvette and Viper got away with using different diameter wheels by having bigger wheel housings on the rear with matching tire proportions, which made them look okay. For the M vehicles, however, 19" fronts with fat tires mixed with 20" rears and skinny tires look absolutely stupid IMO.

I know it's an easy fix, but it still costs $3-5k for an aftermarket wheels set. What gives? What's the reason behind this trend?

Sorry, rant over.

I hate hate that bmw is doing this too. Supposedly it is for better track handling, not sure it is worth it. I would choose 19s all around if given the option. Agree that aftermarket rims (and tires) is an expensive route to fix this.

[Boss330]

Originally Posted by Transfer

Whoa whoa simmer down there sir. Maths proving points is not the way of the interwebz.

lol nice work.

Hahaha 😜😂

Forgot that facts and math are not the recipe for making a case on the interwebz... 😉

[CanAutM3]

Originally Posted by antzcrashing

I hate hate that bmw is doing this too. Supposedly it is for better track handling, not sure it is worth it. I would choose 19s all around if given the option. Agree that aftermarket rims (and tires) is an expensive route to fix this.

They might not have many other choices to balance the handling, given the G8X is carrying more weight on the front axle. I have to say though, I do appreciate the 19"/20" setup on my M4cs...

[solstice]

Originally Posted by lemetier

You have to factor in Manufacturer specific dimensions of the reference sizes to do an accurate comparison. The larger Ø rear has to do with increasing the longitudinal contact patch (circumference) without impacting the necessary max load tire camber (loaded radius).

This is a gem to understand this. Read it slowly and thoroughly.

[jmg]

Originally Posted by Boss330

Have you calculated the contact area for the tennis ball and the bowling ball? (Bad example as the tennis ball has a textured surface while the bowling ball is smooth. Better to use similar surfaces when comparing/calculating).

How large was the difference in %

You might be surprised...

It's easy to disregard the difference in contact area because both are so small compared to the remaining area of the objects. But, even though both still have a very small contact area, the bowling ball will have a much larger area than the smaller ball.

A tennis ball is around 67mm (6,7cm) in diameter
A bowling ball is around 217mm (21,7cm) in diameter

Surface area of a sphere:
A = 4πr2

Surface area of the tennis ball: 141cm2
Surface area of the bowling ball: 1256cm2

For arguments sake, let's say that 0,01% of the surface of a sphere (ball) touches the ground.

Tennis ball area touching the ground: 0,0141cm2
Bowling ball area touching the ground: 0,1256cm2

Percentage increase in contact area: 890%

It has been my understanding that a solid sphere contacts a solid object at only one point and does not have an area attribute. But, that is not taking into account deformation. A larger wheel with the same tire compound might deform more thus increasing the contact patch. So the contact pitch has less to do with the diameter and more to do with the ability of the material to deform due to the structural changes as a result of the larger diameter. The contact area of a solid sphere and solid object with no deformation or either object is always one point regardless of diameter.

However, you can increase contact area simply by increasing deformation by softening the material. On a wheel, this might not be ideal because of decreasing sidewall rigidity so they increase the wheel diameter to decrease sidewall height to allow for tire deformation without losing sidewall rigidity. Or lower pressure and more deformation for increased tire patch without losing sidewall rigidity.

[CanAutM3]

Originally Posted by solstice

This is a gem to understand this. Read it slowly and thoroughly.

Indeed. And it is all accurate when increasing tire rolling diameter...

[solstice]

Originally Posted by CanAutM3

Indeed. And it is all accurate when increasing tire rolling diameter...

I’m getting more and more convinced that lemetier is deeply involved in the development/testing of M cars and I highly doubt this was a generic statement. We asked for more information from BMW about the G8X staggered setup and I believe this is it. I’m not going to expand on how since I risk diluting or misleading the crisp and clear comment made. I leave that to lemetier if he so chooses.

