I was reading last month's Car & Driver and they had an article on the Audi R8 race car where they mentioned that, above 100 mph, it has so much downforce that it could run upside down on the roof of a tunnel! Meaning there are turns you could take at 100 OR 60 mph but not at 80 mph...

Which made me wonder, does anyone know how a bone stock (not lowered) Z06 fares in terms of aerodynamics? Does it generate lift or downforce at speed and any idea how much? I don't imagine it generates too much lift since 171 mph is fast enough for jumbo jets to take off when lightly loaded and we're not hearing about Vettes taking flight...

Just wondering if it's closer to neutral or has good downforce (and how much).

Thanks for the feedback! :cheers:

 

With a Drag Coeff of .29 and its already low stance ... it should have some good down force ...

 

A low CD indicates lack of downforce. I believe the C5 is very neutral, neither lift or down-force.

 

I would agree that less CD = less downforce. I have often wondered this myself, but at speeds the car seems stable.

 

Race cars generate considerable downforce because of their wings, and some generate additional downforce from an underbody "venturi" shape, but this has been outlawed for most classes to limit downforce and cornering speeds. At high speeds a F1, CART, IRL or ALMS cars like the R8 can can generate lift equal to three to four times their static weight. That's why they could run upside down at about 100 MPH on up.

Nearly all production cars generate some lift, and the engineers try to minimize it through the overall shape and some add on devices. The front "air dam" on the C5 serves to both limit lift and force air into the radiator, and road test reports indicate the car is stable and responsive up to top speed. The C5 probably has a bit of lift. Generating any meaningful downforce (without wings) is very difficult, so the effort on most production cars is to limit lift to minimum values. The F360 has a very sophisticated underbody design, which generates some downforce according to Ferrari.

A bit of front lift on a production car is okay as it tends to promote stability - the car will understeer more at high speeds. Rear lift (in excess of front lift) destabilizes a car by increasing the tendency to oversteer.

Little was known about ground vehicle lift until about the mid sixties. Conventional wisdom prior to that dictated designs that appeared "aero", like a teardrop, but automotive designers failed to take into account the ground affect that is well known to pilots. A body will act differently near the ground than in a free airstream, and most shapes generate additional lift near the ground.

The C2 is a good study in poor aero design. Eventhough a quarter scale model was tested in the Cal Tech wind tunnel, the production design exhibited high lift and drag. If a C2 had the power and gearing to make 200 MPH the front tires would exert zero force on the ground - lift would equal weight.

Duntov knew the car was an aerodymanic disaster, but Bill Mitchell wouldn't change the design to improve it.

Duke





[Modified by SWCDuke, 9:54 AM 6/9/2002]

 

Duke,

Very insightful post!

Gleb

 

Thanks for the great post Duke. Yeah, on the rear lift issue, Audi had an issue on the Audi TT; the rounded rear created some lift at high speeds which was exacerbated under hard braking because of the car's natural weight shift. After wrapping a few Audis around trees in the black forest, the engineers, who had initially requested a spoiler but had been denied by the designers, mandated the addition of one to the rear (I think it is a pop up one).

I'm wondering how I could add some meaningfull downforce on the rear of my Z06 without a permanent ricer wing on the back... :confused:

Does anyone know of an aftermarket dealer that handles wings that pop up at speed or on demand (like on the 911 turbo)? I would like to add about 300 lbs of downforce at 70 mph. :cheers:

 

I recall reading about the Audi TT. A respected German magazine declared it unstable. Audi also changed the anti-roll bar(s) to increase the understeer bias, and that included recalling all the cars built with the original bars. A sloping rear deck as in the TT design is notorious for creating rear lift.

The only way you are going to get 300 pounds of rear downforce at 70 MPH is to have a rather large high mounted wing. At 120 MPH such a wing would generate about 1200 pounds of downforce because downforce increases approximately with the square of speed. At 1200 pounds the suspension would probably be on the stops assuming the wing did not tear away; 1200 pounds of downforce will require some structural enginneering. Any kind of spoiler, fixed or pop up will just serve to reduce lift, but such a device will not produce actual downforce.

Also, wings create "induced drag due to lift", so the car would have a lot more drag. Look at a CART car in roadrace trim compared to big oval trim. The wings used on big ovals are rather small (thin, short chord, and a modest angle of attack) compared to what they use for road races, which have slower corners. The bigger wings create more downforce for the slower corners, but limit top speed to well below what can be achieved on an oval (even if the track has a mile long straight) because of all the induced drag. One of the major tuning tradeoffs on both ovals and road courses is the wing bias. More wing means higher downforce for greater cornering speed, but limits straight line speed. The biggest problem the F1 teams had at their first race at Indy was finding the best wing setup. The F1 course has some very low speed corners in the infield, but a very long "effective" straight, which included Turn 1 as they accelerate flat out all the way through it.

Just curious, but why do you think you need 300 pounds of rear downforce at 70 MPH?

Duke

 

Thanks for the great insight. Like much about the Z06, this has very little to do about need. I figured it would be nice to have to help me keep the rears planted if the pavement is a little slippery when I open it up in third (like when doing 50-80 mph at full throttle merging onto a deserted highway).

