Originally Posted by jyasaki
Hmmm. That would imply that if you ran the trailer empty, you'd only need around 10 psi? I don't think your formula works, except for loads in the upper range of the load limit. Required pressure does not extend to 0 psi, so you'd have to fudge in some unknown minimum starting pressure.
Personally, I run the pressure stated on the tire sidewall, or a bit less (like 4 to 8 psi less) if nowhere near the rated load on the tire.
I had a Westfield 7 a while ago. Four 185 section tyres that were probably rated for 450kg each. Max pressure probably 40 - 45 psi.
The car weighed about 650kg and tyre pressures were 16-18psi.
With my trailers over the years I normally inflate until they look right and then check the temperature by hand after a few miles on the motorway. If hot they need more air. If cold then they're not too soft - if the boat's not bouncing than that pressure is suitable.
I guess from a tyre what we're looking for is for it to be as soft as possible to give the boat the best ride without heating up through flexing too much.