Smooth bottom cars, Belly pan??

[fletcherscustoms]

I am in the process of building my full tube chassis 70 challenger and was thinking about completly closing in the underside of the car, so when you look under it there is just a full belly pan. I have seen this done on race cars, etc. But curious if there are certain things that should be accounted for. Seems to me if its completly smooth it would just be a big air foil of sorts. Anyone got any pictures??

[64Chevelle]

I have been thinking about the same thing. I've seen at least one other guy making such a bottom pan in here, but he had to remove it because of heat issues. If you get the heat issues worked out you certainly will benefit from much lower drag, our old cars are really not drag optimized . Also, make sure that you make some kind of diffuser in the rear so that you get the additional downforce benefit.

http://en.wikipedia.org/wiki/Diffuser_%28automotive%29

[Tiger]

I've been thinking about it on my Chevelle but there might be some issues to solve in my car before doing it.

the theory is that if you are able to create underpressure the car will be sucked to the ground, as I have understood it (I'm very far from an expert) you want a flat bottom with some lips on the sides and a diffuser in the rear to create the underpressure. Heat issues are to be solved but once they are you are good to go.

I do indeed beleive that if you make a flat bottom car there will be benefits even if you are not making lateral lips or a diffuser, not due to underpressure anymore but due to better aerodynamics and less turbulence under the car, you will also have less noice.

[64Chevelle]

I do indeed beleive that if you make a flat bottom car there will be benefits even if you are not making lateral lips or a diffuser, not due to underpressure anymore but due to better aerodynamics and less turbulence under the car, you will also have less noice.

True indeed and the i remember the guy who already tried it somewhere in the forum telling he got noticably better mileage and a more silent ride, even without a diffuser. So the car should have the potential for a better top speed and perhaps also better high speed stability?

[Norm Peterson]

But curious if there are certain things that should be accounted for.

Water/dust/dirt/mud/slush?

[Lowend]

If you are really cool you'll make a venturi out of it (a-la Ferrari) and get downforce

[Jeremy]

I tried it and had a lot of problems with heat retention. Even at mild temps (60-70's) the interior was just too warm.

[Hoot]

I've given this a little thought too and wondered about creating a channel around JUST the exhaust system. kind of like a flat bottom everywhere and miniature drive-shaft tunnels for the exhaust piping. the flat part of the belly-pan would probably end up being about even with the bottom edge of the exhaust tubing, and for the most part it runs length-wise to the vehicle, so it seems like any air that does get up into the tunnel would just travel along the piping out to the rear wherever you have your exhaust exiting now. like a big intercooler exhaust tip!

I'm thinking... I'm no aerodynamic engineer, but if the sheet-metal used to create the tunnel is kept pretty close to the exhaust piping, say 1/2" or so, laminar flow will make the air move slower inside the tunnel and the air beneath the car might travel along the boundary layers almost as if the tunnel wasn't there. ...making it seem even smaller than it is as far as drag is concerned.

ok, it wouldn't be a truly flat bottom, but it would be about 80-90% flat and with the exhaust exposed inside the tunnel, no heat increase. in fact, because the exhaust tunnels go completely up and over the exhaust piping, cockpit temps should decrease. trade a very small amount of drag for the solution to the heat issue. seems to me like a reasonable trade-off. what do you guys think?

[TwinSting]

I have had several cars with complete belly pans front to rear. My Vette is smooth underneath except for the driveshaft tunnel. Also many new cars are being built this way also. My new E92 M3 coupe has belly pans front to rear with a diffuser of sorts in the rear. I belive my Viper was pretty smooth underneath too.
Norm mentioned dirt, debris etc.. I think the belly pans make it easier to keep clean. It is easier to clean that flat surface then all the nooks and corners on a normal car.
I would think too that there must be aerodynamic advantages or race cars and many of the high end performance cars would not be doing this.
Thats my 2 cents.

