Metro undertray / belly pan
 

03-29-2006, 08:28 PM
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started working on my prototype front undertray this evening. spent about an hour under the front of the car - box cutter, cardboard and duct tape, of course http://www.gassavers.org/images/smilies/smile.gif

i'm optimistic i'll see a real improvement on this car because unlike the hondas, the metro has NO stock front undertray of any kind. all the guts of the engine bay (oil pan, transaxle, cat, control arms and stablizer bar) just jut down into the breeze, naked.

i stopped by the home depot to see if i could order a black sheet of coroplast, but they can only get it in white & off white. so white it will be. the rear panel will be visible from behind, thus the aesthetic considerations.

in the end, i figure i'll have:

- one piece in the front, from the bumper to the stabilizer bar (about in line with the rear of the front tires);

- in the middle, possibly 3 individual panels running lengthwise: one on each side of the car and maybe an aluminum panel to cover the center tunnel & the exhaust - though i haven't decided for sure if i'm going to cover the exhaust pipes

- one rear panel from the rear control arm area back to the rear bumper

one particular challenge will be whether i can do a proper test to compare the effects of the tray vs. none, since it looks like it will be a pretty labour intensive process to attach & remove it (not to mention designing & making it).

don't have any pics yet, but i'll post some when i'm a bit further along and have something to show.

04-02-2006, 11:01 Pm
 

the front undertray section is installed... cardboard prototype. judging by the amount of cardboard i've used so far for the mock-up, i think i'm going to need another 1/2 sheet of coroplast to complete the "production" belly pans.

on a related note, my neighbour replaced a blown household hot water tank this weekend, so i may have an abundant supply of light sheet metal (his old tank's outer casing) for the parts of the middle & rear undertrays that will cover the exhaust.

i've got a bunch of short highway trips coming up in the next couple of weeks, so i'm motivated to get with the aero improvement program.

04-03-2006, 09:48 Pm
 

some photography of interest:

http://metrompg.com/offsite/undertray-none-front.jpg

above: naked blackfly belly. note the complete lack of OEM front tray of any kind. the control arms, exhaust, oil pan, stabilizer bar all dip down into the flow. now you see 'em ....

http://metrompg.com/offsite/undertray-front.jpg

now you don't! cardboard prototype front undertray installed. mostly duct taped on, but some zip ties and a few screws were used (not visible). it's actually quite secure and firm.

note it overlaps the control arms. the closest attachment points are far enough ahead and behind the control arms, that the tray can move as they do.

i trimmed the tray to permit tire clearance in full-lock with about half an inch extra.

http://metrompg.com/offsite/undertray-front-side.jpg

middle undertray in place. looking forward from the middle of the car. for testing, i plan to cut away the cardboard near the exhaust and tape aluminum foil in its place, leaving a few small openings.

the middle section is done as of tonite. another hour or so will have the rear undertray mocked up, then i just have to wait for a good opportunity to do some test runs, then yank it all off on the side of the road and re-do the runs naked for comparison.

04-04-2006, 10:37 Pm
 

well, screwed up the undertray test today.

i got in two bi-directional runs (of a planned three) with the tray on, when at the end of the 2nd run I heard some cardboard dragging. the rearmost panel had come loose and was hanging down across the width of the car "scooping" up air.

i don't know how long it was detatched since i didn't peek under the car from the time i left the house until the end of the 2nd run when i heard it hitting the ground.

so i pulled all the undertray off, hoping the rear panel only came undone at the end of the last run and that the full set of undertray runs wasn't affected.

i did 2 bi-dir "naked" runs for comparison, which produced very similar numbers:

52.93 mpg avg with the undertray (2 bi-dir runs)
52.38 mpg avg no tray (2 bi-dir runs)

speed was 55 mph.

Conditions at SUPN6 as of (1:00 pm EDT) 1700 GMT on 04/06/2006:
Wind Direction (WDIR): SSW
Wind Speed (WSPD): 6 kts
Wind Gust (GST): 7 kts
Atmospheric Pressure (PRES): 29.87 in
Air Temperature (ATMP): 41.2 °F

all of which means either the undertray has a very small effect on my car, or it was hanging down on both bi-dir runs.

of course, the only way i'll know for sure is to do it again http://www.gassavers.org/images/smilies/frown.gif crap.

04-06-2006, 01:59 Pm
 

in hindsight, i should have just re-attached the rear tray and done another run before pulling them all off.

to be honest, it didn't occur to me that it might have been hanging down the whole time affecting all the undertray runs until after i had removed them all.

big, self-administered dope-slap

09-07-2006, 12:04 Am
 

STARTED OVER...

