Tire Pressure and Brake Bias Initial Settings

[john g]

I am interested in initial tire pressure and brake proportioning valve settings for my 818S.

From what I have gathered:

Tires:

Rule 1: 10psi per 1000 pounds of car weight. This would give 18-19 psi. It does not suggest a front-rear differential.
Rule 2: Pressure from a car with a similar weight distribution. Bob told me 26F and 29R, based on a lotus. Bob also said that I should adjust depending on tire temperate.
Rule 3: My tire man suggested 30F and 32R.

Proportioning valve

Wallace said, 'turn out all the way (counterclockwise) and then clockwise five turns.' Other suggestions?

[Tamra]

I would definitely not run 19 psi. I'd just put it at 30-32 psi for street driving, which is a safe range for most tire/wheel combos. Tire pressure also depends on how well supported the tire is (for example, a 255 series on a 7.5" wheel might require more pressure to prevent it from rolling over, whereas a 225 on an 8" wheel can be run at lower pressures since it is well supported). Are you wanting tire pressure for autocross or just street driving?

That sounds about right for the proportioning valve.

[billjr212]

I haven't done chalk or scrub tests yet, but I think you need to be somewhere between rules 1 & 2. I ran 23 in my roadster. Any more was way too much to get even contact on the ground. I run 23 in the 818 on the ffr wheels and recommended tire sizes. Feels right after 500 miles of driving, but again, I haven't done more formal scrub or chalk tests, so take it with a grain of salt.

[Buzz Skyline]

I'm using the same tires that I had on my WRX, which had a curb weight of about 3000 lbs. My 818 weighs about 2000 lbs. To have the same tire patch as the donor car, the pressure should be about 2/3 the Subaru recommended pressure. That puts it at about 20 psi.

I plan to do a few braking tests to see what's optimal, but stock pressures of around 30 psi would seriously hurt your braking and cornering, I believe. It would be like putting 45 psi in tires on a WRX. That's a difference you can absolutely feel on the road. It would also cause poor tire wear and shorten the tire lifetime.