Quote:
Originally Posted by garysfb
While there have been many threads about tires and tire pressures, I don't recall this point being discussed:
I was at NIRVC today to have new Michelins put on the front as well as an alignment, part of the repairs from my earlier blowout. Pro Tire did the work. I started talking to the tech about pressures as they relate to changing temperatures, especially driving north from Mississippi, or Texas for that matter, it was over 100 yesterday!!!
At any rate he said that according to Michelin the optimum ambient temp to set pressures was 70 degrees give or take 5, and you are good to go no matter the temp changes. I had thought that they should be set according to the AM temp, before the sun hits them, for whatever the temp is where you are.
As an example if I set them at home at 70 degrees and travel north where it's 50 degrees in the morning and pressures are down 5 lbs., I should add 5 lbs. NOT SO HE SAID. If you set them at 70 +/- 5 degrees you are good no matter how much colder it gets. This leaves me concerned about under inflated tires when driving up north. Especially since I'm traveling up to Charlotte, Michigan to have the chassis inspected before our 3 year warranty is up.
Can anybody shed any light on this?
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I think I can shine a light on it.
Pressure advice is to prefent the tire from overheating, when driving the speed constantly , for wich the pressure is determined.
No part of rubber of tire may come above a critical temperature at wich the rubber hardens and beginning cracks are made. Those cracks tear further in time by mechanical forces, untill mayby only after 3 years that far that treath separates or tire blows. Then that once the conditions at wich tire overheated, is long forgotten.
The temperature of rubber is made by the balance between heating up and cooling down.
Cooling down is dependent on temperature, differences between rubber and in and outside tire gascompound.
Heating up by driving only, is dependend on heatproduction a cycle bij the bending of the rubber by deflecting and flexing back every cycle.
But also on the number of cycles a second the tire makes.
If you determine pressure for a certain speed and load on tire, it is for 65 degrF, some say 68, but 70 degrF also is that close that it wont give such difference in pressure, that it will be a problem.
When colder outside, the cold pressure( when temp in and outside the tire is the same) goes lower, wich gives more deflection of the tire, so more heatproduction a cicle, so a second at same speed.
But also the temperature/differences are larger between critical temp of rubber, and in and outside tire's air( or other gascompound), so cooling down is also better.
Within a certain range of outside temperature, the heating up and cooling down stays in balance, so rubber still wont overheat.
So for savety, you dont have to highen up the pressure to calculated advice. For fuelsaving and roadhandling you may do so.
When hot outside, the other way around, higher cold pressure, so lesser deflection, so heatproduction a second, but also lesser tempdifferences between critical rubbertemp and in and outside air, so lesser cooling down a second. Then you need the higher pressure , and never blead down to advice .
You may notice different roadhandling bya harder ride though, but savety first.
So the person at the tireshop was right to the conclusions this " pigheaded Dutch selfdeclared tirepressure-specialist" , so not a professional.
Once you determined the needed pressure with reserve, based on best,weighed axle-end loads in the loading you go on trip, second best Axle-loads, I can calculate a safe highest pressure with still acceptable comfort and gripp for you.
Then this is for 65 degr F outside temperature, cold measured.
I then can make a pressure/ temperature-list for those pressures, so you can look back, what the cold pressure should measure on a hot or cold day, but also, if you have tmps with external sensors, see what the inside tire temp is, because those sensors dont give the temp in tire, but something in between in and outside tire, so lower then real, so unreliable.
These lists then are a better indication then what your tmps gives, but still an indication, because you never can measure the pressure that accurate, and in my list , I assume no vollume-chanche when pressure chanches, wich it will practically be for a already pressurised tire.