Hot and Cold tyre pressures

Submitted: Friday, Apr 13, 2012 at 18:07
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I can already hear the collective groan but I have a question about tyre pressures. I did search the site for other threads but couldn't find exactly the info I was after and some of the posts were quite dated. I'm a newbie so please forgive me :)

Basically, I know you need to adjust your tyre pressures for different types of terrain based on where you are driving ie: sand, corrugations, gravel etc etc and I have basically been using about 20-25 PSI at the front and 25-30 PSI at the back of my 105 series Cruiser as a rule of thumb and seems to be OK. I have gone as low as 10 PSI to get out of trouble in soft sand a few times. I try to make the footprint look equal on all 4 tyres to take into account any difference caused by the load on the car.

When people recommend a certain pressure for a given terrain is it usually based on the hot pressure? For example, if I have driven for 3 hours on the tarmac and pull over before entering a track to drop my pressures to a suitable PSI to get through comfortably then camp the night and head back along the same track (after the tyres have cooled) would I need to increase my tyre pressure slightly to compensate for the drop in pressure due to the tyres having cooled before I set off? Or is the difference negligible?

Thanks

Craig
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Reply By: blue one - Friday, Apr 13, 2012 at 19:46

Friday, Apr 13, 2012 at 19:46
Mate,
Dunno what tyres you are using. I drive a Patrol which has a similar profile to your cruiser and run BFGs.

Cold set my tyres around 40 PSI for unloaded black top.

Gravel don't change.

Gibber / corrugations down to 26-28 PSI and drop speed to 80kms

Sand anywhere between 18 - 10 PSI depends how dry the sand is.

Recent trip from Sydney to Darwin very hot. Checked tyres after a couple of hours and dropped 8 PSI to bring them back to 40 PSI front 44 back as I was loaded. Once set and knowing the same temps would occur the next day left them where they were.

Mud depends on circumstances, bed type of mud etc.

Cheers

AnswerID: 483031

Reply By: Rockape - Friday, Apr 13, 2012 at 20:18

Friday, Apr 13, 2012 at 20:18
Graig,
GROAN, Ha Ha.

Mate, I use my hand for temps and have never had a problem.

Tyres have changed a lot over the years and i always look at my load and road.

I run my fronts at 30 to 32psi on the highway because that is comfortable, the size tyres I use do it easy and my inbuilt temp gauge says all is ok.

Drive tires I vary from between 30 and 40 psi depending on load.

On dirt I will not change until I strike corrugations or stony conditions. Many dirt roads are better than a highway. Then I will come down to 24 front and 28 back loaded with a max speed of 80 kph.

Sand. I have no problems going down to 10psi. BUT! at that pressure it means not giving it a boot full, driving much above 30 kph or changing direction like a V8 supercar driver.

Trust me we have an inbuilt temp sensor in our hands that is free. Drop the hand on the tyre and it will tell the story. Just allow a bit for the sunny side.

RA.



AnswerID: 483033

Reply By: Allan B (Member, SunCoast) - Friday, Apr 13, 2012 at 23:41

Friday, Apr 13, 2012 at 23:41
Hi Craig,

When setting pressure for a given terrain it is usually adjusted during the drive, so the tyre would already be at an elevated temperature. Having said that, the "recommended" pressure is not highly accurate and for any given situation would vary by the recommendations of the person giving it. So hot or cold would simply be another variant in the rather loose equation.

In the example you referenced, there would be little point in readjusting the 'cold' tyres for the return journey because they would quickly warm up to a similar temperature as on the inward journey. In any case, the change in pressure between hot and cooled would not be substantial.

The increase in tyre pressure is caused primarily by the flexing of the rubber with the tyre rotation. When running at reduced pressure there would be more flexing but at a reduced speed (hopefully) so the temperature rise would be similar to that at higher pressures and speeds.

Cheers
Allan

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AnswerID: 483040

Reply By: Pary01 - Saturday, Apr 14, 2012 at 00:20

Saturday, Apr 14, 2012 at 00:20
I have been travelling around Aust with an off road caravan using a 200 series Cruiser and I use the following .
On the Black Top I run 40PSI on the steer and 42 PSI on the drive with 40psi all round on the caravan. usually travelling at 95 to 100 kph
When on corrugation or rough rocky tracks I drop the pressures down to 32psi all round and the speed will vary on the road condition some roads or tracks have had me doing 16kph but as an avg I keep the speed to no more than 65 to 80 Kph depending on the road condition . to date I have had no issues with my tyres . I have driven The Oodnadatta track , Savannah way and other rough tracks and back roads through the Gulf country up north. Mostly we bush camp and spend more time off the beaten track than on I have done 25 thousand plus kilometres this past year and have had one puncture. and that was early in the tyres life on the track to Dalhousie Springs from Hamilton Station .

