Hot and Cold <span class="highlight">tyre</span> <span class="highlight">pressures</span>

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

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.

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.

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.

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

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 ;)

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