Tyre pressures ... what are your thoughts on these?

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Bevanbraves
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Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » April 25th, 2018, 8:32 pm

I'm driving a 79 series land cruiser. Might be 3.2t when loaded up with my camping gear, 180l of diesel, 60l water etc ....

I have 315/75/16 (35 inch) Nitto mud trail grabbers. Doing 75kph at 34 psi (cold pressures) at the rear when the rear drivers side tyre let go in a big way. Both inside and outside side walls full 360 tears and a tyre blow out, saw me a passenger through a big spoon drain and into the bush for an uncontrolled stop.

Some people say tyre failure, others say i had some bad luck and hit a particularly bad rock, and some say 34 psi is too low for a cruiser and i shouldn't be below 42 psi even on the dirt.

So i put a tyre management system on and sometimes any information is too much info. Both back tyres today were 44psi when i left home. Travelled for 30 minutes at 90 kph on the bitumen and passenger rear tyre was 46 psi while drivers side was 49.5. Rig was empty, so i wouldn't think drivers v passenger side was loaded up more than the other. Eventually the drivers side hit 50.5 psi. The "4 psi rule" if its right means the tyre pressure was too low to start.

I can't drive now without freaking out each 1 psi the pressures rise. "Drive to the conditions" i'm told. Well up till now i've done tens of thousands of kms on harsh outback dirt roads with no issue. Now i'd had one bad experience and am not sure if its weight, a bad tyre or poor choice of tyre pressures that resulted in me almost rolling the cruiser due to a blow out.

Any thoughts?

Peter Aawen
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Re: Tyre pressures ... what are your thoughts on these?

Unread post by Peter Aawen » April 25th, 2018, 9:15 pm

3.2 tonne on that size tyre at 34 psi cold doesn't sound awfully low (I regularly get just a 4psi increase from 22 psi cold under 4+ tonnes of loaded Patrol on my Mickey T's?!?) but you really need to share the load & pressure info printed on the side of the tyre for us to comment about the Nitto's with any degree of confidence; AND you really need to get your loaded vehicle weighed, so that you KNOW & can share with us what load each tyre is carrying!! ;) Your local Garden Centre will probably tell you the weights on the front & rear axles for nada, & they'll probably even do a left & right side weigh as well so you can judge how well & evenly you've loaded it - but it shouldn't cost too much $$ even if you hafta pay for it at a Public weigh bridge, altho you might be quite surprised at how heavy it really is!! You've probably already got close to 300kg over tare in fuel & water alone; then there's the weight of you & all your gear & how evenly it's all packed on board too - it all adds up quickly!! :ooh:

Still, if you had the driver's side rear tyre blow out with 34 psi as a cold start pressure, and have since had the driver's side rear tyre pressure go up from 44 psi by over 6 psi when the other side rear tyre, ie, the passenger side rear, has only gone up by 2 psi - I'd be guessing that you've either got the driver's side rear carrying somewhat more than its share of the load than is carried on/by the other side, or there is (or was?) something else pretty wrong back there!! :eek: And you probably also should try to think back to see if you can recall any time when THAT tyre (the one that blew!) either took a solid hit &/or you drove on it for more than about 3m on a hard surface with less than about 5psi pressure in it!! Doing that might have damaged the inner sidewalls enough that at some time in the future (or whenever the event you mentioned actually did happen to you) the increase in temps & pressure thru 'normal' driving proved to be too much for the compromised sidewall!! Full 360 degree tears in the sidewalls often suggest tyres've failed due to having a manufacturing weak spot or a tear/damage introduced some time since.... like the weakness or potentially full 360 damage you'd get from driving on an effectively flat tyre for more than a rotation or two, on a surface hard enough to force the rim into digging into the sidewalls in an effort to carry the load!! But I'd still be more concerned about the potential overload on that side increasing the pressure so much?!? The tyres you've had since going from 44 to 50 isn't too bad, certainly not generally bad enough to worry about (IF that's within the operational parameters of the Nitto's or whatever tyre); but you hafta ask yourself what did the pressure go up to when you only had 34 psi in the 'now dead' tyre back there??

Do you have a tool box or long range or aux fuel tank or anything fitted or loaded on that driver's side rear that could be increasing the load on that side/rear tyre beyond the load on other tyres?? Check things like batteries, fuel load, water tanks, tool boxes, the full fridge, spare tyres, etc, etc - having all the weight loaded or fitted on that side rear can quickly load up that tyre, and even if its pressure is up to carrying the load on it, the extra driving stresses imposed on that tyre can cause it to fail sooner than any of your other tyres, especially if you happen to hit more potholes or road-kill etc on that side.... you really need to try & share the load as evenly on the tyres as you can, and then run pressures to suit that are within the operating parameters of the tyre!!

Good Luck
An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'

Bevanbraves
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Joined: April 17th, 2018, 7:25 am

Re: Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » April 26th, 2018, 12:27 pm

#1 i guess is i was surprised by how much variation there is in psi and temperatures for all 4 tyres over the course of a simple 30 minute drive.

I will try and get to a weigh bridge.

No toolboxes or anything to unbalance the car. Water and diesel tanks are "in the middle". The only thing that is different is that my 80l fridge is on the drivers (problem) side. But on the weekend it was empty so probably weighed what? 30 or 40kg at most, not enough to make a difference i would have thought.

Bottom line: i've done tens of thousands of kms on rough outback tracks. Maybe the blowout was just dumb luck.

Bevanbraves
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Re: Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » April 26th, 2018, 12:31 pm

Tyre looked like this ...

ImageP1100815 by kcpics73, on Flickr

Peter Aawen
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Re: Tyre pressures ... what are your thoughts on these?

