Downforce

[jluetjen]

OK Souk, Take a breath. You've got a bunch of the pieces, but you don't see the forest yet.

Imagine your car at rest, all carefully corner weighted and aligned. It's a stripped down SC which now weighs 2000 lbs with 40%/60% F/R weight distribution, perfectly even side to side. Remember that it is carefully cornerweighted. The rear tires are 50% larger then the front and so with your springs and bars the car is perfectly balanced when going through a steady state corner. Now let's introduce one variable at a time...

1) The car is moving through a perfect vacuum. As your car is taking your imaginary exit ramp at 90 MPH, the only difference will be a 20 lb increase in mass/weight at the back for the whale tail. So this will be an additional (20/(2000*.6)=) 1.7% weight in the back. The car will be slightly loser at all speeds where you are traction limited, but a minor stiffening of the front sway bar and the car is back to neutral.

2) Let's introduce air!
2a) No tail, no spoiler -- Same turn, but now you are losing #50 of weight at the front and #100 of weight at the back. So at 90 MPH your car's traction balance will be #750 front and #1100 in the rear, so your tires will only see #1850 or 92.5% of the traction that they had in a vacuum. And BTW, you now have 40.5% of your traction up front and 59.5% of your traction in the back so your car is going loose. Keep in mind that it will get looser the faster that you go and more neutral the slower that you go. Think of it as a car that gets hairier and hairier the faster you go.

2b: With a turbo tail and a turbo chin spoiler -- -- Same turn, but now you are losing #10 of weight at the front and #25 of weight at the back. So at 90 MPH your car's traction balance will be #790 front and #1195 in the rear, so your tires will only see #1985 or 98% of the traction that they had in a vacuum. (Note the car weight is now #2020) And BTW, you now have 39.1% of your traction up front and 60.9% of your traction in the back so your car is going to pick up a slight push at higher speeds. Keep in mind that it will get "tighter" the faster that you go and more neutral the slower that you go. Many drivers prefer a slight understeer at high speeds since it makes the car more stable. As a bonus you also picked up 10 MPH in top speed from the reduction in drag compared to an un-spoilered car.

Now admittedly I've glossed over the affects of center of gravity, weight transfer, shocks, etc. but hope that you get the idea. For fun you can go back over these numbers and assume an 80% weight transfer from side to side during a corner. Compare the results with your careful corner balance and you see a shift in traction amounting to 10's of lbs easily, which is a big difference when you are at the limit.

[MotoSook]

Well, I get the idea John, I'm just not sold that a few % difference in traction makes a difference for the 90%+ of my motoring, less than 90MPH around curves and in a straight line.

I fully agree that canceling the lift is a good thing, but the benefits are minimal for a street car and a car (like most of ours) that will top out at 130 or so. Heck I bet most of out street cars can't hit 130 at a DE. So is it better to stabilize my car for low speed (less than 90MPH) cornering? That is the joy of our cars right? Cornering? Not top speed runs on the Interstate to show the Mustangs we can keep up with them up to 90 MPH before his 4.9 motor pulls away (not counting the turbo monsters the live on this BBS).

It's all relative. Lift makes our cars marginally "lighter".

I'm I still standing in front of a tree? Someone slap me.

[masraum]

Souk, agreed, 99% of the time the aerodynamic affect of spoilers will not make much difference, but my point is that when you would like any extra stability that you can get to keep you from ending up in the accident that 3 other cars are already in on the freeway you won't have it. Also, this would affect straight line braking if you bumped the speed up to 120 for a bit and then had to bring it down really quickly.

PS Some of our cars (NA) can hit 150, and while I agree the twisties are the primary focus high speed is a bit of a rush too.

[MotoSook]

Steve, I'm telling your mother about your 90 MPH+ driving! Shame on you!

[jluetjen]

Souk - At NHIS (a fairly tight 1.6 mile track) in my old ITB car I used to AVERAGE 67 MPH, including a tight 2nd gear hairpin. I was spending significant amount of time at 80 MPH and above. Now figure what a 911 with 50% more HP will do around the same track, or a faster track like Mid-Ohio.

Sure, for Auto-X'ing or highway driving a whale tail won't mean much since your either driving too slow for aero to matter in the Auto-X or too far away from the limit of traction on the highway. But if you're spending significant amounts of time exploring the limits of traction at 90 MPH or above (say in a DE or when racing), the affects of spoilers is very significant.

