Aerodynamic design question - front fascia

[dave6666]

What is the purpose of the lip under the front fascia? See pic if you still have champagne in your teeth and don't know what a fascia is yet.

I can think of:

--> Stiffen fascia
--> Sacrificial grinding point for your driveway

Would removing the lip, and then replacing the structural integrity of the fascia by another means that does not protrude be aerodynamically bad?

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[plumcrazy]

i think jonB wrote up something on it a while back. i knew Dimitrios shaved it down when he installed bigger wheels and tires and saw no difference. supposedly it does do something for aero at higher speeds though...

 

[GTSnake]

The reason for the lip is to reduce the amount of airflow under the car. As you can see there are a lot of irregular surfaces under the car which would add a lot of drag.

Dimitrios didn't notice any difference because he doesn't have a multi million dollar wind tunnel and scores of aero engineers doing finite element analysis. I assure you there was a difference when he removed it.

 

[dave6666]

The reason for the lip is to reduce the amount of airflow under the car. As you can see there are a lot of irregular surfaces under the car which would add a lot of drag.

I can agree with that, but...

The air has to go somewhere. It could go to the side of the car, which is quite a ways to go from the centerline of the fascia. Or it could force the air to the road surface away from the underside of the car. Or a combo of both.

The Roe fascia kit which I obviously have likely sends some air out the hood vents.

My point in the "but..." statement is that rerouting the air to either the side or down creates drag just like the stuff on the chassis does. Should we assume that the drag from the fascia lip is the better alternative?

 

[powerstream]

it contributes to those stated above but it mainly will reduce lift. At high speeds it will keep car more planted.
As with most street aero treatments it has to be a compromise.
We used to make them long and let them burn off on the track providing the ultimate length.

For street use it would be better if that lip was flexible.
ABS plastic or Polycarbonate would be much less likely to "tear"

 

[GTSnake]

My point in the "but..." statement is that rerouting the air to either the side or down creates drag just like the stuff on the chassis does. Should we assume that the drag from the fascia lip is the better alternative?

Drag is a factor of resistance to flow due to frontal surface area. If you compare the small 2" frontal area of the lip versus all the nooks and crannies under the car it would greatly reduce overall drag. In addition, create negative pressure under the car which as stated reduces high speed lift. So it is the better alternative.

 

[plumcrazy]

i can assure he (dimtrios) drove his car fast enough and often enough to notice

 

[ViperTony]

i can assure he (dimtrios) drove his car fast enough and often enough to notice

I second that.

 

[FE 065]

Given the shape and rather rearward placement of the lip, it could be questioned just how much air is actually forced to the outside of the car by the lip vs how much air caught by the lip and just spills under it anyway.

Which would still have the air bounding off of nooks and crannies under the car.

OE cars not having the screened area as in the picture, the air has to go under the car or out to the side. This lip is tucked well under the chin, and is in fact recessed a bit through the broad center area. Making it a little harder for air in that broad center area to work its' way out to the sides.

 

[Paul Hawker]

Vipers are shaped in wind tunnels. That lip was very carefully crafted to give the ultimate airflow and still give the legal road clearance.

Not only is aerodynamic drag considered, but also aerodynamic lift.

Know on fast downhills at Silverstate and other open road events, with speeds around 165 you can certainly feel the stock GTS lifting as air packs underneath. Steering goes light, and you can feel the front wheels become less effective.

 

[plumcrazy]

i disagree, i've been over 165 WAAAAY too many times to count. I never felt anything but secure in my GTS. it feels like the car squats down even more at 160-170....

 

[Tom F&L GoR]

You can't accurately discuss one (particularly small) part of an aero package and make conclusions. Mercedes is usually pretty clever and their cars still flipped over - on a straightaway, no less. This after many hours in the wind tunnel, I'm sure.

My personal anecdotal evidence is when I added a spoiler to a '70 Duster. At 130 the rust bucket seemed like it needed new shocks and used both lanes. With the spoiler, it stayed on the road and one lane.

