[Max Well]

Originally Posted by Mii

https://f80.bimmerpost.com/forums/sh...d.php?t=921523
https://f80.bimmerpost.com/forums/sh...php?p=22692129

Mii, I've had a chance to review both threads you shared regarding the use of CFRP vs steel drive shafts. For those who don't have time to read the 41 pages in the second one, I've summarized some points:

From 16 Aug 2017 0820PM, post #336, CanAutM3:

Originally Posted by CanAutM3

It is not only about rotating mass, even if I seriously doubt the smaller dimensions of the steel driveshaft will offset the added weight in terms of polar moment of inertia. The CF driveshaft is a one-piece unit and does not have a support bearing and a CV joint in the middle like it is needed for steel shaft. The CF shaft is also quite stiffer than a steel one which means a more direct response from the engine to the drive wheels (less elasticity in the drivetrain). As many have said, most will not notice the difference on the daily drive. However, the F8X is a sum of many parts that makes it a whole.

From 23 Oct 2017 0116PM, post #553, CanAutM3:

Originally Posted by CanAutM3

… a driveshaft is way more complex than a simple annular tube. See picture below, you need to factor the end connecting points, CV joint, bearings, etc... The dimensions and weights are not fully clear either...
Further, you might be confusing un-sprung weight, rotating mass and drivetrain loss. Un-sprung weight affects the ability of the tires to stay in contact with the road when hitting bumps. Rotating mass has an effect on the forces/power needed to accelerate and decelerate the car. Un-sprung weight has no direct correlation with the effects of rotating mass and the driveshaft is most definitely sprung weight. Also, the effect of rotating mass is not considered a loss per se since it can be recuperated; it is simply mass that needs to be accelerated, just like the entire vehicle mass.

From 25 Oct 2017 1039AM, post #566, CanAutM3:

Originally Posted by CanAutM3

As was previously stated, a driveshaft is a much more complex geometry that a simple hollow tube. For the CF driveshaft, most of the mass is located in the steel connecting endpoint flanges, not in the tube itself. So it is only a rather small portion of the total driveshaft mass that is moved farther away from the rotating axis with the increase in diameter. Using the simplified hollow tube formula I=mr^2 for the total shaft grossly overestimates the impact on inertia. Based on the geometry, even without having all the specifications of the driveshafts, one can make a reasonable assumption that it takes a rather large increase in the CF tube diameter compared to the steel one to significantly increase the inertia of the total driveshaft.
Further, to be able to assess the impact of a rotating mass on vehicle acceleration, one needs to assess the effective mass of that component (if you want to find out more on the topic do some research on effective mass, mass factors and equivalent mass. "Fundamentals of Vehicle Dynamics" by Gillespie is a good starting point). Effective mass takes into consideration the combined effect of translational acceleration and angular acceleration of a given component. Simply put, when the car accelerates, the driveshaft not only spins faster and faster on itself but also moves forward with the car. To calculate effective mass, one needs to determine the mathematical relationship between the translational and rotational speed of the component. For the driveshaft example, one needs to consider the rear wheel rolling radius of .337m and the final drive ratio of 3.462 in the calculation. If you start doodling with the formulas for a driveshaft, you'll find out that it takes a rather significant increase in inertia for the effect of the angular acceleration to outweigh the impact of the mass of the component on translational acceleration.
As a tidbit, I also did a little more research looking at driveshaft weight in RealOEM.com, and it seems that the 2.5kg quoted in the AM&S article is more representative of the total weight difference between the CF and steel driveshafts.

From 25 Oct 2017 0254PM, CanAutM3:

Originally Posted by CanAutM3

I studied mechanical engineering with an aerospace specialization. I have been working at an aircraft engine manufacturer for close to 25 years.

From 21 Nov 2017 0333PM, Sassicaia:

Originally Posted by Sassicaia

First person to weigh the steel vs CF drive shaft wins the internet.

To my knowledge no one has yet formally weighed the two side-by-side as Sassicaia requested.

