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Effects of under-body smoothing?
#1
After seeing this thread: Underbody smoothing it seems to have gotten my interest and other members as well, so I thought rather than cluttering that thread, I'd start a new one.


Since the title got me thinking about the aerodynamic effects of smoothing out the underbody of these cars, I was wondering what difference would it make (at high speeds of course) to put an almost complete belly pan on one of these cars? My car is already lowered on stiffer springs and better shocks, I'm planning a front splitter and small rear spoiler (if they'll have a real functional value) and would love it if the front of the car didn't float so much in the 120+ range...

I'm thinking of something (probably a thin sheet of aluminum contoured and bolted in along the frame rails) that goes basically all the way from the front valance to the back valance, with a hole for the bottom of the diff and whatever other pieces stick down that low. I may go so far as to put some of those goofy little fins on the very back (on the bottom) if research proves that to be a valuable addition too. I've had my car up over 130 before but ended up backing off because I had 4 wheel manual drums and it was getting pretty serious lift up front. I have no doubt that the car can get to 140+ if I've got the cojones, and I'd love to see how fast it really WILL go on the 42 year old, un-rebuilt motor.

For those of you more knowledgeable than I, what do you think? I'm not doing this for mileage improvements, only for improved top speed and handling.
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#2
Back in the 60's Smokey Yunick ran a full bellypan on a car at Daytona with good enough results that NAPCAR outlawed it. The following race on a big track he showed up with a car that had all the crossmembers smoothed out with Bondo, they outlawed that too. Big Grin

There are advantages to cleaning up the underside of a car aerodynamically. Todays "stock cars" accomplish nearly the same thing by keeping airflow from going under the car. By directing airflow around and over the car it creates a lower pressure area under the car. This would help keep a lighter car stuck to the ground.

Minimizing frontal area has good effects on aerodynamics. Spoilers create down force which results in more drag. It is a fine line to walk between downforce and reducing drag. Keeping air out from under the long, large hood on our cars would probably reap some benefit.

I would think there are merits to smoothing the airflow both under and over the car. A back to back study with measured effects would be interesting.

There are some very good books and articles on improving aero on production cars. You could experiment with cardboard and sheetmetal to find something that works.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

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#3
I'm always interested in things I can do that won't affect performance and will have an effect (improved effect that is) on my mileage. A full underbelly pan is one of the best ways to increase mpgs for highway speeds at 60+mph. However, you have to have some cooling and airflow so the exhaust system can cool. Partial pans from the front valance to the firewall will make a big dent. You see these on a lot of new cars lately...and minivans now have a lip spoiler at the top of the hatch so the airflow will have a definite termination point and thus fewer eddies behind the back window which makes gleaner glass and increases mpgs.

The ecomodder site discusses these topics and many other ways to increase mpgs. Be warned though some of those guys take it to the extreme and have done some really weird stuff to their cars, but their cars also get 60-100 mpg too!!! The site is much more than a bunch a greenies driving hybrids, but there are a few of those on there too.

http://ecomodder.com/

BTW, the 71-73 has a good aerodynamic tail shape. It doesn't get much better than a 14 degree slope all the way back to the trunk edge!...it's perfect actually. Visualize the hatch area & shape of a prius...notice the similar shape?

[Image: 2010-toyota-prius-modelchange-4.jpg]
[Image: BillDion71Mach1.JPG]


.

[Image: 386_07_10_13_5_58_42.jpeg]
My Mustangs:
71 M-code Mach 1, Medium Blue/White Sport, 4R70W, 3L50, Factory Ram Air.
72 Q-code Mach 1, Pewter/Black Sport, 4-spd, 3L25.
65 Convertible, Britney Blue/White/White, more modified than original.
05 Convertible, Legend Lime/Tan/Tan, future classic??
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#4
whitelouis;14266 Wrote:After seeing this thread: Underbody smoothing it seems to have gotten my interest and other members as well, so I thought rather than cluttering that thread, I'd start a new one.


Since the title got me thinking about the aerodynamic effects of smoothing out the underbody of these cars, I was wondering what difference would it make (at high speeds of course) to put an almost complete belly pan on one of these cars? My car is already lowered on stiffer springs and better shocks, I'm planning a front splitter and small rear spoiler (if they'll have a real functional value) and would love it if the front of the car didn't float so much in the 120+ range...

I'm thinking of something (probably a thin sheet of aluminum contoured and bolted in along the frame rails) that goes basically all the way from the front valance to the back valance, with a hole for the bottom of the diff and whatever other pieces stick down that low. I may go so far as to put some of those goofy little fins on the very back (on the bottom) if research proves that to be a valuable addition too. I've had my car up over 130 before but ended up backing off because I had 4 wheel manual drums and it was getting pretty serious lift up front. I have no doubt that the car can get to 140+ if I've got the cojones, and I'd love to see how fast it really WILL go on the 42 year old, un-rebuilt motor.

