Seeking handling advice
 

Folks, I have a 1988 3.2 targa. GT Racing Carrerra tail and RUF
replica front dam. New rear bushings and elephant bushings
up front. All new Bilstein's all around. Tie rods and ball joints
are good. New Toyo RA-1's. It has been slightly lowered,
corner balanced, and aligned. I'm not concerned it is a
worn component so much - more thinking this is time
for tuning advice from you experts.

Here's the alignment spec's:

Front caster L= 5.1; R= 4.5
Front camber L= -0.7; R= -0.5
Front toe L= 0.15; R= 0.15
Rear camber L= -1.2; R= -1.1
Rear toe L= 0.1; R= 0.1
Thrust angle = 0.0

There is a rake of 10 degrees front to rear (rear 10 degrees higher)

Here is my problem: about 95 or higher, the car starts to feel
lighter - kind of 'floaty'. It isn't wandering or anything, but the
steering feels light and I do not the feeling through the
wheel that the car is as connected to the road.

I notice if I fill the tank to the top, then the car handles fine
at these same speeds. However, as soon as I get near half
tank or less, the light feeling comes back.

Question: what do you think might address this, or is this normal?
I am wondering if I should lower the front some more to
induce more rake (and more downforce), or is there some
suspension setting I could adjust (caster)?

Or would adding a chin spoiler under my current RUF type spoiler
solve this (or a splitter)?

I notice this at HPDC and on the highway. I would like
to address it before I get into DE's.

Thanks.
Spike

Spike,

Your caster settings seem to be low ... and that can affect stability significantly!

My '72 - '73 spec book says 5.5° +/- 15'

Don't have the '84 - '87 spec book at hand, but I believe the specs are similar, if not identical.

Can you post some pics of the car from the side? I would like to see the side profile, This does seem strange, my 69 912 with no dams or spoilers just starts to feel light but only around 120, my 86 carrera seems to suck to the road better the faster I go. 10 degrees seems like a lot, others may have an idea about high speed

You did not specify whether you had toe in or toe out. Toe out will make the car feel unstable at higher speeds, but it may turn in better during a tight handling course. I run a little toe in, maybe too much which makes the steering heavy feeling, but it is really stable at high speed.
Caster angles also seem a little low like Early S man recommended.

My car came with a turbo tail. My mechanic told me during the PPI that the downforce of the tail will actually slow the car down (aerodynamics) and lighten the front end on high speed runs. He actually suggested a full tank of gas for any high speed testing.

The Ruf front bumper should have a rubber lip underneath, acts the same at the factory front lip. If you don't have anything there, you should add something to balance the rear spoiler.

I think there was some discussion of the lip with pictures in the monster Ruf thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/94997-ultimate-ruf-conversion-thread.html

Tom

The rake seems a little excessive to me. 10 degrees sounds like what you'd find on a '69 Nova.

I think you could use a front lip under that Ruf nose.

Yeah, did he mean 10* or 10 minutes? What would 10* translate in inches at the wheel arches? I'm thinking more than the usual recommended 1/2 " (taking into account that the front is higher than the back when the car is dead level).

Thanks for the input guys. Yes it has .15 degrees toe in
on the front on each side, for total toe in of .30 of one degree. Milt, it may be 10 minutes not degrees, as you suggest.
I think it is the equivalent of the 1/2 inch or so
recommended.

I think you may be on the mark with the front lip
under the valance.

The reason I was second guessing this was due to the
change in feel with a full tank of gas. I was figuring
this lowered the car a bit in the front, increasing rake
and possibly altering some suspension settings in
some way unbeknownst to me.

With a full tank, I think the extra weight (8lbs per gallon) is what is making the difference.

Totally noob guess and totally not even familiar, but could the downforce from the rear spoiler be lightening the fronts at speed.

Is your setup proper to give you the additional downforce you need in the front at speed to balance the added downforces in the rear?

Just to add:

You may want to shop around for a different alignment shop. Those numbers should be more closely matched, and do not represent an optimal set-up, even for "aggressive street" driving.

For a primarily/solely street-driven car, I would start with something like this:

Front Camber: -1.0
Front Caster: 6.0+ (max)
Front Toe: 0.05 (toe-in)
(ideal) Cross-Camber: 0.0
(ideal) Cross-Caster: 0.0
Rear Camber: -1.8
Rear Toe: 0.09 (toe-in)

I would definitely add the front chin spoiler as well. It seems that some (all?) of the Ruf bumper reproductions do not include the lip. It shouldn't be that hard to fab something up though. Also, if you want to make things easier, you can set the fender-heights to be the same at all 4 corners, prior to corner balancing. This will give you a slightly more aggressive rake, and puts a little more weight over the front axle-line. Lastly, when corner-balancing, I would make sure to check all tire pressures, have the gas tank 3/4 full, and have a ballast weight (equaling your weight) in the driver's seat. Any reputable shop should be able to give you a computer print-out of the CB figures too. YMMV.

THat's the reason for the suggestion of the front lip. That front lip can make a big difference in front end downforce, and Ruf probably designed it to use a front lip for that reason.

A normal balance with front and rear spoilers is for the rear to be a little more aggressive than the front to give a slight understeer at speed, but I don't think it should feel light at 100.

I agree that the weight is probably making a difference, but not in the rake, probably just due to the weight. Considering how far forward our tank is and how big it is...

I agree w/ Warren, castor is low spec for your car is 6&deg 5' &#177 15' low castor becomes increasingly skittish w/ speed.

Also agree that air under the front could be a problem, ride height and lip are both the factors here.

Most Ruf valances include the lip as part of the piece because that was the way Ruf di them.

The Ruf bumper caps used a separate lip as pictured on my car
http://forums.pelicanparts.com/uploa...1097449958.jpg