Tyre Pressures - Putting Conventional Wisdom to the Test...

Submitted: Monday, Sep 04, 2017 at 07:36
ThreadID: 135516 Views:3836 Replies:17 FollowUps:27
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Seeing a recent post on a tyre that had delaminated prompted me to put “pen to paper”…

"A Faulty Tire?"

The conventional wisdom appears to be to deflate tyres when off-road.

Different reasons are cited including, less prone to punctures and a better ride over corrugations. Although, suspension is supposed to do that work of the latter, not tyres.

In most of the discussions I see on “what tyre pressures should I use” numbers are bandied around without any consideration being given as to the weight of the load it is carrying, speeds being driven, even ambient temperatures. Numbers are often quoted on a “one pressure fits all”. This simply isn’t the case.

Tyres are about providing traction with the surface you are driving on, and if you have traction there would seem little need to deflate tyres. There are circumstances where deflation may assist and is most likely needed, especially with sand driving…

But I suspect more tyres are lost to sidewall damage as a direct consequence of being under-inflated whilst off-road than are lost to punctures through “over-inflation”.

And my own analysis, measured by the wallet in my hip pocket confirms this.

Realistically, most of the tyres used for off-road, across a range of brands, are simply not strong enough in the sidewall to cope with the flexing and overheating generated by under-inflation. And whilst the heat issue can be managed by speed control, exposing the sidewall to “staking” is luck of the draw, but without doubt you increase the risk of damage, perhaps beyond that if you didn’t deflate the tyre.

These days when asked what tyre pressures I use, I simply respond with “ones that suit my vehicle and set-up”.

My couple of bob’s worth on the topic…and it's an opinion of mine, others should feel free to disagree - but my wallet has had the last say for me!

Cheers, Baz – The Landy

Ps: I’m liking this “graduation from work thing (retirement) – we’ve been Out and About enjoying this great country of ours…
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Reply By: MUZBRY- Life member(Vic) - Monday, Sep 04, 2017 at 08:14

Monday, Sep 04, 2017 at 08:14
Gday Baz
I agree with what you said, I did The French Line three years ago with tyres at 25 . I was thinking the trip was pretty easy as compared with other peoples writings.I have also done quite a lot of Victorias High Country at 25 psi and had no trouble.
Muzbry
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Reply By: Member - bungarra (WA) - Monday, Sep 04, 2017 at 08:24

Monday, Sep 04, 2017 at 08:24
Baz

your comments quoted below I thoroughly agree with....

I am a firm believer in maintaining my tyre pressures when off road (sand being the exception)....its all about side wall protection.

and my hip pocket concurs with yours!......Drive according to conditions as in my opinion in most cases its the loose nut behind the wheel that does the most damage to vehicle and tyres !!

"The conventional wisdom appears to be to deflate tyres when off-road."

"There are circumstances where deflation may assist and is most likely needed, especially with sand driving"

"But I suspect more tyres are lost to sidewall damage as a direct consequence of being under-inflated whilst off-road than are lost to punctures through “over-inflation”."

Spot on mate.....now wait for the detractors :)

cheers
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Reply By: Member - torro - Monday, Sep 04, 2017 at 09:17

Monday, Sep 04, 2017 at 09:17
I agree with what everyone is saying. Having done the Birdsvile and part of the Oodnadatta track recently only dropped pressures by 5 psi.
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Follow Up By: MUZBRY- Life member(Vic) - Monday, Sep 04, 2017 at 15:21

Monday, Sep 04, 2017 at 15:21
Gday Toro
From what pressure to what?
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Reply By: Peter_n_Margaret - Monday, Sep 04, 2017 at 09:27

Monday, Sep 04, 2017 at 09:27
I run pressures strictly in accordance with published Michelin data which I have put into a chart to suit my own purposes.
I can not attach it because this site won't let me. I would if I could.
There is a very strict relationship between load and pressure.
I believe that the strict relationship with speed is based simply on heat dissipation and on that score, a thinner sidewall generates less heat with flexing than a thick one.
The Michelins I use have a single steel ply in the sidewalls.
I have never destroyed a tyre due to sidewall heat failure and sidewall damage over the years has been minimal. I have never had anything penetrate the tread area.

