[CanAutM3]

Originally Posted by Moflow

I think you're confusing polar moment of inertia with weight distribution (or maybe you're not confusing them, but we should note that you're talking about two distinct things here.)

You've described polar moment of inertia (how quickly/willing an object is to rotate) however, polar moment of inertia doesn't care if the engine is in the front or back, just how close it is to the center of mass. That's the real reason mid engined cars make good race cars. Yes having more weight over the rear axle helps traction when accelerating, but the biggest benefit is having the engine closer to the middle of the car for quicker rotation. Like a figure skater pulling their arms in to do a triple axle. In practice, this does mean cars with engines behind the driver tend to rotate quicker, as you correctly pointed out.

I personally believe that there isn't an "ideal" weight distribution, anything between about 35 percent to 55 percent front weight can work fine as long as your polar moment of inertia and center of mass height off the ground are set up to make sense and the whole thing matches the drivers preferred style.

As for the 2kg lighter wheels....I dunno. Could maybe be reducing understeer, unsprung weight can make a big difference (evidence here) but the wheels in my example are also carbon fiber wheels which are reducing flex in addition to being really light. Still, 2 seconds a lap over a 2 minute lap is pretty great for a reduction in weight of just 0.5 to 1.5 kg per wheel.

No, I am not confusing anything . Moment of inertia is a bit more complex than what you describe. Your description is only valid if an object rotates around its center of gravity. If the rotation axis is located elsewhere, the Cg having to rotate around the axis also has an inertial effect. See caption below (reference).

In general terms, on a traditional vehicle that is steered by the front wheels, the rotation axis does not pass through the cg, it passes through the center point of the rear axle. Imagine the front wheels steered at 90 degrees and it becomes very obvious.

Mid engines cars are so nimble because they have a lower natural moment of inertia AND because they are rear weight biased (cg closer to the rear axle), both of which contribute to reduce the total inertia when the vehicle changes direction. On the other hand, front weight biased cars are know to understeer because the front tires need to work that much harder to get the car to change direction due to the cg being far way from the rear axle.

This is only entirely true when the front and rear slip angles are equal. When the front and rear slip angles are varied to different degrees, it changes the location of the rotation axis which alters how the car behaves. This is why mid and rear engines cars can become very tricky in oversteer situations. When the rear axle significantly looses grip, the rotation axis moves from the rear axle to the front axle of the car, with the cg being farther back it acts like a pendulum and makes recovering the oversteer that much more difficult. Hence why these cars have the reputation to "snap oversteer".

In race car design, ideal weight distribution is often quoted to be in the 40-60 to 35-65 range. 50-50, or worse, front weight bias, is not desirable for optimal handling.

[Moflow]

Originally Posted by CanAutM3

No, I am not confusing anything . Moment of inertia is a bit more complex than what you describe. Your description is only valid if an object rotates around its center of gravity. If the rotation axis is located elsewhere, the Cg having to rotate around the axis also has an inertial effect. See caption below (reference).

In general terms, on a traditional vehicle that is steered by the front wheels, the rotation axis does not pass through the cg, it passes through the center point of the rear axle. Imagine the front wheels steered at 90 degrees and it becomes very obvious.

Mid engines cars are so nimble because they have a lower natural moment of inertia AND because they are rear weight biased (cg closer to the rear axle), both of which contribute to reduce the total inertia when the vehicle changes direction. On the other hand, front weight biased cars are know to understeer because the front tires need to work that much harder to get the car to change direction due to the cg being far way from the rear axle.

This is only entirely true when the front and rear slip angles are equal. When the front and rear slip angles are varied to different degrees, it changes the location of the rotation axis which alters how the car behaves. This is why mid and rear engines cars can become very tricky in oversteer situations. When the rear axle significantly looses grip, the rotation axis moves from the rear axle to the front axle of the car, with the cg being farther back it acts like a pendulum and makes recovering the oversteer that much more difficult. Hence why these cars have the reputation to "snap oversteer".

In race car design, ideal weight distribution is often quoted to be in the 40-60 to 35-65 range. 50-50, or worse, front weight bias, is not desirable for optimal handling.

Awesome post, yes you're right about rotating around the rear axle without any slip angle vs the center of mass, I stand corrected. Great explanation of vehicle mass dynamics in cornering.

We're getting deep into it now!

