Unstable at high speeds - suspension questions x2

[rolls 912]

My 1968 SWB is unstable at high speeds. Very twitchy. It's running a heavier engine than stock - a 3.2. And yet, the weight distribution is almost perfect.

I have had the suspension professionally set up. It was a real bugger at the time.

Everything underneath the car is new.

My questions to those in the know please:
1/ can I simply adjust the toe in to rectify this issue? Or are there a raft of other knock on factors to consider?
2/ I'm also thinking of adding a rear sway bar. I believe the brackets need to be welded on. Would this be a worthwhile exercise to improve cornering?

I would take it to a Porsche professional but it takes months to get the car looked at & it costs a fortune here in Melbourne, Australia.

Cheers

[stlrj]

Originally Posted by rolls 912

My 1968 SWB is unstable at high speeds. Very twitchy. It's running a heavier engine than stock - a 3.2. And yet, the weight distribution is almost perfect. I have had the suspension professionally set up.

Can you be more specific? Alignment numbers would be helpful.

[mreid]

Caster often plays a bigger role in high speed instability than toe, however if any are significantly out of spec you will have problems. If everything is new as you say and it is aligned properly, have you considered tire balance?

[RSTarga]

what does the professionally set up suspension consist of? + tires/pressure/dimensions?
A rear roll bar would definitely be good.

[fanaudical]

Full suspension specs would be helpful to help diagnose. Also include wheel and tire sizes.

[Trackrash]

Twitchy? Or floaty, darty?

Lots to consider as others have stated. If the shocks and bushings are good, then look at wheel bearings, ball joints, and stance of the car (lower in front).

Are the tires new and the same front and rear? Does it matter if the gas tank is full? What shocks do you have? Alignment settings?

[Matt Monson]

In 1970 the factory introduced a new front spoiler with the intent of giving the nose more downforce. It's not a big gaudy thing but above 85mph it works. Not original on a SWB car, but with a 3.2 swap you don't have an original car.

[DRACO A5OG]

+1 more data please OP

[stlrj]

Originally Posted by rolls 912

I have had the suspension professionally set up. It was a real bugger at the time.

Everything underneath the car is new.

It's the law of diminishing returns. The more you have done to your car, quite often, the poorer the results...or, to put it another way, less is more. Unfortunately, most of us on this forum have violated these laws and have suffered the consequences leaving us paralyzed (due, in part, to the enormous amounts of money we have already spent) and are now unable to climb back up the slippery slope to recover.

I seriously doubt that asking a bunch of amateurs to help solve a problem that the suspension pros screwed up will be of much help other than adding more confusion.


Cheers,

Joe

[Trackrash]

Originally Posted by stlrj

It's the law of diminishing returns. The more you have done to your car, quite often, the poorer the results...or, to put it another way, less is more. Unfortunately, most of us on this forum have violated these laws and have suffered the consequences leaving us paralyzed (due, in part, to the enormous amounts of money we have already spent) and are now unable to climb back up the slippery slope to recover.

I seriously doubt that asking a bunch of amateurs to help solve a problem that the suspension pros screwed up will be of much help other than adding more confusion.


Cheers,

Joe

Possibly true. However some of us do have some experience with setting up a 911 suspension and can possibly offer some help.

[Cajundaddy]

More data. How fast? Suspension or aero?

[RSTarga]

Originally Posted by stlrj

I seriously doubt that asking a bunch of amateurs to help solve a problem that the suspension pros screwed up will be of much help other than adding more confusion.


Cheers,

Joe

I think you'ld be surprised at how much the factory and ourselves have learned in the past 45 years.
Technology has marched on.

[Avanti]

I'm not an amateur, however, I am new to 911 Porsches so I am still learning the differences of these cars.
I would check the front caster and the rear toe settings. A front caster setting too positive will cause steering to be too sensitive or darty over bumps. Too negative and the vehicle will
wander. An Incorrect rear toe setting will cause a steering pull that will require constant steering correction at highway speeds. The vehicle will also feel "squirrely" at highway speeds because the rear tires won't be following the fronts and are steering the car too while the driver is fighting this and correcting with the steering wheel.

[Trackrash]

Originally Posted by Avanti

I'm not an amateur, however, I am new to 911 Porsches so I am still learning the differences of these cars.
I would check the front caster and the rear toe settings. A front caster setting too positive will cause steering to be too sensitive or darty over bumps. Too negative and the vehicle will
wander. An Incorrect rear toe setting will cause a steering pull that will require constant steering correction at highway speeds. The vehicle will also feel "squirrely" at highway speeds because the rear tires won't be following the fronts and are steering the car too while the driver is fighting this and correcting with the steering wheel.

Sounds like you are on the right track.

[DaveA]

The SWB cars had two 25 pound curved weights inside the front bumper to combat this. The weights are at the bumper attach points. Has someone previously removed yours?

[rolls 912]

The settings are on a piece of paper in storage. Ill check my posts from last year as I did publish my findings. I followed the recommendations from the Pelicanites from a post early last year. Although it was problematic as the car is not stock (engine weight) and therefore standard settings don't really apply.

I was hoping that if it was just a toe adjustment i could do this myself. Faffing around with castor is more technical and I will therefore will require professional help. damn it.

Avanti - you made some good points. Its darty over bumps. Sounds like this is more the issue.

Speed - 70 mph +

No aero mods.

Regards

Roland

[haasad]

I suffered instability at speed in my 940 kg 2.8 hot rod. Typically when I was shifting up and perhaps putting in small steering input unconciously. I always felt it was not planted properly.

Carefull checking revealed nil rear toe and no rear camber and a few degrees of right thrust. We adjusted the rear to 2.0 neg camber and 1/16 toe. It shows zero thrust so no rear steer

The car is literally transformed its stable at speed and turns in very predictably.

Just my experience.

[TheSt|G]

Originally Posted by Matt Monson

In 1970 the factory introduced a new front spoiler with the intent of reducing lift. It's not a big gaudy thing but above 85mph it works. Not original on a SWB car, but with a 3.2 swap you don't have an original car.

Fixed.