C4 Aerodynamics

[Guest PURPLE95]

I'm looking for some info/opinions on this deal. I have a 95 coupe and the car feels unplanted at speed. I have an 85 IROC Z that I built for road racing and that car felt the same way when it had the puny factory spoiler on it. I built a spoiler for the car approx. 6 inches tall and the car became planted. On the track the car was confidence inspiring. This is the feeling I would like to get out of my Vette. I have looked all over the web and in many cataloges for a decent looking spoiler, but in my opinion they all look like a piece of furniture bolted to the back of a car. I have heard mixed opinions on the "ricer" style spoiler, but that is what Chevy uses on the C5 & C6R. If in deed that type of spoiler works on those cars, why would you not want to use it on a C4? I'm kinda stuck between the standard old style "NASCAR" type of spoiler or the up level F1 type. Also is the C4 actually designed to make downforce at speed the way the car is stock? I know that there is laminar airflow down the rear window, but the extremely small lip on the top of the rear bumper cover makes me wonder if there is any downforce there. Any help here would be appreciated. Thanks.

[HOXXOH]

I don't know the answer you're looking for, but I'm curious how fast in MPH is "at speed"?

[Manthony]

I did aerodynamics at GM for a summer in college before I went to the aerospace industry. I've seen all kinds of crazy stuff done on cars in the name of aero or downforce, but the reality is that it's actually very difficult to get a car to be aerodynamically balanced at a variety of speeds. What's good at 40mph could be bad at 75mph and downright dangerous at 120mph. Likewise something good for 120mph is likely to be completely ineffective at 40mph. So without having ever actually done any wind tunnel or CFD on a C4 specifically, all one can do is postulate. First, I'd be very surprised if the flow coming down the rear window of a C4 is still laminar. Fluids in general really hate expansion so the chances it doesn't trip turbulent going down the window are pretty slim. I'm not saying your wrong necesarily, I'd just be very surprised. The primary reason that you see rear spoilers on race cars being mounted "ricer" style is because that's the easiest way to increase downforce in a predictable way. By raising the wing away from the rear deck lid (preferably above the roof height if possible), you get the wing out of all the nasty flow caused by the body of the car. This makes the wing behave more like it should in theory without having to do all the analysis to understand the interactions. Makes it a lot easier to figure out what kind fo wing you should use to get x amount of downforce. That said, if you do all the analysis, you can sometimes get the same downforce with an integrated spoiler without as much drag. A good example of this is the C6 Z06. You'll notice that compared to the base model, it has an exaggerated lip where the rear brake light is. My guess is that provides just the right amount of exta rear downforce for the Z06. All of that said, there seems to be a common misconception that adding spoilers to get more downforce is a good thing. The reality is, if you're not careful, it can actually be a very bad thing. Anytime you're creating downforce in the back, you are also creating drag (lower top speed). More importantly, both the additional downforce and drag create a rotational force (moment) on the car that can lighten the front end. The point is, adding a huge rear spoiler without a properly designed accompanying front splitter can be very dangerous. Using your example of the C5R & C6R, they both had large raised spoilers, but they also both had well designed front splitters to keep the car balanced and prevent the nose from getting light at speed. Having both a splitter and a spoiler gives you balanced downforce. In the end, it all depends on what you're going to do with the car. If you're doing road racing, a spoiler/splitter combo would probably be a good idea. If you're on higher speed circuits, a spoiler without a splitter would probably induce understeer (you'd have to play with your suspension set up too). If you're jsut using the car on the street, just a spoiler probably won't hurt you. If you do a lot of freeway driving (read speeding) then you might want to be careful. Too much wing could at best cost you gas mileage (if you care) and at worst cause you to lose steering control. Sorry if that reply got a bit long-winded. Just my 2 cents, take it or leave it. Reality is, most people use spoilers for looks more than anything else :-)

[ChadC]

right on, thanks for the wisdom.

Purple95,

you could always copy these people.

