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? -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?

Paper about vortex generators in a wind tunnel.
http://www.ewec2007proceedings.info/...7fullpaper.pdf

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.

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

delete

A one man thread, keep up the good work!

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

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.

Good thread... Hopefully a lot more people get involved and pass along some good info.

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.

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.

Interesting, so those hood lift kits may actually have some purpose after all.

On some cars.

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.

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.

While it may get dented up, it's underneath a low-slung car, so I put aesthetics on the back burner. :D

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.

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.

+1 for ABS. Easy to cut. Easy to form with a heat gun. Easy to glue with ABS pipe glue. Good stuff for the DIYer that doesn't have a great shop to work with metal. You can easily make your own inlet or outlet vents/louvers in the ABS to move air in or out to help with the cooling issues.

yes, but dents are less aerodynamic than a smooth surface

Tell that to a golf ball ;)

(couldn't find any Homer Simpson speed hole pictures... so you get this.)
http://dotnetworkaholics.com/images/...rdFishCar1.jpg

It's the car from Swordfish....is that John Travolta's very own?

yup! I read about the aero of a golf ball a few years ago it was fascinating. I'm not going to pretend to be an expert or anything like that, but I seem to recall that inducing micro vortecies on the surface makes for a turbulant boundry layer. That boundry layer will consequently stay attached further around the trailing edge leaving a smaller wake and thus less drag.
very cool stuff.

So who's got a hot car with a junker body on it? Ball peen that biotch everywhere. Let us know about the results!

yes, but the space shuttles have flat surfaces, and they go really fast :dunno:

Maybe dimples only help golf balls because they spin?

I think you are correct, maybe the dimples help it grab the air. Kind of like stitches on a baseball.

agian, not an expert. The stuff I read though implied that it was only benificial to keeping the boundry layer attached on a steeply dropping off trailing edge. it was also implied that the resulting smaller low pressure wake was the only reason that drag was reduced. so the increased drag on the leading edges was more than offset by the smaller wake produced.

I'm going to make an assumption here and it may be off base, but I'm pretty sure that it's correct. if you look at the MR? version of the evo 8 there are vortex generators along the top of the steeply sloped rear window. I believe these were put there to keep the boundry layer attached further down the window to adjust the wake in such a manner to make the rear wing more effective.

I saw a subaru today that looked like it had a small wing at the very edge of the roof-line, but it was parallel to the back glass, like it was trying to catch air coming over the car, push it down the back glass, and into the spoiler

All the 05 or 06+ STis have that spoiler on the top of the rear window. I'm sure it's there for the same reason the Evo9s have the vortex generators on their back windows. To smooth the airflow out to get less choppy air behind the window and over the rear spoiler.

laminar flow has a lot of shear in it. it's like a deck of cards being slid on a table--the top one moves over the next one and so on. turbulent flow (at high reynolds numbers) can actually work more like little vortex ball bearings and lower the flow drag of a body.

the stitches on a baseball are to hold it together. but! they are very tightly regulated on official balls. same number and spacing requirements... they do make a difference to pitchers, but they are a design consequence, not the other way around.

golf balls do have dimples to reduce drag. but they also have them to induce lift... backspin creates a pressure differential from top to bottom and helps the ball maintain hangtime.

supersonic jets also have devices to create turbulence along the wing surface.

and check this out... even super efficient cars steal this technique.

http://www.got.net/~davidbu/davidbu/sparrow/costco.jpg

damn hippies and their carrots...

it is difficult to find pictures of the bottom of the LMP1 cars, I have a link at the house that I will put up where someone got pictures of the cars up in the air in the paddock, showing all its underbits :)

hippies dont shop at costco...

note that the dimples on that car are on the trailing edge of the fender only. very cool.

I just bought an Elise mid body undertray and rear diffuser. Aluminum parts. The undertray has Naca ducts in it for transmission and exhaust cooling. Going to try to retrofit these to the Miata. The parts used were cheaper than buying aluminum sheet to fab myself! Here's a pic over at CR that shows what the diffuser looks like on a Miata.

Attachment 207518

You've never seen a fast honda? They normally look like golf balls.. :bowrofl:

Where'd you manage to find that? And how much ducket?

^^ agreed.. How much for the lotus piece?

Lots of good info/links in here, subscribed.