[Robin_NL]

Originally Posted by Boss330

Even the "ideal" 50/50 isn't the optimal weight distribution for a track car.

Don't forget that one of probably the best track day cars, the 991 GT3 RS, has a 39/61 weight distribution! (Kind of like that old beetle... )

Getting close to 50/50 is however as good as it gets for a front engine rwd sedan type of car. Which is one of the reasons BMW's generally tend to be better suited for track day use and fun to drive than the direct competitors with front engine and rwd (even though many have caught up in later years).

But ideally you want less than 50 % on the front axle, more like the GT3 for instance.

BTW: My big 6-series Gran Coupe has a real world 49/51 weight distribution (but at 2070kg it's not quite 911 nimble on the track. And yes, I have tracked it more than once )

Regarding the winter tires making your steering feel lighter, that is more than any other factor caused by the winter tires deeper thread depth and open thread design. This contributes to much more thread squirm, ie the tires «move around» on themselves. This creates the lighter feel that you get when changing to winter tires. Most pronounced effect when the tires are new and have deep threads.

50/50 is alright for an FR car though I'd prefer a fatter ass, like 49/51R


But 911 layout is also 'wrong' is Porsche found out years ago
That's why >2017 911 RSR has a 'midengine' and that's why F1 cars have midengines.

Regarding comment on the wintertyres set I have:
More times than not there are drifttyres on that 18"wheelset I have . It has nothing to do with thread depth of what I'm feeling but all with the weight of the rims + tyres.

You do feel lighter rims.

But this is the internet which means a lot of confusion and assumptions.

Cheers
Robin

[Robin_NL]

About midengines, Lotus didn't(doesn't atm??) provide Exige with an LSD because the car got more into UNDERSTEER than Lotus engineers wanted, but it's in the options list.

Mid engined cars and 911(older ones tell me about it) do have initial understeer too. And sometimes even more than our beloved M2/C on track (been there done that....).

Mainy because of that fabulous diff we have which makes the nose point in even more than you even wish. When tyres are on temp that is


GT4 PDK vs M2CS , GT4 anytime!

And on my 'dreamcarlist' 991.2 GT3RS is number one no worries. Best sportscar on sale today.



Cheers
Robin

[GimmeanM]

Sport Auto's lap on YouTube today:

[Artemis]

Originally Posted by GimmeanM

Sport Auto's lap on YouTube today:
[VIDEO]

Thanks for the heads-up. Now added to the first post of this thread.

Also gets us an idea how the M2 CS sounds inside the cabin space when under load.

02:08: gong signal - M2 CS went into 'limp mode' after the hot lap (or right before finishing it).

[vale007]

Originally Posted by Artemis

Thanks for the heads-up. Now added to the first post of this thread.

Also gets us an idea how the M2 CS sounds inside the cabin space when under load.

02:08: gong signal - M2 CS went into 'limp mode' after the hot lap (or right before finishing it).

or running out of fuel

[Artemis]

Originally Posted by vale007

or running out of fuel

Yup. Text that pops up on the screen 02:06-02:10 mentions that the car was running on empty. You hear the sudden power drop (going into limp mode) in the straight line at 02:06.

So hot lap test result: with almost no fuel weight and with a few moments of limp mode right before the finish line.

[medphysdave]

Struggling to find grip? Kinda looked like a handful.

[FormulaMMM]

Originally Posted by medphysdave

Struggling to find grip? Kinda looked like a handful.

Sure does. Looks like a proper M car and a workout at the limit. Lively handling/balance = actual, lasting entertainment value on track. DSC off mandatory for pace. Sign me up.

Those who love it should get it while they can. Next gens w/ M xdrive will be a different experience.

He’s up one gear and much lower rpm out of a few corners where I would have expected a gear down. Clearly has mega grunt.

Sounds good, will look the part, plenty fast. Drop 1-2 seconds off of that lap time in lower temps. https://fastestlaps.com/tracks/hockenheim-gp

Not sure what there is to complain about.

[AlpsRider]

The performance of the CS should be no secret since the M2C with the M4CS stock tune would have even more power. There are plenty of people running the CS tune. The suspension would be different but the power should be similar.

[medphysdave]

Originally Posted by FormulaMMM

Sure does. Looks like a proper M car and a workout at the limit. Lively handling/balance = actual, lasting entertainment value on track. DSC off mandatory for pace. Sign me up.

