Basic newbie autocross question, Tire pressure Options

[jd66921]

Hi,

I recently started autocrossing my 914. I enjoy it a lot! The car is not really set
up for autcrossing, no sway bars, standard alignment, etc, but it is still fun to see
what I can do.

The tires I am running are Kuhmo street tires. I know they are not optimal,
but I am not ready to upgrade.

The issue is trying to optimize tire pressure. I know the 914 is a very light
car and therefore may not need a lot of pressure. I ran them originally at 35 psi.
That seemed okay. I ran them up to 40 psi, and while fun to slide around, it was
way too much! I am now running 32 psi, and I don't know if I should go lower or
not.

How does the pressure vs cornering curve go? I would think the cornering force
would increase with tire pressure up to a point, and then tail off. Any ideas what
the range should be? 25-35 psi? Higher? Lower?

I'm just looking for general guidelines. Tires are 195/50's on 5.5 inch fronts and
6 inch rears at the moment.

Thanks,

Jeff

PS: Autocross video

[Joe Ricard]

I would run those Kumhos about 36 to 38PSI. Because the side wall is soft you need the pressure to keep the tire carcass in shape.
Otherwise you will be rolling over to the sidewall. (not good)

Plus with no sway bars your body roll is probably significant and you are getting lots of Postive camber on the outside wheels.

Lowering the pressure will allow a street tire to "roll over" and loose grip just as yoiu are really getting through the corner quickly and leaning on the tire. Higher pressure will make the loss of grip a bit more linear and easier to control.

Once you figure out what what you can tune the pressures to achieve more or less grip at the desired end.

Massive grip at both ends is not always the hot ticket. I would bet that this is what Andy is really experiencing with low pressures.
AX cars need to rotate with the rear end quickly.

Only way to find the optimum pressure is to have a G meter and a skid pad.

Best we could get on my car was sustained 1.5 G's with 24 PSI in my slicks.

We found the car would not rotate and we backed off a bit and got the rear end to slide a bit more. Then it got real fast with a bit less ultimate grip in back. And we take temps after every run.