..they knew a thing or two about going fast

[TheCMSH]

I did aerodynamics at GM for a summer in college before I went to the aerospace industry. I've seen all kinds of crazy stuff done on cars in the name of aero or downforce, but the reality is that it's actually very difficult to get a car to be aerodynamically balanced at a variety of speeds. What's good at 40mph could be bad at 75mph and downright dangerous at 120mph. Likewise something good for 120mph is likely to be completely ineffective at 40mph. So without having ever actually done any wind tunnel or CFD on a C4 specifically, all one can do is postulate.

First, I'd be very surprised if the flow coming down the rear window of a C4 is still laminar. Fluids in general really hate expansion so the chances it doesn't trip turbulent going down the window are pretty slim. I'm not saying your wrong necesarily, I'd just be very surprised.

The primary reason that you see rear spoilers on race cars being mounted "ricer" style is because that's the easiest way to increase downforce in a predictable way. By raising the wing away from the rear deck lid (preferably above the roof height if possible), you get the wing out of all the nasty flow caused by the body of the car. This makes the wing behave more like it should in theory without having to do all the analysis to understand the interactions. Makes it a lot easier to figure out what kind fo wing you should use to get x amount of downforce.

That said, if you do all the analysis, you can sometimes get the same downforce with an integrated spoiler without as much drag. A good example of this is the C6 Z06. You'll notice that compared to the base model, it has an exaggerated lip where the rear brake light is. My guess is that provides just the right amount of exta rear downforce for the Z06.

All of that said, there seems to be a common misconception that adding spoilers to get more downforce is a good thing. The reality is, if you're not careful, it can actually be a very bad thing. Anytime you're creating downforce in the back, you are also creating drag (lower top speed). More importantly, both the additional downforce and drag create a rotational force (moment) on the car that can lighten the front end.

The point is, adding a huge rear spoiler without a properly designed accompanying front splitter can be very dangerous. Using your example of the C5R & C6R, they both had large raised spoilers, but they also both had well designed front splitters to keep the car balanced and prevent the nose from getting light at speed. Having both a splitter and a spoiler gives you balanced downforce.

In the end, it all depends on what you're going to do with the car. If you're doing road racing, a spoiler/splitter combo would probably be a good idea. If you're on higher speed circuits, a spoiler without a splitter would probably induce understeer (you'd have to play with your suspension set up too).

If you're jsut using the car on the street, just a spoiler probably won't hurt you. If you do a lot of freeway driving (read speeding) then you might want to be careful. Too much wing could at best cost you gas mileage (if you care) and at worst cause you to lose steering control.

Sorry if that reply got a bit long-winded. Just my 2 cents, take it or leave it. Reality is, most people use spoilers for looks more than anything else :-)

^^^^This^^^^ is AWESOME. This sets the bar high in the advice department. Damn, well done

[Desertdawg]

I have a 95 coupe and the car feels unplanted at speed.

I would like to add.... If your talking over 150mph then just throwing something on is dangerous.

GM designed the C4 to run at speeds above 100 safely.

If you think it feels odd then I would bet you have one or more of the following problems.

alignment issues

suspension problems/or wear

steering linkage issues

mismatched tires

crappy tires

Go see a good shop that knows Corvette's like Doug and let him crawl around under your ride to see if there is something else causing problems.

[Bromley's Corvettes]

Coil overs will make a huge difference. If you like driving fast you will love them and wish you had done it sooner I was very happy when I did my vette years ago.

[Bromley's Corvettes]

I have a 95 coupe and the car feels unplanted at speed.

I would like to add.... If your talking over 150mph then just throwing something on is dangerous.

GM designed the C4 to run at speeds above 100 safely.

If you think it feels odd then I would bet you have one or more of the following problems.

alignment issues

suspension problems/or wear

steering linkage issues

mismatched tires

crappy tires

Go see a good shop that knows Corvette's like Doug and let him crawl around under your ride to see if there is something else causing problems.

