Jim bailey - 928 International

The Enzo Ferrari is said to generate about 756 lbs of downforce at 124 MPH and 1705 lbs @ 186 that is part of what over $600,000 gets you BUT what does it mean ?? At 124 MPH it adds 189 lbs per tire to the actual weight of 850 lbs . That is a 22 % increase in the force pushing the tire against the road BUT that does not mean you have 22% more traction or cornering ability . The increase in traction is greater with more weight force on the tire BUT it is not linear . The 1705 lb @ 186 is a ton of downforce (well nearly a ton) . Drop it down to 80-90 Mph and I suspect the downforce is pretty meaningless . Besides how often do you ever drive your 928 around corners where it is really about to break loose ?

heinrich

Porsche always strived for zero downforce when compensating lift.

Jim bailey - 928 International

The Porsche 962 C as campaigned by Joest Racing was said to make 6,000 lbs of downforce with a drag of 1,200 lbs at 200 MPH . Drag coefficient of around .40 .

drnick

jim,
i havent been making many 'off road' excursions in my 928 after cornering too radicaly so i guess im not going fast enough to break it loose, but i do sometimes feel both under/oversteer when at speed. whether thats because im a s%!t idiot driver or my car is all wrong in its set up is open to debate!

realist,
i agree with you in that we will never generate 'net' downforce on our road cars and my thought was only to reduce the 'net' lift.

i dont believe that aerodynamic aids on pasenger cars are useless because they arent going fast enough, what nonsense! why do the s4 models have an underbody tray or a front spoiler which is aerodynamicaly shaped?

as far as making the underbody into a wing shape, what about appropriate forms attached under the body floor pan sections aprox under the seating area running on both sides?

greg,
maybe a rear diffuser dosent need a complete underbody tray to be effective. even though the air would be more turbulent it would still be flowing it more effectively than without.

Nicole

This is a great thread! It's too bad Porsche never published any data on the 928s aerodynamics, other than the standard drag efficiency numbers.

From experience I can say that I have never driven a car that is less sensitive to crosswind than the S4/GT models - the higher the speed the less sensitive the car gets. I don't think you can achieve this with a lot of lift.

Interestingly, I used to own a car with a roughly similar shape as the 928 - at least at a first glance. It was a Mazda RX-7 and was the exact opposite of the 928. If it was windy outside, I could not drive more than 140km/h (approx. 90mph) - otherwise I'd need at least two lanes and risk to be pulled over for drunk driving.

The different experiences with the RX-7 versus the 928 tell me that it is extremely hard to judge or improve the aerodynamics of any object just by looking at it. Without proper wind tunnel tests, you might achieve the opposite of what you set out to do - and not know it until it's too late...

I consider Aerodynamics not just a benefit for speed and fuel economy, but also an important safety feature. Feel free to disagree, but please don't hit me with a flying object!

John Veninger

As often as possible

heinrich

Jim, if that 6,000lbs weren't compensated for by lift to cancel it out effectively, the car would simply collaps under 6,000lbs of weight.

Imagine you're looking at a Porsche 962.
She's looking fast just sitting there.
0mph.
A real beauty.
Now, a forklift drives up and deposits a waterbed of 6,000lbs on top of her.
What does she do?
What happens to the suspension?
Shocks?
Tyres?

Don't forget that yes, while it may be true that the aerodynamics produces 6,000lbs of downforce, that downforce is created by the designer specifically to do one thing:

Counteract an equal or almost-equal force of lift.

heinrich

Also, when talking about downforce, as it relates to drag ... drag is a necessary byproduct of (not only) the aerodynamic aids added for the specific purpose of combatting lift. So ... you drive your Porsche with no spoiler and no splitter and no bellypan. Her Cd is 0.20. She gets to 160mph really quickly and flips over and summersaults. You return to the drawing board and add those aerodynamic aids. Not you get to 160mph a little more slowly, but once there, in stead of flipping the car stays as Nicloe says, flat. Her suspension doesn't load, but she also doesn't fly away. Why?

Because additional downforce = lift for that speed. I believe it was the 935 langchamps which had zero effective lift, and hence zero effective downforce, at top speed (was it 230mph?).

Either way, here, drag must be increased by nature of the wings and splitters. If wind rushes over the car from the front, and it is interfered with by wings and splitters ...... this is called additional drag.

Toejam

How 'bout bolting a set of aircraft wings to the roof?(upside down, obviously)...

You may clip a few trees, but hey you'd have net downforce.

SharkSkin

I saw a segment on the Ferrari, F50 I think it was... they claimed that it generated enough downforce at 200mph that if the road turned upside down, you could still drive on it... though I'd hate to run out of gas while doing it

I'm not so sure that the intent is to always arrive at a perfect 1:1 relationship between lift and downforce...

D

Jim bailey - 928 International

According to the book "Porsche Excellence was expected" page 847 " It was impressively free of lift , however at its maximum speed of 140+ mph the lift at either end was no more than sixty pounds . " They were of course talking about the early no spoiler 1978 928 . I somehow doubt that Porsche added lift when the added the spoilers or designed the updated S-4 body shape but that is just speculation . So at speeds of over 140 MPH we are discussing "fixing" a total lift of 120 lbs ; I believe our cars are much better than we might have thought . Heinrich the 962 was a truly amazing car it so dominated racing in The US that it was penalized by being forced over the years to add weight . As I recall they ended up being about 50 % heavier and still won !! they were extremely rigid and stiffly sprung . And yes they had huge amounts of downforce .

SteveG

This would be the hook behind the movie "Speed" Let her drop (pun intended) below say 150 mph and she explodes! Wheee

Realist D.

The F50 that creates so much downforce it could drive upside down at top speed is an urban myth. While that may be true of a "ground effects" era F1 car or GTP car it is not true of a current F1 car, and of course GTP is no more (unless you consider those NASCAR-style Daytona prototypes to be GTP cars ;-).

In order for that F50 to drive upside down it would have to make downforce equal to its weight just to get started (a few thousand pounds?). But then as several have pointed out, once the car is at speed its natural shape is creating lift ... so you have to create even more downforce to counteract that lift (probably a few more thousand pounds), providing net downforce greater than the weight of the car and all of the lift it creates.

Oh, and one more thing ... if the downforce is created by a combination of "ground effects" which are unreliable because the ground is not a "constant," and by wings, which are much more reliable and consistent, you have a scenario where you might moementarily maintain enough downforce to drive upside down. But at any moment a gust of crosswind or a pebble on the road will disrupt everything and we all fall down!

SharkSkin

If it's an urban myth, it is being spread by Ferrari personnel....

D

drnick

i dont think aerodynamics are such a 'black art' to be the preserve of people with wind tunnels, nor do i think you need to be travelling extremely fast before any benefit is acrued. look at the wright brothers who built a whole aeroplane without modern design techniques and it lifted off the ground and flew at well below highway speeds.

i 'reckon' lowering my front spoiler slightly and linking it to a bellypan will assist with roadholding, a rear diffuser would probably help too. its only a shame i havent a wind tunnel to prove it isnt all in my mind!