Tire pressure

I once made a spreadsheet for playing with the temperature and see what it does to the pressure. Can be found in next map on my public map of skydrive.in wich also my motorhome-tire-presssure-calculator.
MotorhomeRV-tire pressure download pressurecalculationwithtemp

First download it by RIGHT-CLICKINGand choose DOWNLOAD.
Not leftclick or open in Excel or Web app , goes wrong or to complicated.
After download open in Excell or Open Office CALC on your computer to use it.
This must be seen as a good indication, because you never have the temperatures and pressures 100% accurate, and the calculation assumes no vollume chanche by the pressure chanche , wich it will be about for an already presurised tire.
Play with the spreadsheet and draw your own conclusions.
In part 2 your can play with the temperature and see what it does to the pressure . In part 3 you can fill in cold and warm pressure and cold or warm inside tire temperature to see what the temperature inside the tire will become then.
Here a picture of it to get an idea of how to use it.



Some information.
I once contacted a valve inporter in Holland ( where I live) and normal rubber valves are tested to can stand a Cold pressure of 4.5bar/65 psi to 4.8bar/70 psi, depending on the brand.
Mildest American standard is that tey have to stand 1.8bar/26 psi higher pressure to cover a pressure rising by temperature rising from cold 18dgrC/65dgrF to even 100dgrC/200dgrF? wich is boiling point of water.
This temperature of 100dgrC/200dgrF it can incidentially get by the heat of the breakes , transported trough the rimms to the tire inside.
This is what the valve importer wrote me.

Also the tires can stand a higher pressure. Cold it may be filled to 140% of the pressure given on the sidewall when standing still and then the ambiënt temperature rising can highen it yust a little .
So to my estimation a tire can stand a absolute pressure of 1.5 times the pressure written on the sidewall. Maybe even more, but this is what the tire-makers will stand in for.

A normal warm inside tire temperature of a normal car tire is about 112dgr F . Cold 65dgr F is what they calculate with.
So a temperature rising of 47 dgrF.
mayby for an RV witch rotates slower and cools down more, by the larger surface and more air to heat up, this difference between cold and warm is lesser , mayby only 30dgr F temperature rising above the ambiënt temperature.

so your 20% warning border is only about half way down what the tires can stand. Must be seen as a warning for if your wheel gets stuck so it procuces a lot of heat. If you would calculate from freesing point to boiling point of water , the pressure only rises about 10%.

Popsie wrote:
Note that if you run your tires over inflated based on the load they carry, you will tend to wear out the center of the tread early, opposite for under inflated.

Unless you travel continually odds are your tires will need replacing due to age before they wear out.

The maximum tire pressure on the tires sidewall is when inflated COLD (before being driven on). Manufacturers KNOW they will inflate further under road conditions and this is allowed for in the tires ability to withstand pressure.

Note that if you run your tires over inflated based on the load they carry, you will tend to wear out the center of the tread early, opposite for under inflated.

After 10 miles of highway driving my pressure goes up 18 psi plus or minus 1 degree regardless of the outside temperature. Mind you I have never had the privilege of driving when the ambient temperature is much above 100 degrees F. This has been my observation from when I installed my monitors in 2005.

10 to 20 lbs depending on ambient temp and speed. :)

Moved from Forum Technical Support

Many variables. Our TST system recommends a warning at 20% over ambient pressure. Front axle, 70psi cold/ambient high alarm set for 84, rear duals 80psi, alarm set for 96psi.
Normally the rears will go from 80 to 92 to 94 at about 70degree temps, but one trip, all four hit pressures of 98 to 102 in air temp over 100degrees. As long as all tires are within 5 or 6 lbs of each other and tpms temps similar I don't worry about it.

Beverley and Ken