Front Spoiler....dumb question but...

[Ken911]

I read someplace i think the owners manual that the front chin spoiler ads 200#s of down force to the front of the car and the rear adds 440#s but i forgot at what speed i think 140 but dont remember for sure

[ZOA NOM]

Read this:

It's from here.

Essentially, without making major revisions to the body of the vehicle, there are only two modifications that can be done to increase down force (the main goal of this experimentation). First, small front and rear spoilers can be added to the bodywork. The addition of a front spoiler mounted underneath the front clip of the car can help to reduce under car airflow. In doing so, this can help reduce the drag on a vehicle and even produce some down force. However, critical to this is spoiler length and location. If the spoiler is too long, it can reduce under car airflow too much, and thus increase drag and decrease down force. Therefore, the design of a front spoiler is often an experimental process in which several revisions must be tested. Next, a rear spoiler mounted on the rear deck lid of the car can also help to increase down force. It does this by increasing the height of the rear stagnation line. This will cause a positive pressure buildup on the upper surface of the spoiler and a negative pressure area on the bottom side of the spoiler. The net resultant of these pressures is a downward force. A well designed rear spoiler can even decrease drag while increasing down force. Second, the under car bodywork can be molded and manipulated in order to get it to act much in the same way as an upside down wing. In this second case, the underside of the vehicle body is kept relatively smooth, as is the Porsche stock. However, near the rearward section of the body, the underside is sloped upward at an angle from 0-15 degrees. Past 15 degrees, this slope will create a large increase in drag. The effect of this sloping is to cause an increased down force by creating a suction on this portion of the vehicle underbody. In addition, by simply adding vertical plates along the vehicle sides where this sloping occurs, down force can be further increased. The theory that makes this all work is Bernoulli�s Law. As air flows faster, its pressure decreases. Thus, by increasing the velocity of the airflow, suction can be created. As the tunnel expands the distance from the bodywork to the ground, more and more air must flow at higher velocities to fill the new gap. By adding the vertical plates on the sides of the vehicle, this requires the air to flow from the front of the vehicle to the back, increasing down force. Otherwise, air could simply fill the sloping gap by flowing in from the sides. This would reduce the effectiveness of the aerodynamics.


Looks like Porsche put quite a bit of thought into our cars. Also, the chin spoiler and rear wing seem to perform two different functions, that both result in additional downforce, and reduced drag. The chin spoiler, alone, enhances downforce by "spoiling" the under car airflow, which results in a net downforce. It isn't simply a front end/rear end thing.

[juanbenae]

Originally posted by Wil Ferch
Toby:

Please tell me of a front valance difference in 964's...between the cars using a fixed RS rear spoiler ( Carrera-style) and those using a motorized rear spoiler ....

964 fronts were certainly different than the earlier cars... if that's what you mean..but I don't think there were differences within the 964 series.

..??????

- Wil

i do not know of any differences between the 964 valences, and you are correct in that the valence on the 964 is different than the earlier cars without a doubt. the point i was trying to make, very poorly apparently, was that the 964 valence, on cars with just the pop up wing does not seem to have any sort of shape that would mimic the chin spoiler of the mid-year cars. it does seem to hang lower though from what i have noticed.

admittedly i am no expert on the 964 model, i just was looking to find out if running the smaller pop up deployed wing would result in me needing a chin spoiler. i would suspect that since it is much smaller than the other wings out there, and the fact that it is more vents that solid it might create less downforce. i am sure i will know more after my next visit to the track.

[ZOA NOM]

Toby, the description above tends to imply that your pop-up wing would add some downforce, but since the top surface and bottom surface are not isolated from each other (they're vented), it may produce drag.

[masraum]

My '88 had only a front (the PO removed the rear) and my car felt pretty darn stable up to about 135-140, and even then it was still somewhat stable. And, I found out later that was with really bad shocks. I'd love to have been able to test the high speed after I put the Bilstein sports on.

The 964 aero was completely different than the earlier car because tons of stuff changed. The front and rear bumpers on the 964 made a huge difference, the bottom of the car was different, the way the windows fit the car was different, etc....

