I made a belly pan for my 2009 auto fit sport. Totally home made. The vent/louvre is really easy to make, just cut 3 side of a square and gently bend along the fourth 'side'. Here is a link to my post on Ecomodder where you can see some pics of the panels and other peoples input...

http://ecomodder.com/forum/showthread.php/honda-fit-belly-pan-project-6740.html

Does it work? My gut instict is that it helps a little. I'll tell you where the belly pan really pays of: road debris. After having a oil pan damaged by debris on another car in the past, the added armor means I can run over small cattle and not stress to much about damaging what's covered by the pan. My lifetime mpg average with the fit (every drop of gas that's run through it) is just shy of 39 mpg. Here's a link to my lifetime fuel log:

http://ecomodder.com/forum/emgarage.php?do=details&vehicleid=2177

My overall opinion, if you pay less than $100 in materials and plan on owning the car for at least 3 years, you will probably nearly break even on the cost (after fuel savings) and you probably had a little fun on the project while you were at it.

I've not had any heat problems even with just the single louvre. 110 degrees outside temp and the AC blasting, no problem.

 

I really like your panel that you made there! The belly pan looks great and I hope you actually complete the rear portion.

I have the BeatRush front panel and I love it. I want to be able to complete a rear panel soon but I have no idea when I will actually get around to it.

Keep us updated on your progress!

 

Looks very good but I would expect engine bay temps to increase, significantly. Maybe you can open up the area below the engine bay more?

 

I can't create a second louvre that same size because the oil pan is in the way. As I said before, heat hasn't been an issue so far and I'm over a year in. The bernoulli principal would argue that the air from the engine bay is actually being vacuumed out of the louvre by the fast moving low pressure air traveling under that car.

Additionally, as long as any extra heat isn't overwhelming the cooling capabilities of the radiator, a warmer engine bay actually improves mileage. The large lower grill opening is still dramatically larger than it needs to be to keep temps under control during normal day to day driving. Heat is something to keep in mind during mods like this but it hasn't been an issue for me yet (knock on wood!).

 

I'm guessing you don't have the dimensions handy for your belly pan?

 

I wouldn't mess with louvers on the belly pan if I was to do one... I have louvers on my hood and they have made a big difference reducing under hood temperatures and I am sure they have had some influence in reducing turbulence under the car as well.... I still have some small fender louvers I need to install.

 

Not exactly. I know I bought a half sheet which comes like plywood (4'x8') so a 4 foot by 4 foot square. Each of the two panels is 2'x4' and then the front was trimmed for a snug fit. The rear panel required almost now trimming at all from it's 2'x4' size.

To establish the shape for the end product on the front piece, I took two paper bags, cut them open, laid them flat and taped them together so I had essentially one large piece of paper. Then, laying under the car, I held the paper up the the undercarriage and traced the contours that i wanted the pan to navigate. After that I put the traced paper on the 1/4 sheet of aluminum and then cut off the unwanted parts. Make sure to cut less material than you think at first. Make a rough cut, dry fit for size, trim a little more, dry fit again etc until you are happy with the fit and finish. If your anal like me you can buff a nice shine into the surface... It won't last too long though.

The front panel mounted entirely using 4 bolt locations that were already existing and unused from the factory. The read panel used 2 of the same bolts holes as the front (so the two bolts there hold both the front and rear pieces) and the rear two bolts in the rear piece required some extra work. I drilled and tapped (threaded) two holes into the u-shaped frame member running along each side of the car near on either side of the gas tank. I assume there's nothing vital running inside them as you'd never be able to access broken components for repairs. At any rate I had no issue threading a long bolt with the head cut off into the tapped hole to serve as a mounting stud for the back of the rear panel (panel is supported by the stud and held in place with a nylock nut and a washer).

 

Without any other mods (ie an otherwise stock fit) you will want at least 1 louver in the belly pan if you build it to cover the entire bottom of the engine bay. You need to have SOME vent for the engine bay to ensure you are getting at least a little air flow across your radiator. It doesn't take much but I think it is an important note the consider for those thinking about building their own belly pan.