For those of you more knowledgeable than I, what do you think? I'm not doing this for mileage improvements, only for improved top speed and handling.

In theroy & practice it does work...My son & raced go-karts for quite a few years..I custom made some rear panels to smooth out the chassis on the bottom...with an "air extractor" located 3/4 of the way back in the panel...Our lap times decreased & the kart handled much better..about 2 weeks after that everybody was running them at the track where we were racing & now they are comman place (you can buy them pre-made) To do it on our stangs would be a cool experiment...For now I'll stick to the bondo smoothin game...I'm after looks & any speed /milelage gains are just a bonus..I'll see if i can grab a pic of the kart tomorrow


LOVE OF BEAUTY IS TASTE..THE CREATION OF BEAUTY IS ART
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#5
Thanks for the links/pics and input guys! It seems like it's definitely something to pursue, at least to some degree. I'm thinking sheets of aluminum for weight, but not sure exactly how easy that'd be to attach since welding aluminum to steel doesn't really work.

Would it have noticeable negative impacts if I were to make small brackets and bolt the sheet to the frame/subframe/floor? I'm not going to have much of a baseline other than stock for comparison, so would I even really notice? Drilling holes and bolting it to the frame doesn't REALLY sound appealing, but sounds better than the extra lbs from sheet steel plus the convenience factor of being able to remove it all.

As for heat venting, what is going to be the best method without allowing lots of air to get up in between the pan and the floors?

I'd like to cover as much of the bottom of the engine bay as possible, but obviously that's going to require some venting to release all the heat, it seems like drilling some air vents in the pan would be wise, but then that starts to negate the aerodynamic gains, where do I draw the line?

I need to read up on this some more before I get into it too heavy, I have a general understanding of aerodynamics and get that the less resistance there is, the better, but perhaps there is a specific book which will enlighten me a great deal?

Don - Thanks for the pictures and info, unfortunately I have a coupe Sad I feel like the aerodynamics of my rear end are slightly less appealing for top end and efficiency. But I'll work with what I've got for now!

Thanks guys!

-Louis
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#6
whitelouis;14687 Wrote:Thanks for the links/pics and input guys! It seems like it's definitely something to pursue, at least to some degree. I'm thinking sheets of aluminum for weight, but not sure exactly how easy that'd be to attach since welding aluminum to steel doesn't really work.

Would it have noticeable negative impacts if I were to make small brackets and bolt the sheet to the frame/subframe/floor? I'm not going to have much of a baseline other than stock for comparison, so would I even really notice? Drilling holes and bolting it to the frame doesn't REALLY sound appealing, but sounds better than the extra lbs from sheet steel plus the convenience factor of being able to remove it all.

As for heat venting, what is going to be the best method without allowing lots of air to get up in between the pan and the floors?

I'd like to cover as much of the bottom of the engine bay as possible, but obviously that's going to require some venting to release all the heat, it seems like drilling some air vents in the pan would be wise, but then that starts to negate the aerodynamic gains, where do I draw the line?

I need to read up on this some more before I get into it too heavy, I have a general understanding of aerodynamics and get that the less resistance there is, the better, but perhaps there is a specific book which will enlighten me a great deal?

Don - Thanks for the pictures and info, unfortunately I have a coupe Sad I feel like the aerodynamics of my rear end are slightly less appealing for top end and efficiency. But I'll work with what I've got for now!

Thanks guys!

-Louis

Hello Louis, how are you!???.. I was reading at this thread and started remembering stuff...

You say you have a Coupé?... i red somewhere that coupes sholud behave even better than fastbacks at high speeds.... I dont know for a fact or anything like that but people always complains that fastbacks usually loose the tails at high speeds...

Don on the other hands, told us that our fastbacks have almost the perfect upper tail shape but... could it be possoble that the luck of heigh and some bad air circuit on the floor can loose our tails more than in a coupe??

You say you loose your nose at high speeds?? can it be because of a bad front suspension configuration instead of aerodynamics per se??...

If you are planning on covering the bottom part of the engine bay, don´t think about heat as THE problem... heat tends to go up so if you need a ventilation you need it in the hood... Be careful about the wing effect you can create in your nose...

I´m thinking a good aerodynamic example, similar to our cars are the Chargers Dytona.. at least we are talking about a modified body to perform better... a body similar in shape and heigh to our cars....






Damián Cool

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