I have tyre sensors fitted all round and note that at these pressures and associated speeds the difference form cold to hot is 6 psi the pressure rarely go beyond this variance. when I have to adjust tyre pressures when hot I work on the 6 psi rule I.E if hot at 46psi and want to drop to 32psi equivalent I drop to 38psi when hot and check the next day when cold to confirm. usually it works out to be ok.
All tyres are wearing well and show no signs of damage. I am using Goodyear Silent Armour tyres made in the USA.

Also I find the caravan rides much better when on dirt at the lower pressures.
AnswerID: 483042

Reply By: The Bantam - Saturday, Apr 14, 2012 at 10:39

Saturday, Apr 14, 2012 at 10:39
There is a commonly held idea of "the 4 psi rule", where the tyre preasure is measured cold and then the vehicle is run for a period of time and the preasure measured hot.......suposedly a preasure increase of 4psi is suposed to indicate that the starting cold tyre preasure is appropriate for the load and the driving done.

BUT..sorry its so full of holes to be worthless in my opinion..there are so many uncontrolled variables that it cant be reliaed upon.
I have tried it on my vehicles and it does not work for me.

One single concept that very often people don't grasp is,
Appropriate tire preasure is dependent on the type and size of tyre used and the load that it is carrying.

So....if you have 3 identical vehicles with identical loads but differing type and size tyres the preasure required on each will be different.

There are only 2 things that can be relied upon.

1, The published load V preasure tables for that size and type of tyre...... and it will be pretty much brand independent.

2, Hub centre height.

The tyre and rim association of Australia, Standards Manual, contains load V preasure tables for most tyres, every tyre shop should have one.
OR this information should be available from the manufacturers.

If you weigh the loaded vehicle and get an axle split form a weigh bridge...most tips have a weigh bridge and wont charge you to just drive over and read the display.

take the indicated figures for each axle divide by 2 ( 4 if you are running duals) and look up the load V preasure table for the specific tyre.....this is the appropraite on highway minimum tyre preasure at that load and is good for all road speeds up to the speed rating of the tyre.

Once you have the correct preasure in the tyres, you can measure the hub centre height, with a ruler on flat hard ground.

From this point if you inflate your tires to maintain that hub centre height you will maintain your tires in line with the prescribed load V preasure tables.

This is well documented and very accurate.

ALL this should be done COLD.

The only tyre preasure we can rely upon is cold inflation preasure.

The preasure in a tyre varies, mostly due to heat, this is normal and benificial, if a tyre is corretly inflated cold it should never be adjusted hot.

NOW to tire deflation for terain and speed.

The overriding factor here is SPEED, the only thing that allows us to safely reduce tyre preasures below the minimums from the load V preasure tables is a significant reduction in speed.

Lost of people are looking for some sort of definitave guide to the whole deflation thing, and there is not and can not be one.
Every tire will respond differently and vehicle loads vary so much.

Flotation tyres ( 31x10.5R15s) are typicaly a soft sided carcase by design and belly out more than light truck tyres (255/70R15LT) which tend to stand up straigher in the sidewalls.

You have to rely on look and feel..... as you reduce preasure the tyres will look flatter and feel less stable.

Above all is the speed reduction..if you are traveling above 80KPH there is no question you should be running the at last minimum specified preasures.

typicaly reducing by 20 -25% for rocky ground 30psi from 40 or 24psi from 32 seems reasonable and reducing by 40 to 50% for sand 18-20psi from 40 or 15 to18psi from 32 likewise.

Bit remember the speed reduction, reduced by 25% keeping under 80kph would be wise and reducing by 50% staying under 60kph would be wise.

But you have to look and feel to see how it works on your rig as it is loaded.

There is a guy arround on the forums who after much trouble & dogged persistance managed to get some figures on deflation V load V speed from one of the tyre manufacturers.....
He has pages of well reasoned engineering discussion on the matter...BUT...I believe the figures he was given were overly conservative.

cheers
AnswerID: 483067

Follow Up By: BFreer - Saturday, Apr 14, 2012 at 13:58

Saturday, Apr 14, 2012 at 13:58
Pretty well summed up bantam - the blog you most likely refer to was by Rob D (NSW) labelled " are you operating your tyres beyond their design limit" - a very well researched article that allowed me to get my mind around the black art of tyre pressures.

Like you I don't put much faith in the 4 psi rule - everybody knows about it but nobody knows where it came from, which is a worry. To be fair though I understand that it was reported by a Euro tyre manufacturer for euro passenger vehicles, however I might stand corrected.

