Downforce

1fastredsc · 05-21-2003, 05:11 AM

Do all the tails provide the same amount of antilift, like duck tail, whale tale, turbo tail. And also for the front, if the front valance were to be that of an rs or rsr with the lip extension on both sides of the oil cooler opening, would that be the same as a stock valance with a chin spoiler.

beepbeep · 05-21-2003, 05:21 AM

I don't have time to write right now, so i hope you know how to read diagrams.

Cheers!

1fastredsc · 05-21-2003, 06:25 AM

But what about those aftermarket variants of the rs and rsr style front valances?

MotoSook · 05-21-2003, 07:00 AM

so at 120MPH, my car weights 300 lbs less..wahooo! I think I might be under 2000lbs soon!

Really, how many turns are you going to take at 90+ MPH in our street or DE cars?? Yeah there are those that can go that fast around a large radius sweeper, but I'm still not sold on all the aerodynamic aids on a street car that is not for looks only.

At 90 MPH, my car is less 100lbs in rear and 50 in front..gets me closer to that 50/50 weight distribution, n'est pas?

I'm going to get flamed for this..but it'll start a good heated discussion, yeah?

masraum · 05-21-2003, 07:14 AM

Hmm, Souk, you are exactly correct, the freeway generally doesn't have corners that would upset the balance of our cars at 90 or 100.

Hmm, if you had to do a sudden emergency lane change at 90 or 100 in your car would you like your car to be more stable or less stable in that instance?

successful lane change or spin....

Jamie79SC · 05-21-2003, 07:21 AM

Souk,

In case you're not joking - weight on the road may be closer to 50/50. Mass remains the same regardless of aerodynamics.

That means that at speed, you have that much less 'stick' between the rear tires and the road and just as much inertia.

-Jamie

MotoSook · 05-21-2003, 07:24 AM

Steve, unless my suspension has been greatly upset by the subtraction of 100lbs in the rear and 50 lbs in the front at 90MPH, my car should handle as well as a car that weights 150 lbs less right? So is a 2200 lb car less stable it 90MPH than a 2350 lb car? I know this has not considering all the parameters of the dynamics of motoring, but it is adequate for a discussion about street cars.

As a reference, I ran my car with a Carrera spoiler for years. Recently I took it off and ran my first DE without the tail. I will admit I am not seasoned enough to know the difference, nor drove to the limit at which I would be able to tell, but on the highway going to and coming from the track I did not notice any difference. I drove the route with and without my tail (about 3-4 weeks apart). Actually, I was running late getting to the DE, and it was pouring rain! I was hitting 80-90 most of the time in the rain..I know not smart..but I had no sensation of the car being unstable.

MotoSook · 05-21-2003, 07:28 AM

Jamie, I understand your point. If I corner balance my car at rest, I would be minus the amount of lift at the respective corners at speed right?

My point is, does it make that much difference? We are talking about 50lbs per wheel in the rear at 90 MPH..I think the steaks I eat for dinner may be my problem when driving home from the restuarant. I just can corner as fast..or is it the two bottles of wine?

Lift does not negate all the weight of our cars...there is still weight back there right?

OR am I not seeing the forest?

Dantilla · 07:59 AM

Lift does not negate ANY of the weight.
It just reduces the amount of traction.
In most situations, no, it will not make a difference. When driving at the limit, as many on this board often do, then it WILL make a difference. That may be the difference between parking the car safely in the garage at the end of the day, and a tow out of the weeds.

masraum · 05-21-2003, 08:10 AM

Well, as Jaime was saying, you still have 1400 lbs in the rear, but now since you have less weight on the tires you have less stick.

Imagine it this way. For every hundred pounds lift it is probably like putting smaller tires on the same car. The weight is still in the back and still wanting to pull the back end around, but you have less stick, so it is easier to get the rear started in coming around. Try rotating your tires, put the fronts on the back and the backs on the front, then drive around. That would probably feel an awful lot like having 50lbs lift in the front and 100 in the rear. I'm thinking it could be fine most of the time, but under the right (or wrong) circumstances pretty scary.

That's why I used the emergency lane change as an example, you will want/need the most stick and control, but you may not have it.

carnut169 · carnut169's Garage

As you can see from the charts, lift and speed are directly related. At 90 mph a car with the combo (wing and valance) will be 100% more stable than one without. Huge difference. Lowering your car also helps as the air is forced under the car more quickly the lower you go... the air going over the top of your car is disturbed and a low pressure zone is created... in effect the whole car becomes a wing (an upside-down wing). This type of downforce is really the best as it does not create any additional drag like your tail will.
I had a turbo tail on my first Porsce and removed it. There is a long curving ramp that connects 75 south and 285 east here in Atlanta & the car felt much more unstable without the wing. Big difference. When you consider that the 911 is tail happy anyway more downforce on the rear is really a good thing.

thabaer · thabaer's Garage

I heard years ago that the duck and carrera/early turbo tail gave no downforce, but the turbo tea-tray did provide a few tens of pounds down at the cost of much higher drag. The carrera was supposedly a more significant improvement in lift reduction than the 2nd graph shows.