So, Mii, certainly an interesting topic which was thoroughly discussed in that second thread. My take - like many of our Forum discussions which involve unknowns in data, it's difficult to arrive at firm conclusions as to actual significance (realized benefits) of CFRP in the drive shaft setting, although the theoretical advantages are at least compelling.

I'm still trying to make a case for the CFRP roof, however.

[kozzi]

If I'm not mistaken, wasn't the drive shaft on F80/F82 M3/M4 changed from CF to steel? Why would they include them in the X3M/X4M if they were removed from the M3/M4?

https://f80.bimmerpost.com/forums/sh....php?t=1411862

https://www.autoblog.com/2017/08/11/...r-driveshafts/

[Max Well]

Originally Posted by kozzi

If I'm not mistaken, wasn't the drive shaft on F80/F82 M3/M4 changed from CF to steel? Why would they include them in the X3M/X4M if they were removed from the M3/M4?

That's my understanding as well, kozzi. I don't think it will happen for the X3M and X4M, but I had never even heard of CFRP's application in a drive shaft so was an interesting topic to read. On the other hand, apparently other Mnfrs have been and are currently using so there must be something to it. Whether reason for discontinuation was related to early wear or other failure issues, or it was cost, or ... is unclear to me, but I would rather see that CFRP used in the roof if I have to make a choice as to where to spend extra money to make the X3M and X4M stand out in their class (still making a case for it)!

[Max Well]

Originally Posted by xlover

I think there is a very low chance there will be a carbon roof for 2 reasons -
1. Functional: The cf roof functional purpose is to grab an extra couple tenths on the racetrack by slightly reducing center of gravity ... the only place the minor weight savings will really matter. But i think bmw would say those who really crave those extra few tenths will just get the lighter lower sedan.

Thanks for sharing your thoughts, xlover.

Historically the majority interested in performance vehicles have focused on coupes and sedans. Rightly so, as it has only been the past 5-10 years Mnfrs began designing high-performance SUV/SAVs. So it seems a natural progression of thinking that reduction of roof weight on SAVs might have similar minimal effects as it has on those cars (with relatively small roof surface areas and short heights).

But I would respectfully suggest placing CFRP on the higher and more expansive roof areas inherent to SAVs provides a far greater benefit over coupes and sedans than you and others may indicate. The problem is that analyses to prove such have been hard to find. I’ve tried to put some of the pieces together - we already know the fore-aft CG of the X3 and can compare to the M550i and M850i. I’ve reasonably estimated the weight of the X3’s CFRP roof by extrapolating known weight/surface area of the E90 OEM solid aluminum and CFRP mod (30.3 lbs minus 12.4 lbs = 18 lbs. savings). Although ETK and RealOEM do not list all weights for the G01’s moon roof components, they do record the weights of the F49 X1 and F15 X5 moon roofs (56 lbs. and 103.4 lbs., respectively). Until we can find out the exact weight this is all we have to go on, so using a value between those yields an X3 moon roof estimate of ~79 lbs. So conservatively it seems a CFRP would yield a savings of 66.6 lbs over the moon roof option, and at the highest point of the 66” high X3. If we could then figure out the height of the CG (a really important variable in calculating Roll reactions), then one could at least run numerical estimates to quantitate and compare possible beneficial effects.

Two articles from Car and Driver helped make clear not only the critical importance of the CG in its relation to performance and handling but also how to calculate CG’s height for a vehicle.

https://www.caranddriver.com/news/a1...ravity-height/

https://www.caranddriver.com/feature...tests-feature/

An excerpt from the second link - “CG height will help you grasp why one supercar is able to trounce a similarly powerful and lightweight rival. Dynamically speaking, it is one of the most important differences between a svelte sports sedan and a hulking sport-utility. Total weight, CG height, and its fore-and-aft location are the Matthew, Mark, and Luke of  handling-performance secrets.”

