Keystone RV Forums - View Single Post - How to compare tires for replacement and needed pressure

jadatis · 01-22-2014, 03:25 AM

@ CWtheMan

The system of the tire makers is based on laws of nature.
But it is set up long time ago, when the knowledge about those laws of nature for tires was still incomplete.

You can compare it to company's that already early began with computer systems, and now have a system with many band-aids and suplements to keep it working properly.
Some want to set it up from the beginning, but are risk loosing all the old data.

For instance the system and formula to calculate maximum load for a tire and to calculate needed pressure for lower loads then that, is introduced in 1928.

In time it has been adjusted for instance for radial tires and when problems occured of still blowing tires when the calculations seem to be right, a band aid was put on it by for instance substracting 10% of the maximum load of a P-tire before doing the calculation.
Also In America and Europe different calculations where made , and still are made for calculating that pressure for lower loads.
Or better the other way around is more used, and that is to calculate loadcapacity for a sertain pressure, as the tire makers give for their tires.

As late as 2005 the American TRA swiched over to the calculation of European ETRTO for reasons given of globally using the same system, but the old formula used yust came to to low pressure.
this universal used formula has a power in it and the closer that power to 1 the lower the maximum load for a sertain pressure in the lower pressures.
Europe uses Power 0.8 for all kind of radial tires since decades ( about from 1970) and America used different powers From 0.5( root) for P-tires with Aspect ratio of above 50% and 0.65 for lower then 50% AR.
For LT tires and truck-tires America used and stil uses 0.7 power wich leads to to much deflection in the lower pressures, because higher loadcapacity is given.

also the maximum load of a tire is calculated with a formula in wich also Speed is a factor, as we learned already.
But this formula is generally given to be based on the internal vollume of a tire.
My opinion is , that thats a misunderstanding in the beginning, that is taken over as faxt in time. To my opinion The allowed deflection and the surface on the ground it gives is the thing it has to be based on to laws of nature.
Both for calculating internal vollume and surface for allowed deflection, you need the same data like Overall radius of tire and rimm radius , width and highth of tire.

In the document you gave first, a comparison is made between P tires and Metric tires, and its written that you cant use a list for metric tires for P tire.
But you can even better use the metric list for P tires, it gives lower so saver maximum load.

But that will be another contribution I will make.
The different used system in tires in America and Europe and an overview of the different Pr for the load-kind of the tire.

01-22-2014, 03:25 AM	#16
jadatis Senior Member Join Date: May 2013 Location: Holland ( Europe) Posts: 178	@ CWtheMan The system of the tire makers is based on laws of nature. But it is set up long time ago, when the knowledge about those laws of nature for tires was still incomplete. You can compare it to company's that already early began with computer systems, and now have a system with many band-aids and suplements to keep it working properly. Some want to set it up from the beginning, but are risk loosing all the old data. For instance the system and formula to calculate maximum load for a tire and to calculate needed pressure for lower loads then that, is introduced in 1928. In time it has been adjusted for instance for radial tires and when problems occured of still blowing tires when the calculations seem to be right, a band aid was put on it by for instance substracting 10% of the maximum load of a P-tire before doing the calculation. Also In America and Europe different calculations where made , and still are made for calculating that pressure for lower loads. Or better the other way around is more used, and that is to calculate loadcapacity for a sertain pressure, as the tire makers give for their tires. As late as 2005 the American TRA swiched over to the calculation of European ETRTO for reasons given of globally using the same system, but the old formula used yust came to to low pressure. this universal used formula has a power in it and the closer that power to 1 the lower the maximum load for a sertain pressure in the lower pressures. Europe uses Power 0.8 for all kind of radial tires since decades ( about from 1970) and America used different powers From 0.5( root) for P-tires with Aspect ratio of above 50% and 0.65 for lower then 50% AR. For LT tires and truck-tires America used and stil uses 0.7 power wich leads to to much deflection in the lower pressures, because higher loadcapacity is given. also the maximum load of a tire is calculated with a formula in wich also Speed is a factor, as we learned already. But this formula is generally given to be based on the internal vollume of a tire. My opinion is , that thats a misunderstanding in the beginning, that is taken over as faxt in time. To my opinion The allowed deflection and the surface on the ground it gives is the thing it has to be based on to laws of nature. Both for calculating internal vollume and surface for allowed deflection, you need the same data like Overall radius of tire and rimm radius , width and highth of tire. In the document you gave first, a comparison is made between P tires and Metric tires, and its written that you cant use a list for metric tires for P tire. But you can even better use the metric list for P tires, it gives lower so saver maximum load. But that will be another contribution I will make. The different used system in tires in America and Europe and an overview of the different Pr for the load-kind of the tire.