Bill.e1

Rich,

Thank your for the clear reply. My day job is doing just what we are doing here- getting design groups through the buzzwords and gross simplifications so they can find a problem's root cause and fix it. Unfortunately, if we actually talked in these terms, engineers would have even fewer mating opportunities than we already do ;-) A classic case of "do as I say, not as I do."

What you said this time is a little different than your earlier post even if you ignore the adjective. You state that adding a splitter helped plant your car's front end. I have no doubt that the net effect of a splitter is in the "downforce" direction and the driver would feel it's effects.

I think what is "wrong" is that my zero isn't your zero.

I define zero classically- as the point where the lift vector's net value is zero- the absence of lift or downforce. Your statement reflects a more practical definition- the car's "Zero" is its net vector absent any "active" downforce generating devices.

Who is right?

Me, obviously ;-) After all, I AM A SCIENTIST (insert appropriate insult here).

The different definitions don't change a car's handling. In fact, your definition is more intuitive and may better describe the system. However, if you were giving input to your new race engineer and your "zeros" were 200 pounds apart, he won't be able to fix your understeer since he thinks you already have enough front wing. You have to balance the practical value of a non-standard definition against its confusion-producing potential.

Have fun,
Bill

FWIW, my interest in this subject goes quite a ways back. Our "family hobby" was outboard boat racing and world records. My grandfather built engines for a local racer and, later, Scott Atwater. Racing hydroplanes are boats in name only. They are airplaned designed to ride the ground effect (that term again) a few inches above the water. They are fastest when you have nothing but half the prop in the water. Drivers kneeled at the front of the boat and used body english to maintain altitude. If they felt the boat rising, they lifted a knee to tip the boat and "bleed" the bubble.

http://www.infoblvd.net/sah/OutboardRacers/Page_77.html

This guy was a little slow.

Rich Sandor

Thanks for the reply Bill!

Clearly, the only way to satisfy all parties, is to get some real world data. How could we test this, with an honest budget, in a real 951? As an engineer and a scientist, why don't you propose to me what you would have to see in order to agree that my car does produce net downforce, and not lift?

I'm thinking along the lines onboard GPS for accurate speed and accelleration readings, and an outboard camera mounted in a fender well, pointed at the shock, with some sort of meter on the shock to measure how much it is being depressed.

what do you think?

michaelathome

Late to the party but wouldn't every car produce lift, downforce and drag? Some of the statements seem to state black or white yes or no.

If the car creates lets say, 100lbs of lift but only 75lbs of downforce would you say that the body ONLY creates lift of 25lbs? I think that you need to look at all the #'s.

That in mind I would bet a 944 rear spoiler that the car DOES create downforce (that was the original question). I would also say that the design of the undercarraige would help to produce lift. The only way to determine the amount of each would be with testing.

Just my .02

Michael

Rich Sandor

1: The rubber 944 spoiler doesn't really create downforce, so much as it prevents lift and decreases drag simultaneously... The fundamental difference is that it's designed to reduce drag and zero out lift - not to produce serious downforce, like a big rear wing would. What does create downforce, is the bottom of the car. The low front end and expanding rear end, flattened bottom, and diffusing rear valence.

2: I know where you are going with the "produces both lift and downforce" idea, but it's not really the point... we are talking about the car as a whole, not parts of it. It's possible to measure pressure differentials along different point of the car, and calculate lift/downforce, but the important thing is finding out how much pressure is being put on each of the 4 shocks by the body at equilibrium at a given speed, in order to tune the car for stability. You could have lift on the front and downforce on the rear, depending on where the centre of pressure is for the whole car, but there will be a net total somewhere between Lift and Downforce, and my highly accurate "butt meter" says it's on the downforce side.

odurandina

no need.

just put on a wing almost as big as the damn car.

V2Rocket

odurandina

that 924 will almost float off the ground after 130 mph.

V2Rocket

it won't even get there

ehall

Holy thread necrofilia!

porsche_968

Wow, remember this thread....
So, riddle me this: WHAT EFFECT does this do
1) smooth out the under carriage behind front/before rear wheels?
2) smooth out the under carriage behind the rear wheels?

Side note. Whats up with all these new cars with the little tabs(?) in front of the wheels? some times in front of both front an drear wheels?

John_AZ

In color--pic credit to nasty9r

Richgreenster

Well there you have it. Porsche did the work for you on this. Granted it was a 924 not a 951 but I bet it would be a little better on the 951 but close enough for me.

porsche_968

Good info for air flow over the ar....Any comments on air flow under the car?

For example, if we can slow the air flow under the rear of th car. Does that mean it
reduce lift at the back? but increase drag?

ideola

FWIW, the 924 had better cd than 944/951, and the 931 had better cd than the 924 due to its spoiler.

odurandina

front end lift with the 924 is a real problem.

if you run a big 944 turbo motor, the front end will lift off the ground and you will die.