Underbelly pan: Reducing drag and increasing top speed.

Hey guys...

I've been putting some time into thinking about how you can reduce the drag on a car without influencing the outward appearance! Why? Well... I love the looks of a well crafted rod, but what I dont like is the done at 85 mentality of an unchopped 32 coupe. But I like the top end abilites of late model jelly beans... so the think tank (my cat infested garage...) got to work on some ideas!

The start:

So I was laying under the 75 Vette, looking up at the independant suspension on the rear and noticed the windbag that the body formed going towards the bumper... I was scratching my head at the open areas that ended in a capped bumper skin! Thinking about the venturi effect I started looking at other parts on the car (ok... so its not a trad car..) and I got to thinking about how bad the drag is on the underside of the car...

To start, ya have a not so great aerodynamic body, with a radiator mounted down a tunnel... then immediately behind that is a frame member with some suspension components... then a SBC. Working your way back from the front ya've got a fire wall... then some floor boards and at the back end of those this parachute shaped deal that they call the rear fenders!

So I got to thinking about how ya could fix the drag, and I came up with a couple things:

1. Ya start over with a jelly bean (not an option... I like my 'Merican tin or in this case resin)
2. Ya grab a bucket of bondo an start smoothin stuff out
3. Ya build a sheild that covers the rough stuff and forces the air other places!

For me, I started thinking about number 3... I started thinking about the smooth lines of a track t, to be exact... the formed sheetmetal and rounded nose! I started thinking about belly pans that would smooth out the air moving under the car... held on with wingnuts for relatively easy access to the components that might be hidden under the pans.

So I did some looking around on the net... and read that on late model **** the designs can be improved to reduce drag 5-9% just by smoothing out the underside! I said WOAH: Less power, more speed! Something clicked... so I knew the idea was sound. The reason for bringing it here... Well I though for tech week, as much as these guys help me out with finding solutions for problems I dont quite have... i'd try to return the favor.

Ok... so Flat head guys, ya just finished up the T5 swap... your motors in race trim but yet still isnt pounding the ground, ya need more top end... I wonder how much you could smooth out your baby to make her slicker'n a babys hind quarters to that thing we like to call air!

50's tail draggers with some leaks... maybe a set of underbelly pans would help catch em to stop from markin the territory quite as bad...

Gold chainers... if ya do it in aluminum or stainless... or heck platnum for that matter, you could polish it and when ya throw some mirrors under it at the wax an shine car shows you'll have the most shiny thing around!

Now on a serious note, I'm looking for ideas as to how ya could pull something like this off and increase the down force of the car! I'd love to be able to figure out how ya build a porsche or ferrari killer out of a 50's rod, because power isnt the problem! Getting it to the ground, and keeping the car on the ground is...

My submission to tech week, is less tech, and more brain farts errr brain food, food for thought... or thoughts for those that can think

 

Been thiking about cutting some road cone and taping them over my headlights when running at drags.

 

Hope this shows up. It was robbed from DRD57's site from this past weekends GNRS.
About as smooth as you can make a Deuce Roadster. Channeled with a FULL belly pan. This is the real deal form "the day". Dave Simard restored this car to 100 point type stuff. Showed last year at the GNRS in San Mateo and other venues too.

Thanks Don! Sorry I didn't have a chance to talk with you this past weekend.

 

The blue one looks like he pulled off the slicker then a babys **** approach!

I like it!

 

It can also increase fuel mileage. I have heard accounts of mercedes owners that removed the blly pan from their car for maintence and left it off for a couple weeks and lost substantial fuel mileage(like 4 MPG).

Also if the belly pan is designed correctly it can be used to create large amounts of downforce on the car by reducing the air pressures under the car. Channels are formed that get deeper and wider the further they go back on the car and create an area for the pressures to be reduced. Obviously this works best when combined with a front air dam that is as low to the ground as possible as well as side skirts that are close to the ground. The design of the channel corners is also important and can help create downforce. Just though you may be interested. Shawn

 

[ QUOTE ]
Hope this shows up. It was robbed from DRD57's site from this past weekends GNRS.
About as smooth as you can make a Deuce Roadster. Channeled with a FULL belly pan. This is the real deal form "the day". Dave Simard restored this car to 100 point type stuff. Showed last year at the GNRS in San Mateo and other venues too.

Thanks Don! Sorry I didn't have a chance to talk with you this past weekend.


