Front end floating at speed
 

I have a 70 911 that has the feeling of front end floating or drifting at 60+ MPH. I have period correct 185 x 70 tires on 6 in Fuchs and new turbo tie rods. Shocks seem ok but I dont know their age. Any ideas?

Thanks in advance,

Robo

What bumper? If you don't have it, an S bumper will help plant the nose. It's period correct, and a common upgrade.

I had mine at +130mph on the autobahn which was a hair raising experience without the S front bumper. I had the opportunity of a long autobahn stretch once more last month, now with the S bumper it feels much more stable at these speeds. That said, I do not believe that you should notice a big difference at more moderate speeds. I would first of all check alignment and corner balance.

Its my experience T's start getting light in the nose at highway speeds, I think the design of the bumper really forces a lot of air under the car. I also suspect that if the car is lowered, as mine is, the effect is probably more pronounced, as the same amount of air is being forced into a narrower space. The S or RS bumber will probably go some way to rectifying the problem.

If lift on an airfoil is made when more air is passing over the wing, then why is the opposite not true when you invert an airfoil as in causing more air to flow under a car?

It seems that if you could equal the air flow under the car to what is flowing above, you would cancel lift and the result would be neutral.

Ground effect

The road surface has a huge effect to complicate aerodynamics. Diffusers work with the road surface to create downforce by sucking air out the back at an upward angle. The upward airflow vector causes downforce (and induced drag). More air through the diffuser, more downforce. When the car is not "sealed" to the track (on a bumpy track like the Nordschlieffe-Nuerburgring), the diffuser does not operate in ground effect. Therefore, the air is sucked in from the sides to fill the low pressure area instead of being accelerated and sucked from the front underside of the car. Without the increased velocity, the static pressure rises and downforce is lost.

On the 911, Air is being forced under the car, rather than sucked from underneath a splitter, resulting is a compression and a "spring" reaction forcing the front of the car up.

Ground Effect can be very finicky. Wind tunnels show that there is even a significant difference in aerodynamic measurements when there is a rolling road versus a fixed floor.

Since the profile of the 911, more or less flat at the bottom and curved on top, resembling that of an air foil, why then would you not attribute lift to the upper surface where the air is moving at a much higher speed compared to the ground where the air has more resistance due to drag.

Also, looking at the 911 profile, it would appear that air is being forced above the car instead of below and the addition of a front spoiler would seem to reduce the air flow from below like a 747 deploying its leading edge slats to create more lift at the expense of drag on final.

What is the air speed velocity of an unladen English swallow?

A slightly nose-down attitude, like 1 degree, will help alleviate the problem.

I do attribute lift to the profile of a 911.
http://forums.pelicanparts.com/uploa...1241479580.jpg

http://forums.pelicanparts.com/uploa...1241479598.jpg

Wings are the most efficient aerodynamic devices.

Front splitters make use of the high pressure in front of the nose. They make a barrier to the airflow under the car. This creates a good pressure gradient, pushing the splitter down.

Spoilers cause a clean breakaway of laminar flow. They are usually used to detach flow from a long, sloping tail which would create more drag (skin friction) and lift. The Buerzel perfectly illustrates this.
http://forums.pelicanparts.com/uploa...1241479624.jpg
http://forums.pelicanparts.com/uploa...1241479674.jpg
http://forums.pelicanparts.com/uploa...1241479690.jpg
http://forums.pelicanparts.com/uploa...1241479711.jpg
The ducktail decreases lift and drag while also creating a high pressure area in front of it, right over the engine air intake.

Here is my understanding.

The front spoiler blocks flow from going under the car. Think of the "slot" created in front by the ground and the bottom of the spoiler. What air does go into the slot now expands (diffuses) into the larger cavity behind the spoiler. The air expands into the cavity and drops in pressure. (The secret of the 962's awesome ground effects is the cavity between the front wheels). Since the trunk is sealed, the low pressure on the bottom, is offset by higher pressure on the top of the car. The difference in pressure, top to bottom, pushes the car down.

In the 911, unfortunately, the pressure on the hood is low to start with (lift). I think the projecting bumper (911SC), the large area under the front bumper, and the tall front windshield creates an airflow pattern that creates a low pressure zone on the front hood. A 911 rubber lip spoiler tries to offset the upper lift, with a corresponding suction on the bottom, hence, less lift. It doesn't really fix the bumper induced flow too much.

Also air pressure built up in front of the spoiler, forces some air to go to the side. That decreases the air that is both going over and under the car. No net change in lift or down force.

It seems to me, the evolution of the front spoiler went from a crude lip sticking down, to smooth extensions. Then they got more integrated, and then moved to full vertical surfaces. The Peter Gregg Brumos 911 is a good example in my book as a maximized front spoiler with big splitter. However you can't drive that on the street. The 935 Moby Dick took it one step further and sloped the front for even more air control and smoothness. Can't drive that on the street either.

I just added the front rubber spoiler and early turbo wing. The car does seem more planted. Time will tell how it feels on the track. Lime Rock in the rain was not a good time to try out my new "downforce" ;)

There is no venturi shape, no diffusers underneath early 911s and thus no ground effects as such (negative pressure due to under-chassis airflow).

There is only air that flows under the chassis that tends to contribute, along with the invert-wing, upper body shape, to create lift. Front/rear spoilers and ducktails help decrease lift, and in some cases, actually create downforce.

One should notice the effects of front rear treatments even at highway speeds (+70 mph) or under cross-wind conditions.

Sherwood

Since the spoiler lip sticks below the bottom of the car, it acts like a orifice. Because the flow area behind the spoiler is larger than the slot area between the lip and the ground. The air must expand and fill the cavity.

It ain't pretty, is not efficient, but it acts like a diffuser, and lowers the pressure under the car.

http://forums.pelicanparts.com/uploa...1241503908.jpg
http://forums.pelicanparts.com/uploa...1241503801.jpg
http://forums.pelicanparts.com/uploa...1241503949.jpg
http://forums.pelicanparts.com/uploa...1241503978.jpg
http://forums.pelicanparts.com/uploa...1241504005.jpg

I beg to disagree.

Are you suggesting 911s do not need aero devices to increase downforce?

By your reasoning, negative pressure under the vehicle will increase, in effect creating ground effects, by raising the rear end to increase the air cavity underneath and allow more air to flow under the vehicle.

S

To get back to the original question:

What is the condition of the suspension components ?
Check all the bushings - Have they ever been replaced ?
If not then they need to be changed.

I agree with JCF

There is a lot of 'rubber' about the 911, start there. My 69T was wandering all over the road at 100kph (60mph) when I first got it. I changed out the entire suspension system - A arm bushes, turbo tie rods, strut tower bushes, ball joints, wheel bearings, and finally dampers. I topped it all off with a wheel balance and alignment. Not cheap, but my car drives like its on rails - it is running the original T bumper with no spoilers and I can comfortably hit the 160kph (100mph) at the track with no handling issues.

Cheers

... In order to maintain air-speed velocity, a swallow needs to beat its wings forty-three times every second...

seriously - I had a similar issue on my 73 T which was greatly reduced after adding an S spoiler and is essentially gone now after a suspension rebuild and lowering to Euro specs. In my unprofessional opinion, the best thing you can do to improve the handling of an older car is refresh the suspension; even if it's to original specs. It also has the highest ROI.

Reason I went with the bumper/spoiler is he said it was only at high speed, but I agree the suspension should be checked as well just to make sure. On my first 73 I went from a T to an S bumper and while not a huge change, it was more warm and fuzzy