What we haven't said yet is how much the nose is pushed down at speed, closing the gap between the small "spoiler" and the road.

 

[dave6666]

Vipers are shaped in wind tunnels. That lip was very carefully crafted to give the ultimate airflow and still give the legal road clearance.

Then why based on Sean Roe's scientific study of fascia pressures was he able to make an improvement to the airflow with $10 worth of sheet aluminum? Solving to a point high pressure under the front fascia?

BTW, "scientific" is defined for this post as "instrument used" (manometer), and "smart person doing the measurements" (Sean).

 

[Paul Hawker]

Because Sean was looking at only one issue, additional air flow into the engine compartment for additional cooling.

Did not measure lift at high speeds, additional aerodynamic drag, etc.

I know that on high speed DOWNHILL runs the front end of my GTS got very light as if air was packing underneath. On level or UPHILL runs it felt very planted.

 

[dave6666]

Because Sean was looking at only one issue, additional air flow into the engine compartment for additional cooling.

Did not measure lift at high speeds, additional aerodynamic drag, etc.

I know that on high speed DOWNHILL runs the front end of my GTS got very light as if air was packing underneath. On level or UPHILL runs it felt very planted.

That is exactly what he was trying to solve - air stacking under the front. The result of the $10 worth of sheet metal was to redirect that air through the radiator.

Regardless of the chicken vs egg on Sean's work, if he can figure it out based on bystander observations and then a $5 manometer to analyze and quantify, I question just exactly what DC was looking at in their multi-million dollar wind tunnel.

 

[SVS Turbo]

You can't accurately discuss one (particularly small) part of an aero package and make conclusions. Mercedes is usually pretty clever and their cars still flipped over - on a straightaway, no less. This after many hours in the wind tunnel, I'm sure.

My personal anecdotal evidence is when I added a spoiler to a '70 Duster. At 130 the rust bucket seemed like it needed new shocks and used both lanes. With the spoiler, it stayed on the road and one lane.

What we haven't said yet is how much the nose is pushed down at speed, closing the gap between the small "spoiler" and the road.

I believe this flipping of the Mercedes had something to do with the air being "taken away" or "disrupted" from the cars in front?

When we raced at the Bonneville Salt Flats in 2001 we ran the STOCK Viper GTS body. We added weight to the car and I believe it was somewhere over 5000lbs. We were running the STOCK front fascia. We had a rake in the suspension setup from higher in the rear to lower in the front. We were recording potentiometers on the front suspension. At 4 miles and 240 it started to lift and at 247+ before the chute opened it had lifted 1.5 inches. This was our expierience and it has always stayed with us.

Not sure Where, When, What Weight and Speed the stock Viper GTS fascia will get too much lift to make the car go airborne and I don't want to find out
jR

 

[RTTTTed]

There seems to be a lot of incorrect assumptions in this thread.

The part referred to is called a "chin spoiler" on most high speed designed bodies.

That 'aluminium' cost is not $10 (it probably doubles in price each year). sean's kit is a lot more than the aluminium. There is fiberglas panels that attached properly keep the facias shape and strength.

The opening directs "high pressure air into the front of the radiator and most of that air would have gone under the car, raising air pressure behind the rad which creates a lower flow of air to cool the rad.

Since the hood vents to the sides much of the air pressure under the hood doesn't go under the car for better high speed handling.

The air that goes under the grille openings doesn't all go under the car, even without Seans front scoop. Since the air "stacks" up under the nose and is a high pressure area that redirects air away from the road, into the grille and even over the car. Rear and chin spoilers are designed to cause the air to roll in front and behind the car reducing friction by filling in the vacuum behind the car with a 'roll of air'. At the front some of the air rolls as well pushing the road level air higher so that it goes over the car instead of just pushing on the front. (Hopefully I explained enough tha most can understand those aerodynamics).

One of the purposes of Sean's kit is to allow the radiator to become more efficient and temp drops of 30 degrees are commonly reported. If your car overheats at the track, you don't get to finish the race. If your car runs 30 degrees cooler than the 'normal' race temp of 200 degrees your engine produces more hp!

