eclou

Anyone have any idea how much downforce the sunroof adds when elevated? Offhand it appears it would add a significant amount

sweanders

I am not sure about that it adds downforce since it tends to be sucked out at speed, if you have a weak mechanism it pops out at or lifts at about 240 km/h since it is placed in a low pressure zone.

Rich Sandor

It does not add downforce when elevated, it adds DRAG.

It acts like a spoiler on top of an airplane's wing.

And I can confirm what Anders says above: Around 230-250+ kph there is enough low pressure on the roof area to suck the sunroof open if the gears are weak or the lock is not engaged.

Rich Sandor

KuHL 951

I converted my 86 to a manual sunroof without a restraining strap. I have driven at 145 mph with it tipped up, windows closed, and it did not float, rattle, or lift at all. It's hard to understand how it could get 'sucked out' as stated above unless it was closed, windows were open, and the pressure inside was higher working with the low pressure of the stream over the roof. Once the sunroof is open there is a neglible pressure differential from inside to outside but yes there is drag causing a moderate downforce. I've haven't done the same test with the windows down and don't think I'd like to try either.

Trucho-951

If it did contribute downforce, it can't be much. Those weak sunroof arms would surely snap or at least strip the gears with any significant downforce.

Rich Sandor

You're not quite "getting it" ... "downforce" and "form drag" are two different things:

Take a bone stock 944. Close the sunroof. Let the engagement latches pull all the way into the roof so that the sunroof can flap up and down freely. Roll up the windows and go drive between 130 and 150mph.. At some point, the sunroof will crack open, as the high pressure inside the car tries to equalise itself with the low pressure on the roof area.

Drag does not create downforce or lift, however, downforce or lift ALWAYS creates drag. This is an important concept to understand.

Proper downforce (or lift) is created by pressure differentials. The difference in pressure on the bottom of an airplane wing and the top of the wing determines how much "lift" that wing will create.

Wind in the air and downforce on a car are caused by the same effect, which is air trying to move from a place of High pressure to a place of Low pressure. Air always flows from a High to a Low.

When the sunroof is held up by the latches, the airflow will be "pushing" on the surface of that sunroof trying to close it, but because of the turbulent airflow created by the open rear edge, there will actually be no downforce transferred to the car. There will simply be more drag. This is called "form drag"

lart951

My sunroof pop up open once due to bad plastic gears around 160mph

sweanders

A friend of mine had some problems with his sunroof (993) and it wasn't installed properly, he drove around for a couple of days around town with no problems. Once he got some speed the sunroof was sucked off the car and he could see it in his rear view mirror as it danced it's way back to the ground.

I had a 944S2 with stripped sunroof gears and it always went open at ~240 km/h until I had it fixed. It was scary the first time since I didn't understand what had happened. All I noticed at speed was a very sudden increase in noise after a "thud" of a sound. A bit of trivia is that the reason the 1988 951S cars had all sorts of luxury but no sunroof was because of the risk of the sunroof opening at high speed.

Eyal 951

I would venture to say that an open sunroof adds lift. The car is the shape of a wing to begin with. add the extra arch on top, thats THAT much more distance the air going over the car has to travel as opposed to the air going straight under the car. This is exactly how a wing works. it may have the same effect to an extent as the flaps on an aircraft. cars that are shaped like ours have the most amount of lift. Hatch backs, like my 95 hatch, or like a Scion Toaster have the least amount of lift. (but probably more drag)

Rich Sandor

It doesn't add lift either - it's not a simple matter of adding camber. The tilting sunroof on a car is exactly like a spoiler on top of a airplane wing. It creates turbulence which kills the laminar flow over the top of the wing, and kills lift. It not not create downforce, it just kills lift.

KuHL 951

I might not be 'getting it' but only know how my manual sunroof reacts to higher speed. There is nothing but the weight of the sunroof holding down the rear and the hinges in the front. So once the sunroof is tilted are the pressures inside and outside close enough to not cause lift at the rear? I believe that if I opened the windows it would start to lift very easily. Is that correct?

kevincnc

I learned the hard way never to let a flight attendant ride in my car. I was going about 160 when she reached for the sunroof button. Before I could scream "DON'T DO IT!!" she hit the button. The sunroof raised up, and she was sucked right through it, never to be seen again.

dand86951

Rich you are absolutely correct that a spoiler kills lift and creates form drag at the same time, Thus by raising the sunroof you have negated some lift, therefore the total lift on the car trying to pull the wheels off the ground just when you are getting ready to enter that big sweeper is less...in my books it accomplishes the same thing as creating downforce but it never actually gets into a positive downforce.
If you are in your plane flying along at steady state power setting and you deploy the spoilers what happens...my bet is you start to lose altitude...why...less lift and more drag.

It would be interesting to instrument it and see how much.

RKD in OKC

I was told by a Porsche mechanic to never operate the sunroof above 80 mph as it would strip the plastic gears operating the sunroof.

RUF eliminated the sunroof from the Yellowbird as they could not keep it from being sucked off above 200 mph.

Just because there is a pressure area above the car that is lower than that in the interior of the car does not mean there is not an even lower pressure area under the car.

If you want your car to have more traction at higher speeds, get a front splitter, a larger rear wing, and leave the sun roof shut.