I wonder if we can find more data....I'm pretty sure I heard the downforce claim from a reliable source, I'm normally skeptical about claims like this so I'd like to know why I bought the story.

BTW: in a thread like this, accurate terminology is important. Mass will not change, lift or downforce will affect weight and thus the pressure on the tire contact patch that keeps those nasty walls away...not that we take tighter turns at F1 speeds where the aeros can make a big difference.

MotoSook · 05-21-2003, 08:39 AM

I guess I was using the wrong term. The mass of the car will not change, but at speeds, via lift, the force of gravity (as some have stated) will see some counter action. So at 90 MPH, there is 50 lbs per wheel (in the rear) less "weight". So will 50lbs less weight in the rear make that much difference. You suspension will maintain adequate wheel to road contact, right?

At 90 MPH, with or with out rear tail, the upset from a quick jerk of the wheel will upset a car to a scary end i8n a hurry.

I recall a discussion not so long ago about contact patch - tire pressure - dynamic loading. All that should be considered.

That extra 15 pounds or so of Carrera tail I took off created more negative impact to my lower speed cornering I would guess. It was weight that was higher than my center of gravity and behind the rear wheel.

If I run with my front spoiler and very little ground clearance with side skirts, I think it would be more advantageous than having that 15 lbs hanging over and behind the rear wheel, right? BUT that wouldn't look very cool..esp. if I bought universal side skirts from JC whitney.

Are the charts above from testing of both the front and rear spoilers on the car at the same time? Or were they independent testing?

masraum · 05-21-2003, 08:39 AM

Hmm, I thought that Porsche found in the wind tunnel that the stock style tail actually smoothed the flow of air at the rear of the car and reduced drag (not increased as expected). Also, lowering a car will help reduce the air under the car, but without some sort of ground effects tunnel I don't think it will create a suction, especially not enough to counteract the lift due to air accelerating over the top of the car. Another problem is the bottom of the car is not flat which causes disturbances in the airflow. That's why the Ruf CTR used a second airdam under the car just in front of the engine/tranny/rear susp area, and why Porsche put flat underbody panels under the cars to smooth airflow starting with the 964.

masraum · 05-21-2003, 08:51 AM

Something from and old post with the book right next to me

Quote:

I can tell you that while yes in Paul Frere's book he does list the with front S style spoiler and no rear ducktail the lift over the front axle is cut to .064 (70lb) from (w/no frt spoiler or air dam) .190 (183lb) and the rear increases to .290 (320lb)from .264 (255lb). You can see the delimna, with only a front spoiler the rear sees increased lift which means high speed oversteer which is generally the opposite of what you want at say 130mph.

The same holds true for the other front rear combinations.

MotoSook · 05-21-2003, 08:51 AM

Here is another point. On the same off-ramp, with and without my Carrera tail, I noticed that my suspension compresses more with the tail and I can feel the rear wanting to kiss the wall. I was certainly not going fast enough to benefit from any aerodynamic effect from the tail. So my tail only added to my rear-bias.

Yes, bigger torsion tubes are on the project list, but the point is that that extra weight hanging up there is not good for some type of driving. If I regularly hit Speed Racer speeds on the Autobahn, I might consider more aerodynamic add-ons than just the factory parts.

I know there are some racers lurking out there having a good laugh from this thread.

masraum · 05-21-2003, 09:00 AM

Hmm, suspension compresses more with the tail. I would think that you could be feeling the affects of aerodynamic advantage. Didn't the recent article from C&D from 1974 say that with the duck tail they started to see the effects of the tail at 30mph?

MotoSook · 05-21-2003, 09:06 AM

OK, lets say you are taking a turn at 90MPH, you will be on the throttle right?

I've yet to read that CD article Steve. An I don't think the tail generated enough lift to reduce the compression at the speed I was going. At least I don't think the benefit of having the tail made up for the weight of the tail hanging out there.

But have we considered all the factors from a DYNAMIC perspective?

Dantilla · 05-21-2003, 09:06 AM

Carnut169, the whole car becomes a right side up wing. Low pressure on the top. That's why we need to reduce lift.

island911 · 05-21-2003, 09:46 AM

Quote:

Originally posted by Jamie79SC
Souk,

In case you're not joking - weight on the road may be closer to 50/50. Mass remains the same regardless of aerodynamics.

That means that at speed, you have that much less 'stick' between the rear tires and the road and just as much inertia.

-Jamie

Jamie is right, of course. Maybe if you took a hover-craft out for a spin, you would get his point. Once you start floating the vehicle, "moving the weight bias" is a non-issue.

Also, to the original question "Do all the tails provide the same amount of antilift, like duck tail, whale tale, turbo tail."

While many here will do this, don't lump all 911s as responding equally with a particular tail.

The efficiencey of the tails is dependent on the shape of the car.
From Paul Frere's book the early turbo/carrera style tail work best (is most efficient) on the 74-89 bumpered 911's. . . It's not just the best looking