[MrQuick]

I found this interesting reading when I was building my track car. Touches on aerodynamics basics. http://www.e30m3performance.com/myth...r/splitter.htm

To combat the interior floor heat accumulation some builders have used a double wall floor with a 1/4-7/16" free air gap with front and rear vents and air ports that feed exhaust system channels.

[JamesJ]

When speaking about aerodynamics it is more important to have a large front spoiler / air dam that keeps less air going under the car then a belly pan

[ProdigyCustoms]

Project Prodigy is a belly pan with a open tunnel. The original plan was to have a louvered plat covering the tunnel. I thought the heat may be to much so before we spent all the time making a louvered cover, we tried a quicky solid panel tunnel cover we whiped up. We found that the heat would run you out of the car in the winter. We knew the heat in the tunnel would be a lot, so we framed the tunnel in 1" X 4" and 1" X 2" 11 guage tubing and double walled the tunnel with heavy gauge 11 gauge on the outer and 14 gauge inner, with a 1" gap. We were not worried about weight as the tunnel / spine in this car is strutural Also since the tunnel in the low middle of the car, if there is such a thing as good weight, this is goood weight. Long story short, even with the double floor, After one hour of cruising it started to smell like the SKS coating was going to catch fire. Maybe the louvers in the tunnel cover would have helped, but even if it cut the heat in half it would still be to hot!

I do not really know if their is a aerodynamic advantage we can actually see in street and casual open track play. But I can tell you it sure get peoples attention when they realize noting is hanging under the car. It just looks clean.

To keep it clean underneath the brake lines and fuel lines are run through the rocker. The E brake is a fluid lock so there is no E Brake cables. Probably the biggest nightmare was the headers, and making them exit in the tunnel.

I wish I had better pics, here is somthing I found

[Lowend]

When speaking about aerodynamics it is more important to have a large front spoiler / air dam that keeps less air going under the car then a belly pan

Absolutely... your front spoiler is probably the single most important aero-dynamic feature on the car. Ultimately you want to keep air from getting under the car in the first place.
This roadracer is a prime example

[71OLDS]

Sorry for the thread jack... a little off topic but I'm confused after reading some of the info on this:

http://en.wikipedia.org/wiki/Diffuser_%28automotive%29

For the best aerodynamics, just for the front end, should it be completely flat like:




or should there be a hole in the front to catch the air and send under the car like:



not sure how much I can do to smooth out under the car but would like to at least get it right on the front and back. Here's my front before:



thought I would improve it aerodynamically with something like:




but now I'm wondering if there should be hole in the front about where the license plate is or maybe just leave the 2 rectangular holes to the left and right of the license plate that are already there??

[greygoose006]

well since you dont want air under the car in the first place, i would say that no hole is best.

the only reason for a hole, and the reason the mustang had one i suspect, is that it was a brake cooling duct, or air intake for the engine.

[71OLDS]

That's exactly what I thought until I read the info on this link about aerodynamics with all the pics.

http://en.wikipedia.org/wiki/Diffuser_%28automotive%29

Did you read it and look at the pics?

Here's part of it:

Front
Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. A splitter separates the flow into the upper path, which will increase pressure in the diffuser and the lower path which stays as low pressure or even reduce pressure as the combination of an up looking splitter and the street acts as a Venturi nozzle. The height of the splitter is as low as possible without hitting small bumps, for street legal cars there is a minimum of about 100 mm. But a diffuser reduces the pressure in front of the car and thereby the amount of air pressed below the car. Sometimes a diffuser is used to let the wheel-casing also pump down the pressure below the car. Many small engine-exhausts in the back also help pumping. The exhaust from the engine compartment in the underbody can blows the air outwards by means of convergent nozzles, which reduce the pressure from the high pressure in the compartment to the low pressure below the car (the nozzle is the inverse of the diffuser).

I think this article is recommending a hole in the front, and the diffuser in the back (and I'm sure the smooth under body would help too).