How do you determine air pressure for non-stock tires? Options

[Air_Cooled_Nut]

You've got tires that are not stock dimension, say low-profile. Maybe on a larger rim. How do you determine the optimal -- or even initial -- tire pressure?

I don't care who's running what on their car made by whaterver. I'm looking for a method, rule of thumb, formula, etc. to figure this out.

[lapuwali]

Start out with what you use now. If this is just a street car, and the ride is too stiff, let some air out. If they're squealing at every corner, put more in.

If you're competing with this thing, try measuring tire temps, or air pressure rises cold to hot, or the sidewall chalk test, or all of the above.

There is no hard and fast rule. Air pressures are black voodoo. Sacrifice a chicken and start experimenting.

Me, I just run 30-32psi unless I decide there's some reason not to.

[Andyrew]

25-30.. for a car as light as ours.

Try 27 all around, take it for a spin.. Front end want to push? try 30 on the rear.. Rear end want to come around on you? try 26 on the rear 29 on the front. Dont go less than 23 or more than 35... for street tires...

Also, take chawk or shoe polish, put it on the tire on the front of the tire, about 2 in from the ground in a line.. drive it.. take some corners hard. See how far the tire rolled (this is what the chawk is for where it rubs off) if you went past the tread, inflate the tire more, if you had lots more tread to go, deflate tire..

Thats about all I can tell you asside from using a temp guage.... (and THAT is black voodoo as well)

its a roll of the dice really... lol

[Joe Ricard]

Chaulk the edges.

start with 32 F/R go to a parking lot preferable empty. Go fast turn hard. you should be able to scrub the chaulk right down to the little arrows on the side of the tread.
If you go over the arrow add more air to keep side wall stiffer
can't reach the arrows Turn harder or let some air out.

Should by the nature of tires and cornering forces end up with optimal tire pressure front and back more than likely different.

Push and loose are tough to figure and will be different depending on surface. However the rule mentioned above is how to do it.

[Air_Cooled_Nut]

Ok, thanks!

[bondo]

So THAT'S what those little arrows are for.. (IMG:style_emoticons/default/biggrin.gif)

[Sammy]

I bought an infra-red laser temperature gun at pep-boys last month for $50.
Good tool to have. just shoot the inside, middle, and outside edges of the tiars. Iffn the middles are hotter lower the pressure, iffn the outsides are hotter raise the pressure.
That's a general rule of thumb, sometimes they will act opposite. you gotta play with it to get it right but it's fun so there's no downside (IMG:style_emoticons/default/wink.gif)

It's also a tool to figure out iffn you need more or less camber, or to figure out which one of yer cylinders isn't firing.

[Toast]

(IMG:style_emoticons/default/hijacked.gif) (Sorry Toby)

If you have 32psi F/R and the front feels like it wants to slip out on you, do you add more the the front? I had less than 32 and I was terrified to drive past 35 mph.
I have 205/65. New in back, old in front.

:unhijack:

[Air_Cooled_Nut]

I have a Sears infra-red thermo...I'll have to try that!

[Dave_Darling]

If you have 32psi F/R and the front feels like it wants to slip out on you, do you add more the the front?

Depends on the tire. In general, street tires will stick better at higher pressures. This is far from universally true, though. Try it and see--either pump the fronts to 34-36, or drop the rears to 28-32 and see how the car feels.

Back to the original topic--
I personally would pick a nice "medium" pressure value, like 30 PSI, to start with. That's higher than the stock recommended pressures (26 PSI front, 29 PSI rear) but lower than the "max pressure" listed on the sidewall of just about any car tire you can get nowadays. You can try a couple PSI higher in the back if you like to start with, or even go a bit higher all the way around. Try driving on them and see how the car feels and reacts. Mess with the pressures and see what happens.

If you can find a nice deserted parking lot, and you've got a couple of hours to kill, it could be a fun exercise...

--DD

[Crazyhippy]

ON my Jeep we used to drive through a puddle and adjust based on what part of the tire got dry fastest (all very low speed, someone walking next to you slow) Center dries 1st, less air; edges dry 1st, more air.

Great for maximizing tire life, doubt it would be optimum for performance stuff though.

BJH

[anthony]

I run stock pressure plus 3 psi for the street. So 29/33. For an autocross I run more like 33/36 and then adjust depending on how the car feels.