A different slant on tyre pressures

On dirt I tun 28 PSI cold all round.

Interesting that the fronts @ 28 were OK. Think that is what you wer saying. So maybe you should have dropped the rears to 28 also.

Would be interested to hear what speed you were doing with the fronts at only 28

Hi Rosco

I bet the guy that said that has never seen a corrugated road.

I would love to see the condition or what damage to the vehicle running very high pressures on corrugated tracks. I usually run my Bridgestone LT's at 18 to 20 psi when on such tracks like the Anne Beadell and never had any issues with tyre damage. If I ran even with your rear tyre pressures, the car would be shaken to pieces.

But like all situations, reduced tyre pressure also means lower speeds of travel.

It is either a case of the tyres being faulty, or they were still miles too high.


Just my thoughts.



Cheers



Stephen

Thanks for your thoughts fellas ...
this is what I'm talking about .........

Rosco

Wife's cousin lives in broome and has, still does go quite remote. His view is unless it's sand you run them hard.

Alan

Personally i think the bfg's are way overrated, the best tyres I've owned would have to be bridgestone dueller d697's, bullet proof imo!

Rosco - Those tyres show a classic case of sidewall overheating and carcass failure, due to running too low pressures.
Remember, that as you reduce tyre pressures, the more the sidewall flexes.
Increased sidewall flexing means rapid heat buildup - rapid heat buildup means destroyed tyre carcasses.

I have never reduced tyre pressures for corrugated roads. If you are going to do so, you need to reduce road speed by a substantial amount - and I mean substantial.

Imagine your tyre sidewall is a piece of heavy wire between your hands.
You can move your hands back and forth, flexing that wire, and if you don't do it too fast, you can keep doing that for an hour - the wire will not heat up substantially, and it will not fracture.

Now, start moving your hands back and forth rapidly and that piece of wire heats up to the extent it will burn you, if you put your hands on it - and it will fracture and break in two within 30-45 seconds.

A tyre sidewall performs exactly the same as that piece of wire. The sidewall might be rubber and rayon or steel cords - but it produces heat and failure with excessive flexing, exactly as that strand of heavy wire does.

Years ago, we used to run an old WW2 low-loader that was fitted with 8.25x15 tyres for low deck height.
Truck radials were virtually unheard of in those days (apart from Michelins), and those crossply 8.25x15's would run as hot as hell with regular speeds of 70-80kmh and around the recommended 70-75psi, when loaded. On hot days, those tyres would pop like party balloons.

We had discussions with tyre reps, and they recommended we run the tyres at a minimum of 100psi and preferably 110-120psi. We did so, and those crossplys stopped blowing like party balloons and we even managed to run some through to full tread wear.
The pressure increase was all about stopping the sidewall heat buildup by pumping the tyre up and reducing the sidewall flexing. It worked.
The old WW2 low-loader was designed to be used behind the likes of WW2 Diamond T's and Federals - that were flat out at 50kmh.
At that speed, the 8.25x15 crossplies performed quite satisfactorily at the WW2 recommendation of 70-75psi.

It was only with later model faster trucks, that the tyres started giving trouble. At the low WW2 speeds, the heat buildup from the tyre flexing wasn't really measureable. At higher speeds, it became a major problem.

Incidentally, BFG's are not my favorite tyre, either - I have never had satisfactory performance from any BFG's I've ever bought, or that were fitted to new vehicles I purchased.

Cheers, Ron.

Did you drop your speed as well as the pressure? 40 - 50 km/h max with reduced pressures.

I follow Adam Plate's advice and reduce pressure by 30% on corro and gibber. D694s and D697s driven at up to 80 kmh - never had a problem.

This is the first post I've seen with evidence of one after following common practice among 4WDs. I suspect the tyres rather than the pressure, esp as it's two out of four.

Hi Rosco,

We drove the Kalumburu Track towing a Campomatic up as far as Drysdale River and back and also the Cape Leveque Road and Middle Lagoon Tracks which were pretty bad in parts. The ball weight on my loaded rig is closer to 180kg.
I ran the tyre pressures at 32 rear, 28 front in the Colorado and experienced no problem at all. I run Bridgestone 697 LT AT's and at no time did I feel heat in the sidewalls. The camper tyres were also defated to 32psi from my normal highway pressure of 38psi all around.

