Aero improvement, or roadhugging weight?

[69427]

Had a few moments to kill this afternoon and kicked around some possible aero mods for the '69. Every time I'm under the car doing stuff I'm amazed at how "rough" the underside of these things are. I know a lot of guys complain about the frontend lift (at speed) of C3s, and although I've never had much complaint about the high speed manners of my car, I thought it might be amusing to see if I can still improve the car's manners a bit. I have no illusions that I could actually create downforce with "tack on" mods, but if I can reduce the lift force a touch, it might be worth the effort (and the few pounds of sheet aluminum). Here's some rough cardboard templates of the shapes from the spoiler back to the oilpan (the width was limited by the size of the cardboard):



And under the drivers floor area.

There's a bunch of detail stuff I was also pondering, such as endfences and vanes to try to contain or direct the air. I'm just thinking about possible stuff for the front half of the undercar, as I don't think it's possible to clean up the front enough to keep the back half from turning turbulent anyway.

 

[SmokinBBC]

I was thinking about this just yesterday. I was replacing the heat riser gasket that was leaking. While the car was on the jacks, I was looking at how the air flow would be affected by the oil pan, splash shields, etc. And how effective are the egg crates at moving air out of the engine compartment.


I parked it in the garage after taking it for a ride to heat cycle the gasket/donut(leak fixed). While laying on the ground and listening, I noticed that the engine compartment air(moved by the fan) was coming out almost entirely from the passenger side about the middle of the car. Very little flow through the egg crates and no flow out the dirver's side. The car was in the garage and there was no wind or other external air flow to direct the air in other directions.

I have been thinking about why it is like this. Is it because the engine is offset to the passenger side? Is it becasue the big block takes up so much room that a good strait through flow is not possible. Or, maybe the bell housing collar is forcing the flow to the dirver's side versus going strait through and exiting out both sides and the back. Right now I'm thinking it's a combination of the three and the collar is forcing air to the passenger side.

Anyway, if you put plates under the car to manage the external airflow better at speed, how would that effect the airflow coming through the radiator? Does that airflow coming through the radiator need to be managed at speed? Based on the flow at idle, I can imagine the flow at high speed would be significant.

Or, doesn't this matter at all?

 

[JeffP1167]

Hmmmm is that really a 69? Because I see some coil overs and some mighty strong looking a-arms. Even the back don't look C3'ish ....

 

[Twin_Turbo]

he fitted it w/ C4 susp.

 

[JeffP1167]

he fitted it w/ C4 susp.

That makes him my new best friend hahaha

 

[denpo]

he fitted it w/ C4 susp.

That makes him my new best friend hahaha

He's mine too, he sold me for peanuts those bilstein shocks he couldn't use anymore :friends:

 

[mrvette]

he fitted it w/ C4 susp.

That makes him my new best friend hahaha

He's mine too, he sold me for peanuts those bilstein shocks he couldn't use anymore :friends:

Good man, no question, helped me out too.....

 

[clutchdust]

1) you can't clean up the bottom enough
2) whatever weight you add, it's below the CG so the addition is less relative.
3) I doubt you will notice any difference in stability or fuel economy for anything less than 80mph, but that still wouldn't stop me.

 

[phantomjock]

Rear Diffuser too! I like the approach. What materials will you use - fiberglass sheet with a foam core - or sheet aluminum?
Cheers - Jim

 

[BBShark]

It strikes me that the plates in the front would still allow just as much air to get under the car so, to be effective, the plates would need to cover as much of the bottom as possible.

It might be more effective to barge the air away from the underside of the car and out to the sides.

 

[mrvette]

Totally different car, obviously, but we noticed NO change in fuel economy when leaving the 'fairing' plastic off Linda's Escort....finally tossed it....

:smash:

 

[metalkid]

Rear Diffuser too! I like the approach. What materials will you use - fiberglass sheet with a foam core - or sheet aluminum?
Cheers - Jim

:thumbs:Yeah, I'd like to see a rear diffuser too!

