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Old 02-06-2009, 07:36 PM   #1
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Default Underbody Aero Thread (splitters, panels, vortex generators)

Post links to articles and relevant research in this thread.

Been doing research about underbody aerodynamics. As best as I can tell, the problem with miata under-car aerodynamics is that:
* the front lets in too much air underneath the car
* tires exposed in the front to air flow, resulting in more lift and drag
* the bottom of the car is full of all sorts of jutting edges and nooks that air flows in and out of, resulting in huge turbulence
* inside of back bumper is basically a huge forward facing scoop to catch air

Solutions everyone can agree are good ideas so far:
-ISC Racing air dam. Has built in canards and it blocks a significant portion of underbody air
-underbody panels made out of ABS plastic. To smooth airflow
-rear bottom panel to turbulent area between diff and rear bumper

Questions. Assuming the existence of a panel underneath the car:
- Since airflow would have to curve upward from the rear differential to the rear bumper, would vortex generators here be beneficial?
-if VGs succeeded in reducing separation of air past the rear diff, what would be the benefit besides reduced drag? Would you also get less lift because the flow stays glued to the underbody panel instead of becoming turbulent?
-would this create downforce at all?
-I notice a lot of cars with rear diffusers have parallel vanes that keep the air flowing straight front-to-back. What is the purpose of these vanes, besides preventing cross currents and turbulence?
-Assuming I can prevent the airflow from separating, is there a benefit to chopping away the rear bumper so that the rear panel curves more and exits higher than with a stock bumper?
-Obviously I'm going to have to carve out gaps in teh underbody panel for the wheels. Is it worthwhile trying to steer air around these? What are good methods?
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Old 02-06-2009, 07:36 PM   #2
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Paper about vortex generators in a wind tunnel.
http://www.ewec2007proceedings.info/...7fullpaper.pdf
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Old 02-06-2009, 10:14 PM   #3
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Cheap methods of reducing drag for tractor trailers. 30 percent reduction in drag on a truck with a .90 coefficient. It's mostly not applicable to us because we aren't towing a trailer the size of a house, but it's still a good read. Has some odd stuff about using vortexes to generate low pressure zones.

http://www.solusinc.com/pdf/2003-01-3377.pdf

This has a really nice section at the end with diagrams showing what happens when you have gaps in your undercarriage. It is mostly focused on drag rather than lift though.
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Old 02-06-2009, 11:00 PM   #4
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Found on a hypermiling forum. Basically it makes the boundary layer shallower and more stable- basically creating a wall of air that stays attached to the surface even if it would ordinarily separate. Airplanes have been using these for decades to decrease landing/takeoff/stall speed.

VG's for 2.75 each, dunno how many you would need, probably not more than half a dozen.
What are they?: Airtab
Airtab™, Improve Stability, Reduce Spray, Save Fuel
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Old 02-07-2009, 01:45 AM   #5
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Old 02-07-2009, 03:47 AM   #6
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A one man thread, keep up the good work!
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Old 02-07-2009, 12:33 PM   #7
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At Hybridz.org we put pulled money together and rented a windtunnel. We then tried different air dams, vg's, wings, diffuseres, and even cardboard. This is specific to 70-78 datsuns that were designed in late 60's and aerodynamics were crappy back then so our improvements were large in some cases.

However its a good read for general information on what works and what doesn't work. First link was the info and you have to read everypost sorry no highlights. Second link is the data.

Windtunnel Testing the Datsun S-30 Z - HybridZ

Windtunnel Test Data - HybridZ
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Old 02-07-2009, 12:42 PM   #8
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Ok here are the main highlights that i'm going off of memory FYI I haven't read it in about a year.

The best improvement was with blocking off/in the intercooler. Basically you want all the air to go into the intercooler and not around it. So you build walls above, below, and to the side of it. This not only increases cooling but builds high pressure infront of the bumper decreasing drag.

On z's raising the rear of hood up a few inches help reduce front-end lift and reducing drag (a big problem with them) FYI z's have a reverse style hood. The latch is near the window.

VG's help with decreasing drag and increasing your effectiveness of your spoiler. I can only see vg's being installed on a hard top.

All spoilers help but IIRC one of the best spoiler's we found was the cheap ebay ricer adjustable spoiler.

And of course diffusers and belly pans reduce lift and drag. These are very small highlights. If you are serious about aerodynamics read the thread.
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Old 02-07-2009, 01:05 PM   #9
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Good thread... Hopefully a lot more people get involved and pass along some good info.
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Old 02-07-2009, 01:14 PM   #10
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VG's in my research are iffy. Work well in some ap's actually hurt in others. There was a guy with a Mini S in Portland that does a lot of track days. He tried them all around the back of the car like they do on trailers. Besides looking funny they actually hurt performance. BMW did plenty of testing on the Mini and the VG's screwed up the already mediocre aero. Some cars they have helped, some not.

I've seen tests using VG's on undertray's and they've gone both ways. Some helped, some hurt. Every car's aero is so different it is hard to apply what works for one model to something completely different.

