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Can anyone point me or inform me into knowing how to mess with tire pressure. I have heard and read many times that adjusting teh air pressure of the tires can make a difference in how the car handles on the track and what not. I run at some tracks but have no idea how to mess with the pressure. How far can you go without causing problems, less pressure or more better, which way does what to the handleing, all those good things. Thanks.
 

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Re: Air pressure (pj's 850)

Front wheel drive == raise front tire pressure for stiffer handling. Rear tire pressure does not matter so much, when I autocross I leave it at 36 or 37psi so that I can attempt to swing the back end around a little bit.<br>Running somewhere around 42-44 psi in the front will give you great response, but wont' be so great for the tires...it's also a little risky for road driving.
 

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Re: Air pressure (chopper)

For track/autocross on street tires, I would use a moderate pressure in the front (just enough to keep off the sidewalls), something like 38-40. If the car understeers in the turns, jack up the rear pressure. I don't have much experience with FWD handling, but the rear tire pressure should be even or higher than the front since the car will have a tendancy to understeer unless the suspension has been otherwise modified.<p>With 17" wheels on an 850, you would need a high tire pressure to handle the load and keep the tires from blowing out easily (I'd try 40). For 16" or 15", you could go closer to the recommended pressure for the car. I run the 205/50/15 tires on my 240 at around 34-35psi and I get good wear and decent traction considering how hard the car is driven. For autocross I bump them up to 40psi and they don't roll over.
 

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Re: Air pressure (towerymt)

I have heard the opposite about rear tires in FWD cars, but it can be argued quite extensively either way.
 

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Re: Air pressure (chopper)

Lowering the rear tire pressure gives the tire more grip (wider contact patch). When you add grip to the rear of a car that already understeers, the understeer will become worse. This assumes that there haven't been any suspension modifications that are grossly out of proportion to the rest of the suspension, such as a large rear sway bar only, or stiff rear springs, both of which will help the handling approach neutral on a FWD car.<p>I know of two ways to set up a FWD car. Big front sway bar, small or no rear bar, and stiffer rear springs. The other way is to put on a really large rear bar with a small or no front bar, and put heavier springs in the front. The first way gives flatter cornering and the second is likely to lift the inside rear wheel around turns.<p>Like you implied, there's no wrong answer, and every car is different so it may take some testing to find out what works best.
 

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Re: Air pressure (towerymt)

It's probably important to note what psi the tires are rated for right? Would you put 40psi in a tire that is say 30-37psi rated or something like that? Usually the side wall says, but I guess if your autocrossing your likely to use performance radials that handle higher pressures, so, I guess I kind of answered my own questions... <IMG NAME="icon" SRC="http://********************/smile/emcrook.gif" BORDER="0">
 

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Re: Air pressure (Spearzy)

I've been autocrossing on Mich MXV4 plus recently. Not a performance tire. I'm found best result with front pressure 44-45psi, rear 36/37psi...but maybe it's my driving...or the tight courses my autocross associate runs.<p>Max pressure on those tires is 44psi. You really just want to avoid so high a pressure for highway driving. Don't forget pressure increases as tires heat up <IMG NAME="icon" SRC="http://********************/smile/emsmile.gif" BORDER="0"><p><br>[Modified by chopper, 10:21 AM 10-25-2002]<BR><BR>
[Modified by chopper, 10:21 AM 10-25-2002]
 

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Re: Air pressure (Spearzy)

I've seen some cars running in stock classes with tires that probably were rated for a max pressure of 35psi. For those, I would probably keep it under 40psi. My tires are rated for 44, so I usually go as high as 42 cold and adjust from there. I've gained as much as 3psi after my first autocross run, but we have long courses. Usually I drop a couple psi after each run to keep the pressure where I want it.<p>I would not recommend autocrossing on street tires with low pressure ratings AND low speed ratings because they have very soft sidewalls which allow the tire to roll over when pushed hard. The outside edge will wear quickly and they may even start chunking where pieces of the tread come off.
 
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