Suspension settings for autocross
#12
Just so I'm clear, Norm, do you bleed off pressure between runs to maintain your cold pressure? I had my pressures set at factory starting out a couple of weeks ago. When I checked them hot, the pressure was in the low 40s. I used shoe polish, but I wasn't getting over onto the polish. I've heard that trick doesn't work so well with modern tires and their stiff sidewalls.
That's because when you're out on course - or even after driving on the street for a few minutes - your tire behavior is not going to know or particularly care what their pressure was when you first fired the car up in the morning.
I keep saying pressures in the plural because at autocross or any other activity where you're interested in cornering performance with little or no consideration for ride quality there is no reason to expect the front and rear tires (which get loaded differently) to need the same pressure. They might, but if so it'll be more by coincidence than anything. As a side note, I don't think I've ever run the same pressure all around even for purely street driving.
Pressures up in the 40's do apply in certain cases**, but I don't think that autocrossing a 3500 lb RWD car on tires of adequate size and wheels of adequate width is one of them. Think more like 35 - 37 front and about 4 psi less in the rear, measured right after pulling back into grid. If you want a stronger reference than just mine, I know that Sam Strano ran about 4 psi less in the rear on the Mustangs he was running as well (on R-compound tires).
Near as I can tell, owner's manuals and most other tire pressure references specify cold pressure because "cold" is easier to define than something vaguely between "warm" and "hot". It also provides a fairly consistent basis as ambient temperatures change with the seasons. They undoubtedly are set with the understanding that after the car has been driven a couple of miles or so that the pressures will have crept up a bit, and will then be at the design point for whatever ride quality and handling behavior was intended.
** Rear tires on FWD cars are often run at pressures up at least into the mid-40's range, as a means of getting cars that stubbornly understeer otherwise to rotate. I used to run about 37/47 in smallish RT-615 Azenis on the 626 when I was autocrossing it.
Norm
Last edited by Norm Peterson; 05-28-2012 at 08:07 AM.
#14
But FWIW, I'm running -1.8° camber, Sam's 35/22 mm bars mid-firm front/full firm rear. Konis at about +1.5 turns front, +1 or just a tiny bit more out back for autocross or track, and 255/45 Goodyear Asymmetrics on 18x9.5" wheels (not as grippy as Whiskey's but pretty good as true street tires go). One of the last autocrosses, I ran 35f/31r which are my normal street pressures (cold). Lower rear tire pressure seems to provide a little extra margin between being stuck down at a small to moderate slip angle while cornering and getting too loose when you add throttle.
Norm
Norm
Fwiw, I'm at 35psi cold f & r on 255/45 and 285/40-18 Goodyear F1 Supercars. (OE GT500 tires) The steering feels balanced with the front bar on its softest setting.
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