Following on from Rob D's research, my tyre rules are;
1 : base pressure is taken as cold, ie, first thing in the morning.
2 : I expect the base pressure to increase by 1 psi for each 6 degree c ambient temp increase, so by about midday they will be reading about 2 psi higher just because of the warmer ambient temp.
3 : for LT tyres, I expect psi to increase about 2 psi after 20 minutes running at 80k/h another 2 to 4 psi @ 100 k/h.

So, running @ 100 k/h on a warm day I expect the tyre pressure to be about 6 - 8 psi greater than the base psi. - pretty much as bantam stated Next morning they should be back to the base pressure, and should not be bled during the day.

Reduce the speed and I reduce the psi. about 10% per 10k/h, with lower limits of course.

To get an accurate base pressure relative to load without the luxuary of tables, I divide the actual tyre load by the tyre load rating, and then multiply by the max psi rating of the tyre - seems a good starting point.

Hope this helps,
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FollowupID: 758292

Follow Up By: The Bantam - Saturday, Apr 14, 2012 at 15:19

Saturday, Apr 14, 2012 at 15:19
yeh I ran a preasure rise test on the 235/75R15s on my hilux and on a fairly warm day I could not get 2psi preasure rise out of them at the preasures off the table after 1 hour at 100kmh.

It occurs to me that if the tyre is lightly loaded in comparison to its capacity even though it is at the specified preasure for load, it will not rise as much as a tyre running higher in its load curve

In addition some of the load V preasure tables are very fine and show quite small variations...like the 31x10.5R15 floataion tyre shows a load difference of 75Kg for an increase in tire preasure of 25KPA (3.6 psi).

What is known and published in the way of load v preasure is very well known and quite accurate, far more accurate and consistent than the so called "4psi rule" and all its unaccounted for variables.


Above all we have to listen to what the vehicle is telling us.......how does it feel on the road.

I have had some vehicles that have been very sensitive to optimum preasure....get the preasure just right and they drove and handled significantly better....but some people simply are not good at subjective assessments or don't know what they should be observing.

cheers
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Follow Up By: BFreer - Saturday, Apr 14, 2012 at 16:19

Saturday, Apr 14, 2012 at 16:19
I am sure your comments about the load relative to capacity is right - I also think that the size of the tyre, ie how much air they hold, is also a factor. My vehicle is an AWD VW campervan which runs fairly well loaded.

It runs 215/60 17 LT tyres that hold bugger all air even at 40+ psi, - I am sure that they increase more at sped/temp than do larger tyres
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FollowupID: 758309

Follow Up By: Allan B (Member, SunCoast) - Saturday, Apr 14, 2012 at 16:28

Saturday, Apr 14, 2012 at 16:28
Hey Bantam,

As someone who espouses such knowledge of tyre inflation, I'm surprised that you repeatedly talk of "preasure". What is this "preasure" thing? A "pleasant persuasion" perhaps ? LOL

If you read what others are typing, do you not wonder why everyone else writes "pressure" ?

Cheers
Allan

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Follow Up By: The Bantam - Saturday, Apr 14, 2012 at 19:07

Saturday, Apr 14, 2012 at 19:07
If ya what to nit pick abouts spelling go to school teacrers forum.

I and the vast majority of other posters on forums world wide are not about to spell check every post

If spelling and grammar warries you don't go on internet forums.

cheers
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FollowupID: 758330

Follow Up By: Dennis Ellery - Saturday, Apr 14, 2012 at 19:28

Saturday, Apr 14, 2012 at 19:28
Gaud how complex can you make it.
Go to a Coopers Dealer – they have a free hand out on tyre pressures for different terrains.
The 4PSI rule for sedans and the 6PSI rule for 4WD is a good way to go – also keep an eye on your temperatures. If the tyres are getting too hot - increase your pressure or decrease your speed.
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FollowupID: 758332

Follow Up By: Dennis Ellery - Saturday, Apr 14, 2012 at 19:32

Saturday, Apr 14, 2012 at 19:32
And for Allan - yooz is just a sirry old plik
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Follow Up By: Allan B (Member, SunCoast) - Saturday, Apr 14, 2012 at 20:27

Saturday, Apr 14, 2012 at 20:27
Geez, touchy too Bantam! Not LOL
If I were to question your expressions about tires you would probably try to vindicate your argument, but refer to an obvious spelling error and you trot out puerile responses. By the way, it is "teachers". ROFL


Cheers
Allan

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Follow Up By: The Bantam - Saturday, Apr 14, 2012 at 20:40

Saturday, Apr 14, 2012 at 20:40
Go onto any internet forum and once in a while there will be some nit picking twit that will complain about spelling and grammar, the response is always the same.
Who cares, such complaints about spelling and grammar are simply pointless and irrelivent and nothing more than time and badwidth wasters.

what matters is the subject matter and the discussion.

cheers
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Follow Up By: The Bantam - Saturday, Apr 14, 2012 at 20:47

Saturday, Apr 14, 2012 at 20:47
As far as Dennis' comment.....there are always people who want a simple answer where there realy is not one that is adequate.

