Quote:
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.
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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.
Quote:
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
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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).