Internal Body Aerodynamics

[rtj]

No doubt Mercedes has done extensive tunnel testing and design. Wouldn't it be fun to see details?

Cheers - Jim

In the old days, when the underbody was a mess on production cars, the rule was a low front spoiler. They’ve definitely done some work.

 

[phantomjock]

In the old days, when the underbody was a mess on production cars, the rule was a low front spoiler. They’ve definitely done some work.

That is what I'd like to see. It is "easy" to add panels to the underside - when in the design stage, we have to (get to) make up the difference on a classic, yet dated vehicle. But, there are some significant gains to be made!

My previous was a long post - but should have enough detail that might push someone over to the "Dark Side" of Drag Reduction. Rabbit hole- ANYONE?




Cheers - Jim

 

[vette427sbc]

Besides the transmission, the rest of the underside can be smoothed pretty easily if the crossmembers are "flushed"
I'll work on the belly pan once I've got some miles on the car and my bank account recovers but I made sure the major frame mods were done in preparation. I took the red pill a few years ago!

 

[69427]

A few years back I made a bunch of aluminum panels to smooth the underside of my '69. The first thing I noticed was it's a PITA to retrofit this stuff to our antiques. It's difficult/impossible to hide the undercar exhaust system, so that gummed up the works. In addition, the stock transmission crossmember is an inch or two lower than the rest of the frame. This means either having a semi-circle shaped bellypan (bad), or a much lower pan in front of the crossmember, with an upswept portion behind the crossmember (potentially good), or removing the crossmember and replacing it with a new/modified piece that packages at the same plane as the rest of the frame (my choice at the time).

I eventually removed the panels behind the engine for multiple reasons. I couldn't quantify how much (if any) lift/drag reduction there was after these efforts, the assorted aluminum sheets weighed in the 20-30 pound range (I hate adding weight to my car), and it made driveline maintenace even more difficult/time-consuming than it already is on our antiques.

I do still try to run a pan of sorts under the front portion of the car, as that's where I have some confidence that the air hasn't already gone turbulent, and I'm searching for more front grip (less lift) wherever possible.

Regarding the transmission crossmember, I always disliked the visual nature of it hanging down below the level of the rocker panels. Several years back I kicked around the idea of buying an extra set of panels and slicing them lengthwise, and then adding 2-3" of material in the slice area to hide the crossmember. To see if the thicker look was "acceptable" I cut out some panels from some Lexan I had laying around, and then painted them black. I personally like this look better than the stock rocker look, and, when time permits, I'd like to do some wool tuft measurements to see if this lower panel (skirt) helps reduce air entering under the car from outside.

Just my thoughts and experiences at the moment. I am enjoying this thread. It's a welcome change from the "what thermostat temperature should I run" found too often at other sites.

 

[BBShark]

Regarding the transmission crossmember, I always disliked the visual nature of it hanging down below the level of the rocker panels. Several years back I kicked around the idea of buying an extra set of panels and slicing them lengthwise, and then adding 2-3" of material in the slice area to hide the crossmember. To see if the thicker look was "acceptable" I cut out some panels from some Lexan I had laying around, and then painted them black. I personally like this look better than the stock rocker look, and, when time permits, I'd like to do some wool tuft measurements to see if this lower panel (skirt) helps reduce air entering under the car from outside.

I don't like the look of the crossmember hanging down either. At the very least GM could have angled it up so that it was flush with the bottom of the frame side rails.

Do you have any pictures of what you did?

 

[rtj]

Another over-the-top example of what the ultra high end cars are doing. The “tail” is like the Prius picture (posted earlier) but on steroids.

[ame]https://www.youtube.com/watch?v=wWNjRYOHZts&app=desktop[/ame]

I wasn’t going to post, but the wheels are reminiscent of the old bbs fans. Two bbs fans on eBay for 1000$ right now.

 

[rtj]

As far as the c3 trans crossmember, couldn’t you just cut the bottom off, reduce the height and reweld. Better yet just a chrome moly tube crossmember. The first would still allow factory exhaust.

 

[phantomjock]

Not a happy situation;


But the image shows another clever approach - Boom-Tubes - Totally flat. Make the crossmember the mod rtj suggested - and go side exit! Not so "neighborly" at 3 am I suppose.

