Interview with BMW aerodynamicist Hans Kerschbaum

[Piwoslaw]

In the September issue of Auto, Motor i Sport (Polish version of Auto, Motor und Sport) there is an interview BMW aerodynamicist Hans Kerschbaum. I've translated parts as it may be of interest to fellow Ecomodders:

The aerodynamics of cars is too complicated for pure theoretical reasoning, that's why the field is composed mostly of testing and research. Every detail has an effect on every other and there is no sense in changing something in the body, for example, without keeping an eye on its effect on the wheelwells, everything is interdependent. Let's say you start with cleaning up something in the wheels and wheelwells. This effects the airflow under the car, so you have to do some work there. This changes the distribution of pressure behind the car, so you have to do something on the body, which makes you return to the wheelwells. This is what happens every time.

Every car shape can be characterized by three parameters: coefficient of air drag and the lift force at the front and rear axle.

Getting the right lift is extremely hard and has a lot to do with suspension.

Today it is impossible to separate styling and aerodynamics. The interrelations between them are sometimes good, but often contradictory. For an engineer it is important to have good airflow on the corners and sides of the body, control the turbulence at the A-pillars and the mirrors (which are a small catastrophe from the aerodynamic point of view), and make sure that airflow detaches in the right places at the rear. At the same time you must minimalize the amount of dirt that sticks to the headlights and windshield and make sure that noise stays at an acceptable level. Of course, stylists want an nice body with a long front and short rear, because that shape is regarded as attractive and dynamic. On the other hand, it would be much more effective if the front was shorter and the rear longer.

The classic discrepancies between styling and aerodynamics show up in roadsters. The problem with a car such as the Z4 is the terribly short rear, but it's hard to imagine a BMW roadster of different proportions. This is regarded as beautiful, it's enough to see how people look at the car. But the aerodynamicists have a headache trying to raise the rear and shape it to reduce the lift there.

As much as 30% of drag comes from the wheels and wheelwells - this the critical area, still not understood well. One of the funny things is that the drag coefficient of a car is about 0.01 smaller when the wheels are spinning. The basic principle is to leave as little room as possible between the wheel and wheelwell. If you think that this shouldn't be a problem for stylists, then take a look at SUV's...

Another delicate problem is the wheel's rim. It would be best if it could be covered with something flat, but then the wheel wouldn't look so attractive and may have problems with brake heat, especially in the front. That's why we won't see flat, full hubcaps any time soon.

Airflow under the car is responsible for 1/5 of the drag. It should be smooth, so more and more often belly panels are used. Another problematic area is the engine bay. It can't be totally closed off, but there shouldn't be too much airflow either, so the grille holes can't be unnessesarily big. That's why stylist can change their shape, but their area gets determined in the wind tunnel. This illustrates why the same car with different engines may have different drag coefficients. For example, the Cx of the X6 xDrive30d is 0.33, while for the X6 xDrive50i it's 0.36, mostly because of a larger grille, but also because of wider tires. BMW uses venecian-blind style grilles that close when the engine doesn't need cooling. The wind tunnel also dictates where opennings should be placed. For example, in today's cars the air no longer exits around the transmission but through the wheelwells, where there is a lowpressure zone that sucks the air out.

So what does the future hold for aerodynamics? More research into wheelwells, which cause a lot of turbulence.

Air drag comes from:
40% shape and proportions of the body,
30% wheelwells,
20% under the car,
10% airflow through the engine bay.

[orange4boy]

Very interesting. I will have to take another look at my wheel wells. Thanks for the translation.

[aerohead]

The ultimate solution is to forbid the stylist from having any say about the "outside" of the vehicle,but allow them to go hog-wild with the "insides",where they can't hurt anybody.Vanity is folly.----------- Good article,thanks for sharing it.

[MetroMPG]

Thanks for the translation, Piwoslaw.

[Piwoslaw]

I'm glad you all liked it. Even though we EcoModders already knew most of this, it was nice to see it in a popular automotive magazine. Hopefully it'll get someone thinking. I did like how Hans stressed that the wheelwells are an aerodynamic mess and that so far they are a black hole as far as research is concerned. This is all the more reason to skirt them off and paste a "Do not touch" sticker I also hope that closeable grilles will become more popular, now that a major car maker uses them (and not just some fuelsipping freaks ).

[JasonG]

Originally Posted by Piwoslaw

I also hope that closeable grilles will become more popular, now that a major car maker uses them (and not just some fuelsipping freaks ).

Funny, years ago BMW and Mercedes big diesel trucks as well as some Macks used them to help heat-up in the winter. Wonder why they abandoned them?

[RobertSmalls]

Neat, I hadn't heard of Auto Motor und Sport, but it's a pretty good read if you speak German, with way better coverage of green cars than American publications.

They've even got this list of the 80 lowest-CO2 cars in Europe, and only three of them are for sale in America.

I still think of BMWs as wasteful cars with large engines and high price tags, but lately I've come to realize they actually work hard to minimize the environmental impact of their (still very thirsty) cars.

[daqcivic]

Originally Posted by Piwoslaw

As much as 30% of drag comes from the wheels and wheelwells - this the critical area, still not understood well. One of the funny things is that the drag coefficient of a car is about 0.01 smaller when the wheels are spinning. The basic principle is to leave as little room as possible between the wheel and wheelwell.

. . . . in today's cars the air no longer exits around the transmission but through the wheelwells, where there is a lowpressure zone that sucks the air out.

So what does the future hold for aerodynamics? More research into wheelwells, which cause a lot of turbulence.

This was very illuminating. So not only are wheels and wheelwells the single greatest aero factor besides the overall body shape, but the aerodynamics involved are so complicated that not even the manufacturer aerodynamicists completely understand.

Obviously within the wells there will be low pressure. But then the wheels themselves must create high pressure on their front/inner surfaces, and low pressure behind them. Now the mfrs, at least the ones that spend a lot of r&d on aero design, are using the low pressure in and behind the wheelwells to extract hot air from the engine bay. On my observations I see 3 ways they are doing this: (1) Front underbody deflectors in front of the well openings and front-facing tire surfaces create high pressure in front so as to (a) trip the attached underbody airflow past the wells so it doesn't get sucked inside and (b) open up a low pressure zone to extract engine bay (radiator) air. These also extend to somewhat reduce the drag created by front tire surfaces, but I would argue that this is a less significant benefit. (2) Closing up the wheelwell gaps, especially at the fenders and the front underbody, while opening up and radiusing at the rear underbody. This also helps keep attached airflow from being sucked into the wells and allows air to be extracted from them. (3) Stragegically placed vents from the engine bay into the wheelwells. These can often bee seen as louvered vents in the wells directly opposite the front-facing tire surfaces, where the pressure is likely the lowest. I wonder, though, how much air is drawn through since these are at the front of the engine bay, often even in front of the radiator.

Has anyone designed his own underbody mods with these considerations in mind? Has anyone cut the fender lining to extract air there so the underbody can be closed up more?

I've been thinking for so long that short of a kammback the best thing I could do is underbody trays, but it would seem from this interview that the wheelwells are the most significant aspect to focus on.

[candas1]

Thought of this thread when I saw this video ( airflow in wheelwells ) :

http://www.youtube.com/watch?v=PVN9c...ayer_embedded#

[miket]

Originally Posted by JasonG

Funny, years ago BMW and Mercedes big diesel trucks as well as some Macks used them to help heat-up in the winter. Wonder why they abandoned them?

The chevy corvair and original vw beetle did too, however those two were air cooled engines so there was no seperate radiator to block off with a water thermostat, they had to block the air to keep the engine warm.