Cheers,
Peter
OKA196 motorhome
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Follow Up By: Peter_n_Margaret - Monday, Sep 04, 2017 at 13:52

Monday, Sep 04, 2017 at 13:52
Here it is as a jpeg.
This info is derived directly from data published by Michelin for this particular tyre.
You will notice that a moderate pressure reduction requires a very significant reduction in speed to avoid over heating.
It is also necessary to know the actual loads carried by each axle accurately.



EDIT.... the bottom seems to be covered by the title...
The BLUE line is highway pressures, max 130kph.
The RED line is track pressures, max 65kph.
The YELLOW line is sand and mud pressures, max 20kph.

Cheers,
Peter
OKA196 motorhome
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Follow Up By: Baz - The Landy - Monday, Sep 04, 2017 at 14:13

Monday, Sep 04, 2017 at 14:13
Thanks Peter, and you highlight a point I am making here, there is a very strict relationship between load and pressure, something frequently overlooked in tyre pressure discussions.

Cheers, Baz
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Follow Up By: Sigmund - Tuesday, Sep 05, 2017 at 07:14

Tuesday, Sep 05, 2017 at 07:14
I've found that the pressure-temp-speed relationship varies substantially by tyre model.

My current BFG T/A KO2s run far cooler than the old D693s.

Allan Whiting has also remarked on the KO2 cool running.

They have a relatively stiff sidewall so we could hypothesise that for a given pressure there's reduced flexing - and it's the air/rubber friction that generates heat.
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Follow Up By: Peter_n_Margaret - Tuesday, Sep 05, 2017 at 12:29

Tuesday, Sep 05, 2017 at 12:29
I suggest that it the air that supports the weight, not the tyre.

Cheers,
Peter
OKA196 motorhome
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Reply By: Michael H9 - Monday, Sep 04, 2017 at 13:38

Monday, Sep 04, 2017 at 13:38
Pneumatic tyres were invented to make the ride more comfortable, otherwise we would be running around on solid rubber. My opinion is it's a fallacy to think it is solely the job of your suspension.
AnswerID: 613519

Follow Up By: Baz - The Landy - Monday, Sep 04, 2017 at 14:11

Monday, Sep 04, 2017 at 14:11
Hi Michael

No argument from me on why pneumatic tyres were invented, and for sure they make the ride more comfortable than something made of solid rubber. But I'm happy to agree to disagree that tyres were never intended to do the work of the suspension through pressure variations...

Lower tyre pressures may provide you with a more comfortable ride, but it will likely come at the premature demise of the tyre if relied upon for that purpose.

Cheers, Baz - The Landy

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Follow Up By: Michael H9 - Monday, Sep 04, 2017 at 14:31

Monday, Sep 04, 2017 at 14:31
My experience has been that having the tyre pressures too high can result in the premature demise of the suspension and everything that can possibly rattle off inside the cab. False teeth aren't cheap you know....
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Follow Up By: Member - Boobook - Monday, Sep 04, 2017 at 15:11

Monday, Sep 04, 2017 at 15:11
Getting philosophical in retirement Phil. Keep it up, it's good for EO's post count....

How long is a piece of string.

IMHO 28 at the front and 30 at the back is a good starting point at 80kmph on a dirt road. 32 if towing.

If the road is in very good condition and graded then everything can be a little higher. 30PSI, 32 PSI, 90kmph;.

If you feel the bumps and the car jumps a little occasionally, then go down a few PSI to 24-26, plus 26-28 at the rear and 10KMPH. If it still does it then repeat.

I think we may be saying similar things. The key is to be in tune with what your vehicle is telling you, and be prepared to make adjustments frequently.