[CanAutM3]

Originally Posted by jmg

It has been my understanding that a solid sphere contacts a solid object at only one point and does not have an area attribute. But, that is not taking into account deformation. A larger wheel with the same tire compound might deform more thus increasing the contact patch. So the contact pitch has less to do with the diameter and more to do with the ability of the material to deform due to the structural changes as a result of the larger diameter. The contact area of a solid sphere and solid object with no deformation or either object is always one point regardless of diameter.

However, you can increase contact area simply by increasing deformation by softening the material. On a wheel, this might not be ideal because of decreasing sidewall rigidity so they increase the wheel diameter to decrease sidewall height to allow for tire deformation without losing sidewall rigidity. Or lower pressure and more deformation for increased tire patch without losing sidewall rigidity.

This is true only in theoretical geometry. In the real world, there is always deformation, otherwise the bowling ball pressure on the floor would be infinite. Even steel train wheels rolling on a steel rail deform to an effective contact area.

As I posted earlier, a tire with a larger rolling diameter will provide a meaningfully larger contact patch for the same radial deformation. If you do the (simplified) math for the 991 to 992 rear tire example, going from a 305/30R20 to 305/30R21 yields a ~2% longer contact patch (assuming all else is equal). And if you think 2% is negligible, consider a 315 is only ~3% wider than a 305.

R^2 = (R-d)^2 + (L/2)^2

Where:

• R=Tire unloaded diameter
• d=Radial loaded deformation
• L=Contact patch length

In the real word, it is way more complex than this due to all the other variables at play, but it give an appreciation.

If you want to have fun, have a look at this site. I am not sure how accurate it is, but fun nonetheless: http://bndtechsource.ucoz.com/index/...alculator/0-20

[CanAutM3]

Originally Posted by solstice

I’m getting more and more convinced that lemetier is deeply involved in the development/testing of M cars and I highly doubt this was a generic statement. We asked for more information from BMW about the G8X staggered setup and I believe this is it. I’m not going to expand on how since I risk diluting or misleading the crisp and clear comment made. I leave that to lemetier if he so chooses.

Having exchanged many times with Lemetier I know how knowledgeable he is. He does tend to post things in riddles though . I did ask him a clear specification on his comment to my post though, and he has not yet replied. Let's see when he does.

[solstice]

Originally Posted by CanAutM3

He does tend to post things in enigmas though

I can see that with your thirst for knowing how with detailed scientific validation. Not so for me, I want to know why and I’m satisfied with a short explanation. His response clearly states why and by that I also know what the benefits are.

[CanAutM3]

Originally Posted by solstice

I can see that with your thirst for knowing how with detailed scientific validation.

My quest for a technical understanding allows me to be critical of what I read and hear.

Originally Posted by solstice

Not so for me, I want to know why and I’m satisfied with a short explanation. His response clearly states why and by that I also know what the benefits are.

Sorry to say, but his answer is not clear at all and is very open to interpretation. You are interpreting what he is saying to justify what you believe. Seeking only sources of information or interpreting according to one's own beliefs leads down a very dangerous path...

[solstice]

Originally Posted by CanAutM3

My quest for a technical understanding allows me to be critical of what I read and hear.

Sorry to say, but his answer is not clear at all and is very open to interpretation. You are interpreting what he is saying to justify what you believe. Seeking only sources of information or interpreting according to one's own beliefs leads down a very dangerous path...

Dangerous? We’re talking diameter of rear wheels here...some perspective please.
And for not being clear as for the OPs question on why, how can his comment be any clearer than it is to increase the longitudinal and overall circumference of the contact patch? That is why they staggered the diameter. There is no other way to interpret it.

[CanAutM3]

Originally Posted by solstice

Dangerous? We’re talking diameter of rear tires here...some perspective please.
And for not being clear as why, how can his comment be any clearer than it is to increase the longitudinal and overall circumference of the contact patch? That is why. There is no other way to interpret it.

Tire diameter or rim diameter?