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Old 08-22-2009, 08:46 PM   #1 (permalink)
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A Not So Perfect Belly Pan.

arg. Spent most of the day struggling to fabricate a rear belly pan. In the end it is mounted secure and clear of the rear axle, exhaust, etc.

Worst part was having to work under the car like this. Tiny garage, no ramps, wtf.



Anyway, Here is what got put together. Only addition today was the 1/16" aluminum sheet. From the rear:



From middle of car:



I was initially pretty unhappy with how the piece has to dip down and then back up to clear the rear axle. Was hoping it could be straighter than that. Got annoyed thinking about how it would create lift and maybe some drag at the front half where it forces the air down.

But then looking at it from the rear of the car:





It doesn't appear to hang any lower than the front skid plate, so maybe there isn't too much penalty? What do you guys think? Would the drag from forcing air down on the front side be equally negated from the subsequent attached (presumably) expansion on the rear side? I am rusty on my fluids/physics.

I took a quick road trip just to make sure it didn't make any "lightning" type noises, from the thin panel flopping around but it seemed totally silent. (it was freakin loud in the garage when shook!) Couldn't gauge fuel economy benefit yet but the handling definitely felt a little different. Hard to say. Once I get a week of commuting in the effect should be clearer.

So, curious if you guys think this will help my FE or not.

Before shot:


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Old 08-22-2009, 09:02 PM   #2 (permalink)
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Damn ...... wow.

wow.
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Old 08-22-2009, 09:56 PM   #3 (permalink)
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You and Hermie both are now at risk for flying out of control up and off the road. :P
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Old 08-22-2009, 10:02 PM   #4 (permalink)
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Hi,

You need to look at air getting above the pan at the axle area. My Coroplast rear pan was parachuting, and actually hurt my MPG.
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Old 08-22-2009, 11:12 PM   #5 (permalink)
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there is guages of steel structurally counting on the air hitting it to maintian integrity, stauc disperse, or whtever else is hidden in ther imperfections.. A young car won't notice it for sometime. aluminum is a conductor, there is static from fuel flow, agitated by unpainted exhaust. my lessons are learned from a lightning strike on an old subaru, more than once..the same ridiculously engineered area. Aluminum was a good choice. I bet struts get squishy in the back in warmth, first thing to notice, as my guess. the openness my even be less of a drag, allowing for engine heat expansion, hitting parts that are always busy to get it out of there (exhaust, wheels, rear end)
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Old 08-22-2009, 11:50 PM   #6 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
You and Hermie both are now at risk for flying out of control up and off the road. :P
Uhh, my lift-reducing mods will keep me glued to the road, thank you.



Your underside is already smoother than mine. My exhaust pipe hangs down a bit. I need to get my hands on some sheet metal.

Looks good, though, but I'd replace the cardboard sometime. XD
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Old 08-23-2009, 12:38 AM   #7 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hi,

You need to look at air getting above the pan at the axle area. My Coroplast rear pan was parachuting, and actually hurt my MPG.
Good point Neil. I had a similar problem with my pan around the same place. I found out the hard way because it came loose and started flapping around under there.

( Another thing I screwed up on was when I had my front end pan too close to the suspension pieces. . It tore my front end boots on the CV axle thingee as I turned the wheels.)
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Old 08-23-2009, 04:32 PM   #8 (permalink)
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Thanks for the comments everyone; appreciate your input.

Frank, lol. Honestly in my short test drive the car felt more planted. But we'll see.

Neil, good call. This is a situation I am aware of and concerned about. My pan is really anchored and stretched about as taught as I could get it. So I don't expect it to deform or parachute at speed. That's the main reason I went on my test drive though, to listen for it flopping around. Hopefully I would hear it over road noise. If I had a camera I absolutely didn't care about I might think about strapping it under there... But even in its static shape it could catch some air pressure, so I might look into making some small side panels to somewhat seal it off.

bgd, I don't really follow. I don't think any of the parts hanging out under the car rely on air flowing over them for structural integrity.... static dissipation was not something I considered though, by any means. Your sube got struck by lightning multiple times?!? Please elaborate on what you did so the rest of us could avoid such a ridiculous situation!

Really don't expect the rear struts to warm up appreciably from this mod though. The panel is only 30" wide, a good foot away from each strut. The exhaust runs pretty cool under cruising conditions, after the manifold, turbo and catalytic converter suck most of it up. Being a diesel, and with my aero mods, I'm not requesting a whole lot of power most of the time anyway.

I think this will be the last major mod for the underside. Its pretty smooth now in general. The only other thing I am thinking about is adding a little lip spoiler up front to let less air get under. The curved noise of my car kind of shoves a little more under there than is needed, I think. Maybe this thing:

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Old 08-23-2009, 08:02 PM   #9 (permalink)
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The popularity of air dams to extend the rear low-pressure zone under the car leaves people confused and doubtful about the use of a smooth belly pan to create a venturi effect. If you achieve high speed flow over a surface, such as a rear axle dip, the surface pressure goes down.
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Old 08-23-2009, 11:00 PM   #10 (permalink)
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Hi,

Even if the pan doesn't move or flex, if the air gets above it, it probably will cause drag. You can try to block up the leading edges, so that air cannot flow up above it.

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