The 4 psi Rule

[patch697]

Some time ago I read an article about the 4 psi rule & thought it might be an interesting topic of discussion.
This is the article I read.



Basically, the 4psi rule looks for an increase in tyre pressure from cold start pressures of 4psi after an hours driving (or thereabouts) If you start with 30psi in the morning, then after an hour of driving the pressure should be 34psi. If it increases MORE than 4psi, then your start pressure was TOO LOW, add more air now; LESS than 4psi then your start pressure was TOO HIGH, drop some pressure now.

A rough guide to adjusting pressures on the go (once the tyres are warm that is) is to adjust your pressure up or down as indicated by the 4psi rule by 1/2 the difference between what the pressure actually IS and what it SHOULD HAVE BEEN if it complied with the 4psi rule. So if you started at 30 psi and it only went up to 32 then the start pressure of 30 was too high, drop 1/2 the difference between 32 (what it IS) and 34 (what it SHOULD BE) ie drop 1 psi NOW!! And tomorrow morning, or the next time you start off on cold tyres, your start pressure should be 29psi not 30. If your pressure doesn't go up ANY at all, then drop 2 psi! Rough guide, but it works.

The only thing about this 4psi rule is that it means you need to make your tyre pressure gauge your friend, and use it a lot for a while. And you need to work out what the optimum pressure is for every load condition, road surface, and ambient temp that you drive in. Sounds like an imposition, but it really doesn't take all that long for most to get on top of it and work out the sorts of variations in start pressure that suit them (and remember, what works for you won't necessarily work for anyone else, even in your car - so it means all those 'what's the right pressure??' questions are meaningless cos it's gonna be a little different for everyone!) With a consistent tyre pressure gauge (consistency is more important than absolute accuracy!) most can get pretty good at estimating what is the correct pressure for any surface/load/temp etc with just a couple of weeks of regular checking - even just checking your tyres in the morning then seeing what pressure they get to when you get to work will help!



Ok now you've had a read, whats your thoughts on the subject?


Cheers
Paul

[DX grunt]

Interesting.

'A consistent tyre pressure gauge'..........Do you trust the tyre pressure gauge you buy and treat with TLC? Do you trust the one at the local garage that has been dropped a zillion times, or do you trust the one at the garage that you just plug into yr tyres and set your own PSI? I don't know.

Consistently 2-6psi out (guessing how much some gauges are out), in my opinion, creates more long term problems with tyres than being accurate and consistent.

Inaccurate tyre pressures on a really hot day, can be a major, major issue.

Take care out there.

Ross
DX grunt

[patch697]

Interesting.

'A consistent tyre pressure gauge'..........Do you trust the tyre pressure gauge you buy and treat with TLC? Do you trust the one at the local garage that has been dropped a zillion times, or do you trust the one at the garage that you just plug into yr tyres and set your own PSI? I don't know.

Consistently 2-6psi out (guessing how much some gauges are out), in my opinion, creates more long term problems with tyres than being accurate and consistent.

Inaccurate tyre pressures on a really hot day, can be a major, major issue.

Take care out there.

Ross
DX grunt



Ok that being said. what is the answer? How is the best way to know your tyre pressure at its optimum other than tyre pressure guide lines on the tyre itself & tread wear?

Cheers
Paul

[DX grunt]

I don't know what the answer is Paul. It's one of those 'how long is a piece of string' questions. If we all do our best and know it is our best, we've done our best.
I guess if we know a friendly tyre place we could check our tyres on their gauge - but how long does friendship last?

Great thread. Plenty of food for thought.

Thanks.

Ross

[patch697]

I don't know what the answer is Paul. It's one of those 'how long is a piece of string' questions. If we all do our best and know it is our best, we've done our best.
I guess if we know a friendly tyre place we could check our tyres on their gauge - but how long does friendship last?

Great thread. Plenty of food for thought.

Thanks.