With coroplast. Front sections cut & installed.

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Front portion of the undertray is in place & fastened. I got most of the center section done this evening too.

I - gasp - drilled holes in the underbody to fasten the coroplast. I think that's the real reason I've been procrastinating. I just feels so wrong to drill underneath - thinking of rust.

Once all the panels are in place, I'm going to do a B-A test, where I do some runs with the full tray, then remove it on the roadside and repeat the runs. I'm not doing A-B-A. Too complicated.

After the test, I'm going to have the car professionally undercoated and then put the panels back on.

The trickiest area left to do is the rear control arms. They hang down LOW into the flow - they're located about at the halfway point between the road and the bottom of the rear bumper. I'm thinking I may try to use the coroplast to extend them backwards in a teardrop shape. Complicated, but they're definitely a drag problem the way they are now, all blunt and exposed to the flow.

09-07-2006, 12:27 Am
 

http://metrompg.com/offsite/rear-suspension.jpg

Shows the rear control arms. The angle of the photo actually doesn't show how far down below the floor they really are.

I obviously can't shape them as a teardrop. What I meant was I will try to taper them upwards towards the rear bumper, not exceeding that all-important 10 degree angle.

Anyone want to make any guesses on what the tray is worth @ 55 mph? After seeing the effect of the unloaded roof racks on the top of the car, I'm cautiously optimistic that the tray could be significant - I'm smoothing out some serious "excrescences".

09-08-2006, 08:07 Pm
 

Spent a couple more hours on the tray this evening. Mostly, I'm just laying on my back looking at stuff under the car, thinking. I'm shockingly slow at this project.

At least I've gotten over my aversion to drilling screw holes.

Anyway, the rear portion is in place. The hard part was coming up with a way to extend & taper the control arm surfaces upward toward the bumper and still allow flex in the tray to move with the suspension.

And it's as if the designers were waiting for me and my coroplast: guess what the angle is from the point between the control arms up to the bottom of the rear bumper...

10 degrees. http://www.gassavers.org/images/smilies/wink.gif

Still have to sort out some finishing details. And replace some of the coroplast with aluminum in the areas around the exhaust.

---

Spent a couple more hours on the tray this evening. Mostly, I'm just laying on my back looking at stuff under the car, thinking. I'm shockingly slow at this project.

At least I've gotten over my aversion to drilling screw holes.

Anyway, the rear portion is in place. The hard part was coming up with a way to extend & taper the control arm surfaces upward toward the bumper and still allow flex in the tray to move with the suspension.

And it's as if the designers were waiting for me and my coroplast: guess what the angle is from the point between the control arms up to the bottom of the rear bumper...

10 degrees. http://www.gassavers.org/images/smilies/wink.gif

Still have to sort out some finishing details. And replace some of the coroplast with aluminum in the areas around the exhaust.

09-10-2006, 09:42 Am
 

Here's some info to use for an edumacated guess:

According to: http://www.tfd.chalmers.se/~lelo/rvad/reports/rva2002_gr02_underbody.pdf

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

http://metrompg.com/offsite/audi-a4-with-Cd-deltas.jpg

Assuming my continuous smooth under tray achieves at least that difference in Cd on the Blackflea, the rolling & drag spreadsheet tells me I should see a FE improvement of 4.6% @ 20 C @ 55 mph

EDIT: that's a slightly conservative estimate, since the Audi calcs apparently do not include the large front panel.

09-10-2006, 08:11 Pm
 

Recorded a couple of runs this evening with the undertray on.

This really didn't help much, as you'll see in a minute.

Conditions:

Observed at: Grenadier Island 10 September 2006 7:00 PM EDT
Temperature 14° C / 57 F
Pressure/ Tendency 102.7 kPa↓
Humidity 61 %
WindNE 7 km/h

km/h ... avg mpg (US)

70 . . . 70.0
80 . . . 65.2
85 . . . 61.9
90 . . . 57.0

(Bi-directional averaged runs - one average per speed.)

But what do I compare it to?

1) If I look at the last time I was on the test course, it was for mirror testing. The test speed was 88 km/h. Temp was 67.6 F / 19.8 C.

88 . . . 55.2 / 56.5 - with full mirrors / pass. mirror off, driver's folded back

(Bi-directional averaged runs: 3 runs / 2 runs. These runs were also done on the same tank of gas I'm still on.)

If that's all I look at, it would appear the tray effect is good news. But. ..

