How do engineers calculate tire pressure?

For most of us the go to is to just use the values on the door placard but how are these values calculated in the first place?

I'm assuming we would factor in diameter, width, and vehicle load.

 

They probably go to engineering school for a while.. and then just pick a random number

 

too many variables. pick a number and go up or down depending on results

 

As an engineer for 25 years, can confirm.

 

Lots of testing. Remember the Ford "Exploder" tire scandal from the late 90's-early 00's? Ford discovered the new Ford Explorer they'd blown the R&D budget on was really easy to roll over. Rather than re-engineer the truck, they specified lower tire pressure on the Firestone tires. Which caused the tires to overheat and blow out. Which caused the trucks to roll over anyway. Which led to a massive cover-up, a lot of finger-pointing, and case studies for future engineering and business students alike. Sooo... yeah. Engineers engineer it.

 

Tire manufacturers provide specs on weight carrying capacity of tires at specific pressures. Car makers recommend inflation pressures based on that plus a while slew of other stuff like ride quality and handling.

As I understand it, Ford specified the 90's Explorer tire pressure at the lowest end of the range to make up for ride quality. People proceeded to load up their Explorers with 5 overweight people and all their vacation stuff which put the tires over their limit.

 

Not a tire design engineer but I understand how the process works so I can help with this:
1. Tire design engineers - analyze tire material properties, static and dynamic loading for various sizes, determines maximum and optimum tire pressure per standards.

2. Automotive Engineers- selects tires based on vehicle uses and loads, determines optimum tire pressure for specific vehicle models.

3. Automotive Product Managers - conducts consumer testing, decrease engineers recommended tire pressure for softest ride.

3. Dealers service department - adds 4-6 pounds to pressure listed on door label to ensure low tire pressure warning light doesn’t ever come on resulting in complaints.

4. Driver - notices ride is a lot bumpier and harsher after dealer service, adjusts tire pressure to desired value. Resets low tire pressure warning (sometimes).

 

This " pigheaded Dutch selfdeared Tirepressure specialist" is going to explain it all.

The maximum load of a tire is calculated with the sises and a reference- pressure and reference-speed. So this done for you, is written on the sidewall.
Called service-descriptions is loadindex and speedcode. On European trucktires almost always an aditional service description, is other loadindex for other speedcode.


End 2007 I got hold of the officially used formula in Europe, to calculate pressure for a certain load on tire, and system for higher speed then reference, and alignment camber angle. I went running with it and did administratical research on many side subjects.

In the beginning I declared that formula holy, and went worldwide with it( OK only english writing and Dutch forums).

But discovered that this formula was not that holy as I thought, but the European calculation is pretty safe, and since 2006 also used in US for P-tires. Before that in US a verry bad formula was used, wich lead to lower pressure for the load on tire, or higher loadcapacity for the pressure, so more heatproduction a second when driving the reference speed constantly, so higher temperature of certain parts of the tire material.

The temperature of the gascompound in tire ( mostly air) is somewhat related to that of the tire material, but external factors can make it different.

Goal of pressure calculation is to give the tire-material the same heatproduction a second for lower load on it, as when maxload on tire with reference pressure on it. Long I declared that this is at same deflection, but now not so certain about it anymore.

All so not any part of tire material overheats, when driving the reference-speed constantly.

Enaugh for now, can write a book about it.
But the engeneers do a lot of copy and paste,and converse lots of old lists from kg to lbs and bar/kPa to psi, and lots of double roundings.

In my made spreadsheets I use a tighter formula, and only round up the pressure at the end.

 

Where is this spreadsheet? What are the formulas?

 

https://1drv.ms/u/s!AtzmkuDu4CalhFkiYeQ9jBL3u-Kw?e=Asbgbr
Here link to map " all about tirepressure, on my onedrive that belongs to my hotmail. com adress with username jadatis.
From from that you can navigate my complete public map.

 

My post before was a quick one.
In that map, wich I made to support my contact with IR J.C. Daws, of wich also a pdf about alernative calculation and compare with different official calcs. He gave me much usefull info, but unforunally died febr 2017.

In the map a spreadsheet " pressureloadcapacity lists etc, see it has been used 6 h ago.
It has 4 worksheets.
1 to make graphics
2 to make 3 lists of differen calculation or different tires to compare. But also at the top my universal formula, of wich all the others can be made, and at the bottom the different used calculations in time in europe and US, and the formula I think comes closest to the ever to be constructed ideal formula.

Sheet 3 is to make your own pressure/ loadcapacity list, and conversion to and fro, from kg/ lbs and bar/kPa/ psi.
Must say I have not used that sheet since years, so have to find out how it works myself.

Then you have the cold pressure, is when temp in and out the tire is the same, and that is when nit driven long enaugh.
This last has become the defenition.
My opinion is that this cold pressure is for a certain ambiënt temperature of 65 to 70 degr F ( 18 to 21 degrC).
So dont blead down on a hot day of ambiënt temp of 100degrF, the tire needs the higher pressure to give lesser deflection, so heatproduction, because cooling down is also less, because of lesser temperature differences.
The engeneers in fora dont agree with my idea.

 

Moral of the story: the tire pressure that gives the best cornering grip is lower than the pressure that gives the best tire life.

 

My somewhat informed guess:

It would probably start with what their recommended tire spec i. Passenger or light truck, etc.
Then depending on that manufactures recommended pressure the folks at Yota or others pick a comfortable or "safe"pressure. Depending on the vehicles prupose.

I noticed my trailer tires are recommended for 80 psi cold.

 

It's the tire pressure required to maintain the tire's structural integrity at GVWR/GAWR, with design/test data provided by the tire mfr.

The automaker would need to pass all the federal handling / stability control system tests, etc. at GVWR with the tires inflated to that pressure.

This leads to some interesting specs in the owner's manual. For example, for 2nd gen Tacomas, Toyota says to add 3 psi for "driving at high speeds above 100 mph". For some trim levels, the rear tire pressure is higher than the front tire pressure, even though the front end is heavier when empty. Once the truck is loaded to GVWR, the rear axle will have more weight on it than the front axle.

 

Now you know the formula's, your original question of how carmakers use it to determine the recomended pressures.

Found this article
https://www.google.com/url?q=https:...AQFnoECAcQAg&usg=AOvVaw2MoN15xTcbTXq9kAS_nu8f

Is the google link, did not manage to copy the searchbar.

In that article they write that carmakers add 10% to the pressure searched back in pressure /loadcapacity lists, but I daubt it.

Also that there are no significant differences between calculations of ETRTO,TRA and JATMA, but my opinion is that in the lower pressures it is significant.

One here wrote about his ( or hers) trailer-tires with 80 psi, and for those the calculations are still made with the in 1928 introduced calculation for diagonal tires, wich are yust a little less worse then the old for P-tires. Together with that ST tires are determined in maxload for 65mph, even with safest calculation on the edges, a bit more speed or weight, and tire overheats.

And because trailer-tires often are used to the max, it often gives tire-failure.

In american lists for P-tires, they go as low as 26 psi, but european stop at 22psi.
Below that the calculation gives to much deflection.

The 35psi advice of many cars, is to my opinion not calculated, but yust the reference-pressure of a standard load P-tire.

Nowadays more often the 36 psi reference pressure of Eurometric standard load is used also in US.