Metro undertray / belly pan

[MetroMPG]

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

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.

[MetroMPG]

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.

[MetroMPG]

some photography of interest:



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 ....



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.



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.

[MetroMPG]

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 crap.

[MetroMPG]

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

[MetroMPG]

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.

[MetroMPG]

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".

[MetroMPG]

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.

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.

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

[MetroMPG]

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



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.

[MetroMPG]

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!