The 30% reduction in drag was likely test numbers by the VG manufacturer. I see all sorts of trucking aero devices claiming huge gains. Strangely you don't see trucks using VG's very often. In fact since looking into VG's I've only seen a handful in the last year. A 5% gain in mileage on a semi would be a $4000-$5000 savings on a truck doing 150,000 miles a year. EVERY truck would have them if they really worked that well.
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Old 02-07-2009, 01:18 PM   #11
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Quote:
Originally Posted by cueball1 View Post
VG's in my research are iffy. Work well in some ap's actually hurt in others. There was a guy with a Mini S in Portland that does a lot of track days. He tried them all around the back of the car like they do on trailers. Besides looking funny they actually hurt performance. BMW did plenty of testing on the Mini and the VG's screwed up the already mediocre aero. Some cars they have helped, some not.

I've seen tests using VG's on undertray's and they've gone both ways. Some helped, some hurt. Every car's aero is so different it is hard to apply what works for one model to something completely different.

The 30% reduction in drag was likely test numbers by the VG manufacturer. I see all sorts of trucking aero devices claiming huge gains. Strangely you don't see trucks using VG's very often. In fact since looking into VG's I've only seen a handful in the last year. A 5% gain in mileage on a semi would be a $4000-$5000 savings on a truck doing 150,000 miles a year. EVERY truck would have them if they really worked that well.
On the Z VG's helped the air flow stay attached to the hatch as it went down. I think that is why it helped with that vehicle. So my speculation would be cars with a hatch or sloped coupe should be helped. While a mini or bmw clown shoe (aka m3 hatchback/coupe) would not be.
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Old 02-07-2009, 01:29 PM   #12
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Thanks for participating guys. I'm thinking of cutting and pasting some of emilio's wisdom from the m.net thread on this subject.

My hunch was that most of the non-aeronautical usage of VGs was wishful thinking. That being said, the airplane installed VGs are all tested extensively and do a good job at what they're supposed to- preventing airflow separation due to high angle of attack. So if you have that problem on a car (ie, at the diffuser or at the back of the roofline), they would seem useful. For any other use, they seem dubious and probably ill advised.
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Old 02-07-2009, 07:15 PM   #13
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Interesting, so those hood lift kits may actually have some purpose after all.
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Old 02-08-2009, 03:58 PM   #14
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Quote:
Originally Posted by albumleaf View Post
Interesting, so those hood lift kits may actually have some purpose after all.
On some cars.
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Old 02-08-2009, 06:05 PM   #15
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Here is what I am doing:
-making a frame of 4 aluminum bars (6061 T6) that mount to the PPF and to the two underbody rails that go under the doors. Bars will be 2" x .125" x 48"
-bolting the underbody tray made of 48" x 96 x .125" ABS to this frame as well as any vanes to direct airflow
-the holes in the abs will be reinforced with either steel washers or large rivets and the ABS will be attached to the aluminum bars with JB weld or some similarly strong epoxy. Together, I think that these should actually be extremely strong and hopefully not too heavy
-there will be an aluminum bracket (made from aforementioned bar stock)just inside the rear bumper to hold the diffuser in place. The ABS will be allowed to curve naturally from the rear diff to the rear bumper. Don't know if I need to reinforce this at all beyond the fact that I will be introducing folded down walls to the sides of this curved section (making a 3 wall box basically)
-I will cut and fold down the ABS to form little walls, just before the rear wheel wells. This is to prevent air from spilling into the diffuser area and interfering with its downforce generating properties.

Questions:
-heat from the diff, will it cause problems either for the plastic or for the diff?
-heat from the exhaust, will it cause problems for the ABS?
-the rear suspension arms don't look like they have an enormous range of movement. Can I mount ABS such that it partially covers them without being firmly attached?
-should I go with the heat resistant ABS or just the regular forming grade?


Any advices for forming ABS? Just how soft does it get when heated? If it just becomes bendable around 300F, that is fine. If it turns to goop, that would be bad.
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Old 02-10-2009, 02:03 PM   #16
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Any reason for selecting the plastic over, say, thin sheet aluminum? I'm considering something similar, and always figured I'd use thin aluminum, since I feel that it would be a bit easier to work with. Then again, I've never really worked with ABS plastic like this, so I have no way to compare.
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Old 02-10-2009, 02:31 PM   #17
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Quote:
Originally Posted by the_man View Post
Any reason for selecting the plastic over, say, thin sheet aluminum? I'm considering something similar, and always figured I'd use thin aluminum, since I feel that it would be a bit easier to work with. Then again, I've never really worked with ABS plastic like this, so I have no way to compare.
aluminum would get dented and ugly fast, I think ABS may have more resiliency if it got bent. However I too have thought about using sheet metal.
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Old 02-10-2009, 02:46 PM   #18
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While it may get dented up, it's underneath a low-slung car, so I put aesthetics on the back burner.
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Old 02-10-2009, 03:01 PM   #19
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I would use ABS for the reasons you mentioned. Plus it is easy to work with. You can cut it like butter with a fine tooth circular saw or sabre saw blade. It can be joined together very effectively with epoxy formulated for ABS, or even plastic welded.
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Old 02-10-2009, 04:27 PM   #20
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ABS is fairly cheap (or used to be) and can be rivited together. There's a few threads here about using it to block off the mouth of the car around the intercooler.
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