[Bill Verburg]

Originally posted by Wil Ferch
I find it rather ironic that Porsche supposedly says this vs that as it relates to front/rear aero aids.... but consider this....

With the advent of the 964 series, with their movable rear wings...notice that the RS America of 1993 had a Carrera-like rear wing.... and yep...nothing changed in the front relative to the "standard" cars....

hmmm......

Wil

That's because the front end of a 964 is much more aero efficient than a 911 or even 993

[DG624]

Zoanas, good information. I have driven a plain 86 Carrera at the track up to 120 and had no instability...tires and wheel balance are more critical than spoilers. I can't see how reducing undercar air flow would upset the car more than a full vs empty gas tank which would vary by over 150 lbs. It would be nice to see some numbers on potential downforce for the factory options.

[Bill Verburg]

It would be nice to see some numbers on potential downforce for the factory options.

Do a search or purchase a copy of Frere

[pbflyz]

[QUOTE]Originally posted by ZOANAS
[B]Read this:

... However, critical to this is spoiler length and location. If the spoiler is too long, it can reduce under car airflow too much, and thus increase drag and decrease down force. Therefore, the design of a front spoiler is often an experimental process in which several revisions must be tested. Next, a rear spoiler mounted on the rear deck lid of the car can also help to increase down force. It does this by increasing the height of the rear stagnation line. This will cause a positive pressure buildup on the upper surface of the spoiler and a negative pressure area on the bottom side of the spoiler. The net resultant of these pressures is a downward force. A well designed rear spoiler can even decrease drag while increasing down force. Second, the under car bodywork can be molded and manipulated in order to get it to act much in the same way as an upside down wing. ...



It would appear to be pretty easy to add downforce, but trying to do so while keeping drag at a minimum becomes a whole different picture. This thread reminds me of that racing clip (LeMans?) which clearly illustrates what happens when that delicate line is crossed
I'll try and post a link:

http://www.motorpride.com/5thelement/video/Mercedes%20Benz%20CLK-GTR%20flip%20at%20the%2024%20Hours%20of%20Le%20Man s.mpeg

[pbflyz]

Maybe this will work:

http://www.motorpride.com/VideoGallery/Default.asp?SiteID=1&PageNum=2&Sort=

[Wil Ferch]

Bill:
You misunderstand my point...again, it seems, I'm not posting clearly.

The 964 can be driven "at speed" with the rear spoiler up or down ( regular Carrera, like a C4). It would impart a different "aero-signature" in either position. The RS, in contrast has the more (?) effective Carrera-style whaletail in place all the time. Among the 964 series, then....Porsche didn't add additional front aero devices for the RS.

- Wil

[javadog]

Wil,

Porsche designed the 964 rear spoiler to automaically raise at speeds above 50 mph or so. The rear spoiler can be raised at speeds lower than that using a switch but I'm not aware of any ability to run the car at higher speeds with the rear spoiler down. In fact, I think if the spoiler fails to raise, it turns on a warning light to warn the driver. Porsche did this for a reason. The front/rear lift balance is only important at higher speeds, as the forces are too small at lower speeds to worry about.

The effectiveness of the various rear spoilers on the 964 is pretty close to the same. There is a small difference but it isn't worth talking about. Keep in mind some of the reduction in lift for the 964 is related to what Porsche did on the underside of the cars.

The front bumper/valance on the 964 is much lower than that of previous cars and approximately the same as an earlier car with a chin spoiler.

Porsche hit a home run with the 964 aerodynamics. They haven't really done much better since.

JR

[Wil Ferch]

javadog:

I understand your point about the spoiler not being "allowed" to come down automatically at speed, but this is not the root of my point. The Carrera-style RS tail ( for the 964 RS America, for instance) provided a meaningfully different amount of downforce compared to that provided by the normal C4 / 964 tail in the "up" position.

So...given the 964 platform...the front end of "normal" 964's and the RS were not changed, although this backside difference existed.