I would expand your thought that the purpose of CFRP ‘is to grab an extra couple tenths on the racetrack by slightly reducing center of gravity’ further. Hard to see how removing 66 lbs. from the roof shouldn’t also lead to noticeable improvement even in daily driving involving higher speed maneuvers on highways and their curving and high angle on-off ramps, for example. Not in inner city traffic jams, no doubt, but intuitively, it remains difficult for me to believe that a finely tuned X3M Comp wouldn’t notice a difference in handling with 66 lbs. removed from the roof.

Originally Posted by xlover

2. Aesthetic: probably the #1 reason for most M buyers, carbon roof brings the boy racer swagger and style.... let's be honest the roof on the x3 is too high for most people to notice it

I can’t speak for others but that has nothing to do with my decision to want such, and I would hope those seeking excellence in performance engineering don’t base their decision on that alone. If that’s the case there are wraps available to those interested in mainly the aesthetic.

Once the X3M and X4M are released, hopefully Car and Driver (or someone) can calculate the CG height of both moonroof- and CFRP- (if we’re given the chance) optioned models. Or at least run the slalom and skidpad tests – hard to imagine the CFRP won’t have significantly improved performance in those areas (time will tell, but hope BMW at least offers the option).

[xlover]

Originally Posted by Max Well

Thanks for sharing your thoughts, xlover.

(time will tell, but hope BMW at least offers the option).

All of your analysis around the carbon fiber roof being potentially more beneficial for the suv makes sense, in relative terms. and I am always in favor of more selection.

However when it comes to "is it worth the engineering and marketing investment for the majority of consumers purchasing this product" I think my points still hold. Say a carbon roof gives 10% improvement in in handling for an X3 but only 5% for the M3, the M3 will still be the better handling track car in absolute terms. So in my opinion (and i would suspect BMWs product management) if you are the type of person willing to sacrifice the sunroof for performance on a premium vehicle, you are very likely (but not always) the same person who wants maximum performance overall and would choose the sedan (or coupe).
We also know that there is another group who are "casual" M buyers, purchasing for the badge and style, who would go for the carbon roof purely "cuz it looks awesome" and that use case is negated by the SUVs height..

I think you can add on the market expectation for premium SUVs that they not have a sunroof but instead a giant glass roof, it would seem even more out of character to the average X3M buyer to have the solid carbon roof.

like I noted above would love for BMW to thumb their noses at the above logic and offer it as an option for enthusiats who also want the suv

[Max Well]

Originally Posted by xlover

All of your analysis around the carbon fiber roof being potentially more beneficial for the suv makes sense, in relative terms. and I am always in favor of more selection. However ... Like I noted above would love for BMW to thumb their noses at the above logic and offer it as an option for enthusiasts who also want the suv.

Not sure much ‘marketing investment’ is required for placing a CFRP roof as an option in place of a panoramic moon roof. Sure, Engineering would have to tweak parameters after wind tunnel and track testing as the roof would be significantly lighter, but the production process for a CF roof would be straightforward (as is already being done for a number of their offerings), at least for a CF design based on the current construct (but agree it would be more involved if they attempted an aero re-design of the roof). And they now have years of experience, so not like they’re having to re-invent the wheel.

It seems the old-school homogeneity of Performance Enthusiasts as only coupe and sedan Drivers has evolved as high-performance SUVs have been refined and competition expands. Suspect a fair number of us have had our time in coupes and sedans but now appreciate the practicality this design provides, but we don’t want to lose the performance aspect. Manufacturers understand this as they race to capture a piece of this growing profitable pie - with the introduction of a number of elite high-performance SUV/SAV offerings (Lamborghini Urus, Jaguar F-Pace SVR, Maserati Levante Trofeo, Alfa Romeo Stelvio Quad, upcoming Aston Martin Varekai and Ferrari Purosangue …). While I would agree with much of your discussion in regards to ‘premium vehicles’ (which in my mind is just the more mainstream Acura, Lexus, Infiniti, BMW, Jaguar, MB, Porsche… models), I can’t agree that it characterizes those interested in the world of bespoke high-performance (>170mph) vehicles as seen with SVR, AMG, M (esp Comp and above), Porsche’s upper range, and those Mnfrs listed above. Hopefully in that rarefied air the ‘casual’ buyers you describe are few and far between. Perhaps not.

