High speed stability 1970 911t

[nhs15]

Need some expert advice.

1970 911t - I've been through each system over past 2 years. I had posted in the past about a high speed stability issue and since have had considerable suspension work done. Initially above 70 mph, there was a significant skating or swimming feeling while driving in straight line. Since then every suspension component was replaced, new tires, corner balanced, lowered. Have gone to two reputable local shops. Now it is rock solid till 90 mph, but then the issue creeps back up. Can get to 110 mph but only for a short moments because it gets worse. So its much better but maybe not good enough. Goal is to use it as DE vehicle with the first PCA event coming up at the end of March at Summit Point main. Weight is 2,118 lbs. In the twisties its really great. Only in straight line at high speed do I have issues. Attached is the most recent alignment result. At this point I'm not sure if I should use it at the track or not.

[rxtrom]

It could be your aero or front end weight. Are you running the stock front bumper?

[spyerx]

I can't really tell from above... maybe a few questions. i have a 71T that is a mostly stockish type setup...with a carb'd 2.7 so it's pretty quick.... It's rock solid up to 100+ and will cruise all day at 85 feeling very stable... I haven't taken it any higher and haven't yet driven it on the race track. however, on the canyons, it's very composed and progressive and not at all scary.

What torsions F/R?
What shocks F/R?
What sways F/R?
What is your corner weighting?
Do you have adjustable spring plates in the rear?
What front spoiler do you have?
What is your rake setup?
Do you you have your corner weightings?
How are your engine and trans bushings?
Assume your bearings and drive axles are all in good order?

There are a lot of factors here, so unless your chassis is torqued it's likely setup.

This is mine, just for an example:
21/26 torsions
16mm f/r sways in factory style/position
Bilstein sport f/r dampers
2150 weight
Corner balanced for 200lb driver
Elephant racing rear spring plates adjustable
Ride hight, low
S front spoiler
No rear spoiler
Rake set at slightly more aggressive than factory (just a tick more nose down)
new engine, trans bushings
Bearings all around are new
Rubber is all new
Tires are pirelli cn36 185 f/r on 6" fuchs

[nhs15]

Yes stock bumpers and no spoiler. Stock body all around. No spare tire and the battery is relocated to smugglers box so front end must be very light.

Koni sport shocks
tbars- front 21, rear 27
Yes adjustable spring plates
All else in great order

Hope its an aero issue and not chassis.

[Bill Verburg]

Originally Posted by nhs15

Need some expert advice.

1970 911t - I've been through each system over past 2 years. I had posted in the past about a high speed stability issue and since have had considerable suspension work done. Initially above 70 mph, there was a significant skating or swimming feeling while driving in straight line. Since then every suspension component was replaced, new tires, corner balanced, lowered. Have gone to two reputable local shops. Now it is rock solid till 90 mph, but then the issue creeps back up. Can get to 110 mph but only for a short moments because it gets worse. So its much better but maybe not good enough. Goal is to use it as DE vehicle with the first PCA event coming up at the end of March at Summit Point main. Weight is 2,118 lbs. In the twisties its really great. Only in straight line at high speed do I have issues. Attached is the most recent alignment result. At this point I'm not sure if I should use it at the track or not.

The alignment looks ok and crosswights look ok(is that w/ a driver in place?)

Now it is rock solid till 90 mph, but then the issue creeps back up. Can get to 110 mph but only for a short moments because it gets worse

This hints at an aero issue, w/o at least an S front spoiler bumper you are looking at ~136# lift in front and ~189# of lift in back@ 100mph, at lower speeds it's lower and at higher speeds it's higer, the rat of increase is exponential, a small increase in speed gives a relatively large increase in both lift components

[RSBob]

This is a well documented phenomena. When you look at the profile of a 911, it roughly resembles a traditional aircraft wing. With increased speed, lift increases, especially in the rear. This causes the squirrelly symptoms you are experiencing. To combat lift at higher speeds Porsche developed front and rear aero devices such as a chin spoiler and duck tail. The 72 and 73 911S had chin spoilers to help smooth the air flow under the car, but it wasn’t quite enough for higher speeds. The RS added the duck tail which kept the car planted. You are currently exceeding the cars aero capabilities without going to aero aids.

I have seen stock non-S front ends with later chin spoilers added from impact bumper cars which helped this problem somewhat. You might want to start there. If not enough, then get either a duck or tea tray for the back.

[1QuickS]

Aero induced lift changes toe and camber as a function of speed. You could consider aligning the car after determining how much rise occurs with the lift per Bill's posting (bathroom scales under the car at front and rear axle locations and use scissors jacks to lift the car while monitoring load on the scales) and then lower the car by that amount (front and rear would have different amounts) after static alignment.

According to the info from The Porsche 911 Story by Paul Frere, when a "S" front bumper/spoiler is used without the duck tail then there is an exaggerated pitching and the rear lifts at a somewhat accelerated rate vs. no spoilers at all.

[nhs15]

Yes corner balancing was with my weight in driver seat.

