Question on the 4psi increase in tyres??

Submitted: Wednesday, May 21, 2014 at 16:11
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To the tyre gurus here.
The theory of a 4 psi increase with hot tyre pressures would be for tyres running (say) about 40psi. So that's an increase of about 10%.
Would the same apply for pressures of 70psi cold?? This would be an increase of only about 6% (sorry haven't got a calculator here).
Or is the 4psi increase be applicable no matter what the cold pressures???
Thanks in advance
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Reply By: gbc - Wednesday, May 21, 2014 at 16:16

Wednesday, May 21, 2014 at 16:16
Afaik the 4psi rule applies across the board. Ie no matter what the cold pressure, the hot pressure should be 4 psi more. Do some good research into this as it is only a rough rule of thumb more than an exact science and differs for different tyre construction. Light truck tyres are more like 6 psi if I remember correctly. I have never used it as gospel. You will confuse yourself if you try to start using percentages with it.
AnswerID: 532901

Reply By: Alan S (WA) - Wednesday, May 21, 2014 at 17:16

Wednesday, May 21, 2014 at 17:16
Wombat
The 4PSI rule is a way of associating hot tyre pressures to cold pressures in relation to load and speed to ensure correct inflation.

Being that with pressures increase with heat, and heat is caused by load and speed.

I would agree that using % is probably not a good idea.

Alan
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Reply By: Alloy c/t - Wednesday, May 21, 2014 at 17:26

Wednesday, May 21, 2014 at 17:26
Trying to convert the 4 psi "rule" [it is only a general Guide] to a % is a waste of calculus ,, the general guide is that to have the optimum pressure for any particular tire x load = cold starting pressure + 4 psi when tire has reached operating temp [usually somewhere round 50 /100km IE: starting at 27psi cold pressure should read 31 psi when tire at temp, any higher than 31 at hot temp means tire was to low a pressure when cold .. lower than a 4psi increase means tire was too high a pressure when starting out cold ,, as a "general" rule of thumb it works on most size tires from the tiny 12" boat trailer tires thru to prime mover size …...
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Reply By: 671 - Wednesday, May 21, 2014 at 18:24

Wednesday, May 21, 2014 at 18:24
I asked the manufacturer of my tyres about this 4psi rule on their customer information number. They said their testing had shown there were far too many variables involved for it to be anywhere near accurate. They told me the amount of weight each 1 psi increase would support. The tyres are now nearing the end of their life and it has worked perfectly.
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Reply By: Member - Andrew L (QLD) - Wednesday, May 21, 2014 at 18:24

Wednesday, May 21, 2014 at 18:24
I gotta ask..who decided on 4%..that people seem to quote or blindly follow..?
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Reply By: AlbyNSW - Wednesday, May 21, 2014 at 18:57

Wednesday, May 21, 2014 at 18:57
The other factor that needs to be considered is the ambient temperature
For example say you break camp first think in the morning at around 0 degrees and it is mid 20's by midday you are going to get a different tyre pressure reading even if you have not moved
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Follow Up By: Dennis Ellery - Wednesday, May 21, 2014 at 19:32

Wednesday, May 21, 2014 at 19:32
It’s not going to work in that situation with that time difference.
Measure the tire temperature after no more than an hour’s running – the ambient won’t have much effect.
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Follow Up By: Member - Boobook - Thursday, May 22, 2014 at 05:45

Thursday, May 22, 2014 at 05:45
Yes agreed. One of the reasons I got a TPMS is so I could use the 4PSI rule.

What I quickly found out is that it is far too simple to work and ignores the real world. If you drive on a road with the sun slightly to the left for 20 mins the left wheels will go up several degrees and 4 or 5 psi, then you go on a turn on the road and the pressure goes down and the other side goes up. Then you head into the sun and both front tyres go up and the rear ones down. It doesn't have to be a hot day for this to happen - 18 degrees and sunny will do it.