The belly pan could be "sculpted" around the exhaust, with little loss in efficiency - and good cooling too.

Cheers - Jim

 

[phantomjock]

Following up on the Mercedes testing, here are relative comparisons of several "tail mods":


Porsche went down this route with the 917s; Can-Am, Le Mans, etc:


This has caused me to rethink the rear Kamm approach I was building on Elvira. Ahh too much thinking...

Cheers - Jim

 

[phantomjock]

vette427sbc -

... Id love to do some yarn tuft testing and hopefully some highway smoke testing too. If all goes as planned that should be possible this summer! And the clear VGs are a pretty cool idea... Ive been trying to figure out how to make them look good on the roof. The pre-79 coupes need them badly.

The pre-fastback C3s have a number of ways to work the "drag/turbulence-bubble." The fastback mod I showed on a Miata is one way. I think there is some goodness to be had with VGs too. But I came across another idea or two, you could put in your test plan for next summer.

First comes from Chevy. The 2019 Chevy Silverado did some mods starting at the back of the truck. They realized they had smooth flow over the top the cab, but was then being disturbed by the cargo bed, just like a "notch-back" C3 coupe. This was causing significant drag. Their design-nerds came up with this.



They were working to beat the typical turbulence-bubble formed on a pick-up:



A lot like the sort of bubble that forms on rear deck of the early C3 'vettes. They added two small spoilers. The first is at the back edge of the cab. It directs air up and lengthens the air stream so it then hits the second spoiler on the top of the tailgate. I've looked for close up pics of these, but no luck. Chevrolet found that this and other changes increased aerodynamic efficiency on the new Silverado by 7%.

Vortex generators on the trailing edge might do the same thing. It would be useful to test alone, then with the trailing edge spoiler on the deck.

The second approach uses coroplast and tape:



This cut-and-paste would be cheap, fast, and easy (Yeah - I know - pick only 2). These approaches might be as - or nearly - as effective as the fastback mod. At least you'd most likely see some significant improvement.

Looking forward to your testing!

Cheers - Jim

 

[69427]

Regarding the transmission crossmember, I always disliked the visual nature of it hanging down below the level of the rocker panels. Several years back I kicked around the idea of buying an extra set of panels and slicing them lengthwise, and then adding 2-3" of material in the slice area to hide the crossmember. To see if the thicker look was "acceptable" I cut out some panels from some Lexan I had laying around, and then painted them black. I personally like this look better than the stock rocker look, and, when time permits, I'd like to do some wool tuft measurements to see if this lower panel (skirt) helps reduce air entering under the car from outside.

I don't like the look of the crossmember hanging down either. At the very least GM could have angled it up so that it was flush with the bottom of the frame side rails.

Do you have any pictures of what you did?

Yeah, I got as far as uploading it to VM, but I'm doing something wrong trying to get it onto this post. Not sure what I'm doing wrong.

Edit: Oh, there it is. I thinned out the crossmember arms to allow me to run the bellypan further back at the same height as the pinion bracket crossmember.

 

[BBShark]

So you did modify the crossmember to remove the part that hangs down.

I was thinking you added material to the bottom of the rocker panels to hide it. Any pictures of that?

I inserted the picture in your post. To do it easy, go to "my images" and copy the text in the "BB Code IMG" and insert it in the post. You can insert pictures hosted here at any website.

 

[rtj]

This guy has some interesting fab videos. I liked his floor work. Way better than anything l could do.

[ame]https://www.youtube.com/watch?v=8LM_8ULos6U&index=25&list=PLAxdsA1NmxpHkbfKFFs3CHpcxvbD292fV&t=0s[/ame]



69427,

I like the cross member mod.

 

[69427]

So you did modify the crossmember to remove the part that hangs down. Yeah, that's the old steel crossmember I narrowed heightwise. I eventually replaced that crossmember with a lighter aluminum piece of reasonably similar shape.

