5th generation civic hatchback - improving aerodynamics
 

So in following the footsteps of the pioneer Basjoos before me, I too am setting out on the road of improving the aerodynamics of a 5th generation Honda Civic hatchback.

Goals are as follows:
1.) Improve aerodynamics from Cd: 0.32 to Cd: 0.26 or better (Honda Insight equivalent)
2.) Materials must be Weather, impact and wear resistant with a (projected) life of 5+ years of normal use.
3.) Overall design of components must be removable, with no permanent changes to the structure of the automobile.
4.) Aesthetic considerations play a role, notably in the S.W.M.B.O. test. (She Who Must Be Obeyed cannot object *too* strenuously to the aesthetics of the modifications).

In pursuit of these goals I plan to implement the following modifications, in roughly this order:
1.) Rear wheel fairings (not the most effective, I know, but the easiest, and most forgiving)
2.)Front wheel fairings
3.)Partial front belly pan
4.)Fiberglas front nose/splitter
5.)Boat-tail

And now for a quickly photochopped picture of phases 1-4
http://i239.photobucket.com/albums/f...r/94Civic2.jpg



Now, to kick-off my little project I acquired a full sheet of 4-5mm (?) thick peice of white one-sided-grained ABS plastic. This actually took about a solid month of trying to track down my local plastics supplier. But today I actually found him and for a mere $75 took home a sheet of awesome white ABS.
http://i239.photobucket.com/albums/f...r/IMG_4869.jpg


ABS seems to me to be the ideal material for making fairings and such because of it's malleability under heat and because it can be "welded" with the right equipment. Luckily my local Harbor Freight store has the welding kit if I ever feel spendy, and they had a heat-gun on sale for $9.99 last week which I fortuitously saw fit to pick up even though I had no ABS at the time.

So I got the first of my aluminum supports for the rear wheel fairings attached to the car tonight. It got dark before I could get the second side on though. Oh well, I have a little time tomorrow. So here's some lovely crappy pics.

Here's the horizontal support brace installed; it's pretty sturdy.
http://i239.photobucket.com/albums/f...r/IMG_4870.jpg



On the back side I've attached it with an "extruded U-clip" to the rear bumper.
In the front I basically attached it with a larger, stainless-steel version of your standard drywall anchor. I unknowingly chose to drill into the body where I can't get behind the sheet metal to get a nut on the bolt. So that's where the big drywall anchors come in.
After having it on the car, I've decided that I'm going to try to find a hinge to put on the front part of it tomorrow. A hinge will greatly facilitate tire removal if (when) I get a flat.
http://i239.photobucket.com/albums/f...r/IMG_4871.jpg



The only other thing I need is a much smaller bracket above the tire to hold up the top of the fairing.

So far, I have spent about $8 on the 1"x 1/8" aluminum bar stock and about $15 on various stainless steel hardware, that seems like a lot for hardware, but it is stainless steel, and I have changed my mind about fasteners about 3-4 times now...:rolleyes:

ok, I got all the brackets for the rear fairings installed. There is one horizontal bar running the length of the wheel well opening on the bottom and a small tab on the top of the wheel well opening approximately between the 11o'clock and 12 o'clock position (from the drivers side)
[IMG]http://i239.photobucket.com/albums/f...r/IMG_4877.jpg[/IMG]


Now I just have some template-finessing to do. The initial templates I made for the rear fairings are a little off. I made them before I had the aluminum brackets made or installed, so they're a bit off.
http://i239.photobucket.com/albums/f...r/IMG_4878.jpg

So I got the rear fairings finished up and installed. They need a little bit more coaxing with the heat gun in places. That and the heads of the bolts and the washers need a good coat of white paint.

http://i239.photobucket.com/albums/f...r/IMG_4879.jpg



http://i239.photobucket.com/albums/f...r/IMG_4882.jpg


For all those interested in using ABS in the future please note that it will discolor if you heat it up too much with the heat gun, so heat up the back side of the piece so you wont see the discoloration when it is installed.

So, first off, I'd like to amend my initial "goals" section.

I've decided that I probably won't be converting Harold to an EV. I just don't know what my driving needs will be in the future, and for any foreseeable EV-type range I'd rather just ride my bike.

So, since I probably wont be keeping this car long-term (and it's not worth much anyway) more "permanent" changes are quite acceptable.

So that brings us to today's installment: Airdam Mania!

I've had a crappy cardboard/white-duct-tape grill block on the car for a while now. It's been great, but I decided to make a more permanent combination of grill-block and airdam.