Before I had a TPMS I had no idea how dynamic the tyre pressures are, and tried to roughly stick to the 4PSI rule. It's just folly. Unless you adjust your pressures at every turn.

4 PSI rule Myth - BUSTED LOL



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Follow Up By: AlbyNSW - Thursday, May 22, 2014 at 05:49

Thursday, May 22, 2014 at 05:49
Yes I know Dennis but I thought I should raise it as it often doesn't get mentioned when this discussion comes up
I have a TPMS system and you get to see a lot of other factors effecting tyre pressure even sun on one side of the vehicle on your tyres throws it out
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Follow Up By: AlbyNSW - Thursday, May 22, 2014 at 08:19

Thursday, May 22, 2014 at 08:19
Hahaha Boobook you slipped that post in whilst I was posting mine
Agree with you 100%, until I fitted a TPMS system I was blissfully ignorant of what was really going on with my tyres
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Follow Up By: Bludge - Thursday, May 22, 2014 at 09:40

Thursday, May 22, 2014 at 09:40
Boobook and Alby,

Not everyone has TPMS, and the 4PSI rule is ONLY a guidline. It has been used for many, many years without any issues.

Quite simply after an hours driving, if as you say the left hand side is in the sun and has a tyre pressure 6psi over the start pressure and the right only 4psi then all is right with the world, on average, if the left is 4psi in the sun and the right only 2psi then he tyre presure needs to go up. Not rocket science or a myth.

So the question is now you have TPMS do you stop and change tyre pressures when the sun hits one side or the other side, because the 4psi rule ignores the "real world"?
Or do you just set your tyres to your 'deemed' pressure and ignore the differences, just like I would do with the 4 psi rule?
TonyV

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Follow Up By: Member - Boobook - Thursday, May 22, 2014 at 09:50

Thursday, May 22, 2014 at 09:50
Bludge I understand your point.

Based on what I see, for the 4PSI rule to work, you would need to measure the cold pressures under total shade or if very overcast. Then do the same when measuring the "hot pressure". otherwise you are simply building in temperature based errors. Even when stationary the difference in pressure can change as much as 4 - 5 PSI based on the sun.

To answer your question, Yes, now I set the pressure based on the terrain. You can easily see the 4PSI rule simply doesn't work with the TPMS. However you do see a pressure trend which is more useful.

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Follow Up By: Dennis Ellery - Thursday, May 22, 2014 at 11:35

Thursday, May 22, 2014 at 11:35
Alby and Boobook
I think you guys are over complicating things.
Yes the radiant heat from the sun on the sunny side of the vehicle raises the tyre temperature quickly – more quickly than a rise in the ambient.
Yes pressures are lower in a cold ambient and higher in a hot ambient.
As Bludge says you can’t adjust your tyres every time you change direction.
I set my tyres by the psi rule and unless I have big change in load that will do for a trip or a month or 6 monthly whatever and I leave it at that.
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Follow Up By: Member - Boobook - Thursday, May 22, 2014 at 12:04

Thursday, May 22, 2014 at 12:04
Dennis I don't think anyone is suggesting that you change the pressure every time you go around a corner. The point is that once you see the dynamics of the variables, including load etc you see that the 4PSI rule ignores other, larger variables.

See if you can borrow a TPMS, I am sure you will be very surprised how much the pressure changes constantly. I guess the important part is that you care for your tyres and keep an eye on them. coopers actually recommend cold pressures for various terrain types off road and the 4PSI rule for bitumen so they mix it up in their booklet.
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Follow Up By: Dennis Ellery - Thursday, May 22, 2014 at 12:39

Thursday, May 22, 2014 at 12:39
Boobook,
Wouldn’t be much point.
I don’t need that sort of detail regarding the variation of pressures over a day.
The psi rule is sufficient for my needs.
Thanks for the thought.
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Follow Up By: Dennis Ellery - Thursday, May 22, 2014 at 13:28

Thursday, May 22, 2014 at 13:28
I should have mentioned - when I am in the 10 to 20 psi pressures running through sand, the psi rule goes out the window.
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Follow Up By: AlbyNSW - Thursday, May 22, 2014 at 18:53

