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Old 01-12-2009, 02:15 AM   #1 (permalink)
Fit
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Question Honda Fit Belly Pan Project

This is my first Ecomodder post and I am hoping for a little imput from those who may be more in the know than I.

I recently constructed a belly pan from 0.080" sheet aluminum with some help from my dad (who has a better stock of tools at his disposal). The project took one weekend including time gathering the ~$90 in parts and brainstorming.

The front half is modeled after the Kamispeed/beatrush belly pan used by many other fit owners out there and the back half was freelance and is really just our best guess at good areodynamics.

Here is where I need some input!!!
You will notice in one of the side shots that the trailing edge of the rear section is adjustable. The rear section could probably be raised 1.5" or so and lowered as much as another 0.5". I suspect that my maximum gains in aerodynamics will be found with the trailing edge a bit higher. In the current configuration, the trailing edge is maybe 2" lower than the front edge and possibly causing a high pressure pocket as the air gets channeled into a increasingly narrow area.

Would I see results by raising it or am I better off keeping the trailing edge low in an attempt to funnel air away from uncovered parts further to the rear (muffler, exhaust, etc).







Gains over the single (yeah, lots of error with just one data point) tank of gas indicate about ~2 MPG gain. Eager to hit 40+ MPG as the engine gets conditioned, synthetic oil replaces dino, and summer gas and temps push up efficiency.


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Old 01-12-2009, 04:46 AM   #2 (permalink)
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Hi there! I can't tell you much on the aero aspect (someone else will chime in I'm sure) but I just wanted to say welcome to the site and that's a great looking ecomod you've got going there,
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Old 01-12-2009, 07:59 AM   #3 (permalink)
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that looks very professional, much better than my effort!

as far as the shape goes there's a number of things you could try, but all in all (and i don't want to discourage you from experimenting) i highly doubt you'll be able to detect a huge difference.

one of the things i'd imagine would happen is that while the air will likely accelerate slightly at the point where the reay slopes down it's got plenty of opportunity to escape sideways or perhaps the opposite would happen, a lower pressure area behind the edge of the tray might actially suck air in from the sides... so perhaps sideskirts could maximise the effect... maybe they won't really boost fe but they should give you some downforce, wich might not be what you're after, but if you can improve a car's handling and fe at the same time that's just an added bonus.

another thing i imagine might happen is that the configuration at the corners looks like it might make vortexes... on the bright side one could hope these work as virtual sideskirts, but interferance with the body likely just turns these into drag... so perhaps some (partial) sideskirts or bending up the angles might improve things.

than again i don't KNOW, most of what i've written here is speculation based on what i've gathered about aerodynamics but i think ,at least in theory, some details might give you an even better result
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Old 01-12-2009, 11:49 AM   #4 (permalink)
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Welcome to the site. Your first mod looks pretty nice I must say. I look forward to seeing more.
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Old 01-12-2009, 05:17 PM   #5 (permalink)
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Quote:
Originally Posted by lunarhighway View Post

one of the things i'd imagine would happen is that while the air will likely accelerate slightly at the point where the rear [sic] slopes down it's got plenty of opportunity to escape sideways or perhaps the opposite would happen, a lower pressure area behind the edge of the tray might actially suck air in from the sides... so perhaps sideskirts could maximise the effect... maybe they won't really boost fe but they should give you some downforce, wich might not be what you're after, but if you can improve a car's handling and fe at the same time that's just an added bonus.

another thing i imagine might happen is that the configuration at the corners looks like it might make vortexes... on the bright side one could hope these work as virtual sideskirts, but interferance with the body likely just turns these into drag... so perhaps some (partial) sideskirts or bending up the angles might improve things.
So you are suggesting some diffusers or fins that run the length of the belly pan in order to promote linear flow? I may be able to rivit or bolt some L-bracket aluminum or some of the scaps to the side. If I do that, I will likely have to raise the rear so maintain anough ground clearance over speedbumps and similar that I am not grounding out. I shall have to experiment a bit.

Still accepting input!
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Old 01-13-2009, 02:50 PM   #6 (permalink)
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Belly Pan

Your mod looks great, nice craftsmanship! I think the goal is to have the air flow smoothly with as few disruptions as possible. Do you have any plans to extend it toward the back of the car?

Looking at the pictures you posted, I think I would cut a channel for the exhaust if it would allow the back of the plate to line up directly with the floorpan. (From the side view it looks like the whole thing is angled down to clear the exhaust pipe) A cut-out section would keep the exhaust from rattling or vibrating on the pan and allow dissipation of some exhaust heat.

Keep us posted on the progress.
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Old 01-13-2009, 06:39 PM   #7 (permalink)
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input

My thought would be to load the car with ballast so it has the same rake it will have when your driving.Under this condition,the air should travel level,and parallel with the ground along the underside of the pan.If you complete the pan,to get the biggest bang,you'll want to include a 2 1/2 degree "diffuser" for the rear section.---------------- By sliding an imaginary line,angled up towards the rear,at 2 1/2 degrees,you would slide the angled line forward until it intersects the same plane as your existing pan.The angle will define where the diffuser will end at the back of the car.-------------- From published data,should you exceed the 2 1/2 degrees,the flow will detach,providing zero benefit over a non-diffuser bellypan.--------------------- Your work looks great!
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Old 08-03-2010, 08:42 AM   #8 (permalink)
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Do you have a full belly pan now? Based on your sig you have a 2 piece set.
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Old 08-03-2010, 01:55 PM   #9 (permalink)
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The two piece belly pan I have installed is the one pictured earlier in the thread. It isn't full length but closer to about 2/3rds length (or maybe 1/2). I have raised the rear rake up about 3/4 of an inch from what is pictured in the photos to limit grounding out on short but tall speed bumps when I have passengers.

With just the belly pan and synthetic oil as the only two mods from stock, I pulled 44.2 mpg on my last tank. I realized yesterday when I went out that I had been a bit lax in checking my tire pressure and the fronts were down to about 32 from my usual 44 psi. Topped them back up and have hopes to clear 45 mpg some time this summer before it cools back down.

Have a bunch of gas receipts sitting around and I'll try to update my fuel log shortly.
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Old 08-03-2010, 02:19 PM   #10 (permalink)
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I would think that the trailing edge of the panel would want to be raised up slightly.

With the panel that low, here in Wisconsin it could rub on the snow and possibly build up and pack-in when backing up.



Jim.

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