Tyre Pressures Experts Please

Submitted: Wednesday, Apr 22, 2009 at 21:31
ThreadID: 68120 Views:2831 Replies:9 FollowUps:6
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Here goes. Cruiser is loaded to max legal weight Van weighs 2800kg
Have Cruiser tyres pumped to 38lb Stone cold at 10deg at 8am this morning.
Later in day the fronts are running at 41.5 and backs at 46.2 Fronts at a bit over 25deg and backs at about 36-38 deg.
Caravans are pumped to 50lb at same time
later l/h side pair are at 57-58lb r/h 54lb and are on sunny side.
Cross wind quite strong from right.

Deduction Wind causing more load on far side causing higher pressure.

Coming down the Adeaide hills using gears to slow rig. Back cruiser tyres got up to over 60deg but no increase in pressure.

Coming down hill caused extra weight on rears from van weight pushing forward.????????????????????????????

Interestingly I pumped all tyres carefully when I left Qland and the further south I get the lower the pressures got .
Eg the cars dropped from 38 down to 33 south of Sydney and thru Canberra and Melbourne etc.
However they usually hardened up to about the same pressure.

Also found running with full width stone guard on the car the rear tyres heated up to over 60deg that day thru midly hilly terrain.

Opinions please.
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Reply By: Ozhumvee - Wednesday, Apr 22, 2009 at 22:18

Wednesday, Apr 22, 2009 at 22:18
I used to run the cruiser at around 40 front and 45 rear when running around town lightly loaded, same temp front loaded and up to 55 rear when loaded.
Looking at your hot temps I'd put maybe 5 -10 lb more in the rears with a load and when towing, you might find it a bit more stable with higher pressures in the back.
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Reply By: Robin Miller - Wednesday, Apr 22, 2009 at 22:20

Wednesday, Apr 22, 2009 at 22:20
Hi Graham

You have lost me a bit with all those numbers , but static weight makes no difference (within normal range of our use) , temperature is what does make the difference .

Its reasonable to say that load causes the tyres to run hotter upping the pressure, and wind in turn cools them.

Overall it seems to me like your tyres are to low in pressure , but you don't mention there size so can't be sure.
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Reply By: Lex M (Brisbane) - Wednesday, Apr 22, 2009 at 22:44

Wednesday, Apr 22, 2009 at 22:44
I'd reckon your front pressures are about right and your rear too low. What's the handbook or tyre placard recommended pressure?
AnswerID: 360984

Reply By: Horacehighroller - Wednesday, Apr 22, 2009 at 23:49

Wednesday, Apr 22, 2009 at 23:49
There's a number of questions.

Fronts are about right as the increase with use is about 10%
Backs are clearly under inflated by (as suggested above) about 10 psi.

In undulating terrain you must consider that that brake heat will be dissipated through the wheels & tyres.

As for the van the wind would be an influence but I've always found that the left tyres ( on all vehicles) run hotter (I assume due to the edge of the road generally being more broken/rougher than the middle).

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Reply By: Wayne (NSW) - Thursday, Apr 23, 2009 at 05:19

Thursday, Apr 23, 2009 at 05:19

I would say that the tyre monitor that you are using is giving you false information.

If you are able to quote temperature and pressure and the readings are all over the place, it is not so much the tyre but the unit sending you the information.

AnswerID: 361005

Follow Up By: Member - Graham H (QLD) - Thursday, Apr 23, 2009 at 08:30

Thursday, Apr 23, 2009 at 08:30
They are not all over the place and have checked with a manual gauge to make sure
They get up to what I said and stay there and only vary according to the terrain or driving canditions as coming down the long hill

Sorry forgot to say have new BFG AT's 285 x 70x 16.

FollowupID: 628755

Follow Up By: Wayne (NSW) - Thursday, Apr 23, 2009 at 09:14

Thursday, Apr 23, 2009 at 09:14

I thought you might have been using an in vehicle tyre monitor system.
I had a bad experience with them as they showed a lot of variation in temps and pressures.

Checking the temperature and pressure with a manual gauge is the best way.

I can understand how the pressure can be checked, but how are you checking the temperature of the tyre?


FollowupID: 628764

Reply By: Sand Man (SA) - Thursday, Apr 23, 2009 at 07:19

Thursday, Apr 23, 2009 at 07:19

Tyres should increase by around 4 psi as they heat up.
Anything more than a 4 psi increase and there is not enough pressure in them when cold.
If less than a 4 psi difference, they are over-inflated and you should let a little air out.

I would use a manual tyre pressure gauge while adjusting, then you will get consistency.