Thursday, May 22, 2014 at 18:53
According to the Coopers tyre handbook the 4 psi rule only applies to when on bitumen, they have highlighted that in big red letters.
I am not discarding the 4 psi rule by any means, it has been a base indicator that many have used for years.
My comments relate to with the later technology of TPMS being available, once used it becomes apparent that it is a very basic indicator.
The best thing I like about a TPMS is being alerted early to a puncture, I have only had to change one tyre that was unpluggable all the rest I have been able to plug whilst still on the vehicle and top the air back up
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Follow Up By: Dennis Ellery - Thursday, May 22, 2014 at 21:48

Thursday, May 22, 2014 at 21:48
You have a need for a TPMS I don’t – were both happy.
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Follow Up By: Echucan Bob - Friday, May 23, 2014 at 08:51

Friday, May 23, 2014 at 08:51
Dennis

Changing your tyre pressure by 4 psi every six months might give you a nice warm feeling. What the others are telling you is that is meaningless as other factors change the pressure by more than 4 psi hour to hour.

You mightn't need a TPMS, or a temperature gauge, low coolant alarm, or a dip stick for that matter.

Others value being able to monitor important vehicle parameters, and thereby reduce the likelihood of being stranded when it is least convenient.

Bob
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Follow Up By: Dennis Ellery - Friday, May 23, 2014 at 10:33

Friday, May 23, 2014 at 10:33
Hi Bob,
On this site some time ago an experienced user of these devices questioned the temperature accuracy of the valve stem mounted devices saying in his experience they could be out 20 degrees – I can judge more accurately by hand feel than that.
But I don’t care whether they are accurate or not, I don’t need them.
My tire pressures are based on past experience.
I may change my tire pressures a couple of times a day depending on what I am traveling over, how hot the tires are, the speed I am travelling at, the load etc etc.
But if I haven’t got off a flat hard surface for 6 months and the load is the same I wouldn’t need to adjust the pressures – no point in reinventing the wheel.
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Follow Up By: Member - Boobook - Friday, May 23, 2014 at 11:11

Friday, May 23, 2014 at 11:11
So what you are now saying Dennis is that you base your pressures on experience and the terrain. So do I, but that's not how the 4 PSI rule works is it?

I am sure you have a lot of experience. No doubt. But what you describe changing infrequently and based on terrain is no more the 4 PSI rule than using a TPMS is.

I agree that the temp on a stem based TPMS may not be accurate, though 20 degrees out seems waaaay off, I'd like to see the reference to that source. Try 4 - 5 degrees. I have measured the temp of the wheel and tyre on the side by an infra red thermometer to calibrate the TPMS. It is usually 3 - 4 degrees out, and worst when it is cold with low tyre pressures like in the high country. The temp is probably only useful to tell you of a serious problem, a different situation to the discussion here and one where your TPMS would have alerted you long ago with a low pressure alarm.
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Follow Up By: Dennis Ellery - Friday, May 23, 2014 at 11:32

Friday, May 23, 2014 at 11:32
Hi Boobook
I use the psi rule for flat surfaces – driving through sand for instance I can’t apply it – some sand may need 20 psi another situation may need 10 psi.
I don’t know whether you or the bloke quoted below is correct on TPMS accuracy figures – It’s not important to me as I don’t need a TPMS.
I quote
Olsen's 4WD Tours and Training replied:
I've had my tyre dog for some time now, and because of the externally mounted sensor, I believe the temperature reading is not reliable. I used to have internal sensors, and they read about 15 to 20 degrees hotter than the Tyre Dog. I used to run an alarm temp of 60 degrees with the internal sensors. 60 degrees should not cause too much tyre damage but is high enough for concern. Given that and my feelings about the Tyre Dog temp readings, I run 45 deg as the alarm.
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Follow Up By: Member - Boobook - Friday, May 23, 2014 at 11:51

Friday, May 23, 2014 at 11:51
Thanks Dennis.