2) If I look at the last time I did runs @ 70, 80, 85 & 90 km/h, it was after I changed transmissions. Temp was 73.4 F / 23 C.

70 . . . 76.8
80 . . . 65.6
85 . . . 62.2
90 . . . 58.2

(Bi-directional averaged runs - one average per speed.)

... which would lead me to conclude my MPG has dropped with the new tray!

3) Testing the partial boat tail @ 88.5 km/h / 55 mph, the temps were much warmer: Temperature ... 29°C / 84 F

88.5 . . . 61.2 / 59.9 - with boat tail on / off

(Bi-directional averaged runs - 3 runs / 2 runs.)

So!

I think all this tells me is it's impossible to compare things on different days and temperatures. A-B-A is best, but at a minimum, it needs to be an A-B done immediately before/after.

I'll try to do temperature compensation calcs later, and see what that says, but the other thing I need to consider is that (a) I possibly made things worse by adding the tray, or (b) it didn't help any!

09-10-2006, 10:52 Pm
 

Subjectively... does it coast any better??

Can't really comment on the coasting. I'd like to say it coasts longer, but that may be the placebo talking http://www.gassavers.org/images/smilies/smile.gif I don't have a regular route with "benchmarks" I can compare it to.

I may get a chance to do the A-B before I go. I don't think I'm leaving until the end of the week. I figure it will take me around 10 minutes to remove the 30 or so screws and take the tray apart/off at the side of the road.

The only thing I'm wondering is whether I should do (1) one A&B bi-dir run for a range of different speeds, (2) or stay at one speed, and do a number of A's and B's.

I'd prefer to know the difference at several speeds, but (2) generates more reliable results.

It takes a long time to do these runs.

09-12-2006, 03:23 Pm
 

Excellent weather today (little wind), so took a couple of hours off and did 6 bi-directional runs @ 90 km/h (56 mph).

3 with, 3 without the undertray .

57.78 mpg (US) - with undertray
56.55 mpg (US) - without undertray

1.23 mpg difference, or 2.2% increase with tray

Not really happy with the results. It was a lot of work for such a small gain. But them's the numbers.

A question for the aero enthusiasts: my car rides quite high off the ground - 6 inches or maybe slightly more. Would an undertray help more on a lower vehicle (like the A4, which probably has 3 or 4 inches of clearance)?

09-12-2006, 03:53 Pm
 

Just noticed something:

2.3% = gain from outside mirror changes
2.3% =gain from partial boat tail prototype
2.2% =gain from undertray

Edit: also tested

2.8% =gain from wheel skirts*
2.9% =gain from grille block*

* winter testing - denser air - also I tested at 95 km/h, whereas the ones above were between 88-90 km/h.

02-17-2007, 03:15 Pm
 

Undertray fell off today. Well, I stopped and pulled it off before it was torn off. Guess I didn't really try very hard to attach it properly (wasn't thinking about snow & slush accumulation in the open portion of the wheel well).

EDIT: I only had the front 1/3rd installed after the experiment. So now it's totally naked underneath.

OK, I am looking at your mods here. My Metro Aerodynamic targets will be as follows:

1) Block off grill area and seal around or replace the headlights with flush type.
2) Taller 14" vs. 12" tires and wheels w/stainless steel aero-disk hubcaps.
3) Underpan of aluminum sheet metal or Coroplast. (Prefer metal)

Thanks for the ideas and I'll remember to get pictures!
Cowspots

Glad to see some new improvements Darin ! I'm actually quite surprised that you hadn't had a bellypan installed already.
One thing that I'd really like to ask you about is those mudflaps.
Why do you have them mounted backwards ?
Shouldn't they be mounted in FRONT of the wheels :) ( To act as a wheel spoiler ... of coarse :) )

Besides, the way that they are now, I would think that they would do more harm than good.

This seems to comfirm my suspicion that I had regarding coast down testing as being highly sensitive to subtle variations.

I'd also like to ask how that you go about finding the current atmospheric pressure since it fluctates from area to area. ( For instance, I could check the local weather on the 'net and see what the current pressure reading is, but that would be at the weather station. To have an exact reading you would need to actually have a barometer with you at the time of the test .... right ?

So is a hill really needed to do a coast down test, or can you do one on a flat road ?

Not new improvements, note the date written in post#1: 03-29-2006.

Re: mud flaps - Old picture! :D I've since lost more vanity (wanting to keep the side of the car clean), and the fronts were removed. The rears remain on the car because they're part of the wheel skirt mounting method.