Deflated the tyres at El Questro and left them that way all the way down the Gibb, out to Drysdale R.S. to Broome, then out to Middle Lagoon and back along the Cape Leveque road which is unsealed up to the Middle Lagoon turnoff at least.

I was nearly going to ask you if the tyres were Coopers:-) until I saw the picture of the BFG's.

Really pleased with the performance of the Bridgestone light truck rated 697's which have stronger side walls than the older 693's and 694's and I will continue to use them both on the vehicle and camper.

I cannot see how tyres that do not allow flex in the sidewalls can be considered both easier to drive, perform better on unsealed roads as they "mould" to the changing road surface and give a more comfortable ride can be considered as practical for off road use.
The rigid unflexing sidewalls must cause the tyres to continually bang down on the road surface giving a horrible ride and further damage the track surface as well.

A few years ago we went away from running low tyre pressures, the only time we drop to a lower pressure is in sand..... but on most other surfaces we stay at highway pressures.

So far no funny tyre wear, chipping or damage.

Have had a few friends start doing the same with no ill effects.

And as for damaging the environment....... show the data that says higher pressures cause more damage on harder ground (not talking about sand).

I have used the same tyres towing a C/t or caravan on all the main corro roads..
Plenty, GCR, Tanami, GRR etc & never reduced pressure below 38psi cold. No tyre
damage, no vehicle damage. No need to travel at reduced speed either. Heat kills tyres & the quickest way to stuff them is reduce pressure & not speed. In my last life
as a contract boom sprayer, the tyre damage stopped when I got to 60 psi, & no, I
don't subscribe to that balloon & pin theory. Every tyre troubled poor sod I encountered on outback roads was running lowered pressure.
I don't expect to change the "air up/down" brigade, but I know what works for me.
cheers.....oldbaz.

Howdy - Just back from 18k of touring including Gibb, Kalumburu, Cape Leveque, Tanami, Oodnadatta and Birdsville... and probably one of the worst bits from Birdsville to Windorah. Total casualties were 3 tyres - Goodyear Wranglers Silent Armours 265/70/16 on V8 Landcruiser Ute with Alum Canopy and 200kg ball weight. The ute has an upgraded GVM to 3760 and we were at around 3500kg. Dropped the pressures to around 26 in the front and 36 in the back and I don't drive fast. A mate running Bridgestones dropped tyre pressures even further and went through a similar number of tyres. Again with his Hilux loaded up with a sleeper on the back. My tyres all went in the bead which started to split and then slowly leak air. When noticed I took them off to find the leak and when I saw the split beads they didn't go back on.
I think I am joining those who believe in keeping the tyre pressure up, the tyre temps down and the speed down. I have the drawing from Adam Plate and if I remember correctly, he states on the drawing that the pressures are for high speed dirt driving conditions so my thinking is to keep the pressures up and the speed down and hopefully all will be good for the next trip. I will also be looking at different profile tyres to 265/75 or even 285/75 but the priority when buying will be load rating.

I also have a 79 Series with a "heavy" canopy on the back. My experience on fast dirt has been to run the rears up to 10 psi higher than the front due to the weight on the rear. I experienced the same on a recent 17,000 km trip to the Kimberley in a 200 Series, also heavily loaded in the back. This is borne out by monitoring the pressure increase over all tyres through a Tyre Pressure Monitor System, a must have accessory in my opinion.

I don't believe the brand of tyre has as much to do with failure as inappropriate pressures run (again on fast dirt). Get yourself a good TPMS unit - you will be surprised at the variances from cold to hot.

Re corrugations in gravel roads, here's a webpage with a proper scientific experiment as to how and why corrugations ("washboard" to the rest of the world) are formed.

What causes "washboard" in roads?

Note how the boffins state .. "The fact that our simplified systems produce washboard ripples is important, since it shows that neither tyres nor suspensions are necessary to obtain washboard roads; although of course, adding a spring, a dashpot, a tyre or an engine, would affect the size of the bumps.
In other words, it is not because of the suspension of cars that washboard roads exist. The ripple wavelength is NOT simply the speed of the wheel times the bounce frequency of the suspension, which seems to be a common belief."

