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Discussion Starter #1
I recently acquired a slight amount of oversteer on my 1990 9000CDS. This seems to coincide with adjusting the tyre pressures. I now have 35psi at the front and 32 at the rear. This is with one passenger and a fully laden boot.
 

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Billicar said:
I recently acquired a slight amount of oversteer on my 1990 9000CDS. This seems to coincide with adjusting the tyre pressures. I now have 35psi at the front and 32 at the rear. This is with one passenger and a fully laden boot.
You should get a decrease in oversteer with that kind of setup; (not sure why you would want that..) I've heard from others that tire (sorry, tyre) pressure can make quite a large difference, although I haven't experienced it myself.
 

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That's the problem. Tyre pressure affects grip. Generally, the lower the tyre pressure the lower the grip. So with lower pressure in the rear you get less grip at the rear so you get more oversteer.

Two easy things to remember: oversteer means more grip at the front than rear (and of course understeer means the opposite,) and more spring pressure on the outside wheel of one end of the car increases the loading on that tyre, but reduces the loading on the other tyre on the same axle giving a net reduction of grip relative to the other axle, contrary to popular assumptions. Put another way, if you make a tyre work harder either by increasing the suspension loading through spring rate or roll bar rate (or instantaneously through damper rates) or by reducing tyre pressure, you will induce under or oversteer at that end of the car (under if at the front and over if at the rear)

In my experience the 9000 does not respond well to monkeying with recommended tyre pressures. On the track you could benefit from 2 psi less in the rear than in the front on an Aero, maybe a touch more difference on a standard model.

BTW, you should always increase pressure to change handling balance, never decrease pressure. Tyres should always be at the recommended pressure or higher for safety.

Finally, I find my SAAB 9000 cars like about 2 psi more pressure than specified for light load or low speeds. Always use SAAB's maximum recommended pressures (or even a bit more) for high speeds or fully loaded conditions and use the same pressures front and rear for best handling on the street.
 

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Discussion Starter #4
Many thanks for the replies! I'll try running the rears at the same pressure as the fronts. The manual says 30psi all round, but I find the car's handling quite 'unresponsive' at those pressures.
 

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What size tire are you running? The lowest pressure I use is 32 psi for 195/60x15 stock size on my 86.


The highest pressure I use is 41 psi for high speed or heavy load running on my 97 Aero.

For 195/65x15 tire size I would be tempted to use 32 to 36 psi.
 

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I'd try 34 psi front and rear and see how you like it. I'm using 32 psi on my 86 with 205/55x15 P7000 tires which is the stock size for the 87 model (though the factory fitted the then brand new P600 in place of the p6 on my 86.)
 
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