Thank you. Good to know. Has anyone experimented with adding a generic rubber chin spoiler to a stock t bumper? https://www.rimbladesusa.com/product/razor-lip/

In know its heresy with the purists, but I want to try something low risk before committing to a new bumper or the chin spoiler from later impact bumper cars.

[1QuickS]

Lower car after static alignment by amount car rises during aero lift per post #7 is low risk, low cost and maintains OEM appearance.

[kent olsen]

I've seen 140 at track days and a couple of back roads with no issues. Also moved my battery to the smugglers box and removed the spare. I think my front to rear is 38% and 62%.

[Peterfrans]

I noticed a significant improvement after mounting the S-front bumper, but the car still becomes light in the front a high speeds (took it to its max at the German autobahn).

[Bill Verburg]

w/ an S front bumper the front lift at 100mph goes to ~76# w/ an RS front and duck the #s goe to 50 & 60 respectively

[Trackrash]

Good choice with the Koni's. Have you tried setting them hard? I found that helps quite a bit at speed.

What tires and wheels? Tires can make a huge difference. You may want to play with tire pressure as well.

When my rear tires heat up and the pressure rises above a certain point, my rear end feels less stable at speed. In my case at a certain point the tread is no longer evenly pressed on the ground. Have you used a thermometer to measure your tires' tread to make sure the temperature is even across the tread?

[nhs15]

Bill what would the affect of the g body chin spoiler be? Found one used locally that I may modified to fit my stock bumper.

Gordon the tires are yokohama s drive 205/55 on my 15" stock steelies. Havent measured tire temps yet. Although the stability issues happen to occur right away and it was in the 40s out here when I tested it last.

I guess another question is what speed should I be comfortable at in this car on the straights? I've only ever tracked modern cars (F80 M3).

[Trackrash]

There can be a number of things contributing. But I would put a duck tail and chin spoiler on if it were me.

That said I have seen short wheel base cars at the track in vintage events with no spoilers. Probably had rock hard suspensions.

I really think you shoud play with different tire pressures and shock settings.

You may want to post a shot of your car. Lower is better at speed as is a nose down configuration.

Different tracks may have different affects on your car. My car generally feels more stable at the track since they are usually smoother. However, that said, I found turn 1 at Laguna Seca to be almost scary, with my car feeling light going over the crest.

[Bill Verburg]

Originally Posted by nhs15

Bill what would the affect of the g body chin spoiler be? Found one used locally that I may modified to fit my stock bumper.

Gordon the tires are yokohama s drive 205/55 on my 15" stock steelies. Havent measured tire temps yet. Although the stability issues happen to occur right away and it was in the 40s out here when I tested it last.

I guess another question is what speed should I be comfortable at in this car on the straights? I've only ever tracked modern cars (F80 M3).

Good point above about tire pressures, too high or too low can lead to squirrly behavior

for H body up
w/ front chin and w/o rear spoiler 7 & 176

w/ front chin and whale 4 & 10

w/o front chin and w/ duck 45 & 25

[Mr. Merk]

I noticed this car with just a front S spoiler get a little squirrely at high speeds during the Hill Country Rallye last year. Charles said his car feels way more stable at speed with his windows down, lol. I imagine a duck would even things out a bit.

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A post shared by Charles C. Stanley (@cstanman) on Jan 31, 2020 at 2:33pm PST

I've had my stock suspension whale tail Carrera up to ~150 and it feels like 80. We had the cruise set on 110 all the way across Utah. Aero makes a huge difference.

[Bill Verburg]

Originally Posted by Mr. Merk

I noticed this car with just a front S spoiler get a little squirrely at high speeds during the Hill Country Rallye last year. Charles said his car feels way more stable at speed with his windows down, lol. I imagine a duck would even things out a bit.


I've had my stock suspension whale tail Carrera up to ~150 and it feels like 80. We had the cruise set on 110 all the way across Utah. Aero makes a huge difference.

windows down creates a lot of drag, I've never seen any data on the effects on lift, but drag itself can stabilize a car depending on the geometric relationship between the center of mass and center of drag.

Anecdotally, At DEs the windows are almost always down w/ no effects on track performance other than to slow the cars a bit and create a lot of wind noise that plays havoc w/ an audio recording

[Carbster09]

Had my boosted 930 at 165 and it seemed ok, but things were happening quickly!

More to the point; my 71T was very bad until I started messing with the tire pressures. Just a little low and it was not fun! That is with new and good suspension set up by TRE from Rebel. The tires are staggered 185 / 205 on 7R (rep) wheels. Brand Avon CR6ZZs. Pressures made a big difference. I have a completely stock body T.

[Carbster09]

One item I forgot to mention was that I am considering painting my car. Thus I assumed that more rubber would help high speed stability issues? The 315s I ran on the Turbo were nice to look at my made handling odd to say the least. I am thinking 255s out back would be perfect; comments? Then this means my little narrow body would need to grow flares (SC??)