High speed stability

[pete917]

I wanted to ask you guys a stability question. At high speed my car seems to feel a little light. Its hard to describe, but she feels a little unstable, slightly twitchy even.

What could be the causes of that…. what geometry settings can have that effect?


to help my car is set up as follows:

- RSR Coilovers;
- RSR Spring plates;
- Castor 5.5 degrees
- Camber front left 1 degree neg
- Camber front right 1 degree neg
- camber rear left 2 degree neg
- camber rear right 1.25 degree neg
- toe front 0.5 degree in
- toe rear 1.0 degree in
- thrust angle 0 degree
- front tyres 215x55x16
- rear tyres 245x45x16
- front tyre pressure 31 psi
- rear tyre pressure 34 psi

It has standard front and rear bompers and a turbo rea engine lid.

Thoughts much appreciated....

[Geronimo '74]

What's the ride height? front and rear?
A pic might help too.

[pete917]

Thats one thing I forgot to mention. Doh.

Where's the right place to measure ride hieghts and I'll go and take some measurements.

Cheers

[twistoffat]

Is there a front spoiler. A rear spoiler without the matching front will make it light

Silly me. I just checked your pic....of course it is

How are the bushings. If you have toe out it could make the car light. Is there any uneven tyre wear?

[pete917]

Standard front spoiler, and a turbo engine lid spoiler

[James Brown]

AND what type of tyres are you running. Sidewall flex at high speed makes it "wallow" around when turning slightly left and right (for me) above 110 mph.

[pete917]

That's the kind of feeling im getting james and above 110-120mph

Im running bridgstone S01's

-215x55x16
-245x45x16

[twistoffat]

sorry Pete I edited my post and forgot to ask

How are the bushings. If you have toe out it could make the car light. Is there any uneven tyre wear?

[pete917]

You can see the toe setting in my original post.... its toe in both front rear.

I havent checked the bushings but i think they are fine......

as general question how does ride hieght effect handling

[James Brown]

I'm not sure about the tire sidewall flex but it is the only thing that I can figure out on my car. I have goodyear F1's and others say that they flex alot. You could try a set of other tires (woops tyres) to confirm.

[twistoffat]

I run Goodyear F1´s on my 87 Targa and havent noticed anything. I´ve had it up to 230km on regular occasions and its never been light. I have the front spoiler on and the rear removed. Could it be type pressure related. With the ride hide just right at average speeds it might not be optimal at higher speeds. Just guessing here but with suspension set up it could be a meriad of things

[DanielDudley]

Originally Posted by pete917

Thats one thing I forgot to mention. Doh.

Where's the right place to measure ride hieghts and I'll go and take some measurements.

Cheers

If you look at the sills, the front of the sill should be raked an inch lower than the back. How good was the shop that did your alignment ? If the machine isn't calibrated regularly, it will not align correctly. Also, check your tire pressures.

[pete917]

My tyre pressures are good. 30/34 f/r psi

I'll need to check the ride hieght.

Given its high speed it may well be aerodynamic. I'll fill to the brim with fuel and check again. Maybe add some weight just to check the effect.

It may be the front toe. My front is 0.5mm per side. Porsche recommnd +15' +/- 5', so on a 16inch rim that puts me at the minimum of the posrche spec. More toe in more straight line stability .....

Once Ive checked the ride hieght I'll know more.

I'll measure the difference in height on the side sill as suggested and ee if there is a difference......

Then I suppose there's bump steer to look.

This is a mine field :-(

[HarryD]

Ride heights are nice but corner balance is critical at high speeds.

Have you had you car corner balanced? What were the results?

[pete917]

The diagonal cross weights are as follows. Tey were done very recently:

[Gunter]

Originally Posted by pete917

I wanted to ask you guys a stability question. At high speed my car seems to feel a little light. Its hard to describe, but she feels a little unstable, slightly twitchy even.

What could be the causes of that…. what geometry settings can have that effect?


to help my car is set up as follows:

- RSR Coilovers;
- RSR Spring plates;
- Castor 5.5 degrees I'd like 6 or even 6+
- Camber front left 1 degree neg
- Camber front right 1 degree neg
- camber rear left 2 degree neg I'd like 1.5 deg because it'll be even more with you in it
- camber rear right 1.25 degree neg
- toe front 0.5 degree in O.K.
- toe rear 1.0 degree in I'd like 0, definitely Neutral!
- thrust angle 0 degree
- front tyres 215x55x16
- rear tyres 245x45x16
- front tyre pressure 31 psi
- rear tyre pressure 34 psi

Its a narrow bodied car with standard front and rear bompers and a turbo rea engine lid.

Thoughts much appreciated....

Standard bumper meaning: Stock valance front with the rubber spoiler?

That's what I have plus a small spoiler in the rear.
Castor ~6.1 deg, IMHO, more Castor makes the steering more stable.
Small toe-in front, 0 toe rear,
1 deg neg Camber front, ~1.5 deg rear.
Cornerbalanced,
Bushings all good and tight,
No issues at 100+ MPH.

With Bridgestones.

[chiroracer]

FWIW any bridgestones I have ever had sucked in an undescribable way. S

[pete917]

I orderd a digital inclinomter today so Ill soon be able to measure accurately if the car has the correct angle of attack.

While Im waiting I wanted to ask what peoples wheel arch to ground measurements are. I know this isnt the correct method of measuring ride height, but I know its often used as a way of comparing different car ride heights.

If you have the measurements for your cars wheel arch hieght (heighest point on the wheel arch to ground) front and rear I'd appreciated it.

Cheers guys

[RWebb]

Originally Posted by pete917

...

Its a narrow bodied car with standard front and rear bompers and a turbo rear engine lid.


...std. front spoiler

...

- you may also have an aero problem

if the geometry & tire changes do not fix it, then get a SawzAll and rip that turbo tail off of it - or adjust the angle it sits at, or run a small lip off of it and adjust that angle, or...

[911pcars]

You and no one mentioned what shocks are used, how old they are and if they're in good condition. If the shocks are well-worn, this could produce the effect you're experiencing.

If the ride height isn't set as suggested, air could get under the chassis and produce light steering at high speed.

Sherwood