Tire Pressure

Bob Bidonde

Bob Bidonde

Well-known member
There has been some discussion locally about the tire pressure for touring in Model A cars. Pressures discussed are the factory 35 PSIG, the maximum load pressure on the sidewall of tires, & pressures as low as 28 PSIG. Personally, I run Universal brand tires at 32 PSIG which is the max pressure printed on their sidewalls.


I am curious as to what tires pressures you all run.
 
I run 36psi in my 21” firestones. One thing you need to remember is these aren't a bead lock style rim. They weren't even invented yet then. The bead area of the tire is held outwards against the rim by the air pressure against the sidewall of the tire. The lower the air pressure may Increase the chance of the tire and tube slipping on the rim during hard braking.
This could cause the tube valve stem to tear and potentially a blowout!
It’s something to consider if you choose to run much lower tire pressures.
 
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Remember when you were a kid riding your bicycle and a tire got low due to air leakage? It sure took a lot more effort to keep up speed, didn't it? And you wore yourself out pedaling. Same thing with our Model A's. If the tire pressures are too low, the engine has to work harder to maintain speed, in extreme cases causing the radiator to overheat. The car's handling also gets wonky with side-to-side rolling, wandering and hard steering. 32 psi is too low for me. I run my tires at 34-36 psi. Believe it or not, 30" Model T clincher inner tubes need 55 psi so that the tires don't peel off the wheels! 34-36 psi doesn't sound so radical in comparison, does it?
Marshall
 
I run all tires, of all my vehicles, at whatever the max pressure is called out on the tire sidewall. That being said, the ambient temp here is fairly moderate, no temp extremes to drive tire pressure to high or low, after setting to sidewall pressure.
 
35 psi summer pressure, 40 winter pressure . Station wagons where at a higher pressure,40 psi I believe. Why do I use a higher winter pressure, the car sits and not driven much so too help keep from flat spotting, also if they leak some by the time it’s spring the pressure will be at about the right pressure.
The stated pressure on the sidewall I believe is for the load rating , if you could put tires on that were 10 ply and the sidewall pressure was 75psi would you pump them up to the sidewall pressure ?
This is all good for the standard tire sizes for the A , if your using non standard sizes you can check what pressure you need by using side walk chalk on the tread and drive straight on a smooth surface, if the chalk is removed in the center the pressure is too high, if the chalk is removed on the sides of the thread your under pressure .
Dads work truck we had 6 ply tires on the rear and kept 45 psi all of the time, we loaded the truck at the warehouse with 24 window A/C and the tires looked like radial tires , we put 65 psi in them and drove 2 1/2 hours home . Low tire pressure is much worse than higher pressure with in reason .
Lastly nitrogen molecules are larger and air , so nitrogen leaks out less than air and stays at a more constant pressure. Several years ago we were out on a drive and I hadn’t checked the tire pressures for a long time, the A drove funny and I thought maybe the tires could be low, the filling station had free air but no pressure gauge, I put air in all of the tires so they didn’t look like they were low . We drove another 20-25 miles to home and I checked them and found the pressures were 45 psi . The suspension really go a good workout driving home on bumpy country roads as the over inflated tire sidewalls didn’t have a chance of flexing making the springs and shocks work very hard .
 
The tire pressure is the maximum the tire is rated for, it doesn't mean you necessarily need to run the tire at that much pressure. The tire is made to use on many different weighted vehicles. You check the tread wear and adjust the sire pressure for it, if it is wearing more on the center then you lower the pressure a little until it's wearing evenly across the tire, stay between 28 and 35. There might be a different between wear on the fronts and the backs depending on the weight on the tires. I was a fleet mechanic, at the phone company fuel was the number one cost so they didn't care about how tires wore they cared about fuel mileage. We rotated them at every service.
 
The tires sold today for Model A's are "modern tires" meaning that they are good for 200 psi without bursting, as long as they are not damaged by running flat or other reasons. So it is OK to run higher than the number on the tire.

That said, the tire design and the load will dictate the proper pressure. A radial tire does not bulge out at the center of the tread like a bias ply tire so they can be run at a higher pressure without wearing out the center of the tread. An IR temperature gauge can be used to determine if the pressure is too low or too high. If too low the temperature of the sides of the tread will be hotter than the center after a few miles run. The opposite is true if the pressure is too high. The sidewalls will be too hot if the pressure is too low, especially with radial tires. Too hot is over 140 F in a 70 F day.
 
I use 35psi on my Firestones..all weather. I found that my car tended to be more susceptible to truck grooves (vineyard tractors in my area) when at 32psi, and tended to 'wander". At 35 they track better...with a slightly harder ride.
 
Klattu1929 said:
I run all tires, of all my vehicles, at whatever the max pressure is called out on the tire sidewall. That being said, the ambient temp here is fairly moderate, no temp extremes to drive tire pressure to high or low, after setting to sidewall pressure.
Click to expand...

FYI: The maximum pressure on the tire has nothing to do with the vehicle's optimum requirement. In fact, you are most likely altering the engineered specifications for said vehicle. That can put you and those around you at risk.
The smart money dictates keeping the tire pressure at what the vehicle manufacturer recommends.
 
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