Seriously though, the louver was the easiest part. Cut it to shape first so you know your putting it in the right spot. Draw a square/rectangle where you want your louver (or several smaller ones if you prefer more than one louver). Cut out along 3 sides and bend gently along the fourth. If your louver opens 'in' to the engine bay, you will want the part near the rear of the car to be the 'fold' side (this is how mine is done). If you want your lover opening out and projecting under the car, the fold side will be near the front. If your lover is built opposite than I have described you will effectively be trying to ram air back into the engine bay which is really the opposite of what you want. I chose to have my louver open up into the engine bay (fold at the back) for assumed superior aerodynamics and because I didn't want anything hanging down that could get snagged on road debris.

 

Oh two more things to note. The belly pan I made comes very close to the bottom of the engine. I had a lot of resonance when I first installed it that was definitely concurrent with engine rpm. Come to find out the bottom of the engine block was just barely touching the pan causing it to rattle. I could see where the block was rubbing the pan since I had buffed it to a nice shine because there was one little spot that was scratched. To eliminate the rubbing problem, I put a slight bend in the pan (easy to do if you don't get too thick gauge Al) to that it had a very minor convex shape (bowl shape pointing up). I also found that one roll of thick foam tape (check around weather stripping or double stick tape at home depot/lowes/etc) eliminated all other rattles when going over speed bumps or up rolled curbs. Just put a small amount of the foam tape between the pan the the metal frame and no rattles. You won't want to put the foam on your oil pan because the hot oil pan could cause the adhesive or the foam to ignite or at the very least melt and stink so be sure your belly pan has ample clearance from your oil pan!

 

I was looking around the lower rear section of my Fit and I never really noticed any areas where the bumper would act as a parachute.

Any inputs on why I might be wrong?

 

Lol... as i said before, air does flow very well in the back... just check Apexanimal's pressure testing thread! but if you cover everything without leaving space for the air to escape, then you will make a parachute... a friend of mind is a mechanic in the ICAR series, and they tried to do a full underpanel on a 8th gen Civic, but the air was traped in the rear bumper, giving them slower times and more drag... they cut it inhalf (only kept the front) and it worked very well!!!

 

From what I have been reading and have read in the past (ancient and recent) It is hard for me to believe an improvement in MPG is going to be realized by using a panel to clean up the air flow under the rear of the car.... Hyper milers don't drive at speeds high enough that wind drag and turbulence is that big a thing... I do think that there would be a decrease in drag and turbulence at higher speeds and that higher top end speeds would be improved....I keep thinking back about the Chapparal race cars of the 60s and 70s that pioneered spoilers, wings, louvers and even a car that used a flexible rubber lower skirt and rear mounted fans powered by 2 cycle chainsaw engines to create a vacuum under the car... At Bonneville the stock 53 Studebaker bodied cars that held the world land speed record for years were allowed to place vents along the sides it the rear to lower the buildup of pressure that would cause the rear of the cars to become airborne.... The Duo-Centro Avanti that held the record as the worlds fastest stock production car in late 1962 averaging 179 MPH on the salt... They added a full length under panel and skirted the rear wheels and gained 20MPH just a couple of hundredths of a MPH of of putting it into the 200MPH club at Bonneville..Not bad when you consider that there had been no wind tunnel testing done in advance of the attempts.... I am curious as to whether any one that has installed a JDP hood on their GE-8 cars have noticed a difference in acceleration above 70 mph with it in place since it does flow air that would be directed under the car over the windshield and roof.... Louvers can be used to reduce airflow and turbulence between the road surface and the relatively less than smooth surfaces under the bottom of the car and reduce the buildup of pressure at the rear bumper regardless of how small it may be.

 

Well the nice part about a proper under tray/under body set up is that it increases air evacuation and encourages smoother (laminar) flow, so it decreases the pressure under the car as speed increases.

