Stop using "one-size-fits-all" tire pressures - a science based guide to the right tire pressure

[parb]

I can do all three. Probably tomorrow.

 

[Asnes]

BFG load chart + CAT scale weigh-in is what I use. No guessing required.

 

[globalgregors]

Related to the temp and pressure thing, but something that I ran across last year. Highway driving with 285/70R19s KO3s, a 70-80 degree day, but at 7-10,000 ft altitude, highway driving, probably 75-85mph.. The tires were new and they had been filled to the door plat pressure- 35+ish? I ended up getting pretty high temps, maybe even a high tire temp warning. I'm guessing that the temp would be an issue mainly in highway driving, on hotter days.

What is 'too hot' for a tire like ours? Is there a specific tire inflation target for that? I was loaded, but not over loaded. Two people plus gear.

Yeah, I’ve found the KO3s can accumulate a bit of heat (Atacama Desert, ambient temp mid-30s celsius). Nothing problematic, but noticeably hotter than the KO2s (Simpson/Central Desert, mid-high 40s celsius) for otherwise similar pressure/use on fast gravel with patches of tarmac.

This is the one aspect maybe worth mentioning @parb . Heat accumulation is I gather primarily via side wall flexion when the tyre is subject to dynamic loads, primarily cornering and acceleration/braking however I’ve also noticed it with strong crosswinds. Lower pressures accumulate heat faster (more sidewall movement) and in arid location use this can quickly become a balancing act between optimum pressures for surface conditions versus heat accumulation in the tyre carcass.

On the plus side, the TPMS is a gamechanger in that one has never had a tyre temperature-related failure in a TPMS-equipped vehicle. Have sometimes found it necessary to run higher pressures than one would prefer in the name of keeping temps in check.

Final point worth mentioning maybe is the worst case scenario, which is severly potholed tarmac in high ambient temperatures. Sort of sits outside the highway versus gravel taxonomy. Really difficult to manage pressures and preserve both tyres and rims in this situation, particularly if it persists for 6-700kms and slowing to <40kph or whatever seems unrealistic (perhaps I’m just impatient). Ideas welcome!

 

[TheDocAUS]

Yeah, I’ve found the KO3s can accumulate a bit of heat (Atacama Desert, ambient temp mid-30s celsius). Nothing problematic, but noticeably hotter than the KO2s (Simpson/Central Desert, mid-high 40s celsius) for otherwise similar pressure/use on fast gravel with patches of tarmac.

The Falken Wildpeaks AT4s are definitely running cooler than the KO2s. I just had to put in some more air, the KO2's would quickly go from 35PSI cold to 40PSI. The AT4s only move to around 38PSI after sustained use. Tyre temps are also lower.

My longer review is linked here. My needs are offroad focused, so that influences my review. The second post has just added some observations on highway use.

 

[TCMColorado]

I am running the Mickey Thompson Baja Boss 255/85R17. Love the tire but it heats up substantially when driving at highway speeds, enough to regularly set off the tire temp warning. Increasing pressure does not help reduce the temps.

 

[Tazzieman]

Even though I've lived in a metric country for 55 years, I am partial to PSI ... and pints.
Also I see KO2s are rated for up to 80PSI, so there is some wiggle room for those that chase MPG/Km/L/TikTok hits

 

[MileHigh]

So what is the max temp for a tire like KO3 or Baja?

Why do I seem to get more tire pressure warnings with my IG than with any other car I’ve had? Is it the 17 inch rims and the general tire size? Something to do with the range of pressures that will trigger warnings?

 

[tommydrum33]

So what is the max temp for a tire like KO3 or Baja?

Why do I seem to get more tire pressure warnings with my IG than with any other car I’ve had? Is it the 17 inch rims and the general tire size? Something to do with the range of pressures that will trigger warnings?

Yes. The window in which it alerts you, for both high and low, is pretty sensitive compared to other brands. It's annoying, but it just alternates with it yelling at me about overdue for service.

As far as tire temps, did 575 miles (on k03) yesterday where ambient temp was ~28 degrees F. Tire temp were ~87f when I glanced. Maybe just over 90f in the rears. Started at 28/29 cold, ended up at 33/34. If I start pushing +145f, I'll add pressure. Usually happens when towing something a bit heavy in the rear and using the bags.

 

[Brad@SuperiorCO]

This is great science, I really appreciate it. ou have put a lot of effort into it. Two questions.

Is there some way to allow for tire temperature? Or the temperature doesn't matter , just keep the pressure correct.
A real world example. I leave Denver driving to Moab in the summer with an ambient temperature of 36°C. I have my tires set correctly at 40/42 psi, as I drive the tire temperature increases from 90°F to 140°F and the pressure increases to 52 psi. In your opinion should a driver stop and let pressure out to get back to ideal, or knowing that a higher pressure will reduce sidewall flex, leave the 52psi? The tires are rated to 80psi.

When you calculated your tables, are they based on sidewall and width only, or is volume considered? Example, I have Mickey Thompson pizza cutters 255/85/17. I experience tire pressure swings very quickly, due to the sidewall height, and I believe the volume. How would you calculate the ideal pressure for these tires assuming a 7,000lb Grenadier with the 51/49 weight split.

My experience yields
Highway - 42-48 psi
Around town - 32 psi
Rockies, Moab - 20 psi
yields good results. But could the results be better?

Nice work, this document is a keeper.

 

[parb]

There is a great point made that i glossed over.
The tables are generally oriented around 70F/20C ambient weather. It's the climate i typically have where i live.

What if you live in a different climate or have bigger swings?
I'll add this to the sheet, but yes you should temperature adjust. This is cold tire pressure should be adjusted depending on the ambient temperature,

I'll get this into the doc, i just calculated it out on my tablet while watching fireworks on the TV.

