Ground Effects Needed?

[Mike Thomas]

I've always preferred the look of a "clean" 911, without a tail of any kind, so that's what I sought and bought. I've read before that due to the shape of a 911, however, the ground effects (tail and lip) are necessary because it tends to become a wing at speed and gets light in the front.

When can one feel this effect taking hold? Would it be at a different/lower speed in a targa than in a coupe?

I'm asking because I'm pretty new to 911s. Today I was going fairly fast, say around 65mph on twisty back road that had some "wavy" (not exactly bumpy) pavement. It almost felt like the front wheels were "skipping" over the low spots, and not even in the tightest turns. I'm not sure whether this is due to my car trying to lift off, or some other process that I'm unaware of due to my inexperience. I don't recall having this feeling on this particular road in my BMW 325xiT.

Can someone fill me in on this. Should I go with a wing/lip, or is there something else going on?

Thanks...

[Pazuzu]

My coupe has a Carrera tail and spoiler.

When I hit 85 or so, I can feel the suspension tighten down a bit.

At 100, the entire car drops down to the ground significantly, and the tires really start to bite.

I haven't tracked it yet, so someone else will have to add the 140 data.

65? Not so much.

[Flieger]

Most aerodynamic aids on street 911s merely reduce lift, rather than produce downforce. The better ones like the 964,993 later RS/RSR/GT2/GT3 spoilers produce a little downforce at high speed when combined with the front spoiler and at the correct rake andgle and ride height.

"Ground Effect" usually refers to the underbody diffuser found on F1 cars, 956, 9R6, and RSR (modern) cars.

Your light front end feeling is probably due to... the light front end
911s don't have much weight up there. This condition was magnified when weight was reduced, as in the 909 Bergspyder. This is why they moved the engine and gearbox within the wheelbase. This meant the driver hung his feet out in front of the front wheels, though. A little scary when your only protection is some Aluminum tubing suspect to cracking and fiberglass so thin you can see through it.

Those cars are fast, though

[Jack Olsen]

Just about all production cars generate lift at speed. The 911's shape is kind of like a wing, but I don't know that the 911 generates significantly more lift than, say, an M3 from the same era.

A front spoiler disturbs the airflow as it goes under the front of the car and can also deflect some air off to the side. The lower down you put your front end, the less air will get underneath the car as you're driving. If you put it far enough down, you'll create a low pressure zone under the car. That said, a factory front spoiler will reduce the amount of lift the car is producing at higher speeds -- but it will never result in the car getting pushed down. It will not generate net downforce. The car will still be generating lift.

The same is true of spoilers in the back, and of most of the 911's factory rear wings. You can mitigate high speed lift, to greater or lesser extents, but you're not generating actual downforce. Pazuzu's car is going to feel more 'planted' relative to a non-tailed-and-spoilered 911 that's generating even more lift, but Pazuzu's car is still generating lift. In other words, it's still putting more pressure down on the tires when it's sitting in the garage than it is at 140 mph (or even 80).

'Ground effects' generally refers to the use of a channeled aerodynamic device on the underside of a race car that compresses air in a zone ahead of the front axle, then directs it through aerodynamically clean channels, and speeds it up as it exits through a diffuser in the rear. The sped-up air produces an effective low-pressure zone and can actually suck the car down with a great deal of effectiveness. But cars that generate significant amounts of downforce with an underbody effects kit require a suspension with almost no travel to it at all. For those kind of effects to work, it's important that the underside of the car be very close to the pavement (about 2.5 inches, usually), and that the whole piece be held in a consistent position relative to that pavement. Braking, accelerating and turning would all upset that positional consistency with the torsion bars and springs commonly available for the 911.

Flattening out the underside of a production car like the 911 can reduce the amount lift it generates. Turbulent air will produce drag, and high-pressure areas under the car will contribute to lift. But that's not the same as ground effects, and it's not going to generate net downforce. Using a deep lip in front can also help, as I mentioned above, in reducing the quantity of air getting under the car.

Using a front splitter to harness the high pressure air ahead of the front bumper can help with actual downforce. But that's not the same thing as a front spoiler.

To generate actual net downforce on a 911, you need a pretty serious front splitter, down low, and also an obnoxiously big wing on the back. No factory aero options for street 911s made before 1989 produce actual net downforce at any speed.

