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 right 0.5 mm in
- toe front left 0.5 mm in
- toe rear right 1.0 mm in
- toe rear left 1.0 mm in
- thrust angle 0 degree
- front tyres 215x55x16
- rear tyres 245x45x16
- front tyre pressure 31 psi
- rear tyre pressure 34 psi

Its a 1974 bodied car with standard front and rear bumpers and a turbo rear tail.

Thoughts much appreciated....

theiceman

Pictures of the car would help. Do you have a chin spoiler on your car ? I imagine it is the Turbo tail is causing it on a narrow bodied car . it was not designed for that car at all , and if you have no chin spoiler it will make it even worse.

pete917

This was before the corner weighting and alignment, but otherwise she's the same.

I hadn't thought about the rear spoiler....

whalebird

What do you mean by "at speed"? 60mph or 125mph.

pete917

125mph plus (this is on track of course and not on the public highway)

theiceman

The turbo tail captures quite a bit more air than the Carrera tail which was designed for the narrow body cars but it does look like you have the chin spoiler so tha is good anyway.
You could always take the rear decklid right off and see what happnes for a run

pete917

Given it only happens at speed, I was guessing its aerodynamic. I need to check ride hieght and see how that looks. I assume it has to be slightly lower at the front than the rear.

What are the correct dimensions or difference in front to back I should be looking for?

ivangene

I have a stock deck lid you could run as a test - cheap and easy to ship....

theiceman

Actualy from what i have read the rear actually sits a bit lower than the front. Don't know the exact dimensions though . i tried to check my ride height once and then got told those measurements didn't mean much .. oh well..
Some one posted an excellent graphic once of the effect of the different tails in how they prevent lift. Would be good if someone could repost.

pete917

Thats weird. Given the 911 is quite flat undernieth, you would think that the front should be lower than the back to create some down force....

What is the OEM hieghts for our cars?

ivangene

depends what country the car was being sent to (US or ROW)

rusnak

I'm no expert, but why .75 deg difference in the rear? And why did you only go .5 deg toe in on the front? I might suggest 1/16th inch toe in on both front and rear, and less neg cam on the left rear. Again, no expert here...

Steve Weiner-Rennsport Systems

Your aero package is just fine.

These cars MUST sit slightly nose down to minimize front lift at speed so thats one of the first things to check. 1 degree is perfect.

Your toe-in figures are expressed in degrees: what are they in inches or mm??

rusnak

Other things that affect stability: are the tires front and rear the same brand and same model? Are the rear suspension bushings tight? You can increase front toe I believe by adding more fuel to the tank. That alone should yield a different more stable feel. A lot of neg camber and minimal toe is fine for the race track with tight springs, but makes a street car twitchy.

pete917

My mistake Steve. Those are in mm's. Ive corrected them. Sorry.

Whats the best way o change the angl of attack. Is it OK to measure the distance to the ground from theh side sills at the front and back and then work out the angle?