Internal Body Aerodynamics

[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

 

[69427]

Don Sherman article on corvette adaptive aerodynamics.



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

Just some thoughts/comments on a couple items in the link:

The variable ride height system. Unless the ride height change is just a temporary thing to get over some speed bumps, I've always wondered what the weight/complexity is of a system that allows the ride height to be changed (ie: dropped for track days) without causing the suspension geometry to go all to hell.

The (IIRC) second to the last drawing shows a wing on top of the car. I have kicked around that idea for the past twenty years. I don't care to permanently change the body styling of my car, so a temporary wing that "plugs" into the t-top roof structure appeals to me. What has stopped me so far is any actual data to show that the item would work decently on an older C3, given the "steep" windshield angle (possibly requiring a higher wing height to engage the air coming off the windshield) and the crappy aero shape behind the rear window. Also, a wing that high does cause a moment arm that tries to lift the front end, negating some of the downforce that the centered position tries to put on both axles. IIRC, the Chaparral 2H had a center mounted wing. I recall there were multiple teething pain issues with that car, so I don't know if that wing ever got a real chance to show its worth/potential.

If I had private access (to get away from all the "ricer" comments from others in attendance) to a track or large skidpad, I'd sure like to play with a "bolt-on" wing there to see if it was a productive use of additional weight and drag on the car.

 

[phantomjock]

69427- Yeah the 2H had a lot of problems - mostly one of visibility as I recall. Center-up is a pretty goop position for generating downforce, need to watch aerodynamic pitching moment, but should be favorable to the front wheels. You could tailor that by choosing appropriate NACA wing section.

I like the duct from rear wheel wells to the aft deck. Could be very helpful. Maybe a couple of 12v fans in the duct on a variable speed controller?

If you think of this mirror as the roof, here is a quick and dirty approach:

Well, just use half of the mirror idea.

I just ignore the ricer comments... I'm looking for performance - not acceptance. Here is another guy like that:



I just had a "FLASH OF BRILLIANCE!" pull pressure from the wheel well - a good idea. But you could use the rear deck and a bit of aero-"worK" above it. Then plumb/weld/etc your exhaust through those (or other) vent to fill and provide downforce.
I'll go dig up a few F1 ideas on "Blown Exhaust" diffusers, that are running around.

Cheers - Jim

 

[phantomjock]

Just an idea or 2 to think on for that rear deck:




And remember a wing over a flat deck can have a huge increase in performance. This exhaust would boost that and fill that area (maybe? TBD)

Cheers Jim

 

[rtj]

Just some thoughts/comments on a couple items in the link:

The variable ride height system. Unless the ride height change is just a temporary thing to get over some speed bumps, I've always wondered what the weight/complexity is of a system that allows the ride height to be changed (ie: dropped for track days) without causing the suspension geometry to go all to hell.

Good points, patents can be vague, but I might look it up. Probably optimize geometry for track/road race height settings.

 

[rtj]

C8

https://www.caranddriver.com/amp/news/mid-engine-chevrolet-corvette-c8-wild-active-aero-patents


Phantomjock, your Venturi/ejector exhaust idea is in there.

Google patents is giving a lot of red herrings.

 

[phantomjock]

Right, The GM engineers have had their eyes open!

Here is "sort of" what I was thinking for the rear deck exhaust mod:


You'd probably want it operational "at speed", so an electrically operated (solenoid) and a set of cut-outs could be installed into the normal underbody exhaust. At speed, the switch opens the cut-outs, the exhaust diverted to above the deck, and voila, drag decrease and increase in downforce.

A low, deck-wide airfoil section (wing) could be installed inches above the deck and the exhaust blown under the wing.



DISCLAIMER: My CAD is down, and I lost my box of crayons - so its just in pencil...:nuts:

Some engineering of the exhaust size, flow, etc may be required. Perhaps the exhaust could use design details from the Boom-Tube approach?

Cheers Jim

 

[BBShark]

Just an idea or 2 to think on for that rear deck:




And remember a wing over a flat deck can have a huge increase in performance. This exhaust would boost that and fill that area (maybe? TBD)

Cheers Jim

Is that some kind of scavenging effect?

