Parachute effect, underside of vehicle, undesirable

[electronchaser]

Not sure how many people have gotten under the car to take a look, but my BEV has a rather large triangular cut out below the motor area all the way to the end of the battery pack. I thought it might be for cooling, then remembered our motors are cooled by sealed looped refrigerant.

Anyway I don't see or need a purpose to this opening, other than letting more road noise reflect into the rear of the vehicle (rear seat area is directly above this opening, and creating an open parachute effect when the air goes in and gets trapped by the bumper.

Trying to find some suitable material (plexiglass type stuff). Anyone suggest any other materials that might be lightweight and rigid enough to use to close off the bottom underside of the i3?

[eNate]

I've seen this opening myself, and wondered the same. However, I can't imagine that the Bavarian Aerodynamikers in charge of this project missed this little detail?

Regarding road noise, I will tell you this: I've driven a few trips with the cover removed, and the electric motor is incredibly loud. Earplug-wearing loud, to be exact. I wondered if, as opposed to keeping road noise OUT, this area was left open so as not to keep motor noise IN.

Whatever you try, be sure the front lip is attached well to something of substance. I drove one of my cars with the front splash guard removed and the piece immediately behind the splashguard became a giant wing with its front edge suddenly exposed, and pulled down at 40 MPH making some sort of "oh sh*t" racket. You wouldn't want to send a plexi Frisbee through the windshield of the car behind you!

[alohart]

i3 BEV rear underside showing large triangular opening:



This has to be intentional. Visible are openings in the panel below the bumper that would allow some air that enters the open triangle to exhaust. It would be nice to understand why BMW didn't enclose the entire underside as with many very aerodynamic vehicles. I've wondered about this since purchasing our new BEV in 2014 but decided against closing it because I don't understand why BMW left it open. Not sure that we will ever know…

[electronchaser]

I've seen this opening myself, and wondered the same. However, I can't imagine that the Bavarian Aerodynamikers in charge of this project missed this little detail?

Regarding road noise, I will tell you this: I've driven a few trips with the cover removed, and the electric motor is incredibly loud. Earplug-wearing loud, to be exact. I wondered if, as opposed to keeping road noise OUT, this area was left open so as not to keep motor noise IN.

Whatever you try, be sure the front lip is attached well to something of substance. I drove one of my cars with the front splash guard removed and the piece immediately behind the splashguard became a giant wing with its front edge suddenly exposed, and pulled down at 40 MPH making some sort of "oh sh*t" racket. You wouldn't want to send a plexi Frisbee through the windshield of the car behind you!

hehehe, yes! Definitely no throwing 90mph plexi frisbees! But I do really think the opening was designed to provide cooling to the ICE and associated components.

Makes sense that it lets motor noise dissipate. However I've already addressed the area with sound treatment, so hopefully noise is absorbed and cancelled instead of getting amplified. I'll report back on findings once everything's fab'd and mounted. Nice thing is the triangle opening has predrilled holes for attachment points for a cover. Just need to leave a bit open for that one engine mount that hangs below the motor.

i3 BEV rear underside showing large triangular opening:



This has to be intentional. Visible are openings in the panel below the bumper that would allow some air that enters the open triangle to exhaust. It would be nice to understand why BMW didn't enclose the entire underside as with many very aerodynamic vehicles. I've wondered about this since purchasing our new BEV in 2014 but decided against closing it because I don't understand why BMW left it open. Not sure that we will ever know…

Awesome picture, thanks Art!
Intentional yes, for ICE cooling. I brought a few friends to get under and check things out with me. We couldn't figure out a valid reason for having the opening. If anything I've found the opposite of beneficial when the opening is left open. Obviously water and debris gets into the 'motor bay' and I've seen countless posts of people finding corrosion in the connectors that go in and out of all the components on the motor.

But when all that air rushes into that opening, and only has a few little slits in the rear bumper to escape from, you create a very beautiful parachute effect. by definition

[alohart]

But I do really think the opening was designed to provide cooling to the ICE and associated components.

That would be a reasonable explanation if all i3's had ICE's. It would have been easy for BMW to have blocked off this opening on the many BEV's being sold, but it didn't happen. So there must be another explanation for this opening.

[electronchaser]

But I do really think the opening was designed to provide cooling to the ICE and associated components.

That would be a reasonable explanation if all i3's had ICE's. It would have been easy for BMW to have blocked off this opening on the many BEV's being sold, but it didn't happen. So there must be another explanation for this opening.

All i3's did have ICE. at least per the original design plans. The powertrain and gen unit were designed to be shoehorned into that spot. Cooling was the biggest challenge. better said porting the air to cool down the ICE power plant. BMW just skimped on adding a cover for the BEV models. There's nothing in there in a BEV model which would require that amount of opening/airflow. If we say to keep the cables/electronics/etc cool, you can accomplish the same thing with an opening 1/10 the size. Besides the motor also 'breathes' from the rear fender areas as well. PLenty of openings and airflow. And isn't the motor also cooled by the a/c loop as well?

More cost cutting. Cheaper to leave it as is.....Same reason they left the speaker connections in the back for the non HK systems.

[vreihen]

One person's parachute effect is another person's low pressure cavity for developing under-tray downforce.....

[electronchaser]

One person's parachute effect is another person's low pressure cavity for developing under-tray downforce.....

wouldn't the faster air, with less pressure creating an area of low pressure between the vehicle and road surface, sucking the vehicle to the road.

Instead, in its current form it creates turbulence and vortices. What a drag...

[jadnashuanh]

The i3 spent a large amount of time optimizing it for efficiency. I'd be really surprised if there's not a good reason for that opening to be shaped as it is. At least according to information I've read, they sweated grams and minimized drag. If there were a significant benefit to closing that off, I think they'd have done it. FWIW, I don't think it was for cost savings, but then, that's me. Other things in the car just don't smack of cost savings without benefits in optimizing the thing for range. Every gram extra you're carrying around requires power to accelerate the car and decreases range. If they determined the drag cost was less than the extra weight penalty, there'd be no reason to close that off. Some complain of lack of power seats. I'm the primary driver, so in nearly six years now, it's only been moved a few times...IOW, no big benefit, and when it did need to be moved, no big deal.

BMW has the advantage of lots of test equipment and a wind tunnel along with CAD and system modeling. Second guessing them may create an unforeseen problem if you close that off. May not notice it in the winter, might in the summer. Closing it off could also trap moisture that could create problems.

[Joff]

I suspect the opening does not create a lot of drag, but I hope one of us will get around to testing a block off panel. BEV owners can appreciate a bit of extra highway range!