[Mike Thomas]

OK, good and interesting information, but the responses have me thinking I used the wrong terms . Perhaps I shouldn't have used the term "ground effects". Perhaps I should have said "lip and tail combo".

The graphs that Jack provided (thanks!) lead me to believe that there is a real effect, even at legal speeds, of adding a lip and duck tail (which is what I'd lean toward, if I do anything).

Just as another detail, I'm pretty sure my car was lowered by a PO (I haven't measured yet, but it sure doesn't look jacked-up like the non-lowered cars I've seen, and it bottoms out over pebbles).

Thanks again for the responses so far . This place has taught me so much.

[Peter at Pelican Parts]

Mike-

I'm not disagreeing with anything anyone has said here about aerodynamics, but at that speed and given your description, I would lean toward the weight in the front theory (your 911 has weight distributed differently than that BMW, causing it to handle differently on that stretch of road), or perhaps a shock absorber (dampener) issue. If you have old shocks, your suspension might not follow the contour of the road very well.

[Mike Thomas]

Peter: Thanks for the alternative theory. It's very plausible that it's nothing more than a road feel that I'm just not used to. Unfortunately, it is disconcerting.

I will eventually get new shocks, I think. I'm blessed with a very conservative mechanic who steers me away from replacing anything that's not outright broken or unsafe, and we have a difference of opinion on whether the car needs new shocks. So far I have deferred to his much greater experience.

It might be a bit of a subject change, but I do feel like my car's suspension is not very compliant. I don't expect a 911 to have a buttery smooth road feel, but when I was test driving cars I noticed that some cars seemed to soak up big cracks or bumps with more aplomb than others. The worst ones seemed to "slam" a bit over the bumps. Now that I've driven it for a while I fell my car is more of a "slammer" than some of the others. Could this be related to lowering, or could shocks be the issue?

Thanks again, everyone, for your insights.

[ted]

Are your shocks and bushings in good condition?
Maybe just old hard tires?
Did you apply throttle to soon and make it push?
Is it too low, sitting on the bump stops?
If its a Targa could it just have been some cowl shake/flex?
Aero improvements will not remove vibrations.

[911st]

Any info out there on the C2 3.8 tail w a small wing on it?

How do the Porsche rear and front spoilers effect drag?

Do they brake up the air in the back and reduce drag to some level or do they add to it?

Good point on the tires to. Alignment and corner balance can have effect to.

Also might look at bump steer. If the car is lowered the front steering arms can be in a position where with bumps, there is some toe change. Putting spacers under the steering rack can help some, spacers at the tie rod to strut arm can do more, or just going to stiffer springs that limit movement.

[stlrj]

[QUOTE

Can someone fill me in on this. Should I go with a wing/lip, or is there something else going on?

Thanks...[/QUOTE]

How about an alignment? My 911 feels fine at 100+ mph and I have nothing front or rear.

[burgermeister]

+1 ted. Especially if someone lowered the car significantly w/out altering the bump stops.

The 'slamming' might also be eccentric front LCA bushings, which cause your car to ride on the torsion bars instead of the bushings. This would not affect roadholding much, but definitely ride harshness.

Aside from being quite small at 65mph, the aero effect makes the back end light faster than the front end (check the graphs Jack posted), which should cause the opposite effect of what you are describing - more oversteer, not more understeer.

I did see a plot of steering corrections in one of the Up-Fixin's of a Porsche factory driver performing an emergency stop in a curve at 90MPH, with & without the lip/tail. Without the lip/tail, the car appeared to require lots of correction. With, much less. I doubt I'll find it anytime soon though ...

[burgermeister]

Here we go ... from Volume V. Has the 2 graphs Jack posted in the same article.

[911s55]

I would look at mechanical upgrades (dampers, torsion and sway bars) before considering aero/downforce. Street and or lower speed issues can be better dealt with by stiffer suspension, upgrade shocks/struts, fresh bushings, changing tire and wheel sizes, and especially ride height, corner balance and alignment. And don't forget something as simple as tire conditon and pressure.

[the]

Originally Posted by Pazuzu

My coupe has a Carrera tail and spoiler.

When I hit 85 or so, I can feel the suspension tighten down a bit.

At 100, the entire car drops down to the ground significantly, and the tires really start to bite.

That's all in your mind.