Re: pressure readings - it just so happens the online weather station is within about 1 km of the road I use for testing.

Re: coastdown testing - normally it's done on a level road (if you're trying to use it to back calculate Cd). Hills can be useful just for comparing relative differences because the longer the car coasts, the more accurate the data will be.

Blackfly underbelly
 

Darin,I envy the big-boys,with their climatic wind-tunnels, that can control every variable so as to quickly know what their "tricks" are doing.Back-to-back runs are our best attempt,and even with those,the "environment" can be "drifting".It's very frustrating.In Hucho's book,theres a compilation of results a guy put together on bellypan results,however a lot of this work was done before carmakers started putting front airdams on production cars.It's easily possible,that Suzuki did such a good job with the original design,that they already got the low-hanging-fruit under the car.I do know that all truly low drag form cars use bellypans,and that should you pursue even lower drag,it eventually must become part of the equation,to enable the most energetic flow to the wake.And as low temps affect lubricant viscosities,rolling resistance,pumping losses,air drag,etc.,it may be that as we swing into the summer months,that you may see a upswing in performance.I beat the T-100 to death through one winter trying to get results from it's bellypan,and was only rewarded later,the following spring,when the results finally showed up on the radar screen.Winter testing is a bugerbear! I don't think you'll regret your under-car efforts.I remain only comfortable with long-distance testing for my stuff,as,with the OBD-1,I don't get the luxury of the Scangauge.

Thanks for the feedback, Phil.

The undertray will re-appear on the car at some point, but my efforts will be on the rear of the car again before the bottom.

As for the ScanGauge issue, I know you tend to stick to just the Aero forum here, but there may be good news coming down the pipe in that respect: there's another active thread going on in the Instrumentation forum where members much smarter than me are discussing building an electronic ScanGauge equivalent that would work in your truck (and any vehicle with fuel injection & a Vehicle Speed Sensor). I'll keep you posted.

2% from the bellypan. I'd say that sounds about right. I couldn't really feel a difference on the highway.

When I first got mine installed. I drove around the neighborhood half terrified that my car would burst in flames. Not because of the cost or possible face distorting burns but because I worked so long a failure would have seriously hurt my pride and proved me wrong.

I think the reason I was most disappointed was due to figures I'd seen about other vehicles in "the literature" - the Cd gains claimed from a smooth underside were far more than I experienced.

Anyway, it'll be back on the car. I just have to decide if I'm going to do the lowering first. I think that makes sense, as the control arms' positions will change relative to the car's floor, which will affect the tray design slightly.

So, what's the story with your front air dam? I saw in another post you mentioned you've since removed it. Feel like starting a thread?

I posted a thread about it over at GS. I can post it over here also.
Basically it didn't work.

Please do.

Did I read this right Darin ? Are you going to lower your car ?

:thumbup::thumbup::thumbup:

( P.S. I just removed my entire belly pan from my car, as well as my grill block. )

Yeah, I've got a set of front springs from the Blue ForkenSwift car (it's currently wearing the stiffer springs from the red car). The blue springs should be less stiff than the ones in the Firefly (lighter car), plus they've also got 186,000 km on them (vs. 16,000 km), so I'd expect them to lower the black car without having to cut them. The rears however aren't the same between the 2 body styles, so I'm not sure what I'll do there. But I'd like a 2 inch drop.

Unfortunately, this is a mod that won't be easy to properly compare before/after.

Why have you removed your stuff?

Excuse me for resuscitating an ancient thread (I'm reading up on belly pans in preparation for my next mod...).

Darin, did you ever reinstall your belly pan? Swap out springs?
What are your current thoughts/conclusions?

I know this thread is dead, but this will probably be my first mod. Any recommendations about the cheapest place I can pick up some sheet aluminum? I KNOW its not Home Depot or Lowe's

-shakes fist- darn you, you overpriced superstores

Maybe this google search is useful?
Richmond, VA Aluminum Steel Wholesale - Google Maps

Regarding barometric pressure readings:

When you call your local airport or TV station for barometric pressure, please keep one thing in mind....

These readings are all 'corrected' to sea level.

For example, if your TV station says the current barometric pressure is 29.85 inches Hg, they are 'correcting' the pressure to what the TV station would see IF they were actually at sea level in elevation.

Now if your current elevation is let's say, 1000 feet, then you SUBTRACT 1.0 inches Hg from the TV station reading, so your actual barometric pressure would be 28.85 inches Hg, at your current location, assuming you are also close to the TV station. If you happen to be in Denver, then you subtract 5.0 inches Hg from whatever is broadcast on the news in that area, since Denver is at 5000 feet above sea level.

