Elusive 4psi rule

[HeavyPizzaz]

3 days into a journey, currently in Wyandra QLD and the 4 psi rule is eluding me with my rear tyres. Towing a camper trailer. Got it sorted on the front tyres and camper tyres.

Started the rears at 40psi only coz left late and forgot to put them up. Soon put them up to 44ish, keep adding extra in to put it at what i think should be about right to get a 4psi increase. I think I’m on 50 - 51 in the rears now (cold) and well I must be doing it wrong coz every time I add more in, the difference between cold and hot stays the same!! 7-8psi. The difference is not changing. It has me stumped. The tyres are OPAT2 LT so plenty of room to move still.


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[Seigried]

You need to remember the 4psi rule is a rule of thumb. Diffrent tyre compounds and manufacturing techniques mean diffrent tyres will retain heat at diffrent amount. From what you have written for your tyres its 7 to 8 psi.

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[old Jack]

The 4 psi rule is meant only for passenger construction tyre's on bitumen roads.
There is a 6psi guide that you can use for LT light truck construction tyre's when on bitumen only!
You measure the tyre pressures when cold, first thing in the morning and then drive only 30minutes and check tyre pressures. You are looking for 6psi increase as a guide.
Once you drive for longer than an half an hour you can get higher readings on onside of the vehicle due to sunlight, due to undulating roads where you suspension is moving a lot and even change in road surface can effect tyre operating temperatures and pressures.

The Toyo Load Inflation Tables advise you of a pressure for wheel load, when these are converted into kg and axle loads these are the numbers.
These are cold inflation pressures.
At 35psi Axle Load 1720kg.
At 40psi Axle Load 1760kg.
At 45psi Axle Load 1913kg.
At 50psi Axle Load 2063kg.

As you can see 35psi is more than enough for the front axle and 40psi is enough for the rear axle unless you are grossly overloaded!

Running tyre at too higher pressure (this is a common problem with 4wds) increases the chance of puncturing a tyre through the tread or shoulder area of the tyre and it also puts extra load on the struts and shockers and their bushes and mounts.

If you have a TPMS fitted you can expect to see tyre temperatures 40-50psi higher than ambient when travelling for extended periods in high ambient temperatures. Tyre's can comfortably run at up to 95C with out a problem but once they get to 100C and over then it is time to shed speed and or load.

OJ.

[nj swb]

Tyre's can comfortably run at up to 95C with out a problem but once they get to 100C and over then it is time to shed speed and or load.

Where did you get these numbers? I've been trying to find a "temperature limit" for a few years now, and even Toyo couldn't (wouldn't?) tell me one.

With my Toyo MTs, my experience has been very similar to the OP's - no matter the starting pressure, the pressure rises about 7 psi, and (in really hot weather) the tyres hit 70 degrees. When I rang Toyo's technical department, they told me that any more than a 3-5 psi rise meant my starting pressures were too low, and I needed to add more air. Which made no difference.

I resorted to Toyo's load tables, and concluded that around 42 psi at a tyre temperature of 20 degrees is the highest I should ever need.

I've also noticed my pressures can rise & fall about 4 psi based solely on ambient temperature - from a cold night (0.0 degrees this morning) to a warm day, my rear tyres can start anywhere between 36 and 40 psi.

[HeavyPizzaz]

Many thanks for so many replies. Leaving camp soon. What I did notice is that despite the pressure the temps are all still decent unless my TPMS is way off (will check what temps they get to today) but based on the tables you included OJ, now I’m thinking I can lower them a bit. I notice the TP plate in the door sill says 44 when trailer towing. I could start there. Thinking 50 cold is now too high based on what you’ve mentioned including increased risk of puncture. I’ve been solely bitumen so far but hitting dirt of Plenty Hwy (unless it’s all tarred now) in next few days.

[nj swb]

... the temps are all still decent unless my TPMS is way off

If you're using external sensors take the temperature readings with a grain of salt. How can a temperature sensor outside the tyre on the end of a valve stem, spinning around in ambient air, accurately indicate the air temperature inside the tyre?

[HeavyPizzaz]

If you're using external sensors take the temperature readings with a grain of salt. How can a temperature sensor outside the tyre on the end of a valve stem, spinning around in ambient air, accurately indicate the air temperature inside the tyre?

You’re absolutely right, they appear next to useless.


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[old Jack]

OJ; Tyre's can comfortably run at up to 95C with out a problem but once they get to 100C and over then it is time to shed speed and or load.

Where did you get these numbers? I've been trying to find a "temperature limit" for a few years now, and even Toyo couldn't (wouldn't?) tell me one.

Yes tyre temperature limits are elusive!

From my research race tyres are looking at a normal operating temperature of 80C to 100C.

http://www.dunloptyres.com.au/motorsport/tyre-tips/

Conventional passenger construction tyres I have read that 20C above ambient is a target operating temperature, but is this ambient road temperatures which can easily be 20C to 25C above ambient air temperatures in hot weather?

I have measured tarmac surfaces (runways) temperatures at over 70C when working at Alice Springs and Moomba Airports in the height of summer!

We know that a heavier construction LT tyre will build up heat more and dissipate heat slower than a passenger construction tyre, so maybe 30C above ambient is reasonable?

More telling is the temperature at which tyre rubber starts to fail, this is about 350F/126C to 400F/144C.

https://rubberchemtechnol.org/doi/abs/10.5254/1.3547374

OJ.