Tyre Pressures . . the 4psi rule. I kinda like this

Submitted: Friday, Apr 05, 2013 at 13:06
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I posted this purposely separate to the tyre debate lower down this forum list.

This makes sense to me. But pls note it is for BITUMEN road use ONLY. But that's a good starting point as most of us will start our trek on the bitumen. This rule would automatically work on front and rear tyres & trailers and would also work with the variations in weight loads. And we all have different weights in different vehicles on different tyres.

The rule:
At the starting point, vehicle loaded, inflate the cold tyres to the recommended tyre pressure of your vehicles tyre placard. Then to determine if you have the correct pressure for a given load check the COLD pressure again and note the reading.

Drive the vehicle for several kilometres on bitumen at regulation speed until the tyres have heated up to operating temperature. This temp will also be influenced by the ambient temp of the day and road surface . . . . but that's OK. That's what you're driving on.

Now, say after 15 minutes, check the tyre pressures again and compare this to the cold starting temps. Ideally the tyre pressure should read about 4 psi above the cold pressure.

If the pressure is more than 4 psi above, the tyre is overheating and more air should be added. This is because there is too much friction from the road surface and side wall flexing.

Conversely, if the tyre pressure is less than the 4psi allowable difference, then there is too much pressure, tyre pressure is high and needs to be lowered.

It would make sense that this would also be a starting point FROM which to lower pressures when encountering gravel or sand. My theory is that you lower the pressure FROM a certain level which suited your vehicle on the bitumen. In other words you might drop 10psi for gravel or drop 20psi or even 25psi for sand. Telling everyone that running 15psi is the correct level for sand is not totally accurate . . . . it depends on the tyre brand and weight of the vehicle, and the type of sand, etc etc. Same theory for gravel.

It's a theory, that's all. But a good individual starting point.

Ciao
Cocka




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Reply By: Lex M - Friday, Apr 05, 2013 at 13:12

Friday, Apr 05, 2013 at 13:12
Not as simple as you would think. I've measured tyres on the sunny side of the vehicle as much as 20 degrees hotter than the shaded side. That blows the 4psi rule away.

Do it properly. Go to a weighbridge and consult load pressure tables for the tyres in use.
AnswerID: 508273

Follow Up By: Member - Salt grinder - Friday, Apr 05, 2013 at 15:16

Friday, Apr 05, 2013 at 15:16
Sure. Tyres will be exposed to varying temps as you say. They will also be exposed to day and night time temperature differences, and the sun moves across the sky to the other side. I'm not suggesting that you run around changing pressures all day according to the weather/temps.

As you estimated the temp at say 20 deg difference, it would be interesting to see how much that in fact alters the pressure. Did you check that?? I would find that of interest. Could do it myself maybe.

I'm merely stating this as a helpful guide to some one who has no idea and asks a sensible question. It does need to have a little common sense applied to the application . . . .

I have not yet seen a definitive book like "The Dummies Guide to Tyres"
So maybe you have a better suggestion . . . . . . ?

Ciao
Cocka
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Follow Up By: Member - Mfewster(SA) - Friday, Apr 05, 2013 at 15:53

Friday, Apr 05, 2013 at 15:53
Out of curiosity, was that difference in tyre temperature between the sunny side and the stationary side taken when the car had been driven for some time or had been standing stationary? Stationary, I understand. If you are driving I'd have thought the difference would be negligible.
While anything left standing in the sun will be hotter than something in the shade, once the tyre is rotating a lot of other factors, mainly the heat generated by wall flex, are responsible for temperatures that will be considerably higher than those generated by standing in the sun.
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Follow Up By: Lex M - Friday, Apr 05, 2013 at 16:31

Friday, Apr 05, 2013 at 16:31
Tyre temperature measured after driving, not stationary.
An extreme example where the sun had been on one side of the vehicle only.
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Follow Up By: R Send - Saturday, Apr 06, 2013 at 08:29

Saturday, Apr 06, 2013 at 08:29
LexM,

Re your statement "Go to a weighbridge and consult load pressure tables for the tyres in use".

Can you advise where these tables can be accessed please? The only sources I can find are industry member only associations.

I have Cooper AT3s (LT 275/65 R18) on the LandCruiser and Bridgestone Dueller D697s (LT265/75 R17) on the van, and we've been on a weighbridge so I know what each weighs when loaded for a trip.