Unread post by Peter Aawen » April 26th, 2018, 4:35 pm

Yeah, that tyre does look like it could've been damaged at some stage in its life. Possibly either thru running waaay too low a pressure &/or actually hitting/running the rim on the inside of the tyre; or thru running it a little too low for waaay too long at too high a speed & grossly over-heating it - a proper inspection of the inside of the tyre (or what's left of it!!) should tell someone who knows what to look for which is most likely! Then again, spend enough time running on our rough Aussie dirt roads pretty much means that one day, you are gonna pull the 'bad luck' straw!!

That aside, and quite seriously here, the differences in pressure changes in the rear tyres that you mentioned with a cold start pressure of 44 psi are quite worrying! Have you had a wheel alignment including 'thrust align' done on your vehicle lately? Do you rotate your tyres to even out the wear patterns, & if so, where was that tyre last, & when/how many kms back?? Can you tell us what the Max Load (in Kg's) & Max Pressure (in Psi) shown on the sidewalls of the tyre are? And what your 'loaded for travelling' weights are - front & rear axles is a good start, but that left & right side weights certainly might be interesting.

Normally, with all other things being tickety boo, here in Aust on our outback roads, you should expect the LEFT hand rear tyre to be the one most likely to blow, cos of the camber in our roads & the weight transfer that & driving brings with it, plus the front tyres turning up any rocks or sharp bits for the rear tyres to hit them... but you blew the right hand rear & those pressure changes you mentioned suggest the RH rear tyre is getting a fair bit hotter than the left?!? :petrified: So apart from there being earlier damage to the tyre, there are a few other possibilities, like maybe poor thrust alignment making one tyre drag, a dud shock not keeping it under control properly, or damaged suspension components on that side hammering the tyre that could be possible causes.... or of course there's the cold start pressures being significantly too low or too high to start with & causing over heating or not allowing enough flex in the carcass to soften obstacle damage..... and then there's that 'the luck of the draw' thing too!?! :rolleyes:
An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'

Bevanbraves
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Re: Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » May 2nd, 2018, 8:17 pm

So;
79 series land cruiser with steel tray and alloy canopy, includes 180l of diesel and 50l of water = 3.5t. GVM is 3.3. Canopy is 2/3's empty.

Tyres state when loaded, cold tyre pressure of 65psi.

Door trim states loaded tyre pressure of 55 psi.

I guess 30psi might be ok for < 30kph in sand but not OK for 75kph on a dirt road.

Peter Aawen
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Re: Tyre pressures ... what are your thoughts on these?

Unread post by Peter Aawen » May 2nd, 2018, 9:28 pm

Bevanbraves wrote:
May 2nd, 2018, 8:17 pm
So;
79 series land cruiser with steel tray and alloy canopy, includes 180l of diesel and 50l of water = 3.5t. GVM is 3.3. Canopy is 2/3's empty.

Do remember that the 65 psi cold pressure is the MAXIMUM pressure you should ever run in that tyre when the tyre is loaded to its MAXIMUM - just like the 55psi on the vehicle door trim is the recommended max pressure for the OE Spec & size tyres (but you're not running them anymore, are you?) when the VEHICLE is loaded to its MAXIMUM; and the vehicle max load is usually gonna be a little less than the sum of the maximum load for all 4 OE spec tyres - your tyres need some safety margin! But if your actual vehicle loaded weight is 3.5 tonne & the GVM is 3.3 tonne, then you are already overloaded!! :boggle:

So what did the scales say your vehicle weighed when it was loaded? Did you get the front axle weight & the rear axle weight? What were they? And what does the printed load/pressure info on the Nitto tyre sidewall say? It'll have something like 'Max load xxxx kg at yy max kpa (zz psi maximim)' printed on the tyre sidewall somewhere; so that given that info & the weight of your loaded vehicle (or the weight on each axle) you can do the calculation to work out roughly what pressure you should be running given the actual load you have on each tyre, you just hafta remember to add 4psi to the calc result if you are ever planning on moving & not just sitting there, and don't forget to add an extra 2 psi if you are planning on running at speeds of 100kph or more &/or during periods of high temp!! Plus, you'll need to drop your tyre pressures AND your road speed by anything up to 20% the second you leave the blacktop, possibly more if the track surface you are driving on is loose, sharp gravel or soft sand! ;)
An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'

Bevanbraves
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Posts: 10
Joined: April 17th, 2018, 7:25 am

Re: Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » May 2nd, 2018, 9:40 pm

I didn't get front and rear axle weights, but now i know where the bridge is and how to access it i'll be heading back.

I havent weighed my vehicle loaded. Say 3.5t + 150kg for the family. And another 100kg miscellaneous of gear. Plus trailer at 10% ball weight is 150kg. So i could be 3.9t.

Tyres say max load 1,600 kg at 65 psi cold.

Bevanbraves
Getting to know the place
Posts: 10
Joined: April 17th, 2018, 7:25 am

Re: Tyre pressures ... what are your thoughts on these?

Unread post by Bevanbraves » May 2nd, 2018, 9:46 pm

Its a dual cab cruiser. Curb weight 2.2 i think and 3.3t GMV. So 1,100kg to play with.

180l diesel (180kg)
50l water (50kg)
steel tray (350kg?)
Alloy canopy
Bull bar
winch
driver (80kg)
empty fridge 30kg
two spare tyres @ 45kg each
And i'm already over GVM. Canopy is essentially empty. And add an adult and two kids.

I'm wondering where i could save weight. Alloy tray? One spare tyre instead of two?

Steel tray and alloy canopy feels like where all my weight is but not sure how to lighten it, or that its any different from any of the cruisers with canopies you see getting around.

Like 99% of 4WD's i guess i need the GVM upgrade.

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