 

[TimAT]

Don't forget that the guys back in the late 60's were making speeds in the 160-170 range with pretty simple stuff-=


I look at that space aft of the axle everytime I look under the car. Other than a good starting point at the front edge, a sheet of .030 7075-T6 would be pretty stiff with a couple of ribs, and all sorts of places to mount it.

 

[69427]

Rear Diffuser too! I like the approach. What materials will you use - fiberglass sheet with a foam core - or sheet aluminum?
Cheers - Jim

I'm just using aluminum, as it's quick and simple to cut and weld if necessary. I need to get some more large sheets of aluminum to do the front undertray, but in the meantime I used some smaller remnants and attached them just forward of the footwells to reduce the amount of air exiting the radiator that goes under the car, hopefully persuading some of it to exit the side gills instead. A couple pictures during assembly:





I also bought a low pressure differential pressure gauge recently that I'm going to use to get some idea of the pressure levels under the car. Given the large horizontal surface area under the car I'm trying to get an idea of the lift forces at speed.
Also, depending on the pressure levels under the car (and what some wool tuft pictures look like), I'm contemplating modifying the rocker panels to drop the bottom edge down at least two inches. I think the look will be visually innoticeable, but it might help control some of the airflow under the car.
As I mentioned before, I have no visions of adding great levels of downforce. I'm just trying to reduce the level of lift that these cars are famous for.

 

[enkeivette]

I think it's a fantastic idea, I've thought about doing it myself. I remember the first time I saw a viper at the dealer in the 90s, I couldn't see anything underneath the car. It was all plated off.

What thickness sheet metal would you use?

 

[Twin_Turbo]

Back when I was still tinkering on my heap I actually changed the floor on it, I had a flat floor in the making, both on the inside and with mounting for belly plates. I'd like to see your finished product, keep it going!!!

 

[rtj]

There is a plot in a book called something like "Road Vehicle Aerodynamics" that shows a front air dam reduces drag. Lower is better if I remember correctly. Basically stops flow going underneath. Adding side skirts is shown to aid also. The SAE still sells it last time I checked.

 

[metalkid]

Let's add some 3/8 " or 7/16" thick Delrin strips with a routed leading edge, to bolt on to crossmembers for speed bump frame protection.:thumbs:

 

[69427]

I think it's a fantastic idea, I've thought about doing it myself. I remember the first time I saw a viper at the dealer in the 90s, I couldn't see anything underneath the car. It was all plated off.

What thickness sheet metal would you use?

I just picked up a 4x6 foot sheet of .063" (1/16") aluminum yesterday. This stuff looks to be reasonably light yet thick enough where it won't wrinkle or droop.

 

[69427]

What kind of plastic is this?

I picked up a couple free pieces of 1/8" plastic (about 16"x72" each) at my favorite junkyard today. I'm thinking on using them to fashion some replacement/deeper rocker panels for some aero testing amusement. Is there an easy way to tell what type of plastic it is (plexiglass, lexan, etc)? In addition to dropping down the bottom edge I'd like to heat/bend the bottom one inch of it (outward) in an effort to kickout any air that drops down vertically onto the rockers.

I'm just curious what type of plastic it is. Is there an easy way to determine what this stuff is?

Thanks,
Mike

 

[BBShark]

Is the sheet clear? If it's Lexan (polycarbonate) you can bend it almost like aluminum. If it's Plexiglas (acrylic) it will break.

 

[69427]

Is the sheet clear? If it's Lexan (polycarbonate) you can bend it almost like aluminum. If it's Plexiglas (acrylic) it will break.

The stuff is smoked (I haven't tried to scratch it to see if it's more than just skin deep). I'll try to bend a small section later to see what happens, but it appears to have been part of a display case so I'm guessing it's just plexiglass.

I'll also heat and bend a small section to see if that works out.