Sean's front facia kit is such a great idea that I ordered the kit today. $219. I did look into building the pieces myself since I prefer to do everything myself, but with all Sean's research (heard about his racing experience?) and the low price of his kit I decided that it would cost me ten times $200 worth of time to get my kit as good as his - so I saved money by buying Sean's kit. wwww.roeracing.com

Ted

 

[dave6666]

That 'aluminium' cost is not $10 (it probably doubles in price each year). sean's kit is a lot more than the aluminium. There is fiberglas panels that attached properly keep the facias shape and strength.

There are no FRP parts in the kit. Just aluminum. And since I started the $10 notation, then yeah, maybe it's $20.

But in a heartbeat I would spend the $219 again because it may only be $10-20 worth of raw material, but these are engineered parts like you pointed out.

Back to topic, any more opinions on whacking what we now call the chin spoiler off and making that thing a smooth transition?

 

[Tom F&L GoR]

I looked under the "chin" of my '97 Intrepid and the "Roe-like" hole is there. So at least someone at Dodge had employed this on a previous vehicle. I don't know why it's not on a Viper, but then I have smoothie hood and can agree that at some speed, the hole that I cut may allow more air than desired under the hood. I don't go over 200MPH, either... (!)

The flying MB was said to be caused by a perfect storm - a longish flat bottom ahead of the front wheels, a car in front, a minimum speed, and cresting a hill. There were videos posted online that were filmed (is that word correct anymore?) from the side. It shows the two cars about two car lengths apart, the second car coming over the hill and then very gracefully just lifting off... The ruling body now has a maximum chin length dimension to prevent this.

Dave, I would recommend you leave the lip there to help prevent air from going under the car. There will be enough air going under anyway, but less is better. Some Corvette models have a spring-hinged flap so that they can hang lower but not get damaged over bumps. Them dang clever NASCAR guys usually figure out how to get the nose to scrape the road, so there's plenty of examples where more lower is gooder.

 

[DAMN YANKEE]

lower is gooder.....

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[RTTTTed]

Why would you even want to remove it? The best aerodynamics would be to have the bottom of the front nose that height from the ground, but then it would tear off each time you entered a driveway or went over a speedbump. My car is lowered so that there is less air under the car and the chin spoiler is closer to the ground. Grinding the chin spoiler would be like raising your car 1.5". Never heard of anyone interested in doing that.

My idea of modifying my car is to make it better, not worse. I have only hte faintest of plans for going over 200mph, but I wouldn't even try it unless I had already done all modifications that would make it safer than stock. It would be rather strange for someone to modify their Viper to make it so that a stocker would be a better ride.

Ted

 

[2000_Black_RT10]

Why would you even want to remove it? The best aerodynamics would be to have the bottom of the front nose that height from the ground, but then it would tear off each time you entered a driveway or went over a speedbump. My car is lowered so that there is less air under the car and the chin spoiler is closer to the ground. Grinding the chin spoiler would be like raising your car 1.5". Never heard of anyone interested in doing that.

My idea of modifying my car is to make it better, not worse. I have only hte faintest of plans for going over 200mph, but I wouldn't even try it unless I had already done all modifications that would make it safer than stock. It would be rather strange for someone to modify their Viper to make it so that a stocker would be a better ride.

Ted

Lowering your Gen2 Viper isn't going to do much, if anything it'll lift just as much if not more now, and removing the chin downstanding flange won't have any effect on aero, it's not a spoiler or a deflector, it's already under the fascia.. whereas a functional splitter is forward of the fascia.

The key design criteria for a race car is a small frontal area. Like a F1 car, nose having a frontal area the size of a golf ball.

The Gen2 Viper has to contend with consuming large quantities of air in it's air gulping rad opening, and just as you consume, it has to go somewhere from under the clamshell. This is why there are the large side outlets near your door hinges, to reduce air going under the car, but it still does, so technically, a lower car will reduce the gap to exhaust the air under the car causing more lift due to higher air pressure. The Gen2 has to deal with a balanced lift; front to rear, there's no downforce produced on a stock Gen2 GTS Viper. The comment about lowering a Gen2 Viper is specific, not a generalized statement because lowering has it's advantages on others which do not contend with this lift. The new 08 Viper has more forward venting on the hood which is smart, why not exhaust the air out the top hoping to achieve more downforce from exhausting consumed air over the hood.