This provides better grip, but that means higher rolling friction..

Will the lower effective drag (friction from air) offset the increased rolling friction?

Tough to say, would need some actual windtunnel data to first establish the actual effects of the under tray.

Then it comes time to find a circuit on which to test the vehicle with an without the tray in like atmostpheric conditions.

My guess is for the majority of FF members, there may be a (barely) tangible increase in fuel economy. For those who track and don't care about MPG, it will probably provide more significant benefits.

 

Well, I'm sure there is an impact. ALL well designed aerodynamic WILL either increase your MPG.
The Fit EV uses underbody this panel, i'm sure Honda won't waste their money on things that doesn't work.

 

Wow, where did you get this pic? Anymore of the whole Fit?

 

That's the only pic I found. The ev fit looks exactly like 09+ fit. Just google ev fit under body.

 

exactly my point... honestly i could'nt care less about better mpg

 

You only generates better traction/grip by creating a vacuum at certain area of the underbody... such as the area around the axle/wheel so that the air passing above the car would push down which generate more downforce/traction/grip.

Here: Generating greater downforce for RWD cars towards the end


Here: I believe is the NSX underbody panel, notice the 2 "hump" around the front wheels and the end.


So i believe if the entire underbody panel is flat and smooth, then it should keep the flow smooth which reduce the Cd without generating too much downforce/grip.

 

I agree entirely with the vast majority of your posts in here, however the one main contention is that at any point under the car where you have increased flow velocity, you have decreased pressure. So, because the car is rigid, you have increased downforce. It may not be distributed in an ideal fashion, but the overall amount of force on the contact patches has increased.

And fluids don't flow quite as well over entirely flat surfaces, but certainly better than they would over the stock under carriage of a GE or GD. A slatted, slightly concave design would produce the smoothest flow, like you see in Formula/World Series and LeMans cars.

The diffuser by the front wheel arches and at the rear under the bumper is to keep pressure in those areas low so as to not create an air cushion under the axles, as tends to happen under highway speeds and greater with any given wheel wheel that is not vented in some fashion.

Now if one were to spend the at least $1-2k it would cost to run a complete functional under tray on their Ffit, they would also have to get a stiffer suspension and a low friction skinny tire to take advantage of the tray, and really see any increase in mileage.

The cost of all of this on a Fit would (for all practical purposes) never be regained in fuel savings though.

But a dedicated track car would benefit greatly from a well designed underbody.

Now I can understand why they would do it on the Fit EV, but that is a special case where they have designed it from the onset to return the highest energy efficiency possible, and any sort of tangible gain would be helpful to meet their goals and extend the range on battery as much as is feasible.

But retrofitting an undertray to your GE/GD will not save you much, if any, money in the long run. Your best bet here is to be more conservative on the go-pedal, keep the car out of VTEC and do what you can to maintain momentum where possible.

 

You don't have to retrofit it, just make some panel and smooth out the flow under the car... as long as you keep the entire bottom FLAT and not raise up too much like those race car rear diffuser it will NOT increase the air velocity that much. It only speeds up because the race car rear diffuser creates a greater vacuum.

I believe the point of the complete flat out design of the EV Fit underbody panel is not to gain downforce, but merely to reduce turbulence and decrease Cd.

I would look at it this way, couple of things:
- drag/turbulance
- airflow top
- airflow bottom
- downforce

Race car underbody panel essentially decrease the pressure going under the car by accelerating the flow, which cause downforce, this gives traction/grip.

Now, if the top and bottom airflow is equal, then the downforce will be zero. But the drag and turbulence it can still affect the car's gas mileage/electric mileage efficiency. With a flat underbody panel design, it should be able to reduce drag and turbulence.

Look, if a design can ADD downforce, then by inverting the design can also REDUCE downfoce (by somehow increasing the pressure, not sure if they want to do that...). Anyway, which means if it was designed correctly, it can archive ZERO downforce added while reducing drag/turbulance only.