 

[69427]

Back when I was still tinkering on my heap I actually changed the floor on it, I had a flat floor in the making, both on the inside and with mounting for belly plates. I'd like to see your finished product, keep it going!!!

Made a little progress today. Got the splitter/forward pan roughed in. It's 52" wide by 48 inches long. The angle of the photo doesn't show how far forward the PC spoiler front lip is. It's amazing how much area I had to cut away to clear the tires at full lock.



Still have some clearance cuts to make around the LCA pivots. Also, it looks pretty straightforward to find several areas to position hanger brackets to support the sheet. I'm mulling over riveting some side fences around the thing to discourage the top and bottom air from mixing easily.

I hope this thing earns its keep. While it's only 7 pounds, I hate adding weight to the car, especially when it's centered over the front axle.

 

[phantomjock]

It looks great from this angle.
It would be so kind of you to share a sketch with dimensions in the downloads section - or here before you mount it more or less permanently.
The Notch for the oil pan may be a bit different - but the wheel clearance, etc would be close I'd wager.

Cheers - Jim

 

[69427]

It looks great from this angle.
It would be so kind of you to share a sketch with dimensions in the downloads section - or here before you mount it more or less permanently.
The Notch for the oil pan may be a bit different - but the wheel clearance, etc would be close I'd wager.

Cheers - Jim

No problem with posting up the dimensions of this when I get it finished. Please remember that the dimensions are for mounting under an aftermarket PC spoiler (early C3), and that my front crossmember lower surface is very different shape (flat, versus round) which allows the tray to extend back to the oil pan.

 

[turtlevette]

It will be very interesting to test that. Do you have a place where there is a decently long straight where you get up to 150+? That may trap air underneath the front and make it want to take off.

I took off the lower valance with spoiler opening up the underside of the front nose and surprisingly enough the high speed stability got better. I'm guessing 135 achieved by my wimpy motor on the Mosport straight.

 

[Twin_Turbo]

To stop it from doing that you will need to fab a splitter up front to direct air away from the car, easiest done by extending the lower air dam with something like a garage door sealer strip.

 

[69427]

It will be very interesting to test that. Do you have a place where there is a decently long straight where you get up to 150+? That may trap air underneath the front and make it want to take off.

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

Well, 130-140 (indicated) is all I've seen at the tracks I've been to (I make no claims to the accuracy of my speedo at track speeds).

I don't see where trapped air is going to be a worse issue than before. Given the rough underbody shape of the (stock) volume just prior to the front crossmember, this flat tray ought to reduce the amount of air that gets trapped in there.

Here's an old picture of when I was swapping the body onto the modified frame. This gives a little better view of the front of the spoiler. Note that it has about a 4-5" horizontal section that acts like a modest splitter. The tray I'm putting in just helps improve the function of the splitter.



While I've been messing with the tray installation I've been kicking around putting a couple vanes/fences underneath it to direct a little more air to the rotors. If I can get it right it ought to help brake cooling a touch, and also reduce the volume of air that eventually reaches the underfloor area.

 

[68/70Vette]

What about the rear of the car? It's like a cookie cutter upside down for aerodynamics. I'd don't have spare tire carrier. I've thought about an aluminum floor pan extending from the rear axles back to the end of the car.

Looks like you've a Richmond tranny or maybe the Doug Nash predecessor. Five speed or six speed?

Thanks

 

[69427]

What about the rear of the car? It's like a cookie cutter upside down for aerodynamics. I'd don't have spare tire carrier. I've thought about an aluminum floor pan extending from the rear axles back to the end of the car.

Looks like you've a Richmond tranny or maybe the Doug Nash predecessor. Five speed or six speed?

Thanks

Nash 5spd. Swapped out the original 3.70 rear and have been bouncing back and forth between 3.08s and 2.73s.

I'm still trying to figure out what can be done at the rear. Most of the airflow at that point is probably going to be pretty turbulent. Right now I'm just concentrating on trying to minimize the quantity of air that makes it back that far.