As I’ve not seen studies analyzing the ‘average’ or ‘casual’ M buying population you describe, I don’t know the percentage they represent. Maybe you are right - perhaps company Marketing has shown it represents a sizeable portion which they feel the need to address. I tend to think not (and hope not), but we’ll see how BMW positions the X3M and X4M – we’ll know soon enough.

[kozzi]

Originally Posted by Max Well

Not sure much ‘marketing investment’ is required for placing a CFRP roof as an option in place of a panoramic moon roof. Sure, Engineering would have to tweak parameters after wind tunnel and track testing as the roof would be significantly lighter, but the production process for a CF roof would be straightforward (as is already being done for a number of their offerings), at least for a CF design based on the current construct (but agree it would be more involved if they attempted an aero re-design of the roof). And they now have years of experience, so not like they’re having to re-invent the wheel.

It seems the old-school homogeneity of Performance Enthusiasts as only coupe and sedan Drivers has evolved as high-performance SUVs have been refined and competition expands. Suspect a fair number of us have had our time in coupes and sedans but now appreciate the practicality this design provides, but we don’t want to lose the performance aspect. Manufacturers understand this as they race to capture a piece of this growing profitable pie - with the introduction of a number of elite high-performance SUV/SAV offerings (Lamborghini Urus, Jaguar F-Pace SVR, Maserati Levante Trofeo, Alfa Romeo Stelvio Quad, upcoming Aston Martin Varekai and Ferrari Purosangue …). While I would agree with much of your discussion in regards to ‘premium vehicles’ (which in my mind is just the more mainstream Acura, Lexus, Infiniti, BMW, Jaguar, MB, Porsche… models), I can’t agree that it characterizes those interested in the world of bespoke high-performance (>170mph) vehicles as seen with SVR, AMG, M (esp Comp and above), Porsche’s upper range, and those Mnfrs listed above. Hopefully in that rarefied air the ‘casual’ buyers you describe are few and far between. Perhaps not.

As I’ve not seen studies analyzing the ‘average’ or ‘casual’ M buying population you describe, I don’t know the percentage they represent. Maybe you are right - perhaps company Marketing has shown it represents a sizeable portion which they feel the need to address. I tend to think not (and hope not), but we’ll see how BMW positions the X3M and X4M – we’ll know soon enough.

You make an excellent case. Your post reminded me of a BMW executive "boasting" that the upcoming X3M/X4M would "redefine" the performance standards for SAV/SAC vehicles. I can't seem to find that article/interview but a quick glance back at the BMW-M site mentions such a "marketing" claim.

https://www.bmw-m.com/en/all-models/...w-x3m-x4m.html

"FINAL STAGE OF DEVELOPMENT.
Before the new BMW X3 M and the BMW X4 M are unleashed on the market to set new standards for Sports Activity Vehicles (SAV) and Sports Activity Coupés (SAC), both high-performance models will reach the final stage in their maturity at the Nürburgring. The legendary race track is the perfect venue for BMW M's ultimate testing and tuning programme."

Does the X3M need a CF roof? IMO, I think BMW (CP version) should give its customers the option. Nothing wrong with an "all of the above" approach if you are going to claim setting "new standards" in class. BMW also has no problem stating the new 3 series is a "game changer" that will obviously roll forward into the new M3/M4.

BMW marketing at its best or BMW positioning to be the best? We'll know soon enough indeed.

[Max Well]

Originally Posted by kozzi

You make an excellent case. Your post reminded me of a BMW executive "boasting" that the upcoming X3M/X4M would "redefine" the performance standards for SAV/SAC vehicles. I can't seem to find that article/interview but a quick glance back at the BMW-M site mentions such a "marketing" claim. ... BMW marketing at its best or BMW positioning to be the best? We'll know soon enough indeed.