[/ QUOTE ]


was this car a milestone car in street rodder? I think I read somewhere that this car would overheat quickly as a reult of the belly pan, thus restricting the car to race duty only..(well ok show duty as well.)

if thats true then the real problem would be cooling the car after you "****on" the underside up..

 

[ QUOTE ]
It can also increase fuel mileage. I have heard accounts of mercedes owners that removed the blly pan from their car for maintence and left it off for a couple weeks and lost substantial fuel mileage(like 4 MPG).

Also if the belly pan is designed correctly it can be used to create large amounts of downforce on the car by reducing the air pressures under the car. Channels are formed that get deeper and wider the further they go back on the car and create an area for the pressures to be reduced. Obviously this works best when combined with a front air dam that is as low to the ground as possible as well as side skirts that are close to the ground. The design of the channel corners is also important and can help create downforce. Just though you may be interested. Shawn

[/ QUOTE ]

Yup... I hate the look of the tacked on spoilers... even though they are what "makes" cornering while at speed! So I'm researching the hows and whys of downforce creation... while leaving the top shiny half of the car mostly stock (read: No park bench rice boy wings!)

I wonder about the cooling of the engine... and if its not that ya just cant vent the amount of air the radiator can pull through without having a hole somewhere about the size of a radiator!

 

Can you say louvers?

 

I can say louvers... but I cant do em! Which is why a big empty hole is easier for me to do...

Dont have room for a press... yet

 

I don't remember who it is, but one of the newer Car commercials is showing a Belly pan thats Dimpled like a Golf Ball.

 

Havent seen that one

 

lexus has the golf ball effect.

you are correct about overheating, take a look at current champcars or IRL, the rad exit is actually more critical to cooling than the inlet size. in order for the bellypan to make appreciable downforce you'll have to operate in the ground effect zone, which means (oh damn) you'll have to slam the car to the ground. in my former life in champcars our tunnels would be most effective almost s****ing the ground, we ran skids out of aluminum or jabrock for the times it did s****e.

there aren't many good sources out there to help you design something like this, unfortunately it takes wind tunnel testing to really understand the thing. but for some reference do a web search for a site called Mulsanne's Corner. it's a history lesson on protoype cars, but there's lot of photo's of undertray's and what not.

oh in 1999 timeframe, a champcar made about 5400 lbs of downforce (@ 200 mph) in roadcourse trim, about 60% of this was done strictly from the underbody.

brandon

 

Not only that, but on slammed cars a belly pan works as a skid plate too, and gives the oil pan and trans some protection against big rocks and speed bumps.

I wonder if having a rake to a car with a pan helps with downforce. With the front lower than the rear, and the bottom flat, would you get the same type of venturi effect?

On slightly related a side note, I noticed that almost the entire bottom side of my Beetle is flat. Only the last 1/4 has no pan under it.

-Bugman Jeff

 

Also, our science department has some small wind tunnels. Do you suppose you could use model cars, and get a measurable difference in drag and downforce with different pan designs?

 

rake (we call it pitch in racing aero world) is everything to the underbody. it varies based on the design. flat bottom with an angled kick up in the back lives and dies with rake. 2" difference from front to rear height is very normal. tunnels like a champcar run may or may not be as sensitive to rake. that's the whole wind tunnel problem.

if you read basic fluid mechanics, they'll shove bernoulli's equation down your throat. its the basic equation of flow velocity and pressure with realtion to volume. it works great for sewer pipes because they are a closed volume, the undertray is not, it leaks out the sides or draws air from the sides. that where the complexity starts.

you could go chapparall and build sliding skirts and run a fan ****ing air from underneath. road and track did a skidpad shootout a few years ago and a 100cc go kart out cornered jim hall's indycar on a skidpad. the 45cc leaf blower ****ing air helped a bit though.

 

Dang... that little gocart must really ****! (ok... not funny...)

Thanks for the link to the mulsanne corner! Thats the best place I've seen for semi-basics!

 

There are some very good books out there on this subject. Try Borders or Barnes and Noble- or Go to the SAE website

This is a pretty technical book but is informative

http://www.sae.org/servlets/productDetail?PROD_TYP=BOOK&PROD_CD=B-629

 

If you want to look at how auto designers work the underbody aerodynamics- Lexus is an excellent place to look at what they have done to improve underbody airflow.

