Sorry, the suspension is brand new. It is RSR coilover suspension all round.

I tink you are right. Its an aero issue. I thing the ride height is wrong.... time will tell when I et the inclinometer.....

As RWeb suggests, when the turbo tail "bites" at speed (compressing the rear of the car), there has to be an off-setting down ward force applied to the front, that is, to prevent it from lifting in response to the "compressing" action of the rear. With this being out of balance, in theory, (if you had unlimited horse-power) you could end up "doing a Malcolm Campbell"....of course on land. Cheers

Inclinometer?
The recommended 1 deg nose-down attitude is not rocket science and doesn't tell you what the inclination is at high speed.
Do you have the rubber spoiler on the front valance?
Why is the Castor below 6 deg?
Why do you have a 1 deg Toe-in in the rear?
How about posting some pictures showing the sides, front and rear?

agree with Gunter - but get it right/check it out at rest 1st, then move on to the more complicated stuff

Man, I would think the hardest thing is to diagnose a aero/suspension problem without actually driving it. Like throwing darts at a picture of a 911 and changing whatever it hits. Maybe Pete can drive another 911 and compare or someone can drive your car and compare? it's not like you can take a picture of what's wrong or right with the car. And, for all we know, there might not be anything wrong with it. Were all trying our best but this, I think, is a hard one to correct being several 1000's of miles away. BUT if anyone can cure it, the guys in here can! :)

Thanks, and I agree this is like finding a needle in a haystack. Im sure I'll get therewith everyone on ere's experience.

To answer an earlier question, I do have the rubber lip......

At least with the inclinmoter I'll see where Im at staticly at leats.

I wondered if you guys can do me a favour. It would be good to know what peoples ride height measurements are fro top of the wheel arch mesured to the ground through the centre of the wheel. I know this is not the correway, but it would give me some interesting dat for a comparison of where I am at.

I have to ask again:
Why do you have 1 degree Toe-in in the rear?
Why not neutral ?

Why 5.5 Caster?
More Caster like 6-6.2 OEM gives more directional stability for straight driving.

Camber rear left at 2 degrees (Even more with you in it?) means that you run on the inside edge of a new 245/45. Why not make it 1.25-1.5 ??

Do you have the spacers (Bump-steer kit) installed on the steering rack?
Every little thing adds up to more stability.
Do you plan to change some of the above?

Post a picture showing profile and we'll see how your height looks. :)

Hi Gunter

I just noticed I mad a mistake in my original Post. The toe measurements are in mm not degrees.

I dont have a bump steer kit. I have turbo tie rods, but no kit. How low does the car have to go before geometry changes to require a bumpsteer kit.

I do plan to change the camber at the rear. It was an eight hour session and so that was a far as we got :-)

The inclinmoter arrived today so I can at least measure the inclination...

More to follow.... and thank you for all your comments and advice....

Optimal situation: When on level ground, the tie rods should also be level.
Since a lot of 911's are lowered, the spacers under the rack almost re-establish a level position of the tie rods and reduce bump steer.

Why 5.5 deg Caster?

Why don't you post some pictures? :confused:

Pictures are coming. I havent taken any since the RSR coilovers. Im hoping to be able to get the car out in the next couple of days so I can post them here. Sorry fr the delay.....:(

Regarding caster... I know that more caster increases straight line stability, but doesn't there have to be a trade-off regarding the car's ability to change direction without fighting the caster? Some people have told me to just max the caster out, but at what point does responsiveness take a hit?

Or, am I wrong?

Michael

There is more steering effort with large caster (or rather, with large mechanical and pneumatic trail). On street cars, steering input is not usually force-limited, though. The driver muscles the wheel more but achieves the same front wheel angle. The upside of the high caster setting is an increase in favorable camber when the wheel is turned. Camber is a more efficient/effective way to generate lateral force from a tire than is slip angle so the high caster setting has its benefits if you can handle the high-effort.

Good write-up, Flieger.

I think stability at high speed is important and a higher Caster does that.
When rolling very slowly, there is more effort needed to turn the wheel but, at increased speed, there are benefits.
I like the raw feeling of no power steering.

The rake of the front fork on a bicycle and the angle of the head-set is an example of illustrating differences in Caster angles.
Touring bikes and commuter bikes have a larger rake on the fork giving good stability.
A racing bike (Criteria frame) has a reduced rake.
A mountain bike has no rake to speak of.

Still wonder if Pete917 has the spacers under the rack to minimize bump-steer and if his tie-rods are actually level or almost level with his ride-height?

OK, some progress. I have managed to get a consistent reading on the inclination of my car.