The lotus diffuser is around $300 new at a dealer. (suprisingly cheap for a Lotus factory part) Used they seem to run from $50 to $200 depending on condition, greed, need to sell. I paid $100 each for the diffuser and the mid/rear undertray. The undertray has several naca ducts to direct air up. Lotus does this as it's rear engined. I like the idea to help with cooling the rear diff/exhaust.

Parts haven't arrived yet. Should be hear in the next day or two. As lazy as I am it' likely to take me 6 months to actually install them.

You want a thread with lots of info? Here's one to add to your favorites...

MX-5 Miata Forum - wings and splitters?

Massive thread at m.net that's been running for a couple years. Lot's of good info from Emilio and others out there running the tracks hard.

Can be found here:

Elise SC Rear spoiler (Elise S2) [ESCspl] - €280.00 : Elise Shop, Performance parts for your Lotus Elise


Edit: nvm saw cueball's post, much cheaper at the dealer.

They seem to pop up used fairly regulary on the lotus forums. I just missed a $60 one with minor damage. Ended up buying a $100 unit in good shape. The two parts arrived this morning. Here they are...

http://i184.photobucket.com/albums/x...e/diffuser.jpg

http://i184.photobucket.com/albums/x.../undertray.jpg

why didnt you just use this as an undertray?

Miata BeatRush Aluminum Under Panel 1990-1997

Holy god damn sonofabitch! I have been waiting years for someone to find a relatively affordable rear diffuser of high quality, i.e. an oem part, that translates over to the miata fairly well. IT HAS BEEN FOUND! I would never have thought to check prices on the Elise's unit as I would have assumed it would be $1k.
That is the shit.

Let's get some clear info on how mounting was dealt with, what was fabricated to mount these parts, how that center section covers the rear underside of the miata etc. etc. etc!

-Ryan

Who wants pics of my "undercarriage?"

The beatrush part is a front undertray. I'm DIYing an ABS splitter/ front undertray. The lotus parts I bought are a rear diffuser and mid body undertray. The undertray pictured goes just ahead of the rear diffuser. In fact the 5 holes at the front of the diffuser mate with teh 5 at the rear of the undertray. Those cutouts on the sides are for the rear wheels. Didn't know if I could make it work on the Miata but thought it was worth a try. About the same price as just buying aluminum sheet and this included 3 naca ducts.

Attachment 207398

fabbing up some little blocker plates on the sides to shroud the front tires. 5.5" or so out from the bumper.

Sav - do you have a decent source for ABS?

I've always wondered if propping the trunk open a bit would create the same effect as a rear wing, is that what that red miata with the diffuser did? I would be worried the drag created by letting the air in the trunk would negate any gain from the upwards angle.

No, that's 12mm birchwood. $36 for a 4x8 sheet at Home Depot. TAP Plastics wanted like $130 for a 4x8 sheet of 1/4" ABS (the thinnest I would consider using).

no.

How did you attach it?
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I can't bring myself to use wood on the car...

I think I can get an ABS sheet for $100...

Ripped out the front bumper crash structure and attachment points, then took 1" square aluminum and went down to the R-package lip, then a stretch across the front and back at 45 degree angles to the edges of the lip. Two small supports from the edges up to the frame rails as well. It attaches to the stock plastic undertray location in the back (couldn't reach all the way to the crossbar)

Ryan, you'd be stunned at the number of professional racecars using wood splitters.

side spats:

Attachment 207397

http://img.photobucket.com/albums/v3...r/DSCN1348.jpg

(4052)BLACK ABS 1/4" THICK 48" X96" SHEET IN THIS SALE! - eBay (item 190289521949 end time Mar-27-09 07:45:47 PDT)

Should be cheaper than that other ABS and more than needed.

I know... I kicked the shit around with a guy with a VERY fast civic at the Superstreet Time Attack down at Buttonwillow for a while and his car had a plywood front splitter... I just... it's wood... now I get the cost factor - this guy was sponsoring himself hence the plywood... but then walking over to theTwins Turbo supra that was sitting next to it and chatting with the twins... you got a feel for the difference in quality in the cars, even though the two were almost the same in times. But yes, wood is the poor man's splitter material, and it is plenty effective.
-Ryan

Have you shipped a 4x8 sheet of .250" ABS across the country recently? :2cents::2cents::2cents::2cents::2cents::2cents::2 cents:

now its a track car.

it's real easy to get free composites when you go to an engineering school. just call the big companies and ask for their reject lots. you'll have a 30 yard roll of carbon at your house in a week.