Packard Forum Index » Post-War (1946-54)

Quote:

That is exactly correct. Also, the temperature does not rise the pressure inside the tire, as much as one would expect.

The most common standard (for radials on passenger cars?) seems to be 30-35psi.

28 seems rather low to me. I would think it best to start with the specification called out by Packard which is is probably 32 or somewhere thereabouts.

In the owners manual it says 24 PSI or 26 when loaded. But for bias ply of course.

For what it is worth here is a bit on radial tire pressures from a few years old edition of a Diamond Back printed catalog.

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It seems that this article is a bit over-generalized. Especially about the front tires doing most of the work. This is probably not the same, when we compare a Honda Civic and a Porsche 911

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Yes, and no. In my truck, thanks to its dash gauge, the tire pressure varies no more than 3-4 pounds on a day's journey. On my big motorcycle, however, on a hot summer day, it can climb 10 pounds.

This is interesting. According to multiple sources, for every 10 degrees of increased temperature, the pressure is expected to only increase ~ 1-2 pounds.
To move 10 pounds up, your temperature will need to climb ~100 degrees. And this is twice the typical expected temperature increase at normal highway driving.

Maybe the numbers are different for motorcycle tires, not sure.

The rear tire is higher pressure - 40PSI. Front is 36. I've started out on a hot summer day to go a few hundred miles and seen it peg the sensor, which tops out at 50PSI.


But don't discount an over-sensitive sensor. Still, it makes me wonder.

Ten psi per 100°F would appropriate if, the cold pressure was 35-36 psig, and the volume of the tire did not increase with pressure. Since MC tires tend to be ‘stiffer’ than its automobile counterpart an assumption of constant volume is likely better for MC tires, but not for automotive units where the tread was intended to be in contact with the road over the full width. We know if an automobile tire is over-inflated the tread wears in the middle and that deflection will likely increase the internal volume. This increase in volume will reduce the pressure rise due to temperature. And then there is the rubber compound of a motorcycle tire. They tend to be formulated for traction, not longevity, so there is likely unrecognized motion of the compound at the ‘patch’, hence a bit warmer than the automotive counterpart with a ‘harder’ compound designed for both traction and ‘mileage’. Rear motorcycle tires have a rather short service life. I never got much more than 10000 miles on a larger bike, even less, that is quite a bit less, if the bike had a good amount of power.

I think there are two conditions that apply as to the differences between motorcycle and automotive tires. First the motorcycle tire will likely not expand as much as an automotive tire as addition pressure is applied, and second the rubber compound differences will likely favor the motorcycle tire to achieve a higher operating temperature. Wat however didn’t mention that the quoted data applies for the time he did 25 hot laps on the Springfield mile.

It's a little more complicated than what's been described as the "load" has to be considered as well. The tire's max. rating is at full load which is under the load range letter. You can put your car on a vehicle scale and measure the front and back load. The sum of all your tire max loads should be higher than the GVW.
You can google "tire dynamics" and see all the different things that go into it.
As for wear, there's the cars alignment to be considered. ie, if it's proper or not.
My rule is to check TP if the temperature falls or goes up by more than 20F (~10C) suddenly to make sure at a good value. Usually, that's only about twice a year in the spring and fall. Also before long trip, etc.
Too low and you'll risk flexing the bead and losing the bond and losing pressure and/or blowing out. Not what you want in a high speed turn where the tire is most flexed. I'd not go below about 28 psig front for that reason. Most cars are front heavy, too. This is a common cause of crashes and why the TPMS law came about. If it's not a heavy car, the wear won't be affected as much with a few psi. I've had to run as high as 85 psig(max) in some of my tires to get them to wear flat. Regular rotation is another factor here.
Really go nuts and put a TPMS on your P and get warned. I hate that law. The sensors aren't that robust and they go bad and rather expensive and you have to demount the tire to change. My truck tires are correct but the fault light/alarm goes off all the time and you have to take to the dealer or tire place to reset or have fixed since you can't shut it off yourself. More headache than it's worth. In 12 years, I've only seen that thing work right once. Had a slow leak from a nail puncture.