I placed a block adjacent to the centre of the wheel on both front and back. Both blocks being the same thickness;

I then ran a straight edge from one block to the other. The top of the straight edge now represents the angle of the car at the tyre contact points.

Next I zeroed my inclinometer on the straight edge

Then I transfered the inclinometer to to the centre point of the door sill.

I measure an inclination of 0.4 degrees (down at the front)

So, the distance between wheel centres on my car is approx 231.5cm. Over that distance at 0.4 degrees there should be a difference of 1.61cm. Looking at my original readings of rear 61.9cm and front 63.5cm thats a difference of 1.6cm.

So, Porsche recommend a 1 degree gradient. This means I need a further 0.6 degrees or 2.42cm less onfront. This would make my ride heights as follows:

Rear 61.9cm
front 61.08cm.

So I have some adjusting to do

Just for reference. A 1 degree slope over 231.5cm is 4cm. So the front of the car needs to be 4cm lower than the rear. So clearly the height of the arches at the front and rear of the car are not the same....

More progress anon..

Don't forget that the height of the arches front and rear are different. The fronts are taller. So, it's possible that equivelent heights are correct, though I don't believe that they end up being the correct proportions.

Michael

Hm.......4cm lower in front compared to the rear? That's 2 inches.

And then there is the issue of full tank vs. half full etc. which can result in more than an inch height difference.

No need for a complicated formula on finding the 1 deg nose-down attitude, a simple inexpensive angle finder on the sill will show the angle instantly. :)

So, are the tie rods level?
Do you have the bump-steer spacers under the rack? :confused:

Yes.

S

Two things? I tried to read everything and no where did I see that you have Turbo Tie Rods! I think we all "Assume" you do but wanted to makie sure. Second, the people talking about the Caster and Rear wing. I changed my rear wing to a Carrera and removed the Turbo I bought the car with and then the small Front Spoiler is matching to rear. More Caster is more stability but resistance to turning! Mine is a little over 6 and yours is in the 5's

I also noticed the larger front tires (215 vs 205) this may have raised the car up and as some are saying, loses the down force.

I have a 1976 and a 1982 911. My WIfe wrecked the 1976 and just yeaterday she complained that the 1982 was a little loser at high speed than the 1976 was. The only difference between them is the 1982 does not have the Turbo Tie Rods which I am going to change this weekend. This is the reason I asked. (I do not know if it is good or bad that my wife is very sensitive about the feel of her cars because it keeps me busy making everything exactly right or I hear from her!) :)

There's more difference betw. a '76 and an '82 other than one having Turbo tie rods, which, btw, provide little effect when the car points straight. For one, the '82 is probably a few hundred lbs. heavier, and different ride height, tires, tire pressure and alignments specs, among many other factors, could contribute to the difference she feels.

BTW, did she wreck the '76 at high speed? And what was the "high speed"? Just curious as this could be a factor (nothing personal). There are several variables at work between different MY vehicles and different driving styles.

Sherwood

About 135 through a turn on a Long Off ramp! She was out enjoying herself and came up on a Saturn doing about 40 (SPeed limit on the ramp was 65). A Large tractor trailer was parked way off the side of the road and the woman driving the Saturn got scared and turned left in front of my wife's Car. She was able to turn right but the woman looked in her rear view mirror and then turned right back in front of her again. SHe ran out of space and time! (SHe is now experienced and knows not to go fast when others are around because no one including them know what they are going to do next)

I copied the link to some Pics of how her car came out! She only got a bump on the forehead, Bloody Nose, a Slightly swollen lip and a bruise on her right leg where it hit the shifter. She ended up hitting a tree head on and then flew acorss the tops of more trees before it landed and ended up in the woods between 2 more trees.

http://forums.pelicanparts.com/paint-bodywork-discussion-forum/498163-need-replace-front-clip-1976-911s.html

Already knowing how she drives and was totally on the edge with the 76 as it was stock, I had cut the fenders off and butt welded on the turbo flares, used the 1 inch spacer in front and 2 inch spacer in the rear and had the 225 x 55 x 16 on the rear and 205 x 55 15 on the front. Koni SHocks (Now on the 1982)

Actually I think the Turbo Tie Rods may have a large effect on the 1982 because the tie rods on there are "Original". The car has about 124,000 miles and she feels the car is not as Crisp in feel going straight or in the turns. So my feeling that between the construction difference and "WEAR" difference including the tie rod ball joints maybe being worn, I think she will see a difference. The Ride Height and alignment was done my Piper Motorsports (The guy that put a V10 in the M3 BMW that road Races)