Note also how they find that the speed factor is crucial. However, this is what they find! ...

"The speed of the wheel appears to be crucial. Indeed, there exists a critical velocity below which the road always remains flat, and above which, washboard bumps appear. Typically, for a car, this critical velocity is around 5 mph or 8 km/h." (!)

The bottom line is that you will get corrugations on loose gravel regardless of tyre pressures, speed, or vehicle weight.
It's the grain content of the road surface, and looseness of the material that governs whether corrugations appear.

You will notice how many sections of gravel roads do NOT corrugate. This is because the clay content of those road sections is higher, and the road surface sets hard because of good inbuilt bonding chemicals, and clay, in the road base.
However, road base with higher clay content tends to become slippery when wet, therefore sandy gravels are chosen for road base, for gravel roads.

Finding and producing good road base gravel, is a black art.
Many shire operators and foremen have inadequate skills in these areas, and they always tend to select gravels that are too high in sand content, and which rut and corrugate swiftly.

Another contributing factor is wear in motor grader blade linkages. As someone who is a qualified grader driver, who has owned numerous graders, and who has done thousands of hours of grading - I can assure you that grading at speed with worn blade linkages sets up initial corrugations in the formation. Car wheels then just accelerate the corrugations.

I have even come across sections of bitumen where a grader with worn blade linkages was used to do the final grade work before sealing - and they managed to get corrugations into the final finish before bitumenising - and those corrugations were then sealed into the bitumen!

What no one has mentioned is when they dropped pressures, how fast did they travel ??

Dropped pressures and a steady 70 k/ph would be fine, but get to 80 or 90 k/ph and a hot day as well, then heat is going to kill them.

I had a rear tyre go on me along Cordillo Downs road a few yrs back ....dropped pressures to 28 psi, towing a camper as well, and speeds gradually crept back up to around 80 or 90 k/ph ( the road got that good we could have gone faster )

Other rear tyre ( the good one ) was very hot to touch ( it failed 2 months later just driving along- a blowout ), so nowadays I'm very wary of when I reduce pressures. Went from Yunta to Arkaroola 2 yrs ago and reduced pressures to 30 psi, but set the cruise control to 70 k/ph and all was good, but if I was travelling like the rest of the people in a hurry to get there, I wouldn't have bothered ..

I keep my pressures low when offroad BFG A/T Nissan Patrol loaded up. 24-28 rear, 20-24 Front.

But as a curly one to throw in I talked to a guy years ago and he MEASURED the footprint of each tyre on the road. I can't remember the length but he let the tyre down until he had a set amount of tread on the road.

Would certainly cater for differing loads etc.

One to ponder!!

Oh, and forgot to mention I also think Adam Plate's sheet is excellent!!

Adam Plates diagrams and suggestions are interesting - but they go against nearly all tyre manufacturers recommendations.
Does anyone remember that the multitude of early Ford Explorer crashes were sheeted home to engineers reducing tyre pressures to get a smoother ride? - which caused an increase in tyre failures? (usually associated with high speed vehicle useage).
Ford later revised tyre pressures upwards and the incidence of Ford Explorer crashes caused by tyre blowouts reduced enormously.

Adam has one suggestion there that it's O.K. to reduce 'Big 4WD 15" tubeless loaded' pressures to 20psi front and 26psi rear - and then adds, "These suggested tyre pressures are for high load, HIGH SPEED cruising, on low grip dirt surfaces and rock or sand".

Now, I'm not sure what Adam is referring to, by "high speed" - but to most people, "high speed" relates to highway cruising speeds of around 100kmh.

I would respectfully suggest that anyone who reduces their 15" 4WD tyre pressures to 20 and 26psi respectively - and who then travels at 100kmh with a full load - is asking for tyre trouble, in the exact manner that Rosco has experienced.

I wouldn't have a problem with someone reducing their tyre pressures to those levels to try and improve their ride - but speed of travel would then have to be at a substantially reduced rate - say around 70kmh, or 80kmh at most.

A simple test to check if your tyre sidewalls are under threat from excessive heat buildup, is to place your hand on the sidewall after travelling some 25-30kms with reduced tyre pressures.