Sure, this will increase the CdA of my car, but it will also take care of two (or three) nasty areas of aerodynamic drag, thereby decreasing my Cd.

Alright, here's a pic of good ol' Harold with the temporary grill block:
http://i239.photobucket.com/albums/f...IMG_4807-1.jpg


I went ahead and cut a 82" x 17" strip out of my ABS sheet and grabbed a screwdriver and a few self-tapping screws. Here's what I came up with.
http://i239.photobucket.com/albums/f...er/CXfront.jpg

http://i239.photobucket.com/albums/f.../CXprofile.jpg

http://i239.photobucket.com/albums/f.../CXquarter.jpg


I've got a bit of driving to do this weekend for work, so I'll have to see how this new airdam effects my speed on a few "calibrated" hills on my drive.

Awesome. Great to see another set of wheel skirts join the fleet.

I have a theory that rear wheel skirts help more on cars where the back wheels are set close to the back of the car - meaning, there's not enough bumper cover behind them where flow can reattach before turning into wake. Typically that means small cars.

Anyway, very cool work & looking forward to watching the progres.

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Housekeeping question: some of the photo sizes (big) are wreaking havoc in my browser - do you have an intermediate size option you can link to @ photobucket?

images downsized sir. :)

Good work on the wheel skirts. That air dam is huge. I would be worried about scraping going around bends fast or hitting bumps.

Outstanding. I llike the skirt brackets. Are you planning on a mirror delete? Those are huge. Very nice work indeed.

Looks Great! Glad to see you still making aerodynamic advancements with Harold. (and continuing your GS thread here ;)). I am definately interested if the front spoiler affects your MPG or coasting at all. I am thinking about something like that for the UnNamed Wagon. To be honest, my feeble mind didn't even make the connection to 'ajohnmeyer' until I saw these pics.:o

Thanks. There's likely a function to automatically resize giant ones, even if they're hosted offsite (by dynamically setting width="" in the html). I'll have a look and see, as I'm sure this will come up often.

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That air dam is GIANT! I'll be very interested to see how it compares on your "calibrated" hills. I'll also be very surprised to hear that you didn't scrape it :p.

Sounds like you're going to be using the same methods Coyote X did to determine which bumper style was best for his car - comparing terminal coasting velocities on an often used descent.

I'm envious - "testing season" has been finished here for over a month due to the weather.

PS: smooth wheel covers?

Oh, belive me it scrapes! Eventually it won't be quite so giant as it will be ground down.


Terminal velocity coasting results are in and they're interesting.

So my testing hill starts at about a 3% grade for 1/2 mile then goes to a 6% grade for another 1/2 mile. I usually start by killing the engine at the top going 55mph. By the end of the 3% grade section I've usually slowed to 50mph. Terminal velocity at the end of the 6% grade section is just shy of 65mph.

Today I did the same run with the airdam on and got a surprising result. The terminal velocity at the bottom of the hill was 67mph. But the interesting part was that on the 3% grade section I did not slow down from 55mph.

I think the airdam is a keeper. I'll have some tinkering to do in the next few days to get the final form down.

Any chance of an A-B-A? How many screws on that puppy? :)

I might do an A-B-A on Tuesday or Wednesday. It's only got 4 screws in it at this point, not counting the 3 license plate screws (there were 4 but one was so rusted it snapped off)
I'll probably do some A-B-A with the wheel skirts then too. Maybe front wheel skirts if I can get motivated to get them done by that time.

That would be very cool.

So here's the other big question... do you feel self-conscious driving it?

It took quite a while to forget about my car's skirts. Lots of people noticed, and I even got a few quick rolled-down-window Q&A's. And that's just with rear skirts!

People seem to take notice of the airdam much more than the skirts. I think it's because the airdam is frickin' loud when it scrapes, that and it's a huge white expanse of plastic. I think it gets the attention of the younger guys b/c of it's almost "racer" appearance.

About a week ago I got the first comment about the skirts from an old guy in a store that I frequently work in. I was passing him in the stock room and he stops me "Is that you're little white car out there?" me: "the civic? yeah." him: "those wheel covers are really neat looking!"

I can't help thinking that they're much more likely to be appreciated by the older crowd, those that grew up around all the Cadillacs, Buicks, Oldsmobiles, etc that had rear wheel skirts from the factory.

So I finally got some motivation to do some more work on the car today.

I took off the airdam and shortened it about 2" so hopefully that will resolve the scraping issues I've been having.
I also took the opportunity to add some front wheel spoilers to the airdam.

