gfl

Car - 2000 S track car, that i've just recently completed restoring and building for track. New control arms, bushings, solid toe links, PSS9s, chassis stiffening, good track alignment (as much camber as can be gotten with zero cross camber).

Track street tires (BFG Rivals) 225/45 F 255/40-R. Note this is not stock and is intended to reduce understeer. I get it.

Car drives very well overall.

let's start with i'm an engineer and pretty familiar with the theory, but less so with the model specific black art.

Bottom line, in AutoX, the car showed a tendency to break away at the rear. Not terribly, but clearly the tendency, which seems odd for a boxster. Not just me either, it was very tail happy in the hands of a national-level competitor. He admits he may have simply been over-driving a softer car set up for track not AX ( juts back from WG which is kinda like driving on the surface of the moon).

I did neglect one thing in the rebuild (since fixed) that could explain it - the front sway bar bushings were original while the rears were new, which is in effect making the front softer - oversteer. SO the problem may be fixed - i have nto had an environment to find out since replacing them.

Experience on how others' cars have behaved and what tuning has resulted in maybe surprising effects appreciated.

Grant

DOUGLAP1

In my experience these cars are fairly sensetive to set-up. I have had my car go from neutral to oversteer after just a few clicks on my coilover settings. As you noted too much rear roll stiffness can easily do it too. You might want to try disconnecting the rear bar on one side, and safety wire the end link so it does not flop around to see if that helps.

You also need to check camber and toe settings. Optimum front camber needs about the same rear camber as it is a similar suspension geometry, and of course you want to make sure that you have a bit of toe-in at the rear.

Front bars on a McStrut can have a strange effect - as you increase front bar stiffness you normally get less change in camber due to the reduced roll angle, which counter acts the expected weight transfer effect. So, the front actually sticks better up to a certain point, and after that point if you keep increasing front roll stiffness then understeer comes back very strongly. It is a delicate balance.

Finally, IMHO, you don't have enough tire on the car. I am running 245 front / 285 rear. I can fit a 255 front, but 285 rear is the max I can fit. Your front to rear stagger of about 30 mm seems good if you get everything else dialed in. One caution - check the tire manufacturer's actual data on tire width. Some 255's are closer to a 265, etc.

gfl

Thanks, Doug. As noted, alignment was just done by good Porsche performance shop ( actually, by Pirelli's on-track ALMS/Grand-Am engineer) - so it should not be the issue. Incidentally, i don't much like toe in in the back. Why do you? Any toe ( in or out) results in effective rear steer that changes as you transfer weight. IMNSHO bad in all cases.

Grant

gfl

As noted, alignment was just done by good Porsche performance shop ( actually, by Pirelli's on-track ALMS/Grand-Am engineer) - so it should not be the issue. Incidentally, i don't much like toe in in the back. Why do you? Any toe ( in or out) results in effective rear steer that changes as you transfer weight. IMNSHO bad in all cases.

Also, i don't believe that wider tires are available in 17s, and if so, would likely not fit the wheels properly. Sounds, to me, like you are running 18s. I am not and do not plan to.

Grant

mikew968

A small amount of rear toe-in adds stability under bracking.

insite

my experience w/ the boxster is that it likes zero toe front & rear. at the rear, the car already tends to toe-in under braking & any toe at all adds some push that i don't care for. at the front, toe out does not seem to play well with an open diff under heavy braking. it gets REALLY squirrely. i run zero front & rear whether i'm at auto-x or on track.

to solve your issue, i'm w/ doug. i think this is a sway bar issue. what bars are you running? i think you need to soften up the rear.