I was thinking you added material to the bottom of the rocker panels to hide it. Any pictures of that? Before I went to all the work of sectioning and 'glassing in additional material in the stock type rocker panels to drop the lower edge a couple inches, I just did a quick cut (and paint) of some Lexan sheet I had, to see if I liked the lower look. I do, and the Lexan looks fine enough for my present needs.

I inserted the picture in your post. To do it easy, go to "my images" and copy the text in the "BB Code IMG" and insert it in the post. You can insert pictures hosted here at any website.

Thanks for the help with the picture. (We'll see if I need help once again with this post.)



Hot damn! It worked. (This is just an old picture from a sequence I had to send to my insurance company to get them to understand why my 50 year old Chevy is worth more than $500 if it was stolen.)

The rocker appearance is not NCRS compliant, but I like the proportions better than the stock rocker, and so far no one at any of my track days has mentioned any objection to my alteration here.

 

[phantomjock]

THAT'S NICE!

Guess it also qualifies as a side skirt too. Did you have the belly pan on at that point or when you took it off kept the rocker?

Is that the rattle can Lexan? Lexan is what Jim Hall used on the 2J (sucker-car) side skirts.

Cheers - Jim

 

[69427]

THAT'S NICE!

Guess it also qualifies as a side skirt too. Did you have the belly pan on at that point or when you took it off kept the rocker? I had both the rocker "skirt" and the bellypan for a while, but kept these rockers after I pulled off the pan material aft of the engine.

Is that the rattle can Lexan? Lexan is what Jim Hall used on the 2J (sucker-car) side skirts.

Cheers - Jim

That's the rattle can Lexan. You can beat on or bend this stuff pretty roughly, "and it keeps on ticking". After being impressed with this stuff I bought some more of this for my front pan material (Makrolon brand this time).

Hall's 2J car was the first time I ever heard of Lexan. When I was a kid I was a big fan of his earlier 2E car. While the 2J certainly was "groundbreaking", its older sister was better looking (and sounding! :amused.

 

[69427]

...........

I wish they had shot video from the side views. Would get to see the advantage of a "fastback" mod for the earlier C3s.
OOPS - Just found a "still":


.............................

Cheers - Jim

An item I've thought about for a long time and have wanted to do (but haven't come up with a "clean" way to do it yet) was to somehow connect (through tubing or air channels/ducting) the unwanted air pressure under the cabin floor, to the vents behind the rear window. I've always wondered if allowing some air flow between these two points would reduce lift both under the floor area and the rear deck area. I suspect it would reduce drag a touch also.

 

[vette427sbc]

An item I've thought about for a long time and have wanted to do (but haven't come up with a "clean" way to do it yet) was to somehow connect (through tubing or air channels/ducting) the unwanted air pressure under the cabin floor, to the vents behind the rear window. I've always wondered if allowing some air flow between these two points would reduce lift both under the floor area and the rear deck area. I suspect it would reduce drag a touch also.

Heres an idea... Take the high pressure air from the rear wheel wells and duct it out those vents... close proximity for easy ducting...
Your car looks great with those later aluminum wheels!

 

[rtj]

Don Sherman article on corvette adaptive aerodynamics.



https://www.hagerty.com/articles-vi...uggest-chevy-is-getting-tricky-with-downforce

 

[phantomjock]

Pretty Awesome. I dove after the side skirt (also adaptive) patent. Very good explanations.

SCCA is going to have to come to grips with these adaptive features as the GCR still states:

Active Aerodynamic Devices – No active aerodynamic devices are permitted. These include, but are not limited to, those that allow any degree of freedom in relation to the entirely sprung part of the car (chassis/monocoque), movable or hinged skirts, or that can be adjusted from within the cockpit. Adjustment of aerodynamic devices may only be made by mechanical changes performed from outside the car.

also germane:

Flat Bottom – A race car construction in which the underside of the car is nominally flat and contains no “ground effects” shaping or ducting.

Ride Height – The distance from level ground to the specified portion of the car, with the tires, wheels, air pressure, etc., as normally raced.

They do not address Side Skirts in the Technical Glossary, but add details in various classes that they are permitted if OEM, or Aftermarket as long as ride height minimums are met.

I think - I'll have to check -- that the Super Production Classes (Over/Under 2.5L) do not specify a minimum ride height.

Cheers - Jim