If the tyre sidewalls feel hot enough to the touch to be uncomfortable holding your hand there for a constant 20-30 seconds, then they either need more pressure in them, or you need to reduce your road travel speed by a substantial amount.

A tyre running at a satisfactory temperature should only feel warm to the touch, not hot enough to be uncomfortable on bare skin.

The lifespan of your tyres is directly related to its operating temperatures during its life cycle. A tyre regularly operated at high temperatures will suffer from a shorter lifespan.

This is because not only does high temperatures in tyre rubber affecting the bonding of the rubber to the carcass material - but high temperatures also result in breakdown of the rubber composition.
In tyre manufacturer parlance, this is known as "oxidative breakdown".

Another little understood factor in tyre pressures, is that if you reduce tyre pressures substantially to improve the ride, you are effectively making the tread conform more to the road surface.

Thus, if you have a corrugated road surface, and you're running low pressures, your tyre tread is also flexing at a considerably higher rate, as compared to running higher pressures - thus leading to additional heat buildup - adding to the increased heat buildup from the sidewalls.

Rosco

Do yourself a favor and contact BFG through their customer information service. It is in their interests to set you off on the right track in order to get the best out of their products. That is what it is there for.

I have done that for years whenever I buy a different brand or type of tyre. I rang the manufacturer of my current set of standard size commercial tyres (now 7 years old) and had a 20 minute talk with one of their tyre engineers. He even talked about Outback tyre testing programs that he had been involved in where the object was to deliberately destroy tyres.

Two things that came out of our discussion was do not reduce the pressures no matter what the road surface is like and the weight each 1 psi increase would support. The factory recommended pressures are so low anyway that I would have to be out of my mind to want go any lower.

To my surprise those constant pressures have worked perfectly on every surface from the VHC to the Gunbarrel Hwy to freeways in mid summer without the slightest sign of a problem. Of course the same may not apply to all the tyres in their range so if I bought something else I would be back on the phone again.

Give it a try. Your tyres were not designed to fail when used properly so start with the people who made them

Ran a OKA bus across the Gibb for a couple of years always fully loaded truth is overloaded, had fat radial Michelin tyres on it as it came out of the desert run near Alice, ran 70psi all round had one puncture in two and half years and that was from a bolt. My family ran 9 ton trucks and a single trailer semi on the Gunnawarra Road in Queensland carrying railway sleepers back when it was described the roughest road in Australia (by around Australia rally) always ran 90psi in the tyres. We had blow outs but only on the really hot days but not to many of them then either. It is always a combination of factors, and tyre design and quality will of course have a large bearing.

To all those posing the question of our speed at reduced pressure: I very much doubt we would have exceeded 80 for any appreciable distance with a good deal of the time as low as 10 through the more tricky sections.

Gidday

While the pressure debate will always rage, there seems to be agreement that heat build -up is a major factor in destroying tyres.

A tour guide acquaintance of mine, who is a believer in reducing pressures, carries one of those infra red thermometer devices.

When the convoy stops for morning tea, lunch or whatever, it takes just a few minutes to walk down the line and check the tyre temps and he can quickly pick up who might be running a bit underinflated.

Just a bit more scientific than a hand on the tyre.

Also good to check the shockers after a run on the corrugations.

Cheers

If this is of any use - from my personal experience..
Never have had issues with sidewall or tyre failure by lowering pressure. I generally use higher and wider tyres, have trialled most brands. I do however monitor my speed and tyre temperature. Avoid towing anything in remote areas. I never buy cheap tyres. I look at how my tyres bag out for the conditions and go from there.

IMHO there is a certain degree of gut feeling/nouse/experience required to set up the vehicle in the first place and run it on harsh surfaces. EG I ask myself "Does that look right ? Maybe I overspend or over specify but whatever it is it works.
Sorry can't add more but do see a lot of big heavy vehicles on smallish looking tyres (to my eye) having tyre problems. It is a bit of a black art and from what I read here I must be an exception cause I often use even lower pressures than stated in many of these replies and seem to have reliable tyres. My only suggestion - Go bigger and if you can and buy tyres suited for your worst case off-road scenario. Don't overload them - If they don't look tough enough for the job then maybe they aren't.

