Now here the text I used in another topic.
Dont burn me down on the exact temperatures, see them as an indication. But this already answers that tire inside stays cooler at lower ambiënt temperature.
Why pressure should be calculated back to 70 degrF
That pressure of tires is related to the temperature of gascompound in tire, is fact.
But that for instance you need for 100 degrF ambiënt temperature higher cold filled pressure, then for 70 degrF, and for 40 degrF can do with lower pressure then for 70 degrF, is a conclusion I took, and is the discussion here.
I am going to explain how I came to this conclusion.
I once had a long telephone call with a man from Vredestein , and he explained that goal of pressure determination and maxload of tire, is to not overheat them.
He explained that the molecule-structure of natural rubber can be compared to spagetti, and if you push your nail in it the print remains . Then the “ spagetty-strings" slide over each other.
Tires are vulcanised at the end, and this makes sulfurbridges between the “ spagetti “ wich makes the rubber flexible, so when you push your nail in it , the print directly dissapears, the rubber goes back to its original shape.
The rubber needs a certain amount of sulfurbridges, but to much makes the rubber hard, and if then bending of the rubbber, by the deflection and flexing back every cycle, gives small cracks, wich dont disapear ( logical)
When rubbers temperature goes above a critical temperature, to many sulfur bridges are created,and rubber hardens. Wich temperature this is I dont know, but the vulcanising proces is at 170 degrC is 340 degrF.
Rest is my Einstein way of thinking, and because my average IQ is probly half of that of Mr Einstein, change is 2ce as much that my theory is proven wrong once.
The temperature of the rubber is created by the balance between heating up a second and cooling down a second.
When you begin driving at cold pressure in ambiënt temperature of 70degrF, in-and out-side-tire air and all the rubber of tire is 70 degrF. The heat-transport then is zero. In basics Maxload is calculated for reference-pressure ( E-load 80 psi) and reference-speed ( P- and LT tires mostly 99mph/160kmph) so when you drive that speed constant with maxload on tire and reference-pressure cold filled in tire, no overheating of any part of tire. I practice a bit more complicated.
Yust for the example say you then drive 99mph constant speed, with referencepressure of 80psi ( E-load) and maxload on tire. In the beginning cooling down is marginal, because temperaturedifferences between rubber of tire and in and outside tire gascompound is still zero, but driving long enaugh rubber yust below its critical temperature in the middle of the thickest parts of tire, and at the edges , where rubber meats the gascompound, lets say 300 degrF.
Inside tire gascompound I state as 140degrF. Outside the tire always air 70 degrF
Then temperature difference between edges of rubber and inside tire gascompound 300-140=160 degrF., and between outside tire air 300-70=230 degrF. Also the inside tire gascompound is cooled down mostly trough the rimm to the air outside the tire with only 140-70= 70 degr temperature difference so 160/70 = 2.28 times as much cooling down by the better transport trough the metal rimm.
So coolingfactor then inside tire 160/230=70% of outside tire. In and outside tire Together 230+160=390 degrF
The rubber temperature then stays in balance ,so heating up factor also 390 degrF.
But the deflection , so heatproduction a second at that speed, then is lesser then at cold pressure, because temp in tire 140degrF, wich gives higher pressure. 80 psi filled at 70 degrF becomes 92.9 psi at 140 degrF.
Now situation ambiënt temp 100 degr F.
Temp of rubber still max 300degrF.
Temperaturedifference rubber and outside tire air 300-100= 200 degrF
Temperaturedifference rubber and inside tire gascompound 70% of outside 200= 140 degrF.
Together200+140=340 degrF worth of cooling capacity
So temperature inside the tire 300-140= 160degrF.
So at 70degrF ambiënt temperature 390degrF worth of cooling capacity.
At 100degrF ambiënt temperature 340degrF worth of cooling capacity.
This has to be compensated by lesser heatproduction a second by lesser deflection of tire is 340/390=87.2 % of the heatproduction at 70degrF ambiënt temperature.
Now if 80psi filled at 70degrF
At 100F/85.4psi
At 140F/ 92.5psi
At 160F/ 96.1psi
So 92.5/96.1=96.25% of the surface on the ground then at 70degr F driving 99mph with maxload on tire. Rule of tumb I determined gives 0.9625^2= 92.64 times the deflection . If heatproduction goes lineair with the deflection ( I hope and not more), this gives 92.64% of the heatproduction.
Cooling down is 87.2% , so heating up is even still more then cooling down, if 300degrF is the critical temperature of rubber .
If you then lower the 85.4psi cold pressure at 100degrF ambiënt temperature to 80psi, as is stated to be allowed because cold filled is at ambiënt temperature, the pressure becomes at 160degrF 90.2psi instead of 96.1psi ,wich gives more deflection so heatproduction a second, so rubber of tire temperature goes above 300degrF, and it hardens and cracks.
In fact , because at 100degr heating up factor is 92.64% and cooling down factor is 87.2% at 100degrF ambiënt temperature of the 70degrF situation, you should even pump the tires up a little to give lesser heatproduction a second., so heating up factor goes also to 87.2
Now ambient temp 40degrF.
Outside tire 300- 40= 260 degrF worth of cooling down factor.
Inside tire 300- ( 70% of 260= 182 degrF)= 118 degrF inside tire gascompound
So cooling down factor 260+182= 442 degrF
This is 442/390= 113.3% of coolingdown factor then at 70 degrF.
Then at 40 degrF 80psi filled at 70 degrF becomes
74.6 psi cold and at 118 degrF inside tire temp 88.6 psi
92.5psi/88.6 psi = 1.044 times more surface on ground gives 1.044^2= 1.09x more heatproduction.
So when colder tires rubber stays cooler then 300degrF when driving 99mph with maxload and reference-pressure on tire. So for safety not needed to fill up the cold pressure of 74.6 psi to 80 psi , with difference to hot temps, that you may do so for riding quality and fuell saving.
So I think the lists for filling higher at 70 degrF in a heated garage , to get the 80 psi in this example at for instance 20 degrF is because at those extreme cold ambiënt temperatures the deflection gets to much so for instance snake-bite.
Then its not anymore to prefent overheating, but for riding -quality.
