The Stovebolt Site Forums The Parking Lot Big Bolts Recommendation for minimum tire pressure

Hi - looking to soften my ride a little bit. Truck is a 1954 4100 with 7.50-20 tires. Minimum Ford spec seems to be 40psi front and 60psi rear. I would like to drop the rears down some. My wheels are the lock ring style. I do understand that the air pressure holds the wheel sections out and locked.

I accidently had one rear drop to 35psi with no ill effects so this got me thinking I might be able to drop them down from 60 to maybe 45psi? Since my fronts are spec'd by Ford for 40psi, this should obviously be safe for the rears too?? Seems logical to me.

I never carry more than a ton in her (pellets, mulch, top soil, firewood). Other than that she is my daily driver for errands.

I just wanted to ask the folks here in case I am overlooking something prior to dropping them on down.

Thanks

Scott

I'm no expert, but I think 45 psi should be fine with no load. Maybe a tire expert will chime in.

John

If you could find a load/inflation table for that size, it would tell you what you need, but the only ones I can find are for modern metric size (radial) tires, not the older inch sizes. Maybe someone has an older load/inflation table laying around. For the modern LT sizes, the tire brand doesn't matter. Here's an example,

Thanks for the responses guys. It will be easy enough for me to air up the tires to carry a load. I'll probably drop them to 45 and see how she rides for my around town errands. klhansen - thanks for the load chart.

Sorry, but I have to throw a flag on the play here ...

Kevin -- That chart is for radials on single piece wheels, yes? Should we even be using it for bias plies with tubes on multi-piece wheels?? I think under inflating bias plies on multi-piece wheels could be dangerous ...

John -- Not to put too fine a point on it, but we *are* playing with the man's life here -- we need to be certain, not think or guess when discussing whether or not to disregard the pressure listing on the tire itself. The engineers back then weren't stupid -- they came up with those numbers for a reason. So IF there is a point we can safely underinflate to for a softer ride and still maintain the integrity of the wheel ... where is that point? And what are the consequences of getting that point wrong?

Yes, I too, would like to hear from someone with experience in this. For the moment, and for the record (and to save my butt in these litigious times ...), the official Stovebolt.com position is to maintain the tire at the pressures stated on the tire itself. As the pressure in the tire is what actually holds the wheel together, under inflating tires, especially older, less pliable ones, on multi-piece wheels is dangerous and should be avoided.

I think this is actually another instance in this hobby where you can't have your cake and eat it too -- In the original configuration (wheels, tires, suspension, etc), you can either have a softer ride *or* maintain your load carrying capability -- but not both. There are things you can do to soften your ride but I suspect they will either impact your load carrying ability or be expensive. As is, please do not under inflate your tires! The one time you load the truck and forget to properly inflate the tires ... blowouts do not always occur under optimal circumstances ... There you are, under a load ... at speed ... downhill ... on a curve .... with underinflated fossilized tires ... and your outside dual blows ... followed quickly by it's suddenly overburdened partner ... Do you have seatbelts installed? Ever thought about the solid steel rod pointed directly at your chest? (i.e., the steering column ...)... The ride you will then take will not be enjoyable. And quite possibly, not even survivable. We would prefer that not to occur.

To put it far more bluntly -- if you want a softer riding daily driver -- you have the wrong truck.

Sorry...
John

John, totally agree with the above. When I read the OP's post I was on the verge of sharing my radial tire experiences with inflation based on load. Then I saw the lock ring comment. Decided to stay behind my big oak log and watch a while.

RonR

John, you're absolutely correct. I was only stating my opinion, of which was based on more modern radials with one piece wheels. Something else comes to mind is the pressure rating on most if not all tires state maximum psi at a given load but never a minimum which is why I was so quick to throw out my opinion.

Regardless of my logic, it was a good catch. Advice of this nature is best left to the experts.

John

No worries all. Just an overabundance of caution ..

And as was pointed out to me offline -- if he really must under inflate the tires, do not let them bulge too much (so that the duals make contact with each other) as that will definitely lead to serious problems like a blow out.

I appreciate the comments and caution. Tire bulge has me most concerned, not static but when the tire is "squishing" from an impact. Most likely this is why Ford does not lower rear pressures for lesser ply tires.

This is what was published by the Tire and Rim Association in 1954. It is for tube required bias tires.


https://photos.app.goo.gl/9s6PwtcUaPD1mCwCA


It looks like the minimum for a 7.50x20 8 ply is 40 psi
The minimum for 7.50x20 is 55 psi
You must remember that the old bias tires were not made for the sidewalls to flex like modern radials. Even with no load, bias tires flexing their sidewalls generates heat. I was already riding in two tons in 1954 and Got my commercial license to drive them, as well as any truck in 1959. If you have 10 ply tires, you are asking for trouble with anything less than 55psi when they are cold.

I realize that the table I posted (and stated so) was for radials. It was just an example. I have seen modern ones that had different {lighter) load ratings at a particular pressure for duals than singles. The 1954 one that Ken posted indicates that the older bias ply tires had a much narrower inflation range than radials.

I definitely agree that one must be careful about tire inflation. The Ford "Exploder" issue of a few years back comes to mind.

Thanks Ken! I was hoping you'd weigh in on this.

All you have to do is look at your owners manual. Mine says 40 PSI for the front and 55 PSI for the rear on my 46 2 ton. All ways refer to the factory specs. I've ran 20 PSI in some rock hard tires on the rear for years with cracks so big you could put your finger in, never had a issue.