rmani

Hey Guys,

I hit 130 in my '07 C2S with the top down and noticed the front end getting very light. I was surprised how squirrelly the steering wheel felt at that speed. Has anyone else experienced this? I'm assuming it's just a result of the airflow when the top is down and the rear weight bias, but I've never felt this in any of my other sports cars.

Curious what others have experienced or if I should have something looked at. Thanks

r_towle

Yes, you want to have someone check inner and outer tie rods and all control arms.

it should not get squirrely at that speed

Liste-Renn

I beg to differ.
A coupe 911 in profile strongly resembles an airfoil- which produces lift.
My 997 GTS has an spoiler that deploys at about 75mph (or when engine compartment heat reaches a predetermined value).
I've seen pics of the "roostertail" behind a 997 driving in heavy rain (poor man's wind tunnel) with the spoiler deployed.
It pretty much creates turbulence above and aft of the rear window- which disturbs the airflow that creates lift. It is, indeed, a spoiler.

(Most of the jet aircraft I flew were equipped with them to reduce, or "kill", lift: spoilers on the ground to put weight on the wheels and allow effective braking and speed brakes in the air to slow down or increase a rate of descent when idle power is not sufficient and/or flap extension is not possible due to speed, i.e., existing drag is insufficient.)

It is not an inverted wing, which creates lift in a downward vector, and creates downforce. The 997 Aerokit spoiler and GT2/3 RS model wings do create some downforce, which is matched with more a aggressive lip/splitter/spoiler/dive planes.
The speeds required to create said downforce in any meaningful amount are high. 130 is plenty, real road speeds, not so much.

What I have discovered, on some long high speed runs between LA and Vegas, at night, is my front end does get a bit "light" at 120-130 and gets quite light above that. Especially noticeable in a X-wind, the lift negated by the front OEM front spoiler/lip does not match the lift negated by the OEM extended wing. Neither is creating appreciable downforce, and the car seems unbalanced, to me. Any RWD 911 without a full fuel load is naturally "light" in the front. It's why we trail brake into a turn to shift weight onto the front tires for better turn in.

I've read that Porsche wind tunnel tests all the OEM spoiler/lip and wing combinations to ensure no adverse handling occurs at autobahn speeds. Mismatching F/R downforce/lift with random modifications makes you a sort of test pilot if you drive fast enough to generate a big enough delta between the two to affect stability.

Since the Vmax of my 997 GTS is 189 mph, I can assume that Porsche has wind tunnel tested the OEM aero package to that speed. I would not want to drive anywhere near that velocity, especially with a substantial X-wind though, even on a track.
The factory Aerokit, Speedster, GTS and Carrera front lips are all different designs. Only the Aerokit cars have a dedicated rear wing matched to the front, the rest share the identical retractable spoiler.
The cabriolet version is another ball game, since the roof/rear window/ engine cover line is not an unbroken camber line like the top of an aircraft wing. I'd wager the Vmax is the same as the coupe, but who knows besides the Porsche engineer who performed the wind tunnel tests 15+ years ago? It does have the same retractable spoiler as the coupes, however.


As always, YMMV- but it is an interesting discussion. I believe there is an old thread on this forum about 997 aero devices and high speed stability.

Petza914

There's a reason the upper spoiler wing on a 997 turbo cab raises 1" higher than on a turbo coupe.and it has to do with the difference in airflow of the cab.

JAGMAN1

Anything above 100mph is pretty loud with the top down. 150 never felt squirrely. 170+ felt less stable. I used to live in Stuttgart and spent a fair amount of time in a 997.2 at high speed. Tire pressure should be quite high for those speeds too.

rmani

Thanks for the feedback guys. I should have also mentioned that with the roof up at 130-135 I did not notice any lightness remotely close to what I experienced with the top down. Maybe I'll pop open the manual to see if there are any recommendations for driving with the roof down. That being said I will have the tie rods and control arms checked during the next oil change.

r_towle

it’s also on my list once I get my motor back installed.
I recall reading there is an issue with the inner tie rod bushing or the inner control arm bushing that is frequently overlooked.

my 2006 cayenne turbo (air suspension) I had to replace all rubber on the front end.

I believe there were issues with the rubber that Porsche used during that period. The cayenne stuff just fell apart when removing…some crumbled in my hand.

sad, I have 40 year old stuff that is still fine.

in know the twitchy feeling you mean. My 84 cab is more stable at 120 then my 997 cab.

and, having lived in Germany for a few years, the right lane is 100-120…..
these cars are designed to cruise at 120-150

wiggling is bad.

Doug H

Maybe issue with your car. I have had many 996 and 997 turbo and NA cabriolets above 130 to 150 plus with top down. No problems and wind still not bad. Loved that windscreen. 996 and 997s seemed equally stable top up or top down and I have really pushed them.

Maybe check the suspension. These cars are getting some age so there could be wear.

Another issue maybe damage along the way. Has it been wrecked/fixed what does the underside look like?

rmani

The car came from a reputable shop where it was consigned. Zero accidents and only 48k miles on it now. It’s really mint so I’m baffled. I might need to just test it again on a different stretch of highway.

workhurts

On my coupe the front end felt light on some roads. I dropped pressures a bit which helped resolve.

Liste-Renn

I lived in former West Germany from ‘82-‘84.

Owned two 911SC- one coupe and one Targa. Both had were modded with “tea tray” spoilers and front air “dams” (lower front clips).
Driving at 120+ on the autobahn was drama free, including penetrating the truck bow waves when overtaking with substantial speed advantage.

Have not driven a 997 there however- just a Euro Delivery Macan GTS still in break in. It was stable/solid up to 200kph- (132 mph). Not bad for a box at SUV height.

My description in my earlier post as “light” is more a feeling that airflow is increasing under the car so that rather than squatting lower, it lifts slightly and becomes a bit more susceptible to any X-wind or an overtaken traffic’s bow wave.

The car never wanders or gets twitchy, just moves laterally more rather than less when upset.