My question is what tire pressure to inflate straight-sided, bias-ply tires to in order to get longest tread life. Like on a '26/'27 Model T.
The story starts when I had a 1926 Packard Eight --- a heavy car with 600x21" tires. When I inflated them to, say, 32 pounds they wore very fast in the center of the tread, so I eventually drove on a pressure of 26 pounds which gave me even tread wear.
On a 1924 Franklin with 500x23" tires, I have the same problem. A relatively light car, I'd been running as low as 27 pounds and still get heavy wear in the center of the tread. How low do I dare go without causing a problem like a blowout or valve stem shear?
The question really is, shouldn't tire inflation be a function of load? How heavy your car is? Or how much weight you are carrying on each tire? If so, how do you know? The rating on the tire itself is always a maximum inflation. And tires are so different today than 90 years ago that 1920's car manufacturer recommendations can't possibly apply.
So what do you do for straight-sided bias tires? These things are too expensive to be worn out by 7,000 miles.
I'm certainly no expert, but I would think there is more to it than just tread wear. Underinflation can cause a tire to run hotter than it should due to the excessive flexing of the side wall. Also, there has to be enough pressure to not allow the tire to slip on the rim during heavy breaking or acceleration.
Others, with more knowledge than I, might shed more light on the subject, but I am a little apprehensive to deviate too much from the recommendations.
I have been running 35 lbs on each of my 26 T's. The first one I got had two new tires on it when I bought it and I bought two identical to those very soon after I bought the car. I have had that car 26 years and have put on approx 1500 miles per year. There is still tread and it has worn evenly. Never had a flat tire either.
Tire size Norman?
450x21 INSA tires Hecho en Chili
I have noticed on my '26 touring that if I go over 30 PSI the tires bounce too much.
I run 26 front and 26 to 30 in the rear depending on load.
A model A uses 35 PSI, but they have shocks (dampers?) to keep the axles from bouncing.
He He, If you think you get to much bounce over 30 try clinchers at 65... Sometimes I think a buck board would wagon would ride better.
(Message edited by paulmikeska on February 05, 2015)
Tire size is engineered for each model of car, to have a sufficient contact area on pavement (tire patch) that will support the car in question.
The pressure per square inch on that tire patch will be equal to the pressure in the tires in PSI, that's what supports the weight of the car. This statement disregards any support the sidewall of the tire may give, which is minimal except in the case of newer "run-flat" tires.
Thus a 30x3.5 tire can support a light weight T with a certain tire pressure and patch, but if you put that same tire on a '56 Cadillac it would never hold the weight.
If you over-inflate your tires, then the tire patch moves toward the centerline of the tread, thus wearing that area more than the sides. Specifications and visual should allow you to even out tire wear.