Metro undertray / belly pan

[MetroMPG]

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 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.

[MetroMPG]

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)?

[MetroMPG]

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.

[MetroMPG]

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.

[Cowspots]

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

[Cd]

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.

[Cd]

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!

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 ?

[MetroMPG]

Originally Posted by .Cd

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.

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

Re: mud flaps - Old picture! 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.

[aerohead]

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.

[MetroMPG]

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.