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Old 05-10-2020, 12:09 PM   #101 (permalink)
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With a newfound interest in improving the car (due to the demonstrated efficacy of throttle stop testing), I think I will be putting time into undertray cleanup.

While the car is generally pretty flat underneath, there still appears to be a lot that could be done to smooth flow up front, around the suspension, and the huge triangular gap under the motor area. (The heat exchangers are in the front and only the heat of the range extender exhaust system seems like a legitimate concern.) I honestly think BMW was just being cheap on all counts.

I'd really like to get some professional looking rear skirts done (not down with the coroplast look), but I think the undertray work will be a more productive and easier area to work on.

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Old 05-10-2020, 02:08 PM   #102 (permalink)
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Questionable measurable improvement I have not tested, and I doubt I could even measure it reliably, but really low hanging fruit that may quiet wind noise further.
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Old 05-10-2020, 03:50 PM   #103 (permalink)
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That underbody looks pretty reasonable already, but yes, I can see some room for improvement.
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Old 05-10-2020, 08:57 PM   #104 (permalink)
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A little more gap sealing completed. The OEM design has virtually no seal for the hood to the body beyond the width of the kidney design, so you can see in the first picture here that enough of a gap existed to see completely through to the other side of the car. Unless there was some airflow consideration for the windscreen or cabin air intake involved, I can't imagine why they would have done this.

I used two separate strips of garage door soft butylene seal to close it up.
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Old 05-10-2020, 10:54 PM   #105 (permalink)
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Quote:
Originally Posted by Snax View Post
... I'd really like to get some professional looking rear skirts done (not down with the coroplast look), but I think the undertray work will be a more productive and easier area to work on.
Before making skirts, build your own temporary version and do some A-B-A testing. If you can demonsrate the benefit of wheel well skirts on your car, the incentive is that much stronger. The effect of skirts MIGHT be large enough for the throttle stop technique. Gove it a try.
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Old 05-11-2020, 01:01 AM   #106 (permalink)
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Yeah, I think somewhere back in this thread I have posted pictures of skirts I tried and dust flow testing on a dirt road. The skirts do seem to improve attachment, so probably worth the effort at some point to do right.

I am leaning toward doing some front half fender gap eliminator skirt testing before I jump into that though.
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Old 05-11-2020, 04:32 AM   #107 (permalink)
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Quote:
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I would be hesitant to guess that. I came across this passage in one of the textbooks I read earlier this year and underlined it since I was surprised by it:

"The front wheels contribute the bulk (about 65%) of the total wheel drag in cars because the rear wheels are typically in the wake of the front wheels, only experiencing some of the vortices generated by the front wheel interaction with the car-body air flow, and the lower edge of the trailing stream flow from the front wheel (Fig. 5.8)" [Obidi, T.Y. Theory and Applications of Aerodynamics for Ground Vehicles, (Warrendale: SAE International, 2014), 122].
Just an aside, that is the second-poorest aerodynamic textbook in my entire collection. I am amazed the SAE published it. And, given its price and publisher, I had such high hopes!
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Old 05-11-2020, 09:38 PM   #108 (permalink)
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Quote:
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Just an aside, that is the second-poorest aerodynamic textbook in my entire collection. I am amazed the SAE published it. And, given its price and publisher, I had such high hopes!
Why don't you like it? This is one of the few aerodynamics books I do have.
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Old 05-11-2020, 10:00 PM   #109 (permalink)
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Quote:
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Why don't you like it? This is one of the few aerodynamics books I do have.
It's just distressingly vague and unhelpful.

Take Pages 128, Fig 5.15. Cutting off the tail of the aerofoil does not increase drag only if separation was already occurring at the marked point. The two paragraphs that purport to describe what is going on are just terrible. For example, "the goal of bobtailing is to keep suction in the wake small" is wrong. The goal of bobtailing, as shown in Fig 5.15, is to not bother extending the car past the natural separation point. As the next para says, in each case drag will be the same - a neat contradiction in just two successive paragraphs.

It's all like that. A mishmash of stuff, with very few references made to published papers on real cars (and so having real life examples).

I think a much better overview is provided by Barnard, Road Vehicle Aerodynamic Design.
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Old 05-14-2020, 03:20 AM   #110 (permalink)
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Quote:
I am leaning toward doing some front half fender gap eliminator skirt testing before I jump into that though.
Google fails to find the Edgarwit™ air curtain thread.

With an extruded aluminum section, you could crimp the trailing edge to give it a curve to match the fender gap.

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