Thanks, kozzi. I think the interview you reference may have been with former ‘M CEO’ Frank Van Meel in Sep, during which he introduced the X3M and X4M and stated (starting at the 53 sec mark): “Now these cars are going to set the benchmark regarding dynamics, agility and precision, again. And how are we going to do that? …” https://x3.xbimmers.com/forums/showthread.php?t=1533447 We should know after the release whether this was just M marketing. No doubt their Engineering and Design teams are more than capable of succeeding if Finance gave them the green light.

I’ve spent time researching CG height and ‘Static Stability Factor’ (SSF) - lots online about these and how important they are for determining vehicle dynamics (including Roll Resistance). Rather than listing a number of links, just searching those terms yields plenty for those interested. And although I have no background in Mechanical Engineering, I’ve tried to quantify the beneficial effect on these variables if one options a CFRP Roof rather than the Moon Roof. Considering my superficial knowledge of the subject, I would appreciate input from those familiar with the science as to whether my assumptions and conclusions appear reasonable. I’ve attached jpegs showing my thought process in case I am way off base. As CG heights for BMW’s vehicles are not readily available online, I estimated baseline CG height for the X3M (25”) (and for reference M850i (20”) and M550i (21”)) based on information found here: http://www.accidentreconstruction.co...ers%5b1%5d.pdf, Physics of Automobile Rollovers, by L. David Roper, page 6.

To summarize my results, it seems if one could option the CFRP roof, CG height should lower 1”, and the SSF would improve from 1.264 to 1.318 (approaching car values). If accurate, this could potentially allow for significant improvement in handling, especially with cornering and skid pad dynamics.

[Max Well]

Have adjusted the CG and SSF calculations based on the more reasoned estimate of the X3M's moon roof of 126.6 lbs. vs an OEM Aluminum section in it's place of 15 lbs. Because of the slightly wider track and lower height of the X3M, it starts with an SSF of 1.29 (with an assumption the CG with moon roof comes in around 24.7"). BMW doesn't share that data so estimation is all we have to go on.

As expected, replacing the 126.6 lbs. moon roof with a 15 lbs. Aluminum section suggests the CG could decrease by 1.5", with an increase in the SSF to sedan-range values of 1.38. If these estimations and calculations are reasonably accurate, one should expect improved handling. As previously noted, disclaimer - my background is not in Mechanical Engineering or Physics, so would defer to those experienced in these areas as to whether this methodology is sound. I'm just a lay person trying to understand how options can affect aerodynamics and handling. And CFRP would remove an additional ~18 lbs. from the base Aluminum roof as well.

If anyone can share the measurements of the X4's moon roof and actual roof I can run similar estimations for the X4M as well.

[Max Well]

Continuing the sales pitch to Corporate for a CFRP roof option...

[Transfer]

It's a pipe dream at this point.

[Max Well]

Originally Posted by Transfer

It's a pipe dream at this point.

Oh I know. At least for this go-round. Just trying to keep the seed watered for the LCI ...

[kozzi]

Originally Posted by Max Well

Oh I know. At least for this go-round. Just trying to keep the seed watered for the LCI ...

I'm counting on CFRP for future updates. I am going to hold off until December 2019 / January 2020 to see what changes will be offered. August deliveries and reviews will start trickling in a month or so before my order so there will be much more information by then. I plan on purchasing this one so ID7, laser lights and CFRP are 3 options I'd like to check the box for. Regardless of available options in the US, I won't wait until MY2022 LCI.

Originally Posted by PureModelsOnly

Maybe the X3/4 M CS will have one. The M2 CS is getting one

I doubt they will make CS versions of the X3M/X4M but you never know. Maybe an X4MCS? Will certainly be overkill for my needs. X3MC looks to be the sweet spot so I'm looking forward to real world reviews.

[Max Well]

Originally Posted by Max Well

Continuing the sales pitch to Corporate for a CFRP roof option...