Whack that downstanding chin flange off Dave, it's nothing more than a styling feature, it surely isn't going to deflect air out the sides and it surely isn't going to help reduce lift, and if anything.. it'll contribute to a smaller frontal area.

We do all sorts of aero work, as the original GTS-R wing, splitter, diffuser and complete body we supplied, and as in the previous pic above of the flipping Mercedes, that old race Viper carbon fiber body came out of our shop.. as well as lots of new high end race stuff.

 

[RTTTTed]

Lowering your Viper isn't going to do much, if anything it'll lift just as much if not more now, and removing the chin downstanding flange won't have any effect on aero, it's not a spoiler or a deflector, it's already under the fascia.. whereas a functional splitter is forward of the fascia.

The key design criteria for a race car is a small frontal area. Like a F1 car, nose having a frontal area the size of a golf ball.

The Gen2 Viper has to contend with consuming large quantities of air in it's air gulping rad opening, and just as you consume, it has to go somewhere from under the clamshell. This is why there are the large side outlets near your door hinges, to reduce air going under the car, but it still does, so technically, a lower car will reduce the gap to exhaust the air under the car causing more lift due to higher air pressure. The Gen2 has to deal with a balanced lift; front to rear, there's no downforce produced on a stock Gen2 GTS Viper. The new 08 Viper has more venting on the hood which is smart, why not exhaust the air out the top hoping to achieve more downforce from exhausting consumed air over the hood.

Whack that downstanding chin flange off Dave, it's nothing more than a styling feature, it surely isn't going to deflect air out the sides and it surely isn't going to help reduce lift, and if anything.. it'll contribute to a smaller frontal area.

We do all sorts of aero work, as the original GTS-R wing & body we supplied, and as in the previous pic above of the flipping Mercedes, that race Viper body came out of our shop..

So what you're saying is that if my car is 2" higher or lower is the same? So the co-efficiency of drag is not affected when the car is 4" lower? So a pickup with a bullet nose has the same CD as a Viper since their width is about the same, height is irrelevant? 2.5" or 4" makes no difference?

 

[2000_Black_RT10]

Dave, I would recommend you leave the lip there to help prevent air from going under the car. There will be enough air going under anyway, but less is better. Some Corvette models have a spring-hinged flap so that they can hang lower but not get damaged over bumps. Them dang clever NASCAR guys usually figure out how to get the nose to scrape the road, so there's plenty of examples where more lower is gooder.

Tom - I'm assuming you're talking about the C5 Corvette, which has the radiator encapsulated behind the fascia without a rad opening. This is why there is a flap under the fascia, to rake up air to cool the rad.

As we all know a lower CofG is better for cornering, and the NASCAR body is a different beast than the Viper which contends with lift.

 

[2000_Black_RT10]

So what you're saying is that if my car is 2" higher or lower is the same? So the co-efficiency of drag is not affected when the car is 4" lower? So a pickup with a bullet nose has the same CD as a Viper since their width is about the same, height is irrelevant? 2.5" or 4" makes no difference?

Think of a fighter jet 50,000 feet in the air..

There's no aero advantage in just lowering a Gen2 Viper 2", sure there is some help in lowering the CofG for cornering, but for an aero advantage, not with a Gen2 Viper because of the aforementioned lift issues.

 

[Tom F&L GoR]

Tom - I'm assuming you're talking about the C5 Corvette, which has the radiator encapsulated behind the fascia without a rad opening. This is why there is a flap under the fascia, to rake up air to cool the rad.

As we all know a lower CofG is better for cornering, and the NASCAR body is a different beast than the Viper which contends with lift.

Yes, I am piecing together a few tid-facts.

