I have weighed my fully loaded '87 (with tag axle) -
Front axle weight = 11,760#
The load range G (11r24.5, single) is rated for 6610# at 105 psi . . . . . . that is 13,200# for the axle, which is also the axle rating.
Load range H (11r24.5, single) is 7160# for 120 psi -- 14320#
SO, while a load G tire will work, I'm going to get load range H to have a larger safety margin.
and you can probably run a lower pressure for a smoother ride.
Our tire pressures are best when the tire pressure is set to match the carried load. The only reason to raise them above the pressure needed to carry the load is to have a margin for load and temperature variations. If your load varies little and you regularly check your tire pressures each morning, the manufacturer would say you gain nothing by increasing the pressure. And your ride will be stiffer.
Another point is that as tire grades (G and H, etc) rise the weight carrying capacity of the tires is identical (for the same size tire, eg: 11R24.5) for any given pressure. In other words, at 105 psi both the load range G and H tires will support the same weight according to the manufacturer. In fact, most manufacturer's tires carry the same weight at 105psi.
In order to run at a lower pressure for a given carried weight you would have to change tire sizes, usually to a wider tire. As the tire pressure is spread over a larger contact area the psi can be lower. In fact this is what happens when our tires are underinflated, the contact area increases, but the sidewall ends up flexing a whole lot more and that produces the heat the eventually destroys the tire. The load/psi charts are designed to carry the load (probably keeping the contact patch constant) while keeping the sidewall flexing limited to an allowable level.
While we are on tire pressures... is the recommended pressure when the tire is cold? I have noticed the tire pressure can rise as much as 5 degrees once they warm up.
Yes Mike. Recommended tire pressures are always with a cold tire. Unless you get weights by tire rather than by axle, it is good to add a couple of pounds to the recommended tire inflation to compensate for imbalance in weight between the right and left side of an axle.
Thanks for the good info . I found a place in Edmonton called "Uncle Weiner's" if you can imagine , they have tire prices darn near cheaper than stolen tires ! Don't know how to post their link ?
I had another thought - ( scary, ain't it ! )
Since (for a given tire size), it is the air pressure that determines capacity & more plys allow higher pressure for higher loads . . . Wouldn't that heavier sidewall be stiffer and give a harsher ride ?
So, now I'm thinking that the lowest load rated tire that will safely carry the load will provide the smoothest ride.
Ain't nothin' simple anymore . . .
Kyle, you are right.
I learned this lesson the harsh way on my first motorhome. I was replacing tires, and since more is better, I bought the tire in the higher load rating although the weight of the coach did not require it. The difference in ride quality was easily discernable.