 

[mfain]

Some time back in one of the road race bodies (can't remember which) guys were dumping the exhaust into/under the diffuser with some success. It got outlawed because when you let off the throttle you change the exhaust pressure and ultimately the downforce which resulted in loss of control in critical cornering situations. I kind-of have my diffuser set up that way. Moveable side skirts have, in the past, been outlawed for the same reasons - abrupt changes in downforce when the system fails (skirt sticks) which alters the car's downforce. Changing downforce rapidly or unexpectedly in a near-the-limit cornering situation can get your attention! In the rear I use a spoiler to lengthen the deck, a wing, and a diffuser. They should work together to both create downforce and smooth and reaccelerate the flowing air to blend with the free airstream behind the car, reducing drag. The wing is cockpit adjustable using an aircraft trim motor, but I haven't decided if I will try to mechanize it so it is active. Mary Pozzi and her husband ran an active wing on their pro-touring Camaro for a while, with some success.

Edit: Notice the grill where the license plate goes in the first photo. That is the exit duct for the forced (fan) cooling air coming from the rear end cooler. The inlets for the diff. cooler air are the side scoops just forward of the rear tires. There are also rearward facing louvers in the diffuser that exit air from the transmission cooler. The Corvette race teams have a rear grill (much larger) that exits air from the driver's air conditioner unit. They caught a lot of grief early on because they were using a very large fan and it was putting out a lot of forced air.

Pappy

 

[mfain]

69427-
If you think of this mirror as the roof, here is a quick and dirty approach:

Well, just use half of the mirror idea.

I just ignore the ricer comments... I'm looking for performance - not acceptance. Here is another guy like that:


Cheers - Jim

The gated extensions you see on the back of tractor-trailer rigs were designed by NASA at the Armstrong Flight Research Center to reduce drag and increase fuel economy - they work pretty well. Have you noticed that sometimes a NASCAR car runs faster with the tail panel knocked off? NASCAR won't yet let them have a diffuser that runs from the fuel cell to the bottom of the rear panel to clean up the "parachute" that the hanging rear panel creates. Also NASCAR "squares off" the back of the rear quarters on super speedway cars to reduce drag while they do not do that on short track cars where downforce, not speed, is important.

Pappy

Pappy

 

[mfain]

Here are a couple of shots of the exit air louvers in the diffuser that I mentioned in an earlier post. The larger openings are where the exhaust exits.

 

[phantomjock]

PAPPY:

In the rear I use a spoiler to lengthen the deck, a wing, and a diffuser. They should work together to both create downforce and smooth and reaccelerate the flowing air to blend with the free airstream behind the car, reducing drag.

Another advantage of the spoiler - it makes a significant increase in the wing's effectiveness. WIG - Wing in Ground.

These from Katz: [Multi-element]


[Single element]



Blown Diffusers: Nice comprehensive article here:
http://www.formula1-dictionary.net/diffuser_blown.html

CF Ducting from below cabin to slit exits skip the exhaust if a concern.

Cheers - Jim

 

[phantomjock]

It's a dirty job --- Duct Work

:search:

Here is an idea that looks really well done:


That is on a Porsche 917 - Long Tail. It appears they built these to provide cooling to the transaxle:


The scoops they used on deck were located below the wing:


Nice integration of the ducts to the rear deck shown here:


With a little imagination, you can see how this approach could be reversed to pull air from the high pressure region in the rear wheel wells and dump on the rear deck of the coupe C3s as previously mentioned.


I keep looking at these ideas and continue to "over-think" my C3 bodywork. Ahhh, madness.

Those linkages -- if interest, I'll add details on those too.

Cheers - Jim

 

[rtj]

[ame]https://www.youtube.com/watch?v=k5t2Tt4lWoM[/ame]

 

[phantomjock]

WOW! Thanks for that.

I like their "car-inside-a-car" approach for the rear end. It suggests some interesting opportunities using Carbon Fiber "appendages" over existing shape to further improve overall aerodynamics, by creating some internal volumes and shapes with improvements at the same time. [Time Reference 3;45 to 3:58.]

Thanks for posting that one!

Cheers - Jim

 

[rtj]

Another recent one.


He mentions the wing, but looks like lots of aero work was done that is not discussed.

[ame]https://www.youtube.com/watch?v=P37q2Ju5nr0&app=desktop[/ame]

 

[rtj]

Kind of interesting. Mostly computer generated I think.

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

 

[rtj]

Check out that grill-spitter arrangement. I would have guessed more HP, after seeing the roll cage.


[ame]https://www.youtube.com/watch?v=IDtHjIe13aU[/ame]

 

[rtj]

[ame]https://www.youtube.com/watch?v=7ts_j5Pmwb0&app=desktop[/ame]

Some good views of the car.

 

[rtj]

[ame]https://www.youtube.com/watch?v=gXSL8dPrsJk[/ame]

 

[rtj]

[ame]https://www.youtube.com/watch?v=kBu8oD69IZQ[/ame]

Lots of aero. Those active fender flaps are extreme.