I note that the Coopers have the following info on the tyre wall: Max load 1550kg at 80 psi cold. Can this data be pro-rated to suit the particular loading of my cruiser?

Phil
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Follow Up By: Rockape - Saturday, Apr 06, 2013 at 09:20

Saturday, Apr 06, 2013 at 09:20
Phil have a good read here on tyres. This may help you out.

TYRE INFO
1
FollowupID: 785805

Follow Up By: R Send - Saturday, Apr 06, 2013 at 10:06

Saturday, Apr 06, 2013 at 10:06
Rockape,
Thanks for that - at last something I can read and digest and draw my own conclusions from!
Phil
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Follow Up By: The Bantam - Saturday, Apr 06, 2013 at 19:03

Saturday, Apr 06, 2013 at 19:03
You should be able to get load ve pressure tables for whatever tyre it is from where you baught the tyre.

As a worst case the dealer should have a coppy of the tyre & rim association standads manual very close at hand.

In addition I am sure if you contact the manufacturers customer serice department they will supply you tables.

I note that the 2 tyre sizes lited in folowup 4 are not in my slightly out of date 2010 tyre standards manual they may be in the current one.

needless to say we should be requesting load v pressure tables when purchasing tyres.

NOW here is an interesting thaught.

If you vehicle is fitted with the factory standard tyres as listed on the tyre plackard.....your vehicle is unroadworthy unless the tyres are inflated as specified.....and....you'll like this.....the vehicle is unroadworthy if it is not fitted with a readable tyre plackard.

Cheers
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Reply By: ben_gv3 - Friday, Apr 05, 2013 at 13:17

Friday, Apr 05, 2013 at 13:17
The old 4psi rule...I think Noah used it when transporting material for his Ark.

Cooper Tyres has a little handbook stating that for offroad use then 6psi (or thereabouts) is a better measure. I've got a copy of it somewhere.
AnswerID: 508275

Follow Up By: The Bantam - Saturday, Apr 06, 2013 at 19:06

Saturday, Apr 06, 2013 at 19:06
I have a copy of the Cooper manual somewhere...it specificaly states that the"4 psi rule" is for highway use only.

cheers
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Reply By: Grant L - Friday, Apr 05, 2013 at 15:06

Friday, Apr 05, 2013 at 15:06
Wish someone would put the lid back on this can of worms!
AnswerID: 508280

Reply By: olcoolone - Friday, Apr 05, 2013 at 15:54

Friday, Apr 05, 2013 at 15:54
My consensus on tyre pressure are run whatever you feel comfortable with, most times I don't see the point in running low pressures on good dirt or hard compacted clay capped roads....... unless there are rocky or it is very sandy........ why don't people start adjusting tyre pressures on some sealed road as the road condition can be be pretty poor at the best of times.

As you said all tyres and all vehicles are different and what may suit one may not suit the other.

Plus if bagged out to much the side wall came become very vulnerable to damage.

And as for tyre placards..... our 200 series landcruiser weighs 3700Kg and the tyres are "E" rated meaning at maximum load you can run 80psi...... when we ran Mickey Thompson MTZ's at 25psi they were bagged out a lot but with the new tyres and stronger sidewalls at 15psi they are about the same.

It's funny..... years ago when I had rally cars we never worried about tyre pressures to much for different surfaces and with speeds nudging over 200Kph we never had a problem..... sure we use to destroy tires and in some case within 20 kilometers.
AnswerID: 508284

Follow Up By: Rockape - Friday, Apr 05, 2013 at 17:15

Friday, Apr 05, 2013 at 17:15
Yes!
spot on olcoolone. Pulled into Birdsville a few years back and some how tyre pressure came up at the info centre. The lady said what pressure did you have in your tyres from Windorah to here. I said 38 in the back and 33 in the front. She shakes her head and mutters something about people destroying tyres. I said to her that road at the moment is like a highway why the hell would I drop pressures, the road is better than many bitumen roads.

I know back in my trucking days if it was hot and I will just say I was running heavy, then I would drop to 90k/h to save the tyres. If it started raining then bang straight back up to the dollar if the truck would hold it.

Tyres can actually take up to 100k to reach their equilibrium temp and pressure where the pressure increases enough for the tyre temp to stabilise due to less wall flex. Road camber also changes the pressures and temps of opposing tyres due to the load difference on each side of the vehicle, and so it goes on and on.




