Determining Tire Pressure

regis101 wrote:
This a quick math method for tire air pressure. It can serve as a baseline and on the low end due to axle weight and not tire inflation target(s). I would then put three or four or five chalk marks across the tread of each tire and drive the straight line for a bit. The amount of chalk left on the tire or not helps determine best pressure(s) for your vehicle. I release all liability from myself and have put on the flame suit.
My background with this quick method stems from a more complicated approach to tire pressure from driving the VW Bus and Vanagon for a decade and a half.

I will use the data from our 73 Dodge B30 Kodiak 20'r.

Front max capacity is 3300# Rear max capacity is 5700# Max GVXR as mfg is 8300#

Front - 3300 equates to 1650 per wheel
Rear - 5700 equates to 2850 per pair that equates to 1425 per wheel

Tire is a Load E , 2680 # at 80 psi for single and 2405 for duals at 80 psi

Front - 1650/2680 x 80 equals 49 psi

Rear - 1425/2405 x 80 equals 47 psi.

easy peasy, right?

My two numbers would be a baseline for (that) tire to support the intended weight based on Max GVWR.

If I used a Load D tire with 65 max psi the numbers would be higher for the same vehicle.

One of the keys is to figure baseline tire pressure for the tire you have installed on the vehicle. My numbers may sound low but that's because the tires have a large capacity. The owners manual has tire inflation tables but I find it hard to understand. Plus the table is based on bias ply tires. But, when I narrow down the few numbers I can understand, My quick math come out within five psi so I'm all good.
I'm still gonna do the chalk test but will wait until I put a few miles on the new tires to break them in a bit.
Hope this helps. I welcome more education and replies. YMMV

What you use is what I call the logical calculation and that is part of maximum load calculates part of At-pressure.
And in fact ist not that bad, you even do better then the official pressure/loadcapacity lists the tire makers give.
If you use your calculation on the real weights it would give slighty to high pressure as answer wich is never bad for savety of tires.

But in real live on one tire can get more weight then half ( or 1/4th in case of dual load) of axle load .
So you have to build in some reserve for unequall load R/L.
Also rear GAWR is often overloaded for smaller motorhomes so in combination with that R/L shifting weight on one tire can get higher then you expect.

But if you add to much reserve things go bumping, so comfort and gripp gets lost.
I determined that border if when real weight on tire is below 85% of weight the pressure is calculated for. Example if weight is calculated for 1000 kg/lbs/whatever, then discomfort begins at 850 real weight on tire. Below 80% screws tremble loose from Travel-tailers.
this percentage, I call Loadpercentage Lc, is for calculated for 160 km/99m/h and has to do with the deflection of tire.

Within a large range the total witdth of tires stays on the ground so centre- or sides-wear wont happen within those borders.\
The chalk test so gives even wear within a large pressure/range.
And Best would be then to begin at high pressure and stopp when no chalk wears of the centre anymore , then you are at the upper border of pressure .