Tyre Pressures

[Alex L]

Drag this topic up again...

Looking in my 997.1 Turbo owners manual today I see the following info:

Cold (20 degC ambient temp) partially loaded front - 2.3 bar (33 psi)
Cold (20 degC ambient temp) partially loaded rear - 2.7 (39 psi)

Then there is a chart for how pressure changes with temperature, and also when TPM warns you in 2 stages -3 to -6 psi, and then -6psi plus. The chart uses 36 psi as the target cold temp (20 degC) and 42 psi as the target warm temp (60degC) and has it in a nice straight line (1.5 psi difference every 10 degC)

So how does this translate into front and rears if we assume the same linear relationship?! As a guess I plotted the reference data in excel then added parallel lines for 33 and 39 starting points for cold:

Now any suggestions for tyres filled with nitrogen (and of course not knowing how pure the nitrogen is in the tyre or how much water vapor is in there too)? [8D]

 

[okellyt]

Alex Interesting probably comes from gas law - PV=NRT so there should be a linear relationship (R is a constant). P= Pressure, V = Volume, T = Temp, N I am not so sure form memory (number of moles of the gas I think). Volume once the tyre is under pressure only change a little so in effect the pressure is directly related to the temp, hence the linear chart. I gues though the theory that air holds a lot of moisture that is gaseous at high temp but drops out as fluid at low temp causing a non linear response on pressure should have made more of a dfference and Nitrogen should be more linear over a wider range of temperatures. Try it again in the cold , see what happens.