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Follow Up By: olcoolone - Friday, Apr 05, 2013 at 20:18

Friday, Apr 05, 2013 at 20:18
Talking about trucks...... I don't know many truckies that air down when they hit the dirt, our service vehicles run 60psi at maximum weight on bitumen and dirt and I have never had a tyre destroy itself.

We have just done a trip that had 700K of varied dirt roads, we ran 36psi in the fronts and and 38 in the rear and being new tyres they still had square edges on the tread block....... once on the blacktop we went up to 40 in the front and 44 in the rears.

I'm starting to run higher pressures on dirt then I use to, I have not seen no difference in tyre wear or tyre conditions but I have noticed the Landcruiser is a lot more stable and feels better.

I think this tyre thing is a bit of a urban myth now, before with older technology it may of been true but now not so.

Your comments with dropping speed is a good idea and what we do.... if it gets rough or it's extremely hot we slow down.

I've got better thing to do than worry about tyres pressures unless really needed......

The only time we drop tyres pressures is if the going gets really tough.
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Follow Up By: Off-track - Friday, Apr 05, 2013 at 22:21

Friday, Apr 05, 2013 at 22:21
100% agree. Load of proverbial lowering pressure to prevent punctures.
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Follow Up By: Member - Salt grinder - Saturday, Apr 06, 2013 at 11:20

Saturday, Apr 06, 2013 at 11:20
G'day Off-track

I agree, in general, with your comment "to prevent punctures". However it can still be argued that lower pressures on corrugations is easier on vehicle suspension and the improved footprint better adhesion to the surface.

I bet we have all suffered those corrugations where you are torn between either 15 kph or 60 plus whatever kph up to suicide speed. The prime thought is that what ever you choose . . . . . you have to respect your vehicles capabilities, after all, it has to get you back home.

I reiterate . . . . The comments in my original post at the beginning were and are only, a guide at the beginning of a trip.

Ciao
Cocka

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Follow Up By: olcoolone - Saturday, Apr 06, 2013 at 12:01

Saturday, Apr 06, 2013 at 12:01
Salt grinder..... re tyre pressures and corrugations, corrugations are formed from vehicle suspension resonance, a good suspension system has a dampener (shocker) that controls and reduces the springs resonance.

If you had an under inflated tyre and the tyre was flexing more than the controlled spring then the tyre flex becomes uncontrolled and will just keep on bouncing......... much like a spring with no dampener.

Corrugations usually form by when a vehicle at speed hits a dip or a rise in the road or from sudden acceleration or deceleration, as the vehicle comes back down it starts to bounce compacting the ground.

Corrugations are not only found on dirt roads, many bitumen road and railway tracks have corrugations caused by uncontrolled vehicle suspension resonances.

The real term for it is washboarding.
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Follow Up By: Member - Salt grinder - Saturday, Apr 06, 2013 at 13:45

Saturday, Apr 06, 2013 at 13:45
I'm going to post the following attachment as a separate forum article.
If for no other reason that I find it fascinating that they go into so much detail. Take from it what you will . . . . I never managed to get to the end. . . . . . maybe my mind is not meant for this much science.

Enjoy



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Follow Up By: olcoolone - Sunday, Apr 07, 2013 at 09:10

Sunday, Apr 07, 2013 at 09:10
The paper would of been written for civil engineers to help them build better roads and reduce maintenance costs.

If it helps lower the cost of the road over it's life then it worked.
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Reply By: ozjohn0 - Saturday, Apr 06, 2013 at 10:08

Saturday, Apr 06, 2013 at 10:08
The 4 PSI rule works well for LT and car road tyres at an ambient temperature of 20c.
Hotter days and therefore hotter roads usually allow for an increase of up to 6 PSI.
OJ
AnswerID: 508321

Reply By: evaredy - Saturday, Apr 06, 2013 at 13:08

Saturday, Apr 06, 2013 at 13:08
I was told that when to use the following as a guide. Deflating your tyres, regardless of weight, you should deflate them to roughly one third of the sidewall.
AnswerID: 508327

Follow Up By: ben_gv3 - Saturday, Apr 06, 2013 at 13:55

Saturday, Apr 06, 2013 at 13:55
How does that work?

Do you mean deflate until you see the sidewall height reduce by 1/3?