Here it is all trimmed up with the wheel spoilers, off the car of course.

http://i239.photobucket.com/albums/f...r/IMG_5081.jpg


And here it is on the car.

http://i239.photobucket.com/albums/f...r/IMG_5084.jpg


And one detail shot of the wheel spoiler.

http://i239.photobucket.com/albums/f...r/IMG_5087.jpg


I also deleted both side mirrors, but it was very much dark when I got them done, so pictures will have to wait until tomorrow.

Nice to see you working on refinements.

Any news there? (Clearly I'm a sucker for the data.)

If not, I'll settle for qualified subjective observations :p.

No, still no A-B-A but I'll try to squeeze in some testing on my next day off. I think my data gathering is moving at a Forkenswift pace. ;)

Mirrors.....DELETED!!
 

And here are the mirror delete pics, used a couple of round blind-spot mirrors that I got for $1.45 at the local autoparts store for interior side-mirror duty.

So here's the deletion, I'm kinda in the middle of getting the hardware situation ironed out, need to get that shiny stuff painted black.
http://i239.photobucket.com/albums/f...r/IMG_5152.jpg


And the gory details of the (new) mirror and it's mounting bits

http://i239.photobucket.com/albums/f...r/IMG_5153.jpg

The Data is in.....
 

So I actually got my lazy butt off of the couch today and went out and did some coastdown testing.
Here be the results.

:turtle:Methodology:
using Iwilltrys' instructable "Measure the drag coefficient of your car"
I used a camera mounted on a tripod in the backseat to record the data points. I used my remote shutter control to take pictures of the speedometer every 10 seconds as indicated by my stopwatch.

Frontal area was measured using this technique but with photoshop's histogram tool instead of GIMP.

:turtle:The Raw Data:

Vehicle Weight (with occupant) 2300lbs/1043.26kg

Frontal Area 1.768 m^2

Test run-----V1-----V2----V3----V4-----V5----V6
0 sec---------55-----55----55----55-----55----55 mph
10 sec-------49.5---51-----51----50----49.5--50 mph
20 sec-------44-----45.5---45----44----43----43 mph
30 sec-------38-----40-----40----38----37----38 mph
40 sec-------33-----35-----35----33----32----33 mph
50 sec-------29-----30------------28----27----27 mph
60 sec-------25--------------------23----22----23 mph
70 sec------------------------------------19----19.5 mph

:turtle:The Results

Well, here's where it gets tricky. I used the solver function in the excel spreadsheet, but it keeps giving me really strange numbers, impossible numbers. So I tried my best to eyeball the numbers to get all of the data points to line up, here's what I got.

--------Solver-------Me eyeballing it
Cd------0.062-----------0.20
Crr-----0.0218----------0.018

Perhaps someone who has done this before can lend a hand?

Hi AndrewJ,
MetroMPG contacted me through the Instructables site and asked if I would comment here since I created the spreadsheet you're using. Unfortunately, the lower your Cd is, the less accurate the spreadsheet is since the contribution of Crr dominates the results. I punched in your numbers and the data comes out almost linear which means there isn't enough change in drag force during the test to accurately isolate the contribution of Cd from that of Crr.

The easiest way to improve results would be to determine Crr from a different test and enter that value directly. Then run solver again, changing only the Cd value to match the model to the data.

To measure Crr directly you can use a spring scale (50lb capacity should be enough). Measure the force required to pull your car at a constant speed (a walking pace) on level ground. Crr = Frr/M/g where Frr is the force in Newtons (1lb = 4.45 Newtons), M is vehicle mass in kg, and g is 9.81. Measure the force to pull the car forward in one direction, then turn the car around and measure the force pulling forward in the other direction. Use the average force to calculate Crr. Drive the car for a while first to warm up the drivetrain.

Hope that helps.

One additional comment (hopefully not too critical ;)) now that I've read all the other posts. You said in a prior post that the air dam would increase your CdA, but would decrease your Cd. I just wanted to clarify that CdA is what you should actually be trying to decrease (not Cd). Drag force is proportional to CdA.
Cheers and good luck.

Thanks for clearing that up! Well, looks like I have more testing to do, and a spring-scale to locate. As for the CdA/Cd thing, I basically have no idea what I'm talking about :D, that's why I have all the other smart folks on here to keep me straightened out.

Well, maybe I don't need a spring scale after all. Perhaps a humble bathroom scale will do the trick if I push the car from behind. In that case, I'll have the Crr isolated by the end of the day!

Can't believe my 2 brain cells that are awake thought that up so quick. Alternately, I can't believe it took so long to think of that. :turtle:

Good catch, IWillTry.

