I got the adjustable sway bars and Koni Yellows from Strano and I have to admit, I don't have much of a clue how to adjust for a give autocross/road course. Currently I have the front and rear bar in the middle holes and the front struts set a little firmer than the rears. If the car is pushing, loose, etc what do I do? Car is a 2011 GT with Brembo bbk, Motul fluid, BAMA tune, CAI, etc. Current wheels are 19" SVT's with Goodyear F1's. Our autocross courses usually have multiple tight turns. Thanks for the help.

 

A lot of factors.... First, understand that shocks don't effect steady state balance. They effect transitions. How quickly the car does something.... You shouldn't be using shocks to tune understeer unless you have no other way to do anything at all. You set the shocks so the car responds as a unit the way you want. The front turns in like you prefer, and the rear doesn't over-respond or act all lazy and make the car feel like it's a rubber band in the middle.

Bars. If you are trying to loosen the car up, more rear bar, or less front--or maybe both. Also more rear tire pressure, or maybe (not always you can't run too low) less front pressure. Or more negative camber in front. Or maybe you are overdriving the corners--or braking too late, etc. FWIW this is hugely common and more so on an autocross than a track since the turns are much tighter and harder to setup correctly since they are harder to see and are much more linked to the previous corner than is typical of a track. Example. I'm running a track day this weekend.... the track has 22 corners in 2.2 miles. While it's a super twisty track it's still not an autocross.

Also remember that the car will act different on different tires even with no other changes to the suspension. Typically on street tires the car will understeer more on the way in, and oversteer more on the way out due to the relative lack of grip on a heavy car. Grip rules and fixes a lot.

 

Thanks for the input. For staggered street tires, what kind of air pressure are you running for autocross?

 

Might as well ask me what size shoe should you wear? Pressures depend on the actual tires used, not the stagger... that's based on construction and tires vary hugely on construction. If you want a very generic best guess (maybe assuming the stock P-Zero's) I'd say 38 front, 32 rear as a start.

 

Thanks! I'll try to keep that in mind, too, Sam. Yellows & sway bars are the next things on my upgrade list. Just that pesky little matter of money....

 

Am I correct in thinking that to make the bars less stiff I move to the outer holes (closer the the end of the bar)?

 

That's correct.

And if the outer holes make the effective 'arm' lengths 10% longer, the bar rate will drop by significantly more than 10% (more like 20% softer, actually).


Norm

 

I am not even in Sam's league, and I run my car on road courses only, but...

I am running the svt stagger (285/255) on my 2011 GT with the Goodyear F1s, yellows, sport springs (does that sound familiar?). I like 42 front / 40 rear hot. At Sam's pressures (hot), I find the car very imprecise. If they are cold pressures, the rears seem about what I run but the fronts are higher than I've tried. Based on Sam's advice, I think I'll try lower pressures next time out. I'll bring my pump though!

I do think the stagger matters. The tire shape is different for 255s and 285s on a 9.5 inch rim. The rears "balloon" a bit more which moves the contact patch to the middle of the tire. I believe that rear pressures need to be lower to extend the contact patch, which should improve grip.

FWIW, I find the GY F1s need temperature to be sticky. They work fine for my lapping days where I can take a lap to warm them, but I doubt I could even get them to temp in an autocross run (1 minute?).

 

Lowering the rear tire pressure also improves the mechanical grip back there, which makes the car a little more resistant to getting loose under throttle on corner exit. I normally run about 4 psi difference even on the street, partly for that reason.


Norm

 

Ok, you have to keep in mind that different tires have different constructions. Also the more squeezed a tire is on a rim, effectively the softer you've made it which makes it feel more sloppy.