I don't follow your comment:"Lift makes our cars marginally "lighter"." Mass is evil, not weight! Weight (downforce on the tires) is good since it provides traction.

[1fastredsc]

Really great discussion guys, but no one has answered my question yet. All i want to know is whether one of those front valances that are rs and rsr style with oil cooler openings and spoiler extensions will match up with carrera tails or provide too much downforce/negative lift. And i was also thinking of a duck tail ( i really like the way they look), are they about the same as the carrera whale tales?

[Superman]

Weight and mass are different things. (Duh, I know). Mass is what encourages your car to spin. Weight is what prevents that.

Aero effects don't change mass, but they do increase weight.

[304065]

The 911R set a world speed record at an average of over 130 MPH without a duck tail, with a short wheelbase, only aero was the 1%+ downslope of the body.

So what?

So what? So let's dance!

The question really is how much additional drag and weight is added by a deep front spoiler and splitter and rear tail and how does that reduce corner exit speed, vs. the impact on traction and ability to pull g's through the corner.

I could figure this out if I could just get marooned on an island with Fred Puhn' book and a HP-12c financial calculator I could tell you.

[MotoSook]

Yes, it is understood that weight and mass are different. But is the 2% less traction in John's "2b" example that critical? The -2% is the "lighter" that I am talking about John.

Now when I step on my brakes a get a whole lot more weight up front, and when I'm on throttle, I get more weight in rear right? We all know that lifting in a turn is not good, so we stay on throttle..so the "weight" transfer to the rear probably negates (if not more than makes up for) the 2% I've loss at 90MPH.

And it may be 'til hell freezes over before I will take at 90 degree turn at 90 MPH. At 100+MPH (maybe a lot less) on the front straight of Blackhawk, I'm stable enough without my tail to pass a slower car, and when I stomp on the grippers to slow for turn one I'm in a straight line, with "weight" transfer to the front that can't be compensated by even the turbo tea tray.

I'm just trying to look at this from a practical perspective. I understand the benefits of lift, anti-lift and downforce .. what ever you want to call it. I use to be, and still am part-time, a measurement engineer, so 2% is unacceptable in my world, but I just think that the 2% I gave up to leave my tail at home is better for my short track dynamics.

Fastred...go with the ducktail.

[MotoSook]

a 12C! I thought my 48S HP calc was weak! Hahaha..I was called a nerd by Chris Streit when he saw it in my garage one day...

This is getting too fun..I better do some work.

[emcon5]

For what it's worth, I noticed a diference in the effects of wind after installing a Carerra tail and chin spoiler. Crosswinds and the effects of passing trucks don't seem to effect it as much.

Here is the sidebar from the 74 C&D article:



Tom

[1fastredsc]

I thought so, the ducktail, in my opinion, looks really good on a red car like my sc. Now i just need to figure out which valance would match it, the ones with spoiler extensions or without?

[masraum]

1FastredSC, From reading Paul Frere's book Porsche 911 Story it seems that the factory worked to maintain a balance between the front and rear lift when adding spoilers. In one portion of the book it is stated that they found that the deeper they made the front spoiler the more lift decreased, but since the rear lift couldn't be decreased in a comparable ratio they went with less front air dam to keep from leaving the rear light at speed.

The ducktail and early turbo tail seem pretty close, by eyeball I would say about 20% separates them at around 100mph. The turbo make a pretty big difference after that.

If you go with an RS with a big splitter underneath I think you would be best served with the more efficient tail in the rear, the turbo. Without the splitter you could probably stick with either of the other two. Either way without concrete data it's all a guess.

Souk, I still think in the conditions that you mention there is a small affect from the spoilers, but still worthwhile. It's all probably moot since most of us can't approach the limit where it would matter.

From professional driver who can drive at the limit consistently when they are looking for that fraction of an edge over other's this is important, on the track when we can overdrive 9/10's and or aren't perfectly consistent it is probably immaterial.

I drive my car minimum 5 days a week 25miles each way running anywhere from 60-90 on average. If I am running 90 and have to hit the brakes and swerve to avoid something I would want to have that little extra bit if it helps just a little.

Funny thing is that my car came with a lip spoiler and I have run with that for 2.5 years without the rear (PO removed it rear) I intend to replace the rear spoiler, but money has stood in my way. I have been fortunate so far.

