I learned a ton from Chris back when I was building and racing 993s. He was not my only mentor but he is a great driver, engineer and mechanic and really knew the 993 platform. I do hope it helps some folks. When these cars are set up right, they are truly a joy to drive.

Hank

 

Thanks for posting that Hank, I did look up the original thread and it has a ton of info. The information that Chris posted about the sway bars, answers a question that I have had for a few years now. Thanks again.

 

Thanks all for the excellent detailed replies which I saved for future reference.

 

Yea, I bet all the RS pieces help! Sadly they are not in the budget, not sure they are best for a car set @ ROW height. I run a lot of caster, but don't think it's maxed.

Tire offset should not affect alignment at all. More aggressive offsets can help with stability, but with a C4 it should be very stable. Adjusting the rear bar should not make the car feel like that. You may way to loosen up the front bar. My bars are not adjustable.

Rs is a good start, try running less toe, run higher rear tire pressure, get a lot of front camber. My car (95 C4) is my Auto-X car, & this year I'm undefeated, routinely beat Gt3's. I have almost no understeer, & can have the back end come out when I want. I do have a little low speed tight turning understeer, dabbing the brakes helps as you turn.

What I found with the C4, go into a turn hard, & fast, brake heavy as you turn in, as soon as the suspension sets, roll onto the gas & stay on it as you exit. I'm usually WOT right before the apex.

My specs..
Factory M030 car with LSD, ROW M030 shocks/springs, US M030 bars, RS strut brace, Walrod bushings, 18in TT wheels 225/265 Yoko AD08 rubber.
ROW ride height, 1.25 all round camber, 0 toe F/R, lots of caster, 34 psi front pressure, 38psi rear pressure.

From my last auto-X, came in 1st in class, by over 1.5 sec.

 

Sorry Mike, WOT??

 

WOT = Wide Open Throttle.

 

Thanks Steven and Stealth, really appreciate your experienced information.

Stealth you have PM.

David

 

AWD adds to any inherent understeer in a chassis and all 993 are basic understeerers anyway w/ stock suspension

if you are using really low ET front wheels you have dramatically changed the geometry of the steering, your instant centers and scrub radius are nowhere near where they would need to be for best handling.

The GT2 EVOs use the outer holes for the side members to maintain reasonable scrub radius and instant center positioning. Then they use 10x18 ET32 245/645 tires & 11x18 ET14 325/680 tires to fill the wheel wells

alignment depends on the tire, for Pirelli
front
ground clearance 96mm
toe -5' total
camber -4* 10' each side
caster 4*30' max same both sides

rear
ground clearance 93mm
toe -35' total
camber -3* 45' each side
caster 3.5* max same both sides

camber

 

Very useful Bill, thanks!!

 

So, Maximum Caster is desirable?

I compare Caster to the angle of the head-set on the front of a bicycle: More angle means more stability and resistance to turning the front wheel.
Touring bike: Pronounced angle and long wheel base.
Racing bike: Less angle and short wheel base, criteria frame.

My 911 without power steering shows Caster as 6*5' in the manual for good self-centering and when Caster is reduced, steering is more responsive but also slower self-centering.

How is it that the Caster for 993 with power steering is less than for the older 911 without power steering?

 

generally speaking for most users most of the time, yes

correct

caster doesn't not exist in a vacuum, the entire front end of the 964 was redesigned from the 911, for instance scrub radius was changed from (+) to (-) caster and camber were both reduced and p/s added. The design goal for the 993 was to enhance the sporting feel so steering was quickened over 964, scrub radius increased to -10mm, caster increased to 5*20', track widened by 25mm, camber increased to -20', stiffer steering arm bushes, less toe, quicker steering rack. All these changes add up to fulfill the design goal of sportier handling but still w/ the stability expected at speed. The RS received a slower steering rack to enhance it perceived higher top speed steadiness though I've never had issues even on extremely long straights such as at Monticello.