Using the 4psi Tyre Inflation Rule

Submitted: Tuesday, Feb 14, 2012 at 14:14
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Care must be taken in using the 4psi inflation rule as often the pressures measured 'cold' and 'hot' may be affected more by ambient temperature than tyre heating through vehicle motion.

The 4psi rules states that “if your tyres are inflated correctly then the tyre pressure measured when the tyres are hot (after travel) will be 4psi greater than the pressure measured when they were cold (before travel)”.

See Blog for actual caluclations.

Changes in tyre pressure due to typical ambient temperature changes show:
Example 1. Morning ambient temperature 15c, Midday ambient temperature 30C, pressure change due to temperature change alone is 2.6 psi.
Example 2. Morning ambient temperature 10C, Midday ambient temperature 35C, pressure change due to temperature change alone is 4.4 psi.

I am not saying that the 4psi rule is right or wrong, I am saying that caution needs to be taken in how it is applied.
If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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Reply By: bob smith 1 - Tuesday, Feb 14, 2012 at 14:23

Tuesday, Feb 14, 2012 at 14:23
Yeh I agree. Have tried this theory but it seems to be hit and miss and is not worth the mucking about. After a while you know whats best for your particular vehicle and check when cold and things should be all good.
BS
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Reply By: Robin Miller - Tuesday, Feb 14, 2012 at 15:01

Tuesday, Feb 14, 2012 at 15:01
This is a reason I like the tyre monitoring systems Rob.

You note pressure as you start driving and 15 minutes later you should see an increase of 2-3 psi , you don't need to wait till lunch to do a check during which interval the ambient may have changed as you indicated.
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Follow Up By: Member - Rob D (NSW) - Tuesday, Feb 14, 2012 at 17:38

Tuesday, Feb 14, 2012 at 17:38
If you look at the average pressure changes with temperature in the examples then as a rule of thumb, adding 1.75 psi for every 10C change in ambient temperature will get you closer to the intention of the 4psi rule.
If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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Reply By: ozjohn0 - Tuesday, Feb 14, 2012 at 15:10

Tuesday, Feb 14, 2012 at 15:10
The 4psi rule is for standard car tyres and TL tyres and at an ambianet temperature of 20c.
At higher ambient temps or on very hot road surfaces then 6psi is more appropriate.
Ozjohn.
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Reply By: Crackles - Tuesday, Feb 14, 2012 at 17:08

Tuesday, Feb 14, 2012 at 17:08
"I am not saying that the 4psi rule is right or wrong"
Well if you wont say it Rob I will. The 4 PSI rule is a rediculous theory that doesn't take into account (amoung many variables) ambiant temp, weather, road surface or even what side of the car the sun is shining.
Who in there right mind would bother to stop & let a little more air out of their tyres if the pressure only increased 2 psi after an hours drive or on the other hand pump them up if they had over inflated by a seemingly dangerous 5 psi? ;-)
To follow the rule to the letter you'd need an onboard air inflation system & even then to what end?
In my book the theory has as much credability a Brocky's 'Energy Polarizer' :-))
Cheers Craig................
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Follow Up By: Member - Rob D (NSW) - Tuesday, Feb 14, 2012 at 18:00

Tuesday, Feb 14, 2012 at 18:00
The rule was originally proposed by a Pirelli tyre engineer as far as I can ascertain. If you do a web search you will find many people using it.
If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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Follow Up By: Dust-Devil - Tuesday, Feb 14, 2012 at 22:36

Tuesday, Feb 14, 2012 at 22:36
Craig

Couldn't agree more. The 4psi rule is a 'knob rule' for 'Ruling Knobs'.

I pretty much follow Ocoolone's line of thought and action when setting tyre pressures for the usual four types of terrain that one covers, with special one of scenarios being dealt with as they arise.

DD

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Follow Up By: Crackles - Tuesday, Feb 14, 2012 at 23:08

Tuesday, Feb 14, 2012 at 23:08
"If you do a web search you will find many people using it."
I bet if you do a web search you'll find many people who still think an Energy Polarizer works too :-))
I'm yet to see anyone's explaination of the rule that covers all the varied terrain & circumstances we 4 wheel drivers encounter, in particular how often & by how much the pressures are adjusted in real driving. (Not just the theory)
Cheers Craig...................
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Reply By: olcoolone - Tuesday, Feb 14, 2012 at 17:25

Tuesday, Feb 14, 2012 at 17:25
Out of all the outback driving I have done and and not just me but others on the trips with us have never taken tyre pressures as seriously as some do and we have never destroyed a tyre due to having the wrong pressures.