I've run down to 10psi and hardly see the tyre bagging out due to a very light 4WD and 10 ply rated tyres. I've also seen/measured normal passenger tyres at half the normal road pressures but still look like they're at full pressures. This was due to a small puncture.
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Reply By: The Explorer - Saturday, Apr 06, 2013 at 13:19

Saturday, Apr 06, 2013 at 13:19
Hi

More discussion on same subject in some previous threads for those interested..

Thread 91867 - Feb 2012

Thread 14146..way back in 2004..

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Reply By: The Bantam - Saturday, Apr 06, 2013 at 19:57

Saturday, Apr 06, 2013 at 19:57
The first and most important thing we must understand is...when a tyre is inflated to the correct cold inflation pressure for its load it is a self equalising system....a hotter tyre requires more pressure to support the same weight.

I was reading a research document I linked in one of the other discussions.

One of the tyre manufacturers did some ( quite a lot actally) testing and they consider that a hot tyre may require as much as 15% to 20% more than its cold inflation pressure when running hot....peer comentators in the paper say that may be conservative.....so 20% of 40 psi is 8psi...so is it an 8 psi rule.

This extra pressure is provided by the air heating up inside the tyre...remember the self equalising system.

the reason for this was, trucks fitted with tyre pressure managment systems had the upper limits set too low .....and the systems where deflating the tyre unnecessarily resulting in pressures too low to correcty bare the load..

The use of these tyre pressure managment systems in heavy transport has seen CONsiderable reseach into tyre pressures and the effects.....there are several extensive papers published on the matter.




NOW my problem with the 2,4,6, whatever "rule" is that it is so ill defined and vaguely discribed.....AND....there are so many uncontrolled variables.

tell me.
1/What influence does ambient temperature this 4psi rule.....does it work below freezing, does it work in situations of very high air and road temperature..

People in very cold climates actually have a problem with pressure decrease due to heat being drawn from the tyre by the road surface.
In very hot areas, the tyre pressure will go up just sitting there...inflate the tyre in the shade then park in the sun where the tar in melting

2/ How long do you have to drive to get the tyres up to temperature, some say 15 minutes, others say 2 hours....does 1 influence this.

3/ how fast do you have to be driving for this to work and how does this influence 2 and 1

4/ What influence does the type of driving have on this so called rule....driving along a smooth straight road at a constant speed, puts less stress on a tyre, than hard acceleration and constant hard cornering..

5/What influence does rim type and and heat disipated by that rim or conducted by that rim from the brakes have on this so called rule.

6/ What influence does the size, type and construction of the tyre and how heavily loaded that tyre is in comparison to is loading capacity have on this rule.

7/ how does the proportion of the inflated pressure that the 4 psi represents effect the rule. Is it the same for a tyre that is load v pressure correct at 100 psi as it is for a tyre correct at 25psi.

8/ given a change in speed, how does being away from the pressure from the load v pressure effect how this rule works

The answer to all the above questions is that will all effect the so called rule wildly, so much so that it will be so massivly erronious that it can not be relied upon.

On my vehicle with the tyres correctly inflated,( weighed and inflated to table) in moderate weather, I can drive all day on the highway at 100 kmh and not get 2psi pressure increase....and still people look at my tyres and say they look under inflated.

Consider my mate Mikes Dihatsu...he baught it fitted with big 8 ply tyres....he checked his tyres once and found that one of them had almost no air in it...but looked no different and drove no different to the others and the tyre was stone cold like the others...does 4psi rlule work for him.


Consider too that the some of the pressure tables will show around a 50KG difference in load for 4psi change in pressure...ya recon the 4psi rule is that accurate.

Consider also, many of the load V pressure tables show that load V pressure is far from linear, and different tyre types have different curves.

So If the 4psi rule was to work in the linear portion of the load v prerssure curve, it can not work in the non linear parts of the curve.


There is a very good reason the accepted and approved practice revolves around correct load V pressure measured cold.

OH and remember this rule does not work in aired down situations...Copper say it does not.


If this so called "rule' does not work in every case and under all conditions it is not a "rule"

cheers
AnswerID: 508354

Follow Up By: The Explorer - Saturday, Apr 06, 2013 at 20:14

Saturday, Apr 06, 2013 at 20:14
Hi

Yep - should be referred to as a "guideline" with reference to background reasoning. The term "rule" suggests (to some at least) universal application under all circumstances, obviously not the case here.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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