[solstice]

Originally Posted by CanAutM3

Tire diameter or rim diameter?

Whatever change is relevant to what was done to the G8X if you as I believe his comment was done with the G8X in mind. I’m hesitant to say one or the other due to the possible tolerances and implementation of the tires and wheels. I haven’t measured them myself...statically or dynamically at different temperatures and load.

And again I don’t need or even want to know that. If lemetier say that they staggered the wheels to increase the longitudinal contact patch that is clear as day to me as why and all I need to know.

[jmg]

Originally Posted by CanAutM3

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 way more complex than this due to all the other variables at play, but is 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

That's what I'm saying, deformation has to be taken into account. Contact area is not merely a % of the sphere's total surface area.

[jmg]

Originally Posted by solstice

Whatever change is relevant to what was done to the G8X if you as I believe his comment was done with the G8X in mind. I'm hesitant to say one or the other due to the possible tolerances and implementation of the tires and wheels. I haven't measured them myself...statically or dynamically at different temperatures and load.

And again I don't need or even want to know that. If lemetier say that they staggered the wheels to increase the longitudinal contact patch that is clear as day to me as why and all I need to know.

The problem is one could run with that statement that a larger diameter means a larger contact patch and assume that it is diameter alone that increases the patch. This is not completely right because for the reasons we've already gone into great depth with. It's important because there are car enthusiasts that use this information to improve the handling capabilities of their cars on the track as a hobby. How tall of a sidewall to run, tire pressures, tire widths etc... these all work hand in hand to change how a car handles. We haven't even touched how rolling and tire diameter affects the suspension forces, rotational inertia of the wheels, or camber angles.

[CanAutM3]

Originally Posted by solstice

Whatever change is relevant to what was done to the G8X if you as I believe his comment was done with the G8X in mind. I’m hesitant to say one or the other due to the possible tolerances and implementation of the tires and wheels. I haven’t measured them myself...statically or dynamically at different temperatures and load.

And again I don’t need or even want to know that. If lemetier say that they staggered the wheels to increase the longitudinal contact patch that is clear as day to me as why and all I need to know.

That's what I am saying, you are only seeking justification for your belief without trying to comprehend.

[solstice]

Originally Posted by CanAutM3

That's what I am saying, you are only seeking justification for your belief without trying to comprehend.

And as I said, I was never really interested in the details of how but I did want to know why they did it. So no, I’m not seeking justification that’s your faulty interpretation since it happened to match Porsche’s reasons I mentioned. If lemetier had said it was a fashion move or to improve front end grip, I would have taken that as why without further explanation...but he didn’t

[CanAutM3]

Originally Posted by solstice

And as I said, I was never really interested in the details of how but I did want to know why they did it. So no, I’m not seeking justification that’s your faulty interpretation since it happened to match Porsche’s reasons I mentioned. If lemetier had said it was a fashion move or to improve front end grip, I would have taken that as why without further explanation...but he didn’t

I'll repeat again: You are only seeking justification to your belief without trying to comprehend.

Where did lemetier specify that what he said applied to the G8X?

[solstice]

Originally Posted by CanAutM3

I'll repeat again: You are only seeking justification to your belief without trying to comprehend.

Where did lemetier specify that what he said applied to the G8X?

He didn’t but I can’t see why he as a BMW insider would comment on why the G8X has staggered wheels in the G8X forum if the content in his answer is not related to the G8X.

So no it’s not based on seeking justification it’s based on probability of being the “why” directly from the horses mouth. If you manage to get him to answer to the opposite, well then I’ll go with that.

Btw. I’m writing a question for you in the G8X vs .... section if you don’t mind taking a look over there in a few minutes.

[antzcrashing]

Originally Posted by CanAutM3

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...

Thanks, what is it specifically that you like about it on the M4cs?

[CanAutM3]

Originally Posted by antzcrashing

Thanks, what is it specifically that you like about it on the M4cs?

Improved front end grip (less understeer).

[CanAutM3]

Originally Posted by solstice

He didn’t but I can’t see why he as a BMW insider would comment on why the G8X has staggered wheels in the G8X forum if the content in his answer is not related to the G8X.

So no it’s not based on seeking justification it’s based on probability of being the “why” directly from the horses mouth. If you manage to get him to answer to the opposite, well then I’ll go with that.

Btw. I’m writing a question for you in the G8X vs .... section if you don’t mind taking a look over there in a few minutes.

You see, you are extrapolating. My interpretation of his answer is different.

From what I know of him, he is not specifically a "BMW insider", but an insider to the automotive world in general, which gives him insight to BMW .

[solstice]

Originally Posted by CanAutM3

You see, you are extrapolating. My interpretation of his answer is different.

From what I know of him, he is not specifically a "BMW insider", but an insider to the automotive world in general, which gives him insight to BMW .

If I’m extrapolating you are stretching and being a bit judgmental while you are at it I might add. You say I’m justifying when all I said here is that I trust the manufacturers claims about this. We only had Porsche’s reasons for the 992. Then we got lemetier who I believe have better insight than anyone else here as to BMWs reasons.

[CanAutM3]

Originally Posted by solstice

If I’m extrapolating you are stretching and being a bit judgmental while you are at it I might add. You say I’m justifying when all I said here is that I trust the manufacturers claims about this. We only had Porsche’s reasons for the 992. Then we got lemetier who I believe have better insight than anyone else here as to BMWs reasons.