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AnswerID: 361007

Reply By: Saharaman (aka Geepeem) - Thursday, Apr 23, 2009 at 07:19

Thursday, Apr 23, 2009 at 07:19
Hi Graham,

One of the aspects of tyre care and maintenance that is usually overlooked is the quality of air in the tyre itself. How many service stations or even tyre outlets would service their compressors frequently to remove water. Probably very few. The fact is the quality of air we put in our tyres is probably very poor and varies significantly between inflation points. The quality of air directly effects tyre performance.
A rubber tyre is like a membrane, through which air permeates As a result, the air slowly leaks out through the rubber walls, which leads to under-inflation. The rate of this loss can vary between tyres (even on the same vehicle). As we know underinflation can contribute to increased tyre wear and vehicle performance. In addition if the compressed air contains high levels of moisture this not only effects the maintenance of tyre pressure but can also accelerate the corrosion of the tyre rim (with repercussions later on).
More significantly the higher the moisture content in a tyre's air chamber, the greater the pressure build-up as a tyre increases in temperature. In other words one tyre may have poor air quality (ie higher moisture) than its companions and under heavy load will built up a lot higher pressure than the others. Water vapour also causes oxidation with the rim, which can cause leakage through the valve core and an uneven surface for the bead to seat, and seal. Water vapour can also lead to the eventual corrosion and or weakening of the steel strands in the tyre.
Just another aspect of tyre care to consider


AnswerID: 361008

Follow Up By: Member - DAZA (QLD) - Thursday, Apr 23, 2009 at 07:36

Thursday, Apr 23, 2009 at 07:36

I agree re: Condensation / Water in the air lines, just recently installed several Water Traps in compressed air lines for client with fleet of B-Doubles, they were concerned with water infiltration into truck and trailer tyres.
FollowupID: 628744

Follow Up By: Member - Graham H (QLD) - Thursday, Apr 23, 2009 at 08:37

Thursday, Apr 23, 2009 at 08:37
Thank you I am aware of all of that and the tyres were all pumped up with the same pump when mounted on the rims.

I pumped them up yesterday morning using my own .
ARB compressor.
They have never been pumped up other than that.

One van tyre has been replaced and still seems to hold simillar pressures as the other three.

Am perturbed as to why the rears heated up so much coming down the hill slowly.

FollowupID: 628758

Follow Up By: Horacehighroller - Thursday, Apr 23, 2009 at 10:55

Thursday, Apr 23, 2009 at 10:55
Graham, you say "coming down the hill slowly".

Did you use the brakes or not?
If you used the brakes gently you will find that the landcruisers (and possibly other makes) with ABS will do MAXIMUM braking with the rear wheels until the point of lock-up when the ABS steps in.

After swapping my non ABS 80 for the ABS 100 I was quite surprised to find my rear brakes overheating (2 people - no luggage or trailer) after a long gentle decline.

FollowupID: 628780

Follow Up By: Member - Graham H (QLD) - Thursday, Apr 23, 2009 at 11:09

Thursday, Apr 23, 2009 at 11:09
I did say in original post "" üsing the gears" I would never drive a rig like that down a long hill using the brakes Had it in 2nd or 3rd all the way down with only very light occasional braking.

I have found the sensors to be fairly stable and consistent with no sudden variations They also monitor temperature.
They certainly work as when I copped a nail it soon let me know the tyre had gone down 10lb

FollowupID: 628781

Reply By: Sea-Dog - Thursday, Apr 23, 2009 at 16:02

Thursday, Apr 23, 2009 at 16:02
Apart from agreeing that the initial inflation on the back was a bit on the low side the left side tyres may have been affected by camber of the road as "most" roads will camber off to the left to allow water run off etc... this will effectively put more stress on the suspension and tyres etc on that side as the lean of the vehicles will contribute slightly.

Interesting that the tyres still went up 7 - 8 psi on the left though even when starting at 50psi as I would have thought that the tyres at that pressure would be on the high side if anything.
AnswerID: 361092

Reply By: Member - Shane D (QLD) - Thursday, Apr 23, 2009 at 19:31

Thursday, Apr 23, 2009 at 19:31
It would be intresting to see how much hotter the rears get going UP the same hill.
My theory is that you have MOST of your (braking) being done by the motor thru the rear wheels, if you where going up, all the power would be going thru the rear wheels and I would guess that you may read a increase in tyre temps.

I'd agree with your conclusion, re van, The left side is the low side, plus a crosswind putting more weight on the left may be why they appear to be working harder on y
the caravan.
AnswerID: 361121

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