We run 42-46 depending on weight on the open road at hiway speed..... and across some dirt roads, 28-32 on fast dirt roads, 22-26 over rocky stuff and 10-16 in sand.

I have better things to do then sit down with a calculator, white board and a thermometer doing complex calculations for no particular gain.

KISS and you will not have a problem.

Maybe I should get a white dust coat.... 4 or 5 coloured pens and a note pad so I can look like the rest of the new age 4x4ers!

WOW... antenna gain (dB), battery voltage, battery charge rates and now tyre pressures, hell you'll need a degree in fourwheeldriveology from the university of hardknocks 50 miles east south west of the blackstump
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Follow Up By: ross - Tuesday, Feb 14, 2012 at 21:44

Tuesday, Feb 14, 2012 at 21:44
Im the same ,life's too short to run round sweating on tyre pressures.
Check em once a month and forget.
Modern tyres are much more adaptable to various pressures and a few pounds either way wont hurt.

I often wonder if the tyre pressure brigade are closely related to the constant car polishers and that other mob who buy the best quality oil and change it twice as often as recommended.
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Follow Up By: Dust-Devil - Tuesday, Feb 14, 2012 at 23:01

Tuesday, Feb 14, 2012 at 23:01
Olcoolone

Spot on dude, I do pretty much what you do and have never had a pressure related tyre problem in 45years of driving on all types of surfaces all over Australia.

May I respectfully suggest that you add a 'smartphone' and/or a Ipad/Tablet to your required white dust coat, coloured pens and highlighters whilst studying for your degree in fourwheeldriveology from the University of Hardknocks 50 miles east south west of the blackstump. LOL)

Once out in the Simpson in the vicinity of Poepels Cnr I noticed on the electronic tyre monitor that the tyres on the passenger side of the vehicle were approx 10psi higher than the other side and I was dead sure they were the same when we started off earlier that day.

The 'worry dagger' in the guts started to stir, however the psi didn't rise any further so I just ignored it. On our return to camp later on, the drivers side did exactly the same with the passenger side returning to the orig setting. It took a while, however it finally dawned on me that it was Sun. Never thought it would make such a difference. Just glad I wasn't wearing a white dust coat that day. (LOL)

Yep the 4psi rule is definitely a knob rule for 'ruling knobs'

DD
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Follow Up By: olcoolone - Wednesday, Feb 15, 2012 at 07:51

Wednesday, Feb 15, 2012 at 07:51
When we were selling TyreDogs TPMS we had them on a vehicle and we stopped for about 1/2 sightseeing.... jumped back into the Hilux and the TyreDog was going of it's tree saying the temp in the front right was way to much, ended up being we were parked under some trees and the suns narrow beam was hitting the sensor dead on causing a funny reading.

We would also get strange reading out in the desert in the mornings.... same thing sun hitting the sensor.
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Reply By: AlbyNSW - Tuesday, Feb 14, 2012 at 17:28

Tuesday, Feb 14, 2012 at 17:28
It should be called a guide rather than a rule, after using a TPMS system you realize how the varying conditions effect the tyre pressures and temps greatly
AnswerID: 477778

Follow Up By: Member - Rob D (NSW) - Tuesday, Feb 14, 2012 at 17:49

Tuesday, Feb 14, 2012 at 17:49
I am not advocating the use of this rule at all. But if you do a web search on the 4psi rule you will find many people recommending it and even on this forum, as I recall it, you will find people who have company procedures mandating this rule.

My intention was to alert people to the fact that, if you are going to apply it then you need to understand some of the other variables affecting it.

If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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Reply By: Rockape - Tuesday, Feb 14, 2012 at 17:59

Tuesday, Feb 14, 2012 at 17:59
Rob,
I have a tpms system and it is great for a slow leak or sudden deflation.

I agree with you about the 4psi rule as Crackles said the amount of variables goes on and on.

Many mornings we have started of in temps below 5 degree c and driven at different speeds and road conditions up to 30 degrees +. That is a fair temp change and especially when you consider one set of tyres is on the shady side and the other on the sunny side.

Whenever I pull up I just use my hand (no smart arse comments please) to check all is well. It is amazing that the body has its own built in temp sensing system, although this come at a cost. Mine requires the addition of alcohol in the evenings and depending where you are it can be quite expensive.