A TPMS is mandatory, it helps you get in tune with the tyres. and it will also tell you that the 4PSI rule is garbage on dirt, especially if the sun is to the left or right.



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Follow Up By: Baz - The Landy - Monday, Sep 04, 2017 at 17:35

Monday, Sep 04, 2017 at 17:35
Hi Tony

Given the reference to "retirement" I'm assuming you are referring to me (Baz), not Phil...

Actually, we aren't saying similar things...

I am highlighting one crucial thing in tyre pressure discussions that is lacking is a reference to the load being carried. You've quoted "numbers" above that are meaningless to the extent they don't reference a load.

Pressures need to be set referencing a load chart for the weight being carried, not adjusted by an arbitrary amount based on the road surface...

And I'm also putting to the test the wisdom of pressure reduction simply because you are off-road, whilst acknowledging there may be times where this is prudent (sand driving for example).

Cheers, Baz
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Follow Up By: splits - Monday, Sep 04, 2017 at 21:58

Monday, Sep 04, 2017 at 21:58
" Pneumatic tyres were invented to make the ride more comfortable, "


That was most likely true when cars were little more than a horse drawn vehicle minus the horse. That purpose became outdated by the 1930s when even basic family cars could easily exceed today's freeway speeds but their handling was woeful. Extensive research into suspension design soon discovered the vital role tyre design, pressures and slip angles played in keeping the car stuck to the road. Tyres and their pressures then became a integral part of the entire suspension design. If you change the size of the tyre or the recommended pressures then you are altering a multi million design and are now on your own.

It is not hard for suspension design engineers to design a suspension that does the work of a suspension. They do not need owners deflating the pressures to assist them in doing their job. If that was necessary, there would be instructions in the owner's handbook.

Soft dry sand is different but there is rarely if ever any real speed involved in that type of driving. Things like understeer, oversteer, slip angles, braking distance and sidewall temperatures don't come into it. The length of the tyre footprint is the main issue here.


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Follow Up By: Michael H9 - Monday, Sep 04, 2017 at 23:36

Monday, Sep 04, 2017 at 23:36
Yes, I follow those design engineers and use the recommended tyre pressures for my vehicle as stated in the manual on the highway....which means I usually have to let around 10psi out of the tyres if I've been to a tyre shop. I've driven a gazillion kilometres on all manner of surfaces with umpteen different brands of tyres and I know what I like. You blokes can drive with whatever pressure you like but my logic says that if I feel comfortable and in control then the car likes it too. It's important to be nice to your car.
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Follow Up By: Baz - The Landy - Tuesday, Sep 05, 2017 at 05:26

Tuesday, Sep 05, 2017 at 05:26
Michael, no issue from me, and I did say...

"My couple of bob’s worth on the topic…and it's an opinion of mine, others should feel free to disagree" and...

"Pressures need to be set referencing a load chart for the weight being carried, not adjusted by an arbitrary amount based on the road surface..."

Cheers, Baz - The Landy
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Follow Up By: Member - David M (SA) - Tuesday, Sep 05, 2017 at 11:11

Tuesday, Sep 05, 2017 at 11:11
Boobook makes a very valid point with his reference to the effect the sun has on tyre temperature. When heading north I stop for lunch around midday and adjust tyre pressures to take into account the East/West movement of the sun.
Dave.
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Reply By: Robin Miller - Monday, Sep 04, 2017 at 16:30

Monday, Sep 04, 2017 at 16:30
"But I suspect more tyres are lost to sidewall damage as a direct consequence of being under-inflated whilst off-road than are lost to punctures through “over-inflation”. "

Hi Landy , when you make a statement like that (which I tend to agree with)) , all it means is that the user has failed to appreicate the factors involved and not used the right pressures - or knows its not right , and can't be bothered to correct them.

Making it easier to adjust one's pressures would save a lot of tyres and suspension wear !

The first thing to know is the real weight load .

