PandaBear

Hey guys,

When I was taking a class in vehicle stability they say at high speed air drag is the most important factor ( not weight ), and belly pan can make a significant (20%) difference by reducing air drag.

So I was wondering if anyone has ever got a belly pan installed? or if anyone made them?

Mr. Softee

real time uses them on their race cars...

Daemione

I've thought about this a little bit. The underside (at least on my Prelude) isn't actually all that disruptive. The only component that protrudes into the airflow is the exhaust. So unless we sacrifice a good 2 or 3 inches of ground clearance to completely cover the underside of the car, our options for improvement are limited.

One thing I had considered doing was taking the stock plastic splash guards, and spending a little time with some fiberglass to smooth them out. But I'm not sure whether the time & effort spent would be worth the small gains - especially since my time spent at speeds where I'd notice a difference (80+ mph) is almost negligible.

Another concern lies in downforce (or lack thereof). If we manage to accelerate the air passing under the car too much, the car turns into a large wing. Which is the reason for front under-spoilers - they limit the amount of air passing under the vehicle. If limiting underbody airflow is the most beneficial way to go (and I could be wrong, I'm no expert), it doesn't seem like it'd be all that worthwhile to speed up the air that's passing under there.

Just a few thoughts.


[Modified by Daemione, 10:33 AM 7/29/2002]

PSU-TEG

You are correct... but at the same time wrong.

If we manage to accelerate the air passing under the car, yes it will act like a large wing.... but.... it will act in our favor. The more we can accelerate the air passing under the car (or limit the drag that slows down the air passing under the car), the lower the air pressure will be under the car... and thus, the lower the pressure under the car with respect to the air pressure on top of the car, the more down force (and the more traction) we will have. And that is exactly what we want!

There reason we have wings and spoilers in the front bumpers is to keep the air from moving under the rough uneven contour of the bottom of our cars, where the air will get traped and create drag under the car (which will cause higher pressure under the car, and inturn the higher presure under the car will decrease to down force, traction). But buy directing the air way from the underside of the car, we are also slowing down the velocity of the air under the car, which will also create higher pressure, and decrease down force (although, this is assuming that the bottom of the car is smooth). However, the by directing the air way from the bottom of the car using spoilers, does not create as much of a high pressure air under the car as the rough, unever bottom of the car will by trapping air and creating drag.

It is so much more efficant and effective to decrease the drag under the car (and lower the air pressure) to increase the down force on the car than it is increase the drag on the top of the car, to create higher air pressure on the top of the car (also creating greater down force).

Why?

Becuase that way you have just increased downforce by decreasing the drag (on the bottom), and as a side effect of decreasing drag... you will also increase your top speed and accelerating potential.

But if you increase to down force by increasing the drag on the top of the car, you have just increased the drag, and as a side effect... you will decrease your top speed and accelerating potential.


Now the only thing with this is that it is much easier to increace drag on the top by using wings and spoilers, and you also able to increase the drag alot more on top than you are capable of decreasing the drag on the bottom, therefore, getting a much more dramatic resulting downforce and much more dramatic increase in traction.... but at the expence of some top speed and acceleration.


Oh, well... have a nice day fellas.......


[Modified by PSU-TEG, 7:52 PM 7/29/2002]


[Modified by PSU-TEG, 2:17 PM 7/31/2002]

PandaBear

I know, it is very hard to build a belly pan that doesn't get scratched all over the place. One thing the UC Davis future car has tried is to put a honeycombed plastic sheet with alumnium reinforcement as a belly pan. It is bolted all around the car covering the whole area from below the firewall to the rear bumper. Scratches and dings happens frequently as it reduce the ground clearances.

Another thing I heard about the front spoiler is the theory that it is better to not get air into the bottom in the first place, because it is easier to be done and it reduce the air much better than just a belly pan. Of course, you can do both if you want.

PSU-TEG

But to be honest with you, we typically don't drive our cars fast enough, and hard enough to benefit (or notice a difference) from the use of a Belly Pan.

Yes, in AutoX you drive the car hard, but you don't drive fast enough to creat a noticable effect from the aerodynamic improvment.

For drag racing, you don't need that added down force. There are no turns, and it won't impove tration at all where we would need it (the launch)

And as far as racing down the free way.... it will add a little bit of downforce.... but if anyone is pushing there car hard enough down the free way, at 100 MPH, in a daily driven car (not one that is an all out race car, with the appropriate safety equipment) that they would notice the difference with the small added downforce of a Belly Pan, they are going to be dead before long.

Throw in the added weight issue... and I don't think it is anywear near worth the money or effort!!

And yes, it will be scraping all the time!



[Modified by PSU-TEG, 3:08 PM 7/31/2002]

grocerygetter

you do need the added downforce in drag racing. think about it. High horsepower applications when drag racing will cause the car to pick the nose up a little. More down force on the top end will keep you from blowing the tires off if you're making serious power at the end of the track. I've seen this before.

PSU-TEG

I agree.... for SERIOUS drag racing downforce is a hugh factor.... and should not be overlooked.

but.... as far as a Belly Pan is concerned..... and for the majority of the people on this board....

I don't think the again you will see in downforce from adding the Belly Pan will not be significate for someone who is runing 11+ second quater mile (which 98% of the people around her fall into).

Plus, the addtional weight will probably hurt these cars more that the Belly Pan will would help.

So like I said.... unless you are building an all-out, trailer queen, drag car.... In my opinion, it is waist of money.

But hey, like I just said.... this is only my opinion.

brucepick

Interesting reading - thanks people!

I spend a fair bit of time on the fuel economy sites including ecomodder.com where this topic has been discussed at length. Belly pans are definitely worth building. The same reasons that make them worth it for mpg also make it worthwhile for speed/acceleration etc.

I drive a 55 mile commute (each way) and can clearly see the effect of even a 10-15 mph head wind, tail wind or cross wind in my mpg for the trip. With a tail wind I can get about 46-47 mpg for the trip - but with a headwind I get more like 40-42. I do mostly 60-65 mph by the dashboard speedometer. MPG data is from a MPGuino gauge which I find is pretty darn accurate.

No I'm not BSing about the mpg my car gets, but that's only because (a) the HX is tweaked for mpg and (b) I've adjusted my car and my driving for mpg. Other people driving my car get more like 35 mpg on similar trips.