Have a good one,
RA.

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Reply By: Effie C (Ex NT now ACT) - Tuesday, Feb 14, 2012 at 20:06

Tuesday, Feb 14, 2012 at 20:06
Tried the 4psi rule here in winter when it was lucky to get above +9°C, tyre pressures did not change at all, was very disenchanted with this "rule" :)
Use the TPMS to monitor for any major changes
Live One Day at a Time for Tomorrow May Never Come.
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Follow Up By: Will 76 Series - Tuesday, Feb 14, 2012 at 20:31

Tuesday, Feb 14, 2012 at 20:31
Far to much mucking around, I run a very simple system which has never let me down. It is a rule of thumb and can be adjusted for extreme conditions.
Highway = 40PSI
Dirt=30 (pending conditions)
Sand=20 (start pressure)

Basic, yes practical yes.
Good luck.
Will
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Follow Up By: bob smith 1 - Tuesday, Feb 14, 2012 at 23:56

Tuesday, Feb 14, 2012 at 23:56
Yep same here....and a good kick off the tyre would give a more acurate reading.
BS
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Reply By: Off-track - Tuesday, Feb 14, 2012 at 21:56

Tuesday, Feb 14, 2012 at 21:56
Tar, dirt, gravel, rock - 40-42psi
Sand - 20psi or thereabouts.

Its that simple. Hasnt failed me yet.
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Reply By: gbc - Wednesday, Feb 15, 2012 at 07:01

Wednesday, Feb 15, 2012 at 07:01
Has noone even heard of the tyre and rim association?
"40, 30, 20 its that simple" what a crock.
Take the same sized tyre (265/75/16 for example). Take the Oe passenger rated one vs the Lt version we all are so fond of. To carry an average load, the LT will need 10 psi MORE than the passenger rated tyre. At touring weights, the oe will top out at 32-34 psi, whereas the LT will need to go nearer to 65 PSI to carry the same weight.
The manufacturer of each tyre has already worked out load vs pressure rating for all of our tyres - who knows what their's is? The tyre and rim association has the generic ones.
I've never understood why we Australians totally discount everything the experts tell us and try to come up with our own theories, then simplify them so far as the make them dangerous.
Every vehicle is different. First find out What tyres you are running, then look them up - not hard. Find out how heavy you are when touring. Put them together. Not hard, and most of all ACCURATE.
Oh and you won't find any mention of the 4psi rule (6psi rule for LT construsted tyres) anywhere.
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Follow Up By: Member - Rob D (NSW) - Wednesday, Feb 15, 2012 at 09:31

Wednesday, Feb 15, 2012 at 09:31
Thank you for that.

I must confess I was a little surprised by some of the responses, given that I was trying to make things safer for those who do use the 4 psi rule, and there are a lot of them.

As you have observed there are a number of members who are "often wrong but never in doubt".



If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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Reply By: fisho64 - Wednesday, Feb 15, 2012 at 17:52

Wednesday, Feb 15, 2012 at 17:52
when I first read this I thought it was a windup!
As said above you do need to make sure you have around the correct pressures-but to stress over 1 or 2 psi? If that difference really was critical thered be cars spinning off the road all over the place with blowouts-the vast majority of people would be lucky to check them once a month let alone 1 hour up the road into a long journey.

40-30-20 is a pretty good rule of thumb, and is what Ive used though never put it down like that.
Theres little need to add stress to your trip by panicking over a couple of psi?
AnswerID: 477865

Follow Up By: Geoff H (Q - Friday, Feb 17, 2012 at 13:22

Friday, Feb 17, 2012 at 13:22
I've always used the 40 - 30 - 20 guideline in the past. Even though it may not be exactly correct it has served me well over the past 40 Years. Maybe a few PSI either way doesn't make much difference ?.

I think I'll check out my current tyre requirements and adjust things up or down as required.

Regards
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Reply By: Will 76 Series - Friday, Feb 17, 2012 at 20:48

Friday, Feb 17, 2012 at 20:48
People seem to be getting a little excited here. The 40 - Hwy, 30 - Dirt and 20 - Sand are only a rule of thumb and can be adjusted for the conditions.
Sand for example most occasions 20 is more than low eneogh however if need be you may need to go down to 16?
Acknowledged about the size of the rim and all that crap but in most cases we are referring to 15 or 16 inch rims and probably AT tyres.
I would suggest if you havn't tried the theory give it a go and see how you go.
Regards Will
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