Your explanation highlights the reason why active/adjustable downforce producing devices are not authorized in racing applications. You make a good point about the 1200 lbs at 120 mph and I guess that's why a variable geometry wing would be the ultimate. You could vary the angle of attack of the wing by mounting it with a motor that turns the wing slightly at various speeds (or with a manual control). This could adjust considerably the downforce generating capacity of the wing so it is never too high or too low.

That being said, it would probably be huge, look dorky and cost a bundle to get it just right so I guess I'll pass for now. I guess with the really low CD, the car should at least be fairly close to lift neutral at all but the highest speeds. Just want to make sure I don't pull an Audi TT move (pre spoiler) if I ever have the car well into the higher parts of 5th gear...

Thanks for the great posts! :cheers:

 

I would like to hear more ... so TTT

 

There is some good information out there on vehicle aerodynamics. Millikan's "Race Car Engineering" is probably the best. It's used as a textbook in some upper division/graduate level engineering courses in universities. It's not a subject for the faint of heart!

You guys are lucky to be driving C5s and not have to wrestle with C2s. At least they were stable because the front lift caused them to understeer at high speed, so you just didn't want to have a big crosswind or hit a serious corner at 150 MPH.

By comparison modern cars are very stable.

A few years ago when I had an opportunity to spend a couple of days with Chuck Jordan I asked him why the aero data from the Cal Tech tests of the C2 did not result in changes. Chuck explained that back then there was no feedback loop from design back to engineering. Mitchell ran design like a Medieval king, and what he said the the final word. Duntov had a lot of pull (he was hired by Ed Cole), but he couldn't butt heads with Mitchell and win.

Nowadays the designers in the studio know more about aero, and they have engineers to work out the details. They test both models and full scale prototypes. Models have their limitations.

Ford claimed, based on model tests, that the original GT-40 with the all-aluminum 4.2 liter small block V8 developed for Indy would hit 205 MPH in still air, but it was only capable of 190. It turned out that they had not sufficiently accounted for the drag horsepower of the air that was going THROUGH the car - radiator, engine compartment, and the very exotic driver ventilation system, which was later deleted.

The original Ford GT-40 design also produced a lot of front lift, so the nose was modified - made lower to the ground. The car also had a problem at the rear. At speeds approaching 200 it developed a rotary moment in the transverse plane - like an arrow without feathers. It was this instablity that led to the crash at LeMans practice in April 1964 that took the life of Walt Hansgen.

Shortly thereafter the rear "ducktail" spoiler was added, which cured the problem. According to what I have read, "invention" of the rear spoiler is attributed to the late driver Richie Ginther, who first suggested its use on the Ferrari 250 GTO circa '63.

This was the point when the automotive engineering community really began to pay attention and learn about so-called "low speed aerodynamics" as applied to vehicles. It's interesting that in this era, jet fighters were capable of Mach 2 and the X-15 was capable of Mach 5 in the outer reaches of the earth's atmosphere, but little was really known about the aerodyamic characteristics of ground vehicles until speeds got up to about 200 and serious problems developed. They THOUGHT they knew, but didn't.

The C5 doesn't really need a rear spoiler for stability because of the high tail. It's been criticized for "bulky" hind quarters, and it looks that way for a reason - aerostability.

Duke


[Modified by SWCDuke, 9:49 PM 6/10/2002]

 

Thanks!

 

Duke, I have been enjoying your very interesting and informative discussions on aerodynamics. Keep it comming. I am searching for the most effective location for trans and diff coolers besides in front of the radiator. You seem to have a background in engineering or aerospace? Am I correctly recalling that Jim Hall used an adjustable rear wing, opperated by a fourth pedal, on the first chaperell ? On a more current note I have seen a small rear deck mounted spoiler from DRM but have not talked with them to find out how they came up with the design and how it really works. Maybe some of those who are running it can give us some feedback.

 

Jim Hall's Chaparrals with the moveable wings had automatic transmissions. Information was just released on this recently (Road and Track), and it turns out the auto was just a dog-clutch crash box with a torque converter instead of a clutch - not some exotic planetary box as was thought all these years.

The car shad three pedals - throttle, brake, and wing - what would normally be a clutch. I think pushing the pedal in decreased the angle of attack (but it might have been the opposite), so upon braking, the driver released the third pedal with his left foot, which increased the angle of attack for downforce. Once through the corner the driver pushed the pedal in to reduce the angle of attack for low drag down the straight.

Hall is a very bright guy, but he got a lot of help from the GM Tech Center. He certainly brought some serious engineering, particularly aerodynamic, to racing. This was the era when automotive engineers really began to understand ground vehicle aerodynamics.

Cooler placement is fairly tricky. Up front you look for a high pressure area upstream and low pressure down stream. The cooler does not have to be directly in the wind. It's the pressure differential across the matrix that counts. The radiator is a good example. The front air dam and horizontal openings just forward of it create a high pressure area in front of the radiator, which forces the air through, but the air can have a hard time finding it's way out. I recall seeing a photo of a Lingerfelter TT at two and a quarter and the whole hood was bulged up a good half inch at the boundaries with the fenders.

