Tire pressure question

[Dale Davies]

Thanks for the responses, I had no idea that a tire pressure question would evoke such emotion. I agree that tire pressure is important. I am sure that one would never find tires of the vehicles that I am associated to 'under inflated'.
My question is driven by the fact that this car with these tires will be driven hard in performance venues like drag races and autocross events. The car is far from stock. That is why I found the thought that I should be referencing the 1968 OE stock sized tires and 1968 OE load capacity amusing.
I feel much the same way concerning using the calculations mentioned in the above post, as mentioned due to the vehicle weight vs tire capacity with modern OS tires. To me it is obvious that to just carry they 'load' not much air pressure will be needed. And that would not be a good situation for handling.
I am wanting the air pressure required to flatten the tire tread to the road surface and to properly stiffen the side wall to minimize heat build up and minimize side wall tire flex in sharp turns. Doing the chalk test as described in post #2 to me is a direct route to the flatten the tire tread to the road surface half of the question.
I can get the other half answered with some track time.
thanks

For Autocross you generally need higher pressures in the tires. This helps stiffen the sidewalls and control flex. Probably up 5PSI to maybe 10PSI from the street driven pressures. If the event is a parking lot event, the possibility of breaking bead seal is not a concern like a track event where if you come offtrack sideways, a small dip in the turf can catch the tire, break the bead seal and all pressure is lost. A not happy outcome then transpires, and insurance companies are unwilling to cover.
For drag racing the front tires can be fairly hard, but the rears need to be as flat on the track as possible. Sidewall flex sideways is not generally a concern.

 

[MoparLeo]

Pre 70's rims are too weak in the center. Radial tires because of their ability to maintain traction on cornering at much higher speeds put a lot of stress on the wheel and they were prone to cracking in the centers.

 

[MoparLeo]

Tire dimensions are spec'd on a certain rim width.
There are differences in the load capacity of Metric, P metric, Alpha Numeric, LT sizing of tires.
P-Metric Tire Load and Inflation Table - TirePressure.com
What is a Hard Metric Tire? Critical Tire Buying Tips | DrivingLine
Tire Width for a Wheel/Rim Size Chart
Never use less pressure than the tire sticker on the drivers door jam. It gives the proper size and type of tire recommended for that vehicle.

 

[Dale Davies]

Pre 70's rims are too weak in the center. Radial tires because of their ability to maintain traction on cornering at much higher speeds put a lot of stress on the wheel and they were prone to cracking in the centers.

To a point true, but autocross occasjonally will not stress the wheel too much. That said, regular inspection for cracks developing is mandated.
Years ago a friend who raced a Vauxhaul Firenza bought a set of takeoff DTV whels from their race car. These guys had found that the speeds and cornering they were doing every weekend during the season fatigued the wheels. Rogerbrought them through customs with brake dust on them. When asked their value he said old wheels he bought for $60CDN. Each wheel was £400 new. He raced club racing for years. Another Volvo racer broke a wheel coming into our corner at speed where the frost fence stopped him. 4 new wheels for Sunday.
Tire rim spec is generally the median of about a 2" rim width spread. The tread width is based on that.