With regards to tyre manufacturers, it is worth noting that for legal reasons they will ALWAYS tell you to run them at their recommended pressure for the load you are carrying. These recommended pressures however do not take into account road conditions.

To my knowledge Cooper Tyres are the only manufacturer that acknowledge the lower pressures argument. Not sure though what their take is on corrugations.

This is pretty much the oldest 4x4 argument in the book. Experiment and stick with what works for you. I have only had to replace one tyre in the past 10 years, and that was a sidewall that got spiked at 40 PSI. For the record I reduce to 30 PSI for corrugations, because that is my personal preference.

There is a lot of mis-understanding over tyre pressures.



The first thing that must be understood is.....if you are running at highway speeds, you need to be running highway pressures......regardless.




That means at least minimum inflation pressure for the load carried as found in the tyre standards manual or in the manufacturers specs.

80KPH is pretty much highway speed...the lower end of highway speed...but highway speed all the same.





The second thing that so many fail to grasp is.....the minimum normal inflation pressure varies for the load carried and the tyre fitted.

AND...it can vary quite a bit as the load varies.




If you have not weighed your vehicle ready to travel and looked at the load V pressure tables for the specifc tyre you are running......frankly.........you have no idea what pressure you should be starting with.


Combine the above with the fact that many touring 4wds are overloaded on one or both axles..........but the owners are completely oblivious to the fact.



Then there is the whole idea of airing down.........the whole idea and concept revolves arround low speed 4wding over various ground types and for relativly short periods.

You woud get away with running pressures well below the tyres ability to carry long term if you where not doing so hour after hour day after day...and in hot conditions.

It may be well and good to drop pressures considerably when traveling at low speeds and for relativly short distances......but once the speed and duration gets up...you realy need to think about how much if at all you reduce your tyre pressures.

If you where starting arround 40psi for highway work it may be suitable to reduce to 30 psi for generall off road and 20PSI in soft sand.......IF you are traveling in low range or in lower gears in high range.........AND at relativly low duty cycles...

some off road trips may work out to be less than 100Km in reduced tyre pressures over a weekend.....a trip up the beach may be a couple of hundred Ks..all week end.

so.

just for an eye opener

here are the tyre load limits at cold inflation pressures for the 265/75R16 light truck tyre from the standards manual.
figures in psi converted from kpa to the nearest 1psi

890kg at 36psi
1010 kg at 44psi
1120 kg at 51 psi
1240 kg at 58psi
1360 kg at 65psi
The table goes further but very few would be using thres capable of running above 65psi.
The table is fairly coarse in this group of tyres, but if you graph the figures provided you can work out what is in between...just remember the grap wont be all linear.


so at 36psi....the rear axle loading should be less than 1780Kg...consider the maximum rear axle loading on a 70 series is arround 2300kg...you would have to be arround half a tonne under under GVM to run that pressure in the rear long term.
BTW...toyota commercials are designed to carry the majority of the payload on the rear axle.

The other thing to consider is that at 2300kg, the maximum rear axle loading is near the maximum load capacity of the tyre....at 65psi.

There would probably be more latitude for pressure reduction if the tyre was running in the lower part of the load curve.




There are a lot of well meaning comments about tyres.......but untill the rig is actually weighed.....as it travels with all aboard..AND..the load V pressure tables referenced......big chunks of it mean nothing.

All to often people have the tyre pressures wrong, even before they start talking about airing down.

cheers

G'day Rosco,

we did a Kimberley trip similar to yours a couple of years ago.
We were running Dick Cepek FC11s. I was dropping and raising pressures whenever we changed surfaces. The rear tyres were stuffed when we finished our 13,500 kms and one front one didn't make it back along the Tanami.

I spoke to a few tyre blokes - Bedourie, Cunnamulla, Borroloola, Mt Isa, here - and they all say they don't touch the tyre pressures unless they're in sand. All of them.

Last month we were in Qld and NT, a lot of dirt roads, same load, same vehicle, didn't touch the pressures. No catastrophic failures, two stakings in the tread, some chipping but nothing like the Kimberley trip.

Speed is an important factor but different brands also vary greatly. I monitor the tyre pressures with tyre dogs. I played around with cold pressures a bit early in the trip. Whether I started with 40, 50 or 60 in the rear they still went up 10 psi in the first hour on a variety of surfaces and pretty much stayed there.