HP 32S (same one since 1990) here, still prefer it to many of the modern calcs. Besides, almost no one else can use it, so it never wanders off.

[1fastredsc]

Thanks for the info, now i gotta raise the cash for the oil cooler to go along with it!

Just my opine, but I think this is the perfect combination.

[jluetjen]

Souk, Assuming that about 80% of the weight transfers to the outside wheel (in a 911 it's more like 100% at the front), your talking about that 2% equaling almost 20 lbs. Try scuffing a tire and wheel across your driveway with #20 (4 5 lb bags of sugar for instance) sitting inside the rim. You'd be surprised at the difference that it makes in the traction.

As soon as you started to talk about braking and such you started introducing other variable which I took out to simplify the equation. Trying to take into account the affects of braking and throttle usage add another order of magnitude to the model. A lot also has to do with the drivers use of the brake and throttle. Novice drivers often over brake and then use a lot of throttle in mid-corner and exit. I suspect that you'll find the fastest racers are not braking in a straight line and carry a lot more speed through the middle of the corner. They are still hard on the throttle, but the car has a lot more side loading on it and may be accelerating at a slightly slower rate, but is travelling at a significantly higher speed. The set-up which will make the car happy is different for both of these situations.

At the end of the day, I think that we're agreeing, if you're not travelling fast nor are approaching the limits of adhesion at speeds above 60 MPH, the reduction in lift winds up being essentially meaningless.

[MotoSook]

LOL.

Steve, Reverse Polish Notation is what has kept mine by my side (no it's not in a holster or in a plastic pocket protector you punks) since Sophmore year at college

As fun as it has been playing the otherside in this thread..I do have plans for a ducktail for car and those my latenight blasts through the countryside at Speed Racer speeds. And I hope, HOPE!, that ducktail helps me avoid the racoon that has been playing chicken with me since high school. Hehehe.

[1fastredsc]

Mark, your car is sexy as hell man. Still though, a nice rs valance would be nice.

[MotoSook]

John, I braked into corners at my DE, but that was only because I couldn't get my car into 2nd gear before the turn!!!! Damn sychros! On my motorcycle, I make all gear changes and set up before I enter a turn (mind you I have a redline nearly double that of my car and the power band is wider so I can get away with more). I think it's a habit I'm likely to continue in my car on the track. A smooth throttle through the turn is what I'm used to. I realize that trail braking is good for some corners, but I ain't nowhere near doing all that and trying to force my trans into 2nd gear.

Oops, did I just throw this thread into another whirlwind?

I agree John, we are splitting hairs with a topic we all agree on, but just merely standing on opposite sides of the hair! It's been fun.

[Bill Verburg]

As I originally posted those lift charts several years ago I feel that its only fair to post their original source, which is Bob White. The original article was published in the 5/78 issue of "Panorama" and is currently available in "Up~Fixen" Vol. 5 p309. There is a load of other interesting info in the article.

The body configurations being compared are as follows

• '73 RS front spoiler and ducktail
• '76/77 turbo tail(similar to 84 Carrera tail) and p/u front chin spoiler
• '78 3.3 turbo tail(oterwise known as the "tea tray") and matching front 3.3 turbo p/u chin spoiler
• std 74 on no spoiler body work

The spoilers all reduce lift in varying degrees, none create downforce(the 993RS club sport tail is the only factory tail that I know of which does create down force). As such the tails affect what is called the normal force on the road. In that the spoilers affect f/r normal forces differently there is an effect on steering which is also graphed in the article(the effect of steering inputs is increased w/ spoilers and their period reduced, ie the car is harder to catch). Since the spoilers reduce lift they allow the normal forces to remain as high as possible thus increasing available traction(i.e. the friction circle is kept as large as possible

Weight , of course, also affects the normal force but is a constant as is the polar moment which is determined by the weght distribution. in another thread the issue of where to reduce weight was addressed the weight at the ends(far forward and far aft)are critical in this regard because this weight has the most effect on polar moment. The more the mass can be concentrated near the center of mass(or reduced at the ends) of the car the better will be its response not only to yaw inputs(turning) but also to pitch(up/down motion)

The net mass(or lack thereof) is felt primarily in acceleration and in braking but as mentioned does also contribute to normal force, but in general less is better as the reduction in normal force is more than compensated for in other areas.