Every Lexus has numerous tricks on them to help reduce turbulence because turbulent airflow is noisy (and Lexus wants it to be quiet)
they have little spats stuck in many places to improve airflow- or check out a Honda Insight- that car is aeroed out to reduce drag tremendously so it can get maximum gas mileage

the LS430 from Lexus has a drag coefficient Cd of 0.26 when it has air suspension (in 2001 when it was new it was the lowest of any production car)- the Cd was 0.28 on non-air suspension cars because the air suspension cars drop about 1 inch lower at freeway speeds so less air under car= lower Cd

 

the mulsanne website goes into some pretty gory details on aero from those prototypes. i used to be an engineer at Gurney's but i missed out on the GTP days. i was only around for the return to champcars.

katz's book from the previous post is pretty good, i haven't read through it in a long time. i know it profiles undertray designs but there's really not going to be any easy formula to design this thing. the worst scenario is to end up with a design that puts lift on the front axle or rear axle, nothing like losing contact with the pavement at speed.

brandon

 

Amen...

The old challengers that tried to lift off at speed... got some friends that have talked about just how loose the steering gets!

 

Sweet! http://www.mulsannescorner.com/vortexlift.html

I'm a person that learns by seeing... and that... that is just AWSOME!

 

I had thought about the cooling effects a full belly pan would have as well. I am planning on running a full pan on one of my cougars. what I was thinking was a combination of things. by extracting air from the engine compartment to the sides of the body (behind the front tire on the fender) by using tubes it would put high pressure air from the compartment to the low pressure area along the side of the car. I was also considering a channel between the belly pan and the floor pan of the carthe high pressure air would flow between these two to the back of the car where it would also help to reduce drag on the unaerodynamic tail end of the car as well. This air current would also help to cool the transmission and rear end and kepp the exhaust at somewhat of a more livable level of temperature. I believe the exhaust would also need to be coated with a heat reflective coating inside and out to keep as much of the heat in the pipe. This would alos contribute to horsepower but that is a different topic.
Another option I had considered was using oval exhaust tubing and using the exhaust pipes as an integral part of the belly pan using them to form one side of the channel on each side of the car. just food for thought. Shawn

 

man, i just read an article on boundary layer blowing to reduce drag turbulence but i can't for the life of me remember where that was. they were testing it on big trucks (they called 'em lorrys so it was UK written). sorry, try googling something to the effect of "boundary layer blowing" and see what turns up. i'm not familiar with the exhaust heat theory for anything other than a turbo.

 

Thanks for the hint in the right direction with the boundary layer! I've been thinking up until now that leaving the transmission tunnel alone and just smoothing out the floor boards (pan running from the level of the frame to the inner half of the floorpan...) all the way back to the rear suspension.

This makes me think that that may not be the best way to pull it off

 

Another method to reduce parasitic drag is to clean up the back end of the car, aerodynamically. By reducing wake turbulence, you can reduce the amount of HP necessary to push through the air.

Essentially what you want to do is allow the displaced air to rejoin itself behind the car as smoothly as possible. Real world methods include tapering the trunk area or a small trunklid spoiler.

Whooda thunk I'd use my Masters Degree on the HAMB?

 

hi guys the golf ball with its dimples travels further than asmooth ball with the same amount of force, the chap that invented the buncing bomb inww2 did tests on it ,**** ive got a head ful of usless **** thats why the new car might have it?

 

plug in aerodynamics on www.autospeed.com and theer is some good info. Only a few articles are full, though, as this is an e zine. They feel like a pan doesn't offer benefits beyond what a splitter and side dams would do. A splitter being a horizontal extension in front so air hitting the front of the car can't go under.

 

I hear ya on the spliter and side skirts...

For me the big deal is dropping the drag down to a relatively reasonable number instead of the .47 - .50 that I've heard qouted... without impacting the view from the outside of the car!

My internet connection is not working real well at the moment... only letting me click on hamb links! Guess this is a good thing

 

with your vette- it probably is going to be easier (and cheaper) to ust run a very low chin spoiler and side skirts-

just think like every oval track race car- low clearance in front- alot more on back to let air out from under car

see picture attached

 

the drag is a function of frontal area as well as the CD number. if you reduce frontal area without even affecting CD, drag will go down. so lowering the car out of the air is the first step. the skirts are the next step like the above pic. taper the side skirts into the rear of the front wheel arch so they will extract a little harder. make them out of some type of plastic so they will be "self machining". then cleaning up the floor and transitions will be your next step. stegert, do you work on that dash car?