2ndhom wrote:
I know tires have recommended pressures listed on the sidewall and always assumed it is the hot number. Is there an average rule of thumb as to the temperature swings between hot and cold. 5, 10 or 15 degrees? 110 - 114 seems to be the hot numbers. This, for talking purposes would be for a 40', single axle class A.
To get back to the original question.
The pressure written on the sidewall mostly like this "maximum load xxxx lbs AT yyy psi( cold)" already says what it means.
Its when the temperature inside the tire is the same as outside the tire.
This pressure is based on an outside temperature of 18 dgr Celcius wich is 65 degrees Fahrenheit but if someone gave 68 degr F my conversion shal be wrong.
Once made a spreadsheet to play with the temperature and see what it does to the pressure , here the link to a map on my public map of skydrive that belongs to my hotmail adress. with the pressurecalculationwithtemp at the bottom.
download it first by
RIGHT-CLICKING then open after eventual virus-check (you never know what those Europeans put into it), open in Excell or Open Office CALC on your computer, to use it.
The map motorhome-tire-pressure calculator with pressurecalculationwithtemp When you then give in part 2 higher cold and warm temperatures, you see that the needed cold pressure wont be that much different.
But in practice the tires are filled in the garage at that 65 degrF and in use the outside- , so inside- cold-temperature is different , so you have to include that.
play with the spreadsheet and try some situations to see for yourselfes. Normal cold temp 18dgrC/65dgrF and warm 45dgrC/112dgr F so difference between cold and warm = about 27dgrC/47dgr F.
I asume that for other cold temperatures the temperature rising will be the same , so that 47 dgr F.
The pressure given on the sidewall of most LT tires ( used for motorhomes) is not the maximum pressure .
Its the pressure needed for the maximum load ( that xxxx in the example ) to give the tire a deflection that the tire-maker intended.
Also saw it called the maxloadpressure and in the formula sheet I once got hold of it is called the reference-pressure shortened Pr.
This Pr is not the maximum pressure of the tire ( Pmax).
On standard load/P-tires and XL/reinforced/Extraload mostly used on persons cars the Pmax is given on the sidewall of tires after 2000 .
The Pr of SL/P tires is for american tires 35 psi and for XL// 41 psi
So lower then that Pmax. Difference is used for higher speed then reference speed of mostly 160km/99m/h and alighnment camber-angle above 2 degrees ( tires like this on the axle /-\ not used for motorhomes)
From C-load/6PR tires and up the Pr is written on the sidewall, and even stiffer TRA system allows up to 10 psi higher then Pr for LT tires. TRA even supports higher maximum load for verry low speed with that higher pressure.
So tires and valves can stand a higher pressure then given on sidewall , even the P- and XL-tires higher then Pmax(cold) , for higher inside tire temperature.
For valves I once asked a valve importer, and even to mildest american standards a 4.5bar/65psi max pressure normal valve ( or 4.8bar/70psi) is tested to stand a 1.8bar/26 psi higher pressure then cold so 96 psi for higher temperature. Tire inside can get up to boiling point of water ( 100dgr C /200dgrF?) incidentialy by the heat of the brakes, transported trough the rimms, the tire-importer wrote me.
So for this reason it is unreliable to measure warm, because you dont know how hot the inside tire temperature is at that moment. Can be that 200 degrees by using the brakes yust before that intensively, or can be cooled down to fi 112 degrees during tanking and paying.
the inside tire temperature determines the pressure rising.
Cold is always secure, if not in the sun , so also be ware of the conditions.