[StevieG]

2nd on coilovers, that will stabilize the car a whole lot more than attaching some aftermarket wing on

[Guest PURPLE95]

Thanks for the info guys. I used to build super cars for Dennis Mecham. We primarily worked on Trans Ams and Z28's. When the demise of the F body came and went we started building the C5 DuMANS. I have the prototype for the Mecham WS6 which uses the standard Mecham wing. It starts to build downforce at approx 55 miles per hour. The front of the car is stock with no additions to it. The car feels way better at freeway speeds with this wing. I have had the car up to 155 MPH with no adverse effects noted. The front of my IROC Z is stock as well, but the spoiler on it made the car more stable. Because of the smaller tracks here in town this car has only seen about 140 MPH, but at that speed it is rock solid. I understand the balance of front to rear, but I just wanted to say I have driven cars without the front "treatment" with wings that performed well. I have recently read some posts that mention suspension adjustments. I think what I am feeling is some worn out suspension bushings. I think the car would do better with a performance alignment and also a better set of tires. The tires that are on it now are ok, but not what I would consider a high performance set. I have looked at the bottom of the car and it appears to be ok, but I'm sure that there is a couple non visible components that are worn. This particular car is an FE1 car, (soft ride). I usually set up with soft suspension but put more aggresive shocks in the system to dampen the movement of the suspension better. Have any of you put Z51 bilstiens in the mix with this suspension calibration? Also if and when I rebuild the suspension I would probably want to put urethane bushings in. What I don't want to do is turn this thing into a lumber wagon and have a really crappy ride. Some would say that you can't have both, but I have achieved this with a 3rd and 4th gen F bodies. I can only surmise that I can do the same with a car that is more suited to performance driving. I gotta run work calls. Thanks again for your input.

[Guest PURPLE95]

I don't know the answer you're looking for, but I'm curious how fast in MPH is "at speed"?

[Guest PURPLE95]

"At speed" means 55 MPH to the speed limiter. At least that's my definition.

[Guest PURPLE95]

Hey thanks for the thourough response. As far as I am concerned, tech stuff can't have too long of a response. I used to work for GM for Buick, Olds, Cadillac out at the GM Desert Proving Grounds. I really wanted to work for CPC because thats where all of the high perf cars were, but I still learned volumes about car engineering there. The place was like area 51 for cars, pretty cool stuff going on out there. I really enjoyed my time on the 5 mile "oval" track. I had the GNX on my platform and got caught on the high bank with no helmet on. I had just changed out the frame and felt that "it was my duty to ring it out on the round track". Unfortunately my supervisor didn't think so. I have been thinking about the aero on the cars. I think the front stays planted because of the air direction change from the hood to the windshield. Is that true? At what speed do you think that the stock aero with a modest wing in the rear would need the front splitter to control rotation? Over 150? Also do you know of anyone that makes a good looking rear spoiler/wing that actually adds to the performance of the car? Thanks again.

[Desertdawg]

I may be off base here,but unless your just truly in love with the design of the C4,and before you dump a few grand trying to make a C4 into something spectacular, maybe you should look at a newer model that will have more of what your looking for already. there are plenty C5's out there for sale that will run circles around a C4, or if your budget allows I would even recommend stepping up to the C6.... (nothing against the C4, but your driving a 17 year old car with 25 year old technology...)

[Shifty]

2nd on coilovers, that will stabilize the car a whole lot more than attaching some aftermarket wing on

They made a big difference on my 89, at a larger window of different speeds, and improved handling as well.

[FNBADAZ06]

Probably WAY beyond anything you're willing to spend money on, but this active wing is the shizzle ....

http://aeromotions.com/

Watch it in action on a GT-R

http-~~-//www.youtube.com/watch?v=p7l70Q6P-vU

Installed on a C6 Z06 Carbon Editon

Full down position for aero-braking

[Desertdawg]

thats cool until one of the actuators fails and you go into an uncontrolled barrel roll. Actually if your going to have a wing, wouldn't you want full downforce on the rear when turning, using just half the wing is only giving you half the downforce. It's not like a lot of cars roll over in a turn ... right!