Maybe it is brand specific. I guess the main thing is that keeping an eye on the tyres and being aware is the main thing. Regardless of how we do it.
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Reply By: Dennis Ellery - Wednesday, May 21, 2014 at 19:14

Wednesday, May 21, 2014 at 19:14
I have a brochure from Coopers that quotes 4 PSI for sedans and 6 PSI for 4WDs.
For sedans around town I have always used the tyre supplier’s recommended pressures.
For heavily loaded 4 WD I have always used the 6PSI rule.
It’s a rough guide but it’s simple and it suits me - I get an even wear out of my tires.
AnswerID: 532915

Reply By: kcandco - Thursday, May 22, 2014 at 22:05

Thursday, May 22, 2014 at 22:05
As mentioned by others, the sun shining on tyres can lead to a 4% increase in tyre pressure without it even moving. As an experiment I placed a tyre in the shade and then in direct sun. Within the space of an hour the temp of the tyre went from 30C to 80C (yes 80) and pressure increased by 5psi. Like you I was investigating correct tyre pressures. After much reading, I found on a tyre related site that the right way to calculate pressure is to calculate as follows. Load on one tyre divided by maximum load rating of tyre multiplied by max pressure of tyre at max load. IE load on tyre 600kg. max load rating 800kg. max psi 65psi. 600 divided by 800 X 65 = 48.75 psi.

regards Kc
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Follow Up By: Echucan Bob - Friday, May 23, 2014 at 08:55

Friday, May 23, 2014 at 08:55
Kc,

you need the whitewall tyres - they absorb less radiant solar energy.
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Follow Up By: Echucan Bob - Friday, May 23, 2014 at 09:11

Friday, May 23, 2014 at 09:11
"The pressure of a gas of fixed mass and fixed volume is directly proportional to the gas's absolute temperature."

Kc, when the temp went from 30C tom 80C it went from 303K to 353K. The volume of the tyre and the mass of air in the tyre were unchanged. So you would expect the pressure rise by 353/303.

If you started with 30 psi (about 2 bar, or 206 kilopascals) it would rise to 34.9.

Bob
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Follow Up By: Bludge - Friday, May 23, 2014 at 09:55

Friday, May 23, 2014 at 09:55
Kc, slightly tongue in cheek, but is that for crossply, Highway or Light truck tyre?

Both the 4psi or the load calculation is and always will be subject to ambient temperature, direct sunlight and running tyre temperature.

Which is why motor racing warm up their tyres and manufacturers provide COLD tyre pressure figures only.

So like the 4psi rule, the way you load the vehicle will effect the load on the tyre and change the required pressure.

Empty the front may be heavy (engine weight) so front tyre have more load, then a long range tank behind the rear axel will require tyre adjustment from full to empty.

As said earlier this, like the 4psi rule can only be a guideline, in Australia there are too many variables to be absolutely accurate.

TonyV

Cairns FNQ.

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Reply By: Alan S (WA) - Friday, May 23, 2014 at 10:36

Friday, May 23, 2014 at 10:36
Hi All

But is there another 4psi rule?. Because the one I am aware of takes into consideration speed and loading, as both these effect temperature which then also effects pressure.

If at what ever pressure you start at and you travel at a set speed, and the tyre pressure increases more than 4psi then you either need to slow down to reduce heat, reduce load or add more air to reduce tyre flex and heat build up.

Its olbvious to me that this would only work once past initial warm up. As a general rule i use it as a way of determing a suitable speed for the pressure I am running, and being able to reduce heat input into the tyre. You could use what ever pressure difference you wanted, and if you went higher you would have an associated higher speed for the pressure, and a lower pressure differential would give you a slower speed for the pressure.

Alan
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Follow Up By: Norm C (WA) - Saturday, May 24, 2014 at 18:45

Saturday, May 24, 2014 at 18:45
Well put Alan
That's been my understanding of the 4psi
Norm
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