Round bottom versus flat bottom aero

[Tom Bryant]

My son Jeff quizzed me about the round bottom versus flat bottom comments. I am glad he did! I had not been on that site yet. It is a very interesting read. I also enjoyed the pictures and downloaded them for future reference.

I am not an engineer, aerodynamics expert, or even a good student sometimes, but I do have some experience in this arena. Our car was first designed with rounded lower sides from the belly pan to the side panels. I ran over 230 mph with it at the World Finals in 1992, the first year it ran. It seemed to handle well, but when Jeff started driving in '97 (on worse salt I would add) there was a big problem. It would spin the wheels @ about 215 mph and immediately spin. I personally believe that the spins were induced by over correction, but I was not in the seat so that is just my analysis. Never the less it tried to fly when side ways. I have video of the second spin which I slowed down and watched frame by frame. when the car was broadside to the course, the front wheels were two to three feet off the ground. (The first two spins were snap spins, once around and on down the course) 

For the World Finals, we added 500 lbs of weight just behind the front axle to move the center of gravity forward and stabilize the car. I made the first run at the World Finals on a very bad course. The car moved around on the course when spinning the tires, but stayed straight .  After a couple of runs, we broke a front suspension arm when it hit a hole in the turn out area. We repaired the arm and removed half the weight. That was a mistake...this time when it spun, it was a lazy spin and the second time around it went over twice.

On the rebuild, after it crashed in '97 on it's third spin, I got rid of the rounded lower panels and put skirting on the sides. This created enough down force under the car I had to reinforce the belly pan because it was being sucked  down forward of the engine. At the same time I added a spoiler to the rear. The car now sticks to the course like glue, does not spin the wheels and goes straight, even on the 2005 course. Things may change, but at present I vote for the flat bottom with skirts.

[JackD]

You discount the effect of the spoiler ?
Get the suck down a little sideways to break the seal and see how it reacts.

[Harold Bettes]

I think that it is a foregone conclusion that a rounded bottom vehicle probably has a higher roll rate, once that it is clear of the friction of the tire/salt interface. However, it is a better path to achieve lower wetted Cd in some circumstances. The nasty influences of the undervehicle air is easiest to trap/keep from occuring with skirts. The skirts or body parts that function like them are easier to apply.

Having said all that drivel, I still prefer round bottoms and skirts! LOL

Regards to ALL,
HB2

[doug odom]

I like round bottoms in skirts also.   Sorry, just had to be said.
Doug Odom being bad in big ditch

[Rex Schimmer]

I think that we may have two different methods of generating down force with low and flat bottoms that we are talking about here and we need to clarify. One method is the type used in todays F1 and sports car design in that they are required to have a "flat" bottom but are also allowed to have a rear difuser area. On this type of "flat" bottom design the air is allowed to come in from the front of the car and is accelerated which increases the velocity of the air and therefore drops the pressure and generates down force. With this type of arrangement if the car gets side ways the air entering the front of the car is reduced or cut off entirely and the down force is reduced or goes away completely so this design is very dependent on the car going straight. Also with this design if the front of the car is raised, by a large bump say or it is shielded, say by following another car to closely, then the down force can go away and if you remember the Mercedes coupes at Le Mans a few years ago, they will do back flips.

The second type of method, and I think the one that Tom may be using on his car but is also used on Mann's Ecotech liner, is where you run a skirt around the complete perimiter of the car, except the back end, and the idea here is that if you prevent any air from entering the area under the car and it will become a low pressure area and generated down force. This method was very popular with Can Am cars back when they were single seaters and usually made from a converted Lola T330 Foumula 5000 car. With this method when the car gets a little sideways down force is still being generated because the system is not dependent on the air coming under the car from the front. Of course if the car hits a bump of something that causes it to allow air under the car then the results are the same as above. Sky-ground, sky-ground, sky-ground, etc.

Rex

[JackD]

That is just part of the secret to make the bottom of a door slammer faster is to let out more air than you let in.
Do it without add on DO DADS  and keep the car small.
Ground clearance and body angle in relation to the wind is as important as anything else.
How you route the exhaust can be a free air spill / dam nobody thinks about.
Zero gapping the body panels is another freeby that just takes a lot of cheap labor.

[Harold Bettes]

Jack,

You make some excellent points. Normal stuff that has to fit on the car can be used in your favor to "cheat" the air. The attention to the details is often much more labor than just buying parts. There have been some great examples of very successful vehicles that didn't "look trick", but were very trick with details like the zero gaps that you mentioned. The underhood stuff and undercar stuff must work together. Afterall, the components stack into a system that must work together.

Sure hope that I don't forget all that stuff on my own junk! LOL

Regards to All,
HB2

[jdincau]

in 1962 Norm Thatcher told 3 green kids running a brand new dodge 413 Ramcharger "just because you have a radiator dosent mean you have to let air go through it" I still treasure that lesson

[JackD]

in 1962 Norm Thatcher told 3 green kids running a brand new dodge 413 Ramcharger "just because you have a radiator dosent mean you have to let air go through it" I still treasure that lesson

That would have been Mr. Norm to you kid.  LOL

[doug odom]

Years ago we got a super speedway cup from Petty. 72 Dodge Charger they had run the year before. Boy, you would not believe how much Bondo they had put on the bottom of that body.   LOL   
Doug Odom in big ditch

[Tom Bryant]

Rex is correct. We don't intentionally let air under the car. This is not a "ground effects" car. If you inspect the nose of our car, you will see that the lower edge of the rubber shirting is formed by the salt. I still won't change what works until it doesn't!

[dwarner]

Sorry Rex,

"Mann's Ecotech liner..."

Actually, I think you are referring to Ron Main's car. Ian Mann is one of the computer geeks that is a part of Ron's crew.

I was the guy that did the original mounting of the mentioned skirts on Ron's car. This car was rebuilt in Rich Manchen's shop after the blowover (who can recall that year?)

Rich and Jim Miller turned that car into what it has become.

I am proud to say that I sweep the floor in the shop while that project was going on,

DW

[Glen]

Dan
I remember the blow over as it was right in front of us in the timing stand, I also remember all of the bags of lead shot that was held down in the nise with MC ties downs and they came loose in the drivers compartment
. Driver was lucky. However there are lots of stories and lessons learned in these incidents.
Glen

[JackD]

That is sorta like the guy that got beat up by his own fire bottle.
The medics said "You are gonna get some bruses and hurt for awhile but otherwise you seem OK."

[Glen]

Jack as I remember that became a instant rule as well, hose clamps and nylon ties won't hold the mass after the first hit. The one I was on was a Studibaker and the bottle was still attached to the hose and it hit everything inside the car including the driver. Another lesson learned the hard way.