And thanks for signing up to clarify the coastdown testing with your spreadsheet also.

I don't know if you have a stat counter at your Instructable on this subject, but if you do, I bet it's seen a lot of traffic in the past week.

A bathroom scale may or may not work. Some scales will only work in a horizontal orientation. They also can be difficult to balance on a car bumper (depending on the car). If you do use a bathroom scale, be sure to "zero" the scale while it is in a vertical position to calibrate out the weight of the scale platform. Good luck.

The Data is in: part II
 

So, using the bathroom scale trick (zeroed at vertical) I found that my car takes 35lbs of force to keep it moving on a level surface.

*a note to people who try this method*
smoothing out your stride seems to be an important part of getting an accurate measurement. Just pushing normally produces quite an oscillation of force, making the measurement "jump around"
So focus on the dial of the scale and smooth out your power delivery to keep the dial steady.

So, 35lbs = 160.2 newtons, so on an so fourth. Voila! Crr for my car is 0.0169

Plug it in to Excel, set the solver to solve for Cd (cell F41 only) and it spits out a Cd of 0.194

Hmm, still seems low to me. If the "stock" car is 0.33 and I managed to knock that down to less than 0.20 with a mirror delete, airdam/radiator block , and rear skirts? Seems unlikely.

I think I'm gonna take all the aero goodies off the car tomorrow and run another set of coastdown tests in "stock" form. That way I'll have some idea of how accurate the 0.194 actually is.

Interesting update. *applaudsyourtenacity*

Clarifying some terms. Cd, A, CdA and such.
 

What I *should* have said was:
"Sure, this will increase the frontal area (A) of my car, but it will also take care of two (or three) nasty areas of aerodynamic drag (Cd), thereby decreasing my coefficient of drag (CdA).

Basically the coefficent of drag is CdA which is comprised of the Cd multiplied by A. See wikipedia for further detail.

So the big idea behind airdams is that they will improve your aerodynamics IF you can improve your Cd without adding to much A. If you tip the balance the other direction, by adding a lot of frontal area without streamlining the shape much then you will actually be hurting your aerodynamics.

This is why an airdam like mine would actually be a bad idea for basjoos's CX. He has a full belly pan, so adding an airdam wouldn't improve his Cd any, and it would add a bit to his A, thereby raising his CdA.

As for the original post, "You have to hear what I'm thinking." :D

The testing drags on....
 

I went out to do some more testing today and while I didn't get to do all of the testing I had planned to do, the intrigue continues.

I did 2 of 6 runs of coastdown testing in "stock" form to verify that the Excel spreadsheet was giving me decent numbers. By comparing the Cd that the spreadsheet gave me with Honda's Cd of 0.33 for a stock 5th generation Civic hatchback, I can see how far off (if any) the numbers in the spreadsheet are.

Now, I only got to do 2 runs, one in each direction, before I had to quit due to the sudden appearance of a fierce crosswind. I didn't want my data corrupted by any wind, so I just used those two sets of data to plug into Excel.

I know, I know... two points of data isn't much, in fact, it's probably safe to consider it flawed.

But then again...

Guess what Excel came up with based on my 2 data points? :confused:

Cd = 0.33 :eek:

Grrr....more testing....damn wind. :mad:


Given the windyness situation I headed on over to my "calibrated" hill for a little session of A-B-A. The hill is usually windy, but today it was on the leeward side of the wind, so it was unusually calm on the downhill side.

I brought the car to 55mph at the top of the hill, and at the same point in all 3 runs, cut the engine reset the trip odo. I then kept track of speed and at what distance on the trip odometer those speeds were reached. The runs ended when I coasted back down to 55mph at the bottom of the hill.

---------------------------A(stock)----B(aero)---A(stock)
Initial speed-------------55mph-----55mph-----55mph
Slowed to----------------52mph-----55mph-----52mph
Miles@lowest speed-----0.375------0-0.4------0.375
Top speed---------------66mph-----71mph-----65mph
Miles@top speed--------1.1---------1.05-------1.05
Miles@55mph (end)-----1.35-------1.45-------1.25

Also, on the "B" run, 66mph was reached at 0.75 miles.

Looks like this aerodynamics stuff might actually work ;)

Haha, interesting results, ;)

Stick with it though, I'm glad you have the patience to be so methodical about it, :p

Welcome to the club!

Nice work. I envy you people and your winter time testing weather.