Those who love it should get it while they can. Next gens w/ M xdrive will be a different experience.

He’s up one gear and much lower rpm out of a few corners where I would have expected a gear down. Clearly has mega grunt.

Sounds good, will look the part, plenty fast. Drop 1-2 seconds off of that lap time in lower temps. https://fastestlaps.com/tracks/hockenheim-gp

Not sure what there is to complain about.

I agree with your comment on gear selection. Looked like a lower gear was in order, but maybe traction was not allowing it.

[Artemis]

Originally Posted by Artemis

M2 CS and M2C dimensions (length/width/height) and wheelbase are identical.

And so it appears that the length of M2C + M2 CS and 718 GT4 are virtually identical (difference of only 5mm/0.5cm or 0.2in), unlike the wheelbase (difference of 20.9cm or 8.23in).

Length and wheelbase:

• 981 GT4 = 4438mm (174.72in) | wheelbase: 2474mm (97.40in)
• 718 GT4 = 4456mm (175.43in) | wheelbase: 2484mm (97.80in)
• M2 Competition = 4461mm (175.63in) | wheelbase: 2693mm (106.02in)
• M2 CS = 4461mm (175.63in) | wheelbase: 2693mm (106.02in)
• original M2 = 4468mm (175.91in) | wheelbase: 2693mm (106.02in)
• E46 M3 = 4492mm (176.85in) | wheelbase: 2731mm (107.52in)

[Robin_NL]

Sport Auto's lap on YouTube:

At the end of the lap enginepower is gone, because (stated by Sportauto) in long right turns with a tiny amount of fuel in the tank power cuts off: limp mode. Time difference 0.xxxs(?) I can't read.

0.5s according to Sportauto( Comments below YT video)

So this CS had not a full tank of fuel thus being even lighter than the M2 Comp(?)

*Conspiracy Theories galore*



Cheers
Robin

[Robin_NL]

Seeing the video over and over again I'm under the impression, that M2CS testcar had a bit more than '450PS' in that kind of hot weather, I guess >480PS.

Cheers
Robin

[FormulaMMM]

Originally Posted by Robin_NL

Seeing the video over and over again I'm under the impression, that M2CS testcar had a bit more than '450PS' in that kind of hot weather, I guess >480PS.

Cheers
Robin

I'm under the impression that my GTS, not a test car, has a good chunk more than 493 bhp.

[N & M]

This car is shaping up to be epic. Can’t wait to drive.

[bjcarls]

What I would expect from a car with M-performance bits attached. They just aren't trying hard enough but will probably sell them because its still somewhat reasonable.

[M 4EVER]

Originally Posted by N & M

This car is shaping up to be epic. Can't wait to drive.

Originally Posted by ///M TOWN

BMW is Definitely starting to mean... Bring My Wallet.

It will be hard to get one
Called so many dealers and no luck

[///M TOWN]

Originally Posted by M 4EVER

It will be hard to get one
Called so many dealers and no luck

Check your PM ‼️

[Blader]

Originally Posted by Robin_NL

About midengines, Lotus didn't(doesn't atm??) provide Exige with an LSD because the car got more into UNDERSTEER than Lotus engineers wanted, but it's in the options list.

Mid engined cars and 911(older ones tell me about it) do have initial understeer too. And sometimes even more than our beloved M2/C on track (been there done that....).

Mainy because of that fabulous diff we have which makes the nose point in even more than you even wish. When tyres are on temp that is


GT4 PDK vs M2CS , GT4 anytime!

And on my 'dreamcarlist' 991.2 GT3RS is number one no worries. Best sportscar on sale today.



Cheers
Robin

The reason for the optional LSD on the Exige is that for road driving you don't need it.

However on the track the LSD is definitely an advantage. I frequently track with other Exiges that are virtually identical cars apart from the LSD and I get out of the corners harder.

[chris82]

Originally Posted by PLF69

This car is really aimed at ppl who want a more special or modded M2 but don't want to touch the aftermarket or risk voiding there warranty..

Nothing so unique about it. Looks like BMW marketing team discovered that they can make even more $$ by splitting models in more sub-models because today most people just want to impress others and can't appreciate what they have. This phenomenon can be observed in every other consumption markets and is fueled by social media behaviours.

Not even. people who are willing to spend the additional money couldn't possibly be concerned about voiding warranty, it's entirely about the exclusivity of the car.