I also doubt.... and respectfully disagree ...that the front end of a 964 is "lower" to the point of being equally low to a front-lip spoilered early car. I would be welcome to real measurements. Besides, the early car's lip spoiler provided downforce not just by minimizing the airflow under the car ( i.e....by being "lower")..it did so also by the angle or slant of the spoiler itself. 964 doesn't have this feature.

Here is some interesting reading about the effectiveness of the up/down rear spoiler, in another engaging thread --> Track experience w/ C2 spoiler on earlier car

- Wil

[javadog]

Wil,

I'm not aware of any information available to the armchair experts (us) that shows that the Carrera style spoiler was significantly more effective than the standard 964 spoiler. The data I can find gives a rear lift coefficient of .02 for the standard car and .01 for that of the 964 turbo. Granted, the tail on the turbo is different than the Carrera tail but I'd argue that it is probably slightly more efficient due to the side fences present. That's a pretty small difference in lift factors, no matter how you look at it. I suspect that if there had been a significant difference, Porsche would have done something different to the front ends of the two cars.

As for the front, the lift coefficient for the 964 was -.01 and the one for the earlier cars with a front spoiler was .01. I can't say that the slope of a lip spoiler makes it more effective but you can't say the 964 doesn't have less lift. However they got there, it's a better design.

I don't have a 964 laying around to measure but if you look at a photograph of each car, compare the relative height of the bumper/valance assembly from the bottom of the front fender to the bottom of the valence/lip. You'll see that the later car has a little deeper front end, which is probably why so many people don't like the "heavy" look of the 964 bumpers.

I remember the thread you quote. I am also reminded of an earlier article by C&D, wherin they looked at the effectiveness of Porsche's aero add-ons in the '70's. Pretty soon, someone will want to revisit which tails came on which cars and I'll get to "negotiate" with Bill Verburg again as to what what was optional on the late 911SC... All in fun.

JR

[vanwyk4257]

One other issue that I don't think has been addressed in this thread thus far, at least not at length, is the effects of different ride heights on downforce and drag. I don't know if there was a ride height difference between an SC, Carrera, or C2/C4 964 other than US vs. Euro spec. I would think simply lowering the car would reduce both lift and drag, but then again I'm not an aeronautical engineer, so I may be wrong. I would be interested to hear some of your thoughts on this issue.

[Wil Ferch]

Drag is pretty much directly proportional to cross sectional area ( think.... projected image of a front view of a car against a wall)...so slightly up/down won't make any meaningful difference. Total drag = frontal area x Cd. Drag coefficient (Cd) is a function of the car shape.

Now rake will affect where center-of-pressure occurs....

Slight downward rake may improve downforce...but maybe over a portion of the car only....there are threads on the rake question posted here..

You need to get to crazy levels of lowering to really start seeing grounds effects or drastically reduced airflow under a car.

- Wil

[juanbenae]

Originally posted by Wil Ferch
Here is some interesting reading about the effectiveness of the up/down rear spoiler, in another engaging thread --> Track experience w/ C2 spoiler on earlier car

- Wil [/B]

thanks will for the link. thats exactly the kind of info i was looking to find out.

[javadog]

There are several differences in ride height over the years but the biggest difference was found in the 964 RS, compared to the rest of the 964 series. It also had a slightly more pronounced forward rake. Porsche quoted the same top speed for the RS as the standard car so the difference in drag was probably negligible.

I have run SC and Carrera models with spoilers and several 3.3 turbos. Over the years I have tinkered with ride heights and have run cars back to back with different heights. I like the cars best when slightly lowered and with a little more rake than standard. I don't like to lower them too much, as the suspension geometry gets worse. They seem to run best with new front tires, not a lot of front camber and a full fuel tank. It's not scientific at all but after driving these things for 25 years, in all kinds of weather, you get a feel for what works and what doesn't.

JR

[vanwyk4257]

Good info, thank guys!

[masraum]

Originally posted by rgrimm
Makes sense, I guess. You certainly wouldn't want a car that's light at the front to become even lighter at speed. But the front works just fine for anything other than serious high-speed track use.

Actually, I think the opposite is true. Generally you would prefer understeer at speed to oversteer.