In my opinion, an airdam is just a band-aid to use if you have a aerodynamically dirty underside you want to divert air away from. In a max FE application, we're not overly concerned about generating added downforce, which is the other main purpose of an air dam. On my car, I have a totally smooth underside and the only "airdams" on my car are the two air diverters (wheel spoilers) that sit ahead of my front wheels to split the airflow around the wheels. This split airflow is maintained via the double side skirts to the rear wheels and then ideally (future project) should have a small boattail fin behind each rear wheel to reduce its eddy. If you look at the Loremo and the Daihatsu UFEIII, they have airflow splitters in front of each of their front wheels and the UFEIII has a small boattail behind each rear wheel. If you insist on going with an airdam, you can make it a variable height airdam that is deepest in front of each wheel and highest in the section between the wheels. Some airdam-equipped Audis and BMW's go this route.

I agree that the airdam isn't optimal FE-wise. However, I'm finding that it is a really nice thing to have as part of a work-in-progress car.
I will be modifying the airdam further as I continue to work on the car, eventually resulting in it being just a grille block/wheel spoiler type thing.
But for now, while I won't be able to do a full bellypan anytime soon, it's helping to staunch the aero losses.

I like the mini-boattail idea behind the wheels, I may have to work that into my car somehow. I've been thinking of trying sideskirts next, maybe work them into the mix there?

And after re-reading IWillTrys post I think I'll have to look for a new place to do my next round of coastdown testing. It looks like the tests might be more accurate if I start out with a higher top speed, where the areo-drag is more pronounced.
My current testing circuit is a 50mph speed limit, and right next to the Highway patrol headquarters. Probably not wise to speed ;)

basjoos: How did you get your Cd numbers? Any tips?

The full belly pan/undertray thing is not that hard to do. You need a couple 8*4 sheets of 0.6mm aluminum, probably some sort of heat resistant padding to stick in the middle somewhere. Jack stands, your regular jack, a friend, a drill, screws with big heads, some metal shears and about 5 hours is all you will need.

Airdam or Airdamned?
 

One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.
So far we know (at least in theory) that a bellypan is better because of the lower frontal area, and also the probable decrease in Cd from all the hanging bits on the underside.

But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study
"Hoerner 1958 discusses the underbody as the biggest single avoidable
aerodynamic drag component and gives an old example where drag goes
from 0.3 for a smooth underbody to 0.6 for an open and rough as
customary car!"

Still no word on how much an airdam would knock off that 0.6

So, I hope eventually when I do get myself a jack, some jackstands and a new (working) drill I can get a belly pan together to test it out and get some numbers.

This list of things to do is getting quite long. :(

Well, one thing Phil Knox has said in terms of the lowest extension of an air dam is that it shouldn't go lower than the lowest hanging suspension/underbody bit, or else you're just adding frontal area at the expense of Cd.

That said, your tests so far, and Coyote X's, seem to suggest that more can in fact be better.

I'd be wary of relying on 1958 data. Cars back then would have had MUCH worse undersides (basically being body-on-frame truck like construction) than today.

Here's a couple more modern references:

• Ford's hydrogen fuel cell Bonneville racer was lowered and had a full undertray added. They claimed a 0.01 reduction in Cd with the pan

• Audi improved the Cd of the A4 with a patchwork of panels applied to the underbody. The sum difference was Cd -0.019

Hoerner Dr.-Ing. S. F., Fluid-Dynamic Drag, 1965
 

Technically, Hoerner is referencing "Sawatzki-Weiss, Autom.tech Zts. 1941; (2,d)" in Figure 9 (page 12-6) that is paraphrased in your qoute (assuming I'm reading the bibliography correctly). . The 0.3 is for a "completely smooth model" while the "1/2 faired (Tatra or Volkswagen)" is Cd = 0.5, and "Open and rough as customary" is Cd = 0.6. However, I don't think these are meant as overall coefficients. Instead he (seems) to reference the "completely smooth" (with no cut outs for turning of the wheels) is worth a DeltaC_d_o of 0.3 over the open/rough bottom.

Hoerner doesn't describe airdams in particular, but the paragraph on "Cooling Air (also page 12-6)" he seems to blame most of the cooling drag on the turbulence of the air exiting the bottom of the engine bay. There is a factor of C_D_box = 0.4 where "box" represents the "circumscribed area" of the open radiator with that number coming from Kamm's "Automobiltech Zeitscrift 1939, page 447". He also mentions achieving thrust from the cooling air via the proper "exit nozzle" and "energy transferrred to the cooling air by the engine fan (and some heating.)" Which may be harder for us to realize with fans no longer being attached to the crankshaft.

I heard this recently went out of print, so I'd go find a used copy if you can find one.