Hope this helps, Jim.

http://metrompg.com/offsite/audi-a4-with-Cd-deltas.jpg

Anybody know more info on this audi? Specifically im looking at the very small panels located behind the front tires rated at a decrease of 4 counts of drag for each side!

Theres got to be more to those than what the pic suggests.

I know this sounds like a lame idea, but I'll post anyways - I will sometimes go by car dealerships and act interested in a certain car, with the real intention of just seeing the underside. :D
If you can find a used A4, just act like you want to see if it has any leaks and climb under there.
Post pictures if you take 'em !

Whoops - super delayed response.

No, I never did either. But both of those are still on my to-do list. (Which never seems to get done...)

If you're good, you can get them to put it on a lift. Don't ask on a busy day, though. Just tell them that you want to get a good idea of what under-car maintenance will involve.

Why not just tell them the real reason you want to look under the car? - to check out the aerodynamics because you know it affects fuel economy.

If enough people did that, the word might get back to car manufacturers that some people car about this stuff & it could potentially influence your decision in buying a new car (well I'm sure it would if you were in the market for one).

Hi. Does anyone have a template for an undertray for a 96 firefly? Thanks

Hi. I didn't make a template for mine. But it's not that hard to just get underneath the car and figure it out.

The area that needs the most creativity is the rear suspension, since the control arms stick down into what would otherwise be relatively clean airflow. And there's not much distance to taper back "up" before you get to the end of the car. (Plus you don't want to taper up at too aggressive an angle or you cause flow separation anyway.)

Nice thread. It looks like the potential gains from a full belly pan are modest, but it appeals to me as it is the only aero mod I can think of that is more or less invisible. I don't want my car to attract too much attention. (Well, removing the longitudinal roof bars is an almost invisible mod, but that probably won't make much difference to drag at all.)

The longitudinal bars aren't going to make enough difference to get outside static noise, but it will help unless they serve some other innocuous function to aerodynamics [there's a car that removing the outside mirrors actually makes the aero worse, etc].

On some things, you have to choose between utility and economy to some extent. If you never use the roof rack bars, remove them. If you honestly will use them at some point, leave them there. They ain't gonna hurt anything.

Belly pan is a good mod that is, as you say, basically invisible. It will also help with long-term resale value in some cases, if that's a concern to you, since not only does it protect the engine bay from underneath, but also the entire underside of the car, from dirt, rust, and corrosives.

Wheel spats are also 'basically invisible' because they install under the car behind the tires, and a modest lowering and/or a front 'lip' on the bumper also provide reasonable gains for little expense and will be 'basically invisible'. Between a modest lowering, a belly pan, wheels spats and maybe going up one size on tires [due to reduced aero load] you may see up to a 15% overall gain in fuel economy [depending on the driver and overall configuration, of course] without any/much expense and very little 'noticeablility' increase.

The reason for going up a tire size is because you're reducing aero load and your car was designed with a certain tire size both for aesthetics and to account for average load while cruising. You've altered that load, so your engine will be operating in a less efficient range of the BSFC map while cruising. Going up a tire size will increase load without increasing rolling resistance [generally speaking], and also fills the wheel well slightly better, for added aerodynamic benefit.

Going up a tyre size is an intriguing idea. Of course that would put the speedometer and odometer out, but perhaps the different tyre sizes can be compensated for in the ECU options? (via VAGCOM or similar?) My tyres are all new though, including my winter set, and are all of the standard size, so I won't be making any changes there for quite a while.

I've had this car for two years and have never used the roof bars yet, and I don't own a roof rack (attaching luggage to the longitudinal roof bars without transverse roof bars is pretty difficult) so my guess is I will never use the roof bars. I didn't in my last car either and I had that three years.

I reckon it would be possible to remove the bars in a temporary and easily reversible manner by cutting neat round holes in the head lining (with a sharpened length of pipe in an electric drill?) to access the six securing bolts, and using PVC electrical insulating tape of a matching colour (black) to cover the six holes in the top of the roof. (I reckon that covering the holes with tape will make corrosion on those holes LESS likely than having the holes hidden under the roof bars, where corrosion can develop undetected.)

As you say the difference that removing the bars will make to mpg will be real but probably unmeasureable. The non-estate version of the Golf has no such bars so I doubt they contribute in any way to aerodynamics.

I'd also like to try a front air dam, as it seems that can have a surprising effect on mpg, but I am nervous that it would look too weird. That's certainly less work than a full belly pan.