I took the advice of the tyre guys and had a minimum of 40 psi front and 50 psi rear, unless we were in sand or rock crawling.

I probably haven't answered your questions but I hope I helped a little.

Steve

"quote I ... like most here, have been a great believer in reducing tyre pressures when traversing corrugated/rough dirt roads..end quote."'

I have never subscribed to lower presses on hash / rocky/ corrugated roads.....I have travelled those roads you mentioned and many more in harsh rocky goldfields country as well and always maintain high pressures that is > than 50pis Always I have been very heavy as well.......The tyres have mostly been BFG or Coopers.

My reasoning is simply the lower the pressure, the greater foot impint (hence the reason for low pressure in sand)
The side walls also bulge making them more susceptible to side wall damage from rocks...the tyre flexes more generating heat etc.

I wont change my view based on my experience and many km's doing this........I do however, control my speed and am very conscious of nurturing the suspension and not using the tyres as the cushion

The picture of the side wall bulge looks to me like heat generating from over loaded ( there are 2 different specs for BFG ...look closely as its not easily apparent)....or under inflation or both

Geez after reading all of this it is wonder I don't have a headache. All I can say is thank heavens I have stuck with stock size 205Rx16 LT tyres and tubes on factory pressures of 25 front at all times and 25 to 34 rear depending on load. There have been no problems at all over a seven year period everywhere from Blue Rag, to the Gunbarrel to the Sydney Harbor Bridge. I reckon Toyota got it right when they specified those tyres at those pressures and the tyre manufacturer told me not to change a thing.

If I had a Cruiser instead of a Lux, it would be a 7.50x16 on factory pressures and I am sure I would get the same results.

So, we have samples here of folk who don't air down on corro, gibber or other rock, and those who do. Assuming we're comparing apples with apples we can conclude that tyre pressure doesn't matter that much except on sand and then only for traction.

Without rereading the whole thread there's only one first hand report of failure. The data in the OP is a bit limited though. The weights were estimated not measured.

So it it may be fair to say that it wasn't due to pressure but to something else. Dud tyres? Overloading? Factor X?

...

For myself, I'll take the advice of the roadhouse mechs I've spoken to for who this stuff is their bread and butter and continue to air down. Have done that over three outback trips on two varieties of LT construction keeping under 80 with no failures or chipping out. Less chance of staking, a smoother ride over corro and much better grip on sand and gravel.

An interesting topic that highlights differing approaches by many.

What it tells me is that it is a “trial and error” scenario and that there is no one size fits all approach.

Those that air down perhaps have sufficient evidence to suggest it is the best way to go, others who don’t might also argue the same based on their own individual experiences…

Perhaps it is a bit like the “shoo roo” whistles on the front of some vehicles. If you have one and never hit a roo you can argue they are most effective; if you don’t have one and haven’t hit a roo, you might be entitled to suggest they are not worth the money. But neither scenario tells us much unless it has been tested under controlled conditions over a long period.

I suspect the tyre argument is much the same, remembering that changing one thing, or nothing at all, usually has impacts on other areas, and not always in a positive way…

Deflating or not deflating tyres will have benefits and drawbacks.

But plenty of food for thought in this post and it has given me some things to research and think about – and that is what this forum should be all about!

Cheers, Baz – The Landy

It would appear I have opened a can of worms here and a tad more than I bargained for.

A shame it would appear reasoned sensible debate must be coloured with that which approaches invective and is, in my opinion unnecessary and uncalled for.

first time I travelled the Kimberly I ran 50 psi in the tyres and barely made it to kunununurra with 2 wrecked tyres and one leaking one.
just recently I returned after having my wagon in the Kimberly for 5 months doing trips from kalumbaru. to kakadu and everything in between.
I had the cheapest tyres money could buy. road stone roadians and ran low pressure low speeds for the tracks.
I lost one after slicing a sidewall after not re inflating after going to 10psi to get out of a bog
besides that yes there chipped but after many of the rocky tracks all tyres were covered in sidewall scuffs but there still fine

Here's a thoughtful piece that I think is worth a read:

http://outbackjoe.com/macho-divertissement/macho-articles/tyre-pressure-guide/