Gearbox and axle coolers on the C5 would be tricky to locate, unless you put them up front and run transfer lines. It's tough to tell what the pressure distribution is under the car because the airflow is so dirty. It's one of those things that has to be worked out with a little aero engineering intuition and a lot of trial and error.

Most of the aftermarket add on aero devices are just "eyeball" designs, and I doubt if vendors have any real engineering data on how they perform, but you can always ask.

Duke




[Modified by SWCDuke, 11:37 PM 6/11/2002]

 

In many respects, the early aero attempts were actually superior to the modern versions. The early Chaparrals and F1 cars with the high mounted wings were good examples. Back then, the wing supports were mounted on the car's hubs rather than the car's body work. The advantage to this setup was that the downforce was transmitted directly to the wheels and did not load the suspension. The cars benefitted from the downforce, but could still use soft springs and shocks to maintain good mechanical grip.

A series of wing support failures caused the governing bodies to outlaw the high, hub mounted rear wings. (Just another case of a motorsport body mistaking the cause/effect. The failures were caused by metullurgy issues and not any intrinsic issues with the wing layout.) Since then, wings have been mounted to the car bodies, and cars have had to run very stiff mechanical setups to accomodate the aero loading.

Years later, the F1 guys started using underbody aerodynamics and moveable skirts to channel airflow and generate downforce. The skirts were also banned (for being too effective) and many series went to flat bottoms in an attempt to reduce downforce and slow the cars in the corners. We ended up with even more compromises to circumvent the rules. Without the skirts to hold the air under the car, designers had to run the cars extremely low and use all manner of barge boards and vortex generators to control the airflow beneath and above the car.

Millions of dollars were spent in the development of three spring suspension setups and fully active suspensions. The purpose of all these suspension mods? To better control the ride height so that the underbody aerodynamics would work efficiently. (This was never a problem when the skirts sealed the car's underbodies.) The reason the ride heights varied so drastically? That the wings were mounted to the chassis and the cars were literally being pushed into the ground as the speeds and downforce increased. (This ride height variability wouldn't have been a problem if the aero load was transmitted through the hubs.)

In sports cars, the rules requiring flat underbodies led to some spectacular accidents. Witness the multiple incidents of Mercedes CLRs and Porsche GT1s blowing over and taking flight. The problem for the Mercedes cars was so bad that Mercedes actually withdrew their cars from the 1999 Le Mans race when THREE cars took flight. Next year, the cars go back to full underbody venturis to prevent cars from flying into grandstands. (Remember that the flat bottoms were actually supposed to slow the cars down and make things safer.)

The point is that race technology is sometimes some of the least elegant engineering solutions you can put on a car. Governing bodies make sure that engineering break throughs get abolished if they become too effective. I still believe that some of the most innovative race car technology came in the 60s.

The Chaparral 2A with the moveable wings was way ahead of its time. (We wouldn't see it's contemporary until the late 90's, when F1 teams developed rear wings that would intentionally bend under extreme aero loading. The wings would effectively lay down at high speeds to reduce aero drag. The FIA killed these off when rear wing failures started raising eyebrows. Sound familiar?)

The Chaparral 2J actually used a snowmobile engine to suck the air from beneath the car. The vacuum added downforce without adding drag. It was called the sucker car, and it too was abolished after only four races. (But not before Jim Hall tried to justify the fans as cooling fans for the radiator. Nice try.)

The idea was later used a 1976 Brabham F1 car. The designer of the car was Gordon Murray. Why is that important? Because the only road car to use the principle was the McLaren F1, a car designed by (you guessed it) Gordon Murray. The McLaren F1 actually used underbody fans to control the aerodynamic center of pressure as the car pitched and yawed.

And that's my useless trivia for today.


[Modified by Racer281, 2:43 AM 6/12/2002]

 

To give you an idea of what you are asking for, a World of Outlaws car generates about 800 pounds of downforce at 65 mph. The main wing area is 25 square feet. As you decrease the size of the wing, it will become progressively less efficient. That means that ot get 50% of the downforce, you will need more than 50% of the area for an equivalently shaped wing. How would you like wings like this on the top of your Z06?



If you're looking for a way to keep the tires planted from a 50-80 mph, you may want to look into more aggressive rubber. The performance will be better and you're car will look much less ricey.


[Modified by Racer281, 2:37 AM 6/12/2002]

 

Good point! My car would look very ricey with anything close to what! Wouldn't really want that to such a beautifiul car! :nono:

In terms of the rubber, I don't know what is much more aggressive than the stock F1s for road driving. I guess I'll just have to keep the revs below 4500 in 3rd and 4000 in 2nd if the road if the road is at all wet. Or ride out the limited slip diff 'wiggles'.

Oh well, the burdens of driving around a 400 hp car! :cheers: :seeya

Maybe I should just get a WRX sti to toss around if the road is wet.

 

This months Road&Track has a good article on aerodynamics, it explains downforce, drag, etc.

 

Yeah ... I read that ... its really good ...