Originally Posted by Transfer

It's a pipe dream at this point.

Originally Posted by Max Well

Oh I know. At least for this go-round. Just trying to keep the seed watered for the LCI ...

Originally Posted by PureModelsOnly

Maybe the X3/4 M CS will have one. The M2 CS is getting one

Originally Posted by kozzi

I'm counting on CFRP for future updates … so ID7, laser lights and CFRP are 3 options I'd like to check the box for … I doubt they will make CS versions of the X3M/X4M but you never know …

While waiting for the US website - have wondered what a CS (or just LCI) might add (including kozzi’s ideas).

1-CFRP standard (no rails, no moon roof)
2-ID7
3-535HP +/- 10HP
4-Laser lights
5 +/- adjustable air suspension (? weight difference – if adds ‘significant’ weight, is that offset by improved performance and handling from a lower CG and better SSF?)
6 +/- Bowers & Wilkins Diamond Surround Sound (? weight difference c/w HK – would keep HK if ‘significant’?)
7-With lower CG and improved SSF (if #’s 1 & 5 produce such) can wider wheels be optioned?
8-If #7 is true, wheel wells would be flared more (as a number have already requested)

A rough idea of how that might appear, c/w the original -

[Max Well]

Originally Posted by PureModelsOnly

I really doubt BMW will do a CF roof for the X3. It's simply too large of a piece they'd have to charge a prohibitive cost to make a profit. And no one will see it. B&W sound always reserved for 5/7 er doubt we'll get that in a lowly X3. Air suspension more for cushy ride not for sport intentions.

I know the chips are against us on the CFRP roof, but wanted to at least research the pros and cons and put it out there. The X3 and X4 will be popular M models which reside in the fastest-growing performance SUV size class. With other upper echelon Mnfrs producing elite models in this class, just hope BMW doesn't always view them as the 'lowly' X3/4 and will give them a chance to stay even with the competition. Granted, not the DBX, or the Trofeo, or the Urus, but they should be on par with Porsche. And they won't get there by bottom-feeding the components 'not worthy' of the higher-numbered siblings.

Just to clarify - the CFRP is desired for it's weight reduction at the top of the vehicle where it makes the most difference in CG and SSF (not because it can be seen); and air suspension has been touted by Mnfrs for years as a way in which to improve aerodynamics at speed by it's ability to improve Cd (which is why I mentioned it - not in reference to comfort as you mention, at least for the intent of my discussion).

Unfortunately it becomes a self-fulfilling prophecy if there isn't an upper echelon version one can option into... Wouldn't be cost-effective for them to have fabrication for a small number of parts, I get that. And BMW has obviously assembled market research to know what their demographic is willing to spend. But I suspect there is a subset out there, especially in the M world, who'd be willing to ante up for a refined X3M. In the end it does come down to economics, though, and you may be right - perhaps they don't see a profit in marketing top-of-the-line components to the demographic they're trying to reach for the X3 and X4. For the minority who desire more 'fit and finish', even if BMW is preferred, the boutique Mnfrs are there.

[paliknight]

Originally Posted by Max Well

Today I finally got around to trying to find the thread(s) you implied in your post, glennQNYC. I'm assuming it was the Performance Parts thread ( https://f87.bimmerpost.com/forums/sh....php?t=1492934 )? I spend some time loitering around the M5 Forum but have never been to the M2 series. It would have been nice to have had the CliffsNotes version, as I still don't know what the answer was/is for the M2C and the CF roof, despite reading through 32 pages.

What was the end result - does the M2C get it from the factory, from the VDC, from the Dealer, or ?

Not that it has a definitive bearing on what we'll see, but it might help to anticipate and understand Corporate's (CFO's) directive on how this might play out.

This is a little late, but CF roof on M2C will be installed at dealer for additional cost.

[Max Well]

Badges removed and CFRP Roof:

[Kevin_The_Clean1]

@Max Well - I'm going to submit a request too!