Cooper are one of those espousing the 4 psi rule, none of the major manufacturers do.

its not that hard.

Weigh the vehicle and use the load V preasure tables for the tyre in use to get a reliable and consistent result.

If you want you can use hub centre height to work from there.

You need to reduce speed if you reduce preasure.

cheers
0
FollowupID: 758358

Follow Up By: Member - Cruiser74 - Sunday, Apr 15, 2012 at 14:48

Sunday, Apr 15, 2012 at 14:48
Hi,

Once again I am overwhelmed with the amount of people on this forum who are so willing to spend their valuable time sitting down to write a detailed response to help out. I can't tell you how much it's appreciated and I hope at some time I will be able to help out as well.

The whole tyre pressure thing is a bit confusing for a newbie like me but it's becoming a lot clearer. The main thing being the adjustment in speed with relation to pressure. An important point! I am glad I have not dropped down to 10 PSI on a flat straight stretch of beach and floored it yet :) Looking forward to weighing the Cruiser when it's loaded and figuring out what works best for my vehicle, tyre combination.

Again, a huge thanks for all your advice (whether spelt correctly or otherwise!)

Craig
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FollowupID: 758423

Follow Up By: The Bantam - Sunday, Apr 15, 2012 at 16:55

Sunday, Apr 15, 2012 at 16:55
Thanks I am glad it helps someone.

remember there are two major issues when reducing tyre preasure.

If we reduce tyre preasure below recommended we both, reduce the tyres ability to carry load and its stability on the rim and thus the stability of the car.

in the worst case an underinflated tyre may roll off the rim in an agressive cornering situation.
Once the tyre rolls off the rim it will deflate suddenly and the wheel may dig in.

At high speed this can result in a diagonal end for end roll over, possibly the most vicious and damaging roll over in vehicles.

This sequence of events has been implicated in some very serious beach accidents.

So as well as reducing speed when running reduced tyre preasures we should also be less agressive in cornering.

It is reasonable to run very low tyre preasures in soft sand, but it becomes very easy to roll a tyre off a rim, so driving gently at lower speeds is required.

cheers

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FollowupID: 758441

Reply By: Member - Paul B (WA) - Sunday, Apr 15, 2012 at 12:00

Sunday, Apr 15, 2012 at 12:00
If your 105 series is full time 4wd, when you're just running about town unloaded, maintain your front tyre pressures and reduce your back tyre pressures. That way you'll get a better ride and better wear on the back whilst looking after your front tyres which are doing almost all the turning, most of the braking and providing a fair bit of the traction.

On my Prado I got nearly 100,000 km out a set of Wranglers at 35 psi on the front and 30 psi on the back unloaded, around town, which was a fair bit of its use. Laden, 40psi all round.

On tracks 20 psi all round 60 km/h max

Worked for me. Cheers

PS. Hope the spelling & grammar meets your exacting standards Allan ;)
Paul B Kalgoorlie

Do your best, have fun & s/he with the most friends wins!

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AnswerID: 483176

Follow Up By: Allan B (Member, SunCoast) - Sunday, Apr 15, 2012 at 12:54

Sunday, Apr 15, 2012 at 12:54
Paul, go read again what I said. I made no reference to "spelling".
Seems that I need to set it out in one syllables for you.

Bantam was expecting us to give credence to his deep and lengthy expression about tyre pressures and I questioned his knowledge and expertise (whoops, that's three syllables) as he was not even able to get the core word "pressure" correct even though all the replies did so and presumably all his sources. I was being critical of his "facts" not his spelling.

However, it does seem that anyone who has some difficulty with spelling immediately becomes offended and defensive when a correction is offered rather than accepting it as learning information. If a criticism of the choice of say tyres or fridges is made then discussion ensues but not if you say "you spelled that one wrong mate". It is like an assault on his manhood!

Cheers
Allan

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Follow Up By: Member - Paul B (WA) - Sunday, Apr 15, 2012 at 13:34

Sunday, Apr 15, 2012 at 13:34
Who's touch now!
Paul B Kalgoorlie

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Reply By: Member - Bucky - Monday, Apr 16, 2012 at 06:24

Monday, Apr 16, 2012 at 06:24
Girl's
Settle down



AnswerID: 483249

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