[911st]

I have heard of some 911's squating with speed.

All I can think is it takes factory spoilers and the front set low enough, and a fair amount of front rake (1.5" front the back?). The underside if set right I suspect and others have noted can have some areo effect.

Max caster and some toe in I think can add some stability. Toe out makes it a little darty but makes for faster turn in.

+1 on stiffer suspension, alignment and CB. Reduces the bump steer issue, roll center losses with lowering, and the car can be set lower for a lower CG and possible areo effect.

Not an expert, just my thoughts.

[masraum]

My guess is that it's just a characteristic of 911s. They feel different in the front. But compounding that could be your shocks. How old are the shocks on your car. New shocks can make it feel VERY different. My old '88 felt good, to me, but then I put new Bilstein shocks in. Wow! Enormous difference. You can't tell if you have bad shocks in a 911 with the old "bounce test" like you can with a '76 Monte Carlo.

Also, my '88 had a front lip but no tail. It felt good up until about 130-135, and then it felt a little floaty, but that was with the bad shocks. I suspect that after the new shocks it would have felt much better.

I wouldn't expect you to "need" the aero aids unless your spent a lot of time north of 125mph going around curves and such.

[Zeke]

ted has nailed it. Do some investigating on the age of the shocks. A well balanced shock and torsion bar system will out handle anything you can do otherwise. A wise and very well respected person on this BBS once told me I could do a little better in ONE specific turn at Willow Springs if I had a duck tail or a Carrera tail. This turn is 120 MPH plus. Jeez, how many times do we need that extra margin?

[Jack Olsen]

Yeah, I went on at length about aero stuff, but ted and milt are right -- that probably isn't a factor in the symptoms you're talking about. I'd look at alignment settings first, and then your (consumable) suspension components. A 911 has a light front end, which is what gives it such sensitive steering -- and crosswinds can make the front end wander (somewhat) at autobahn speeds when there aren't any factory spoilers in place. But what's most likely with your car is that your camber/toe/caster are out of spec and your bushings and shocks are due for replacement.

Originally Posted by 911st

Any info out there on the C2 3.8 tail w a small wing on it?

How do the Porsche rear and front spoilers effect drag?

Do they brake up the air in the back and reduce drag to some level or do they add to it?

I think the 3.8 wing might have been the first one to actually provide downforce, but I'm not sure if that was without extensions and a wider wing. And I don't know if it has the same effect when you're running it with a pre-964 body. The 964 had a lot of aero improvements in its design.

The ducktail and whaletail spoilers reduce both lift and drag. The actual wings are more effective with the lift part, but bring a slightly higher drag penalty.

Originally Posted by 911st

I have heard of some 911's squating with speed.

All I can think is it takes factory spoilers and the front set low enough, and a fair amount of front rake (1.5" front the back?). The underside if set right I suspect and others have noted can have some areo effect.

Outside of the rear coming down and the front coming up a little under initial throttle, no 911 squats at any speed without big aftermarket aero pieces in place. Rake will mitigate some lift, but it won't eliminate it.

Originally Posted by milt

a person on this BBS once told me I could do a little better in ONE specific turn at Willow Springs if I had a duck tail or a Carrera tail. This turn is 120 MPH plus. Jeez, how many times do we need that extra margin?

Well, at Willow Springs, you need it every minute and a half or so (says the guy with the big home-made wing).

[DanS911]

I love this thread....it brings back memories of......ummm this thread.........ground effects, downforce, lift, spoilers and wings OH MY.
The last time we visited with these concepts the thing that clicked for me is........spoilers.......as said by all above any car moving at a decent speed through the air is going to produce lift. The shape of a 911 (esp. one without a wing) is really good at it. A spoiler, including the duck tail, will to a greater or lesser extent "spoil" that lift by disturbing the air as it flows over or under the car.
While this is certainly different from actual downforce, it is exactly what you want to do to get your 911 to be more stable at speed.

[Mike Thomas]

Wow, thanks everyone for your contributions to this thread. What I'm taking from it is:

a) To some degree just a different car design feeling different in my hands
b) Most likely a need for suspension/steering adjust/replace/refresh
c) I can keep my clean no-spoiler look unless I intend to drive at extra-legal speeds

What's cool is I expected to do b) next spring anyway. I also want to replace the POs basically new (but generic, or no-name) tires.