Important ST tire SPEED information

[Tireman9]

Originally Posted by billrehberg

your tire size is ST225/75R15 scale reading is 3550# on the axle. Add 20% to get a good margin and you get 4.600 or 2,330# for each tire. An ST225/75R15 needs a Minimum of 60 psi to support 2330#.
Now since I advise you add 10# to that inflation so you do not need to mess with inflation when the weather cools down (2% pressure change with each change of 10°F) you would have a target inflation of 66 psi when using your hand gauge.

My trailer weight is 3480; close to your example weight above; when I run through the calculations from you, I get 2088 / tire. So I should still go for 66psi even though slightly lower weight / tire?

Don't forget we were talking the minimum to support the load, not the inflation get best tire life.


If you review my posts on "Interply Shear", the forces trying to tear the belts off the tire, you would know that I suggest you lower them as much as possible which means that trailers should run the inflation on the tire sidewall.
Yes this might give a rougher "ride" for the trailer. It's up to you to decide how to balance that rough ride against decreasing the chance of suffering a tire failure.


You can Google the term if you want to know more.

[Tireman9]

Originally Posted by ITSNO60

My post wasn't directed to anyone in particular, I was just wanting to bring it up since reading this thread yesterday, I felt that it would be pertinent here. I first tried to find Roger's revised advice from a while back but was unsuccessful.

Sorry but I am not allowed to post direct link to my RV tire blog. best you can do is Google "Interply Shear tires". That will lead you to some technical articles on the subject but also to some that the average person can understand.

[BlackAces]

Tire inflation pressures for original equipment tires have been set by the vehicle manufacturer. Therefore, the recommended cold inflation pressures depicted on the vehicle certification label are minimum inflation pressures. That statement is valid in accordance with current government regulations and supported by tire industry standards. Why? You might ask. Because in the government standards, the vehicle manufacturer MUST follow the directions given there and they are considered minimum. When certifying your trailers the vehicle manufacturers are directed to provide tires that are appropriate for the vehicle they are fitted to and then provide a recommended cold inflation pressure for those tires. The building regulations trailers are built from support minimum standards. Therefore the vehicle certified cold inflation pressures for OE tires are Minimum.

When replacement tires of the same designated size are installed, there is room for optional inflation pressures between recommended cold and tire sidewall max. When plus sized tires are used as replacements the first step for the installer to accomplish is setting a new recommended cold inflation pressure for those tires. That inflation pressure MUST cause the replacement tires to provide a load capacity equal to or greater than what the OE tires provided. Again, options are available from recommended to sidewall max.