[FNBADAZ06]

thats cool until one of the actuators fails and you go into an uncontrolled barrel roll.

Actually if your going to have a wing, wouldn't you want full downforce on the rear when turning, using just half the wing is only giving you half the downforce.

It's not like a lot of cars roll over in a turn ... right!

The split wing design can load the inner tire when making a turn, which assist in keeping the tire in contact with the pavement and improve grip and cornering.....think of it like an aircraft's ailerons.

[Desertdawg]

Yeah, an airplane banks but uses an increased angle of attack to turn... A stiffer sway bar would load the inside tire way better than that whacky wing. If you want to turn better then mount a rudder, use a wing for downforce!

[Desertdawg]

If we had 2 cars with equal lap times but mine has sway bars and yours has that flappy wing thingy, I'll outcorner you everytime you get in someones slipstream or a crosswind blows.

[FNBADAZ06]

If we had 2 cars with equal lap times but mine has sway bars and yours has that flappy wing thingy,

I'll outcorner you everytime you get in someones slipstream or a crosswind blows.

The active wing is not used exclusively for turning (not a replacement for bars), it augments.. silly

If we had 2 cars with potentially equal lap times but yours has sway bars AND a fixed wing....versus mine that has SWAY BARS and that flappy wing thingy....the flappy thingy car would have less aero drag at top speed, greater aero drag when braking (from speed), AND would be able to generate additonal weight on the inside tires for potential better corning grip.

Active aero is not new.....it's just generally outlawed in most forms of autosport.

[Desertdawg]

....the flappy thingy car would have less aero drag at top speed, greater aero drag when braking (from speed), AND would be able to generate additonal weight on the inside tires for potential better corning grip.

Active aero is not new.....it's just generally outlawed in most forms of autosport.

equal lap times would mean drag and braking from your flappy wing thingy wouldn't benefit you, but any advantage you would expect from it would be removed in traffic.

And I'm putting my foot down now, now active aero from a flappy wing thingy allowed at my autoX!

[FNBADAZ06]

....the flappy thingy car would have less aero drag at top speed, greater aero drag when braking (from speed), AND would be able to generate additonal weight on the inside tires for potential better corning grip.

Active aero is not new.....it's just generally outlawed in most forms of autosport.

equal lap times would mean drag and braking from your flappy wing thingy wouldn't benefit you, but any advantage you would expect from it would be removed in traffic.

And I'm putting my foot down now, now active aero from a flappy wing thingy allowed at my autoX!

That's why I edited your original comment by saying "potentially" equal lap times...which would be cars based on equal power, chassis, tires, etc....minus aero. The car with active aero would definetly have the advantage of less drag at high speed versus a fixed wing car....definetly would have an advantage when braking from high speed over a fixed wing. How much more cornering force could be selectively applied with an active aero over fixed ? Don't know.

Active aero is coming into production cars, from the electronically controlled wings of the Veyron and Porches, down to the elastic "aero winglets" on the Ferrari 458.

Their website shows a video of a BMW with active aero in an auto-x......don't know how much benifit it offers during a relatively slow speed event, but when you're competeing where winning is measured down to fractions of a second I'm pretty sure it's advantageous there as well.

Concerning airflow while in traffic........I would be willing to bet you a paycheck that if NASCAR allowed active aero on the bricks those guys drive in close quarter, you'd see pitot tubes and active aero on every car out there.

[Guest PURPLE95]

I may be off base here,but unless your just truly in love with the design of the C4,and before you dump a few grand trying to make a C4 into something spectacular, maybe you should look at a newer model that will have more of what your looking for already.

there are plenty C5's out there for sale that will run circles around a C4, or if your budget allows I would even recommend stepping up to the C6....

(nothing against the C4, but your driving a 17 year old car with 25 year old technology...)

[Desertdawg]

That's why I edited your original comment by saying "potentially" equal lap times...which would be cars based on equal power, chassis, tires, etc....minus aero.

See I said make 2 cars equal and then race them in traffic, you say make 2 cars equal, then add the advantage of the flappy wing thingy...