Viper fascia causes lift. Intrepid fascia has hole. Intrepid's hole relieves air pressure to radiator. So a Viper fascia hole should relieve air pressure into the radiator area also. Somewhere I saw the Viper "grill" opening was marginal in size, so the under-chin hole sounds like it should be good. Having any air dam behind the hole (as Dave has a hole now) would help add air flow to the radiator and lessen air flow under the car. This is why I thought leaving the strip in place would help.

I can see that in the absence of the hole, the small spoiler might not make any difference. Assuming also that the Cd of the undersides of any car is poor, a chin spoiler should always reduce drag - but depending on the position, may not reduce lift. Maybe this is where we are getting our comments mixed up.

And the NASCAR reference was to point out that they try to reduce lift and undercar air flow, and manage flow via ducts and dams - just as Dave was asking. If Dave's new hole is a duct, then the teeny "spoiler" could push a little more air up rather than allow it under the car.

I guess I am not clear whether you are hinting that the Roe hole still allows the front end to lift - I would assume not.

 

[2000_Black_RT10]

Yes, I am piecing together a few tid-facts.

Viper fascia causes lift. Intrepid fascia has hole. Intrepid's hole relieves air pressure to radiator. So a Viper fascia hole should relieve air pressure into the radiator area also. Somewhere I saw the Viper "grill" opening was marginal in size, so the under-chin hole sounds like it should be good. Having any air dam behind the hole (as Dave has a hole now) would help add air flow to the radiator and lessen air flow under the car. This is why I thought leaving the strip in place would help.

I can see that in the absence of the hole, the small spoiler might not make any difference. Assuming also that the Cd of the undersides of any car is poor, a chin spoiler should always reduce drag - but depending on the position, may not reduce lift. Maybe this is where we are getting our comments mixed up.

And the NASCAR reference was to point out that they try to reduce lift and undercar air flow, and manage flow via ducts and dams - just as Dave was asking. If Dave's new hole is a duct, then the teeny "spoiler" could push a little more air up rather than allow it under the car.

I'm just babbling Tom..

The hole would relieve pressure under the car.. may be good for rad air at a lower speed.. don't see any advantage for high speed.. but it's hard to say, a flush duct may not do anything at high speed depending on it's location, it could even pull / siphon air out at high speed.. all depends on the pressure under the fascia, surface velocity / pressure.. you / we know all this stuff..

Heck.. that flap (in a perpendicular shear plane) could be viewed as an upsidedown gurney.. but I'm just babbling without a proper study.. as you know, there's always a surprise in results.. such as..

Gurney flap - Wikipedia, the free encyclopedia
"Some copied the design, and some of them even “improved” it by pointing the flap downwards, which actually hurt performance"

Regardless, I really don't think Dave will see any changes with whacking the flap off. Adding ducts and holes.. just need to be aware of the resident air pressure & velocity, otherwise it may be something in a theory that sounds & looks good, but has opposite results..

 

[dave6666]

Why would you even want to remove it?

My idea of modifying my car is to make it better, not worse.

Ted

Well Ted, as I said at the beginning, one of the reasons I might want to remove it is to get rid of sacrificial grinding. The other reason, I am contemplating building a splitter that could possibly replace the entire underside on the fascia from the frame forward. Just a thought, and wanted some opinions on the purpose of the lip we now call the chin spoiler.

And on the second bit of your post I left above, I have to pull a Texas size ***?

Doesn't asking questions first and hacking parts second, qualify as an attempt to make my car better? And not worse?

 

[dave6666]

Tom and 2000 Black...

I think after reading all of your posts, including the ones by others, that my plan might be formulated as this:

- Trim the spoiler down just enough to even it out. Make it purdy under there for some fresh paint.

- Redesign my thoughts on the splitter to keep the nose otherwise intact in present form. Which a splitter should maybe compensate for a loss of downforce, if there is any, from trimming up the lip underneath.

 

[Paul Hawker]

Just a side note, that Chevy has mentioned that their new Z1 is having a problem with lift near top speed. While Viper ACR produces massive downforce, Chevy is getting light in the front.