Basically at these temps
Under 20F add +4 psi over what the cheat sheet says
20 - 40F add +2 psi
40-85F follow the table
85-100F reduce -2 psi
Over 100F reduce -4 psi


I'll get it in tomorrow in pai, bar and kpa.

 

[Logsplitter]

There is a great point made that i glossed over.
The tables are generally oriented around 70F/20C ambient weather. It's the climate i typically have where i live.

What if you live in a different climate or have bigger swings?
I'll add this to the sheet, but yes you should temperature adjust. This is cold tire pressure should be adjusted depending on the ambient temperature,

I'll get this into the doc, i just calculated it out on my tablet while watching fireworks on the TV.

Basically at these temps
Under 20F add +4 psi over what the cheat sheet says
20 - 40F add +2 psi
40-85F follow the table
85-100F reduce -2 psi
Over 100F reduce -4 psi


I'll get it in tomorrow in pai, bar and kpa.

And Celsius please
Happy New Year.

 

[Rok_Dr]

There is a great point made that i glossed over.
The tables are generally oriented around 70F/20C ambient weather. It's the climate i typically have where i live.

What if you live in a different climate or have bigger swings?
I'll add this to the sheet, but yes you should temperature adjust. This is cold tire pressure should be adjusted depending on the ambient temperature,

I'll get this into the doc, i just calculated it out on my tablet while watching fireworks on the TV.

Basically at these temps
Under 20F add +4 psi over what the cheat sheet says
20 - 40F add +2 psi
40-85F follow the table
85-100F reduce -2 psi
Over 100F reduce -4 psi


I'll get it in tomorrow in pai, bar and kpa.

EDIT. Dropped the max to 40 deg C that’s more than enough.

Great stuff! And for us in the warmer climates I’d take it up to 40deg 45deg C or even 50deg. Some towns in northern WA have already had a few scorchers in the high 40s this summer.

 

[parb]

i updated the document. Formatted it for easier reading
For the cheat sheets i put in entries for extreme heat and extreme cold.
3 versions of the cheat sheet, Bar / KG, PSI / LBS and kPA / Kg.

Thank you all for your helpful feedback. Sometimes you get blind when you stare at a document for too long, and what seems obvious to me is convoluted to others

 

[globalgregors]

There is a great point made that i glossed over.
The tables are generally oriented around 70F/20C ambient weather. It's the climate i typically have where i live.

What if you live in a different climate or have bigger swings?
I'll add this to the sheet, but yes you should temperature adjust. This is cold tire pressure should be adjusted depending on the ambient temperature,

I'll get this into the doc, i just calculated it out on my tablet while watching fireworks on the TV.

Basically at these temps
Under 20F add +4 psi over what the cheat sheet says
20 - 40F add +2 psi
40-85F follow the table
85-100F reduce -2 psi
Over 100F reduce -4 psi


I'll get it in tomorrow in pai, bar and kpa.

My other suggestion here in warm environments is to let your tyres warm up a bit (eg increase in pressure 2-4 psi) before hitting the cold tyre pressure reset, so that you stay within the +/- 4 psi operating range of the TPMS throughout the day.

 

[globalgregors]

So what is the max temp for a tire like KO3 or Baja?

Why do I seem to get more tire pressure warnings with my IG than with any other car I’ve had? Is it the 17 inch rims and the general tire size? Something to do with the range of pressures that will trigger warnings?

In my experience temperature is more critical ultimately than pressure as with high temps you can get tyre carcass (usually side wall) delamination without seeing a pressure spike.

This shows up as a prominent blister on the tyre sidewall, which AFAIK is unrepairable (irreparable?).

This based on experience driving too fast for too long in midsummer Saudi (in a non TPMS-equipped Toyota Prado). Took me longer than it should have (4 tyres) to work out what was causing failures.

 

[Rok_Dr]

i updated the document. Formatted it for easier reading
For the cheat sheets i put in entries for extreme heat and extreme cold.
3 versions of the cheat sheet, Bar / KG, PSI / LBS and kPA / Kg.

Thank you all for your helpful feedback. Sometimes you get blind when you stare at a document for too long, and what seems obvious to me is convoluted to others

Thank you Par

I know exactly what you mean and I have the same challenges with writing the Rok_Dr manual. This and your other white papers are valuable contributions and I’ll be referencing you with links to the documents in the next edition of the manual.

Thank you again.

Cheers
Steve

 

[parb]

Thank you Par

I know exactly what you mean and I have the same challenges with writing the Rok_Dr manual. This and your other white papers are valuable contributions and I’ll be referencing you with links to the documents in the next edition of the manual.

Thank you again.

Cheers
Steve

I complete feel the same, i have truly enjoyed the Rok_Dr manual -its how i have traced all my relays and fuses for my high load circuits and modified them to suit how i wanted them to operate. A task that would have been significantly more effort without your incredible body of work. Thank you!

 

[C-Mack]

@parb on the pressure/load tables in your document it shows the KO2 255/70 18” tires with a Load Range of “E”. My factory installed KO2s on the 18’ rims actually show a Load Range of “D” on the sidewall? Do I still use the same recommended pressure/load settings in your table or is there other pressures you recommend for the “D” range tires?

 

[parb]

ITs a bug, let me fix it. i don't have the K02 tires and i think i got some bad information what stock tires are. i went and looked it up directly right now and i'll update the tables with that.

 

[parb]

i updated it.
here is a summary:
Made sure i picked the right Grenadier tires.
Re-did the calculations for larger tire volumes = lower tire pressures needed.

Also, redid some sidewall calculations to ensure that speed and lower pressure is compatible with side wall structures leading to an minimum speed floor for PSI.

made a point that these are minimums.