Again here you are extrapolating...

[VIERsr]

275/30-20 wheel has the same diameter as OEM 275/35-19, putting lighter 20" rims in the front we can have a better look and keep the same ratio F/R without adding weight, but I don't know how the lower sidewall only in the front could impact on dynamics.

[jmg]

Originally Posted by VIERsr

275/30-20 wheel has the same diameter as OEM 275/35-19, putting lighter 20" rims in the front we can have a better look and keep the same ratio F/R without adding weight, but I don't know how the lower sidewall only in the front could impact on dynamics.

275/30-20 is 673mm
275/35-19 is 675.1mm
(Edit - corrected diameters)

CanAutM3 stated earlier:

Originally Posted by CanAutM3

Most likely to reduce understeer. In general terms, a larger sidewall tire provides greater ultimate grip at the expense of handling response.

The same thing was done on the F8X M3/4[SIZE="1"]CS[/SIZE] and M4[SIZE="1"]GTS[/SIZE].

So a lower sidewall would probably increase understeer all other things being equal.

[VIERsr]

Originally Posted by jmg

275/30-20 is 590.5mm
275/35-19 is 578.85mm

It is 26.50 vs 26.58 inches, or 67.30 vs 67.51 cm, it's a negligible difference




Anyway, if it can cause more understeer it's not a good option.

[CanAutM3]

Originally Posted by jmg

275/30-20 is 590.5mm
275/35-19 is 578.85mm

CanAutM3 stated earlier:



So a lower sidewall would probably increase understeer all other things being equal.

When calculating the tire diameter, you need to double the sidewall height...

[jmg]

Originally Posted by CanAutM3

When calculating the tire diameter, you need to double the sidewall height...

Corrected!

[DDS2015]

Originally Posted by CanAutM3

When calculating the tire diameter, you need to double the sidewall height...

Now that we have the 19's/20's figured out....

Any idea why they switched to a floating rear brake caliper from the 2-piston in regular form and 4-piston with CCBs?

I doubt that a floating rear caliper has as much clamping force as a 2 or 4 piston caliper. Or is it all for weight savings and they've offset it with a larger rotor?

One thing I find with my M3CS is that on track the brakes aren't the most confidence inspiring... I'm completely stock down to the pads/lines/fluid, standard brakes, no CCBs.

Since you have a near identical car and infinitely more knowledge about these, figured I'd ask!

[CanAutM3]

Originally Posted by DDS2015

Now that we have the 19's/20's figured out....

Any idea why they switched to a floating rear brake caliper from the 2-piston in regular form and 4-piston with CCBs?

I doubt that a floating rear caliper has as much clamping force as a 2 or 4 piston caliper. Or is it all for weight savings and they've offset it with a larger rotor?

One thing I find with my M3CS is that on track the brakes aren't the most confidence inspiring... I'm completely stock down to the pads/lines/fluid, standard brakes, no CCBs.

Since you have a near identical car and infinitely more knowledge about these, figured I'd ask!

, but anyhow :

The single piston rear brake calipers are for the e-parking-brake. A real pain IMO, since you can no longer do quick track side rear pad swaps in the paddocks without having to carry a laptop with you to reset the e-brake. Plus they are paired with puny little pads .

Single piston vs multiple piston is not about clamping force, as a single piston can apply sufficient force to lock the brakes. It is a question of distributing this clamping force over al larger area for a more even application, which can also accommodate larger pads for more swept area.

I find the iron brakes on my M4cs to be plenty sufficient on track, but they do need proper track pads. PF08 are my favourite thus far. Stock fluid works decently fine, but you are better off with Castrol SRF.

[DDS2015]

Originally Posted by CanAutM3

, but anyhow :

The single piston rear brake calipers are for the e-parking-brake. A real pain IMO, since you can no longer do quick track side rear pad swaps in the paddocks without having to carry a laptop with you to reset the e-brake. Plus they are paired with puny little pads .

Single piston vs multiple piston is not about clamping force, as a single piston can apply sufficient force to lock the brakes. It is a question of distributing this clamping force over al larger area for a more even application, which can also accommodate larger pads for more swept area.

I find the iron brakes on my M4cs to be plenty sufficient on track, but they do need proper track pads. PF08 are my favourite thus far. Stock fluid works decently fine, but you are better off with Castrol SRF.

I admit that I'm not a track junkie and at most I feel comfortable to do a wheel/tire swap.


So floating rear caliper is more of a pain since not as easy to swap pads, but not a huge difference in terms of performance? Also, they definitely don't look as cool (because that's what it's all about after all!)

Ya I know off topic, seemed close enough though!

[CanAutM3]

Originally Posted by DDS2015

I admit that I'm not a track junkie and at most I feel comfortable to do a wheel/tire swap.


So floating rear caliper is more of a pain since not as easy to swap pads, but not a huge difference in terms of performance? Also, they definitely don't look as cool (because that's what it's all about after all!)

Ya I know off topic, seemed close enough though!

The single piston rear calipers are not as good in terms of performance because of the tiny pads and less even force application on the two pads.

[VIERsr]

The same happens on heavier and more performing cars than the M4 (M5 F10, M5 F90, M8, Audi RS6/7, Lamborghini Urus, MB 63AMG), all with electric parking brake.
This would have been the alternative.