[Max Well]

New as a Registered Member to this informative community, but we've been reading for some years now. We're trading in our 2011 X3 xDr35i ZAP, ZCV, ZPP, ZTP for a '15 xDr35i ZMM, ZDA, ZPP, 5DL, 609, and 688. This will be our tenth BMW (three 3's, three 5's, one 7, and two X3 xDr35i's), but our first with the M sport package. Although BMW indicates the aerodynamics are wind-tunnel tested, we haven't been able to find technical data to indicate if the Cd (drag coefficient) with the M Sport and its aerodynamic package is lower than the base design (at highway speeds). We take a fair number of long-distance trips and are curious if anyone is seeing improved mpg and/or higher speed performance c/w the base, or is it just a great-looking X3? Thanks.

[FSociety]

I think the M-Sport pkg is probably more aesthetics than anything (anyone please correct me if I'm wrong) but the DHP (Dynamic Handling Package) is what makes the difference

[BMW-X]

Originally Posted by ROBNYC

I think the M-Sport pkg is probably more aesthetics than anything (anyone please correct me if I'm wrong) but the DHP (Dynamic Handling Package) is what makes the difference

I agree, it's mostly preference of what one looks better, rather than functionality. Someone could argue the vents let more air in. But even if you ran the damn thing on a track I found it hard to believe it would make a significant difference in performance. The Dynamic Handling Package is what really makes the difference, which can be purchased with the Xline as well. I myself, got the Xline with DHP because I like the way it looks better.

[Max Well]

Thank you for the replies. After 2-3 years experience with our X3 xDr35is in a number of highway conditions, we're averaging roughly 24-25 mpg, which is not bad for what this SAV brings to the table (handling, carrying capacity, performance, and to a certain degree, off-road abilities). But when I noticed the new X5 xDr35i, which weighs roughly 550 lbs more and has a larger frontal surface area, is actually rated slightly higher in Hwy fuel economy (27 vs. 26), I started trying to learn about the complexities of automobile design and the forces involved. It seems BMW's engineers must have come upon some important design changes for that model allowing for this seeming improvement from its 2011 EPA est of 23 Hwy.

Fluid dynamics (and Physics for that matter) is way over my head, but still intriguing to read about, specifically in regards to our BMWs' engineering. A Motor Trend report in June 2009 discussing the 750i xDrive in wind tunnel testing mentioned aerodynamic resistance of the bodywork 'accounts for only about 40%' of the total, with the wheels, wheel wells, underbody, and internal air flow accounting for the rest.

It will be interesting to see design changes for the third generation X3. If it follows the X5's (and other models') path, it seems aerodynamic efficiency will undoubtedly be further refined.

[FSociety]

Originally Posted by Max Well

Thank you for the replies. After 2-3 years experience with our X3 xDr35is in a number of highway conditions, we're averaging roughly 24-25 mpg, which is not bad for what this SAV brings to the table (handling, carrying capacity, performance, and to a certain degree, off-road abilities). But when I noticed the new X5 xDr35i, which weighs roughly 550 lbs more and has a larger frontal surface area, is actually rated slightly higher in Hwy fuel economy (27 vs. 26), I started trying to learn about the complexities of automobile design and the forces involved. It seems BMW's engineers must have come upon some important design changes for that model allowing for this seeming improvement from its 2011 EPA est of 23 Hwy.

Fluid dynamics (and Physics for that matter) is way over my head, but still intriguing to read about, specifically in regards to our BMWs' engineering. A Motor Trend report in June 2009 discussing the 750i xDrive in wind tunnel testing mentioned aerodynamic resistance of the bodywork 'accounts for only about 40%' of the total, with the wheels, wheel wells, underbody, and internal air flow accounting for the rest.

It will be interesting to see design changes for the third generation X3. If it follows the X5's (and other models') path, it seems aerodynamic efficiency will undoubtedly be further refined.

That is indeed a very interesting research and thought you have going, indeed very intriguing!

[Max Well]

Thanks, Rob. There is a fair amount of information online, most of it above my understanding. But it seems one of the formulas for air resistance against a vehicle as we drive is - '1/2(Cd)(A)(rho)(V squared)', where Cd is the drag coefficient, A is the frontal surface area, rho is the density of air, and V is the velocity [I think I have that correct]. Since the velocity is squared, one can see why the resistance increases so much as we drive faster, and why more horsepower is required to reach top end speeds in less aerodynamic models.

For a given vehicle type, engineers have to design around certain height and width parameters (and length, but that is independent of the front areal calculation) depending on their vision of the vehicle's purpose. So by definition, an X3 is going to have a much different frontal area than a 3 Series. Variations within certain margins may occur as the model type matures with successive iterations, but it must be difficult to make wholesale changes to 'w x ht' for a defined series without its purpose changing.

Since Designers can't affect air density or how fast we drive (except for the speed limiters), that leaves the drag coefficient as a prime target for their aero expertise. This doesn't even consider other areas of energy loss they can exploit (from drive train, etc). BMW's website provides a great short course on some of these aerodynamic issues they are focusing on, such as the 'active rear spoiler', 'air breather', 'air curtain', 'air blade', 'aero wheel rims', and 'air vent control' ( bmw.com/com/en/insights/technology/efficientdynamics/phase_2/technology/aerodynamics_2013.html ). It appears the new X5 has incorporated at least some of these into its design, and judging by their new hwy numbers, they must be on to something.

So if these new aero-modifications (I guess not the type to be found in our M sport 'aerodynamic kit' currently) can add 5 mpg Hwy to our X3 35i (improving from 25 to 30 mpg), that would extend our useful range about 90 miles, and on a 900 mile round trip, we would use 30 gal instead of 36 gal. And that would be without reducing HP, interior room, performance or handling it seems. Not sure they will do a mid-cycle design change, but if wind tunnel testing on the '3X3' (third gen X3) confirms similar improvements, it seems likely they should continue to own a reasonable share of this market.

[Booty]

What he said ��

[Max Well]

After finally reaching an elementary grasp of the equations, a spreadsheet was made today for most BMW model types (2 Series through X), so a rough idea now provides as to how Cd, front surface area, and weight influence horsepower (HP) requirements at defined speeds. If interest is there, a summary table showing est HP needs for speeds from 20 mph through 150 mph in 5 mph increments can be shared. And its been interesting changing the variables to see which ones matter most at different speeds.

This is a rough approximation, but after running checks on a few racing websites where unseen equations provide HP results for weight, Cd, frontal area in square feet, and speed, this estimation doesn't seem that far off. Assumptions have had to be made, though, with the major one being the Area (and also the rolling friction to a lesser degree). Although some sites provide Cd information, Area figures have been difficult to find. As this is not about encroaching upon BMW’s trade secrets, but rather trying to better understand how our BMW's operate, those numbers were est by taking greatest frontal 'width x height' measurements as found on BMW's site multiplied by 0.85 as some racing sites recommend.

This is simply about trying to better understand the dynamics of what we drive.