Ross

Thanks Ross

Yeah I thought this might bring about some interesting points of view & personal expediences & where all on a quest for knowledge so I hope this brings about some of that as well.

Cheers
Paul

P.s Mate I would love to here some more of your thoughts on this as the thread progress's.

[DX grunt]

Thanks Ross


P.s Mate I would love to here some more of your thoughts on this as the thread progress's.

Happy to do it. PM me with how we are going to start the new thread we were talking about.

Ross

[patch697]

PM me with how we are going to start the new thread we were talking about.

Ross

Will do mate I'll get onto that this week.

Cheers
Paul

[the evil twin]

We just had the WA Product Manager for some of the leading American Tyre Manufacturer's as a guest speaker at our Club and he is of the opinion that if after years and years of research the Manufacturers endorse the 4 PSI rule as an acceptable method then that is good enough for him... managed to convince me as well. Don't get me wrong, I'm not saying it is the only way, just saying it is a widely accepted way that the Manufacturers are well aware of.

The other 'method' of setting pressure that I have heard and also makes sense is use the 4 PSI rule to get the initial "ball park" pressure and then adjust the pressure so that all the tyres have an equal footprint including your trailer. Makes sense in that it compensates for loading.

Personally I just run my Tyres at the high end of the manuf recommended pressure range as my truck is pretty heavy. I was also running remote Tyre Pressure Monitors for a while and it is amazing how much a tyre temperature reacts to pressure. It was nothing to see temps rise 10 to 15 degrees for just a couple of PSI difference. This was also mentioned in his presentation when he stated that at least 90% of the failures (not stakings etc) but failures could be directly attributed to under inflation therefore high temps leading to delaminations.

[patch697]

Did he explain it though? Being the state product manager I thought he'd know the reason behind it. I would be interested to know that's all.

I wouldn't mind knowing the reason behind this ether hence the reason for starting the thread. I will also do some more investigation & see what I come up with.


Totally agree. A tyre is deflated to increase the surface footprint on the ground. As a tyre rotates it flexes when at the bottom (bearing the weight of the vehicle) then stands tall when unloaded (at the top of the wheel). This continual flex/unflex heats the tyre up and if the temperature gets too high (whatever temp that is?) delamination begins to occur from the inside of the tyre. You can't see the progressive damage until POP! That's why it's important to keep speed down when running lower pressures. Saw this on a tyre segment on a 4x4 DVD, presented by Cooper Tyres.

I also have the same dvd of the Fraser island trip & know what your referring to but I think the 4psi rule was only intended to be applied to driving on bitumen surfaces.

Cheers
Paul

[Col.T]

Just a thought on using inaccurate guages ( as opposed to totally stuffed ones).
Even if the gauge is say 3deg. out, the error within each degree will be extremely small, so small in fact that it could be safely ignored.
Accordingly, if you work the pressures according to this 4deg. rule, once you get it right, the pressure will be correct for your vehicle/ conditions regardless of what the guage reads.
I think
Col

[patch697]

Just a thought on using inaccurate guages ( as opposed to totally stuffed ones).
Even if the gauge is say 3deg. out, the error within each degree will be extremely small, so small in fact that it could be safely ignored.
Accordingly, if you work the pressures according to this 4deg. rule, once you get it right, the pressure will be correct for your vehicle/ conditions regardless of what the guage reads.
I think
Col

My thoughts were as long as you used the same gauge to check your tyres each time in accordance with this principle you should be right which is along the same lines I think.

[Silver]

I had heard the rule first as 2 psi for passenger car tyres - noting the comments above about gauge accuracy, I guess it is about consistency from reading to reading.

I have travelled to Weipa with a mate at speeds consistent with conditions in his fairly heavily laden tojo ute, towing a boat. He ran the tyres a lot lower than I would have thought - 16" Yokahamas, to cope with the corrugations, and it seemed to work quite well. I asked a tyre bloke about this and he made the point that with lower pressures, the starting cold pressure is likely to turn into a higher warm pressure due to flex etc.