Re: Tire Pressure

Posted by Tim Cole On 2013/3/25 19:57:34

So that everybody has a firm grounding on the engineering involved here let me summarize.

The bias ply tire succeeded the old Baloon tires by virtue of one of the fundamental discoveries from hydraulics. One of the companies, maybe Goodyear, discovered that when cord plies are integrated in a hose, when that hose is pressurized the cross sectional area decreases and the hose becomes smaller. That is why brake hoses don't explode under 2500 psi. I don't remember the angle but it is something like 53 or 57.35 degrees.

So, they started making tires that used this angle bias which allowed for superior ride and performance because when inflated the cross sectional area became smaller.

For ride quality nothing beats those old bias ply tires, especially those beloved Martins which also ran 40,000 miles.

The reason cranking up the pressure in a bias ply tire only makes it ride lousy is because the cross sectional area doesn't change much above a certain pressure which is engineered into it. So over inflation of a bias ply tire doesn't achieve much except a harsh ride.

Cranking up radials does change the cross sectional area of the tire significantly and the pressure on the door card is selected as optimal. On my modern car I keep the pressures where they say because otherwise the ride is lousy and I don't like doing suspension work.

Tires are the first pillar in the suspension. If you like superior ride then use the car makers recommendation. If you don't mind a harsh ride and accelerated vehicle wear do what you want. The old low pressure bias plys are very easy on the car.

For these classic radials, there is no data except for what is specified for a modern car of similar weight and axle loading or what the manufacturer says. My modern car says 33 psi front and 29 rear and that is what I use. When they fall more than 10% I add air to around 34 and 31 because they go back down.

One day when I get stuck with one of those dumb, stupid, and moronic TPMS systems that doesn't go out unless I over inflate the tires I will simply take the dashboard apart and smash the light out.

Hope this helps