I have just towed a small camper 15k thru the kimberlys etc in which I specifically set up my tyres to act as part of the campers suspension which meant 9 psi was just right to take the load of 500kg (250kg/wheel) whereas some thought tyres would fail and they would have if load was 500kg/wheel.
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Reply By: Member - Roachie - Monday, Sep 04, 2017 at 19:04

Monday, Sep 04, 2017 at 19:04
You raise a particularly interesting subject....one that I have always had firm views on, but also one that I am now a bit confused about.

Up until 2 or 3 years ago, I'd owned various Japanese 4x4s....mainly Nissan Patrol wagons, and then a single Toyota 100 series. Each of these wagons weighed around 3,000kg +/- and I towed a 1200kg Ultimate Offroad Camper trailer.

Venturing onto corrogated gravel roads such as Birdsville Track or up to Cape York was relatively easy...just drop the pressures down to about 25psi all round (from the bitumen pressure of 40psi). Never had any issues with tyres on any of our extended trips.

In January 2015 I decided to buy the Chev Silverado that tips the scales at around 5,000kg when loaded up and hauling the 3500kg Bushtracker.

We haven't ventured up the Birdsville Track or anywhere equivalent yet....only along the excellent dirt road from Wave Rock to Norseman, and didn't need to alter tyre pressures....that dirt road is better than a lot of the bitumen roads I usually travel on!!!

So, I am now a bit perplexed as to what I will drop my pressures to when I do actually get away "properly" again.

The Chevy runs 60psi on the black top and the caravan 45psi.

I'm thinking of experimenting with 45psi and 30psi respectively....but it will be a case of suck-it-and-see.

Roachie
AnswerID: 613522

Follow Up By: Member - Scott M (NSW) - Tuesday, Sep 05, 2017 at 08:43

Tuesday, Sep 05, 2017 at 08:43
Roachie,

My 40 series has a canopy and a fair bit on the back. I've upgraded and strengthened the frame and have helper springs to keep the load level. As Baz said, a lot of things (temperature etc.) can affect the tires.

I've gone to simply throwing the recommended out the window and simple adjust the tyre pressure by looking at the profile of the tyres to suit the conditons. More bulge for sand, less bulge for dirt, less again for blacktop. When it looks right, that's where I stop.
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Follow Up By: mountainman - Wednesday, Sep 06, 2017 at 05:17

Wednesday, Sep 06, 2017 at 05:17
Scotty
look at a 80series brake booster mod on your 40series
I have a 40 and at max weigh the brakes are average.
I have spoken with my engineer and this is the next mod im doing on my 84hj47 tray back
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Reply By: 9900Eagle - Monday, Sep 04, 2017 at 19:33

Monday, Sep 04, 2017 at 19:33
Baz, I don't think there is an answer to this at all. I guess it all comes down to what country you are travelling in and what the conditions are at that time.

An example of this is, I have done a lot of K's on corrugations in heavy vehicles. On a short run, I will leave all pressures alone but on a long run I will drop the steer pressures down from say 90psi to 65 psi just to keep me in one piece. I don't drop the drive or trailer tyres at all. I do similar with my vehicle and I just back off and try to find a good pace for the country.

I guess there is no answer to this end. I just know what makes me happy, one of those things is not pulling a wheel and changing a tyre.

AnswerID: 613523

Reply By: Member - Phil G (SA) - Monday, Sep 04, 2017 at 20:51

Monday, Sep 04, 2017 at 20:51
Theres too many variables and tyre pressures is just one of them.
My list of what contributes to tyre damage and punctures
- inexperience of the driver in offroad and offtrack travel
- overweight vehicle
- too much speed
- too little tread
- not using TPMS
- chunky sidewalls getting staked
- too much unsprung weight
- Tyre brand

Thick sidewalls can be good and bad. They can resist stakes but they also generate heat at lower pressures and can more easily delaminate.

Lower pressures are important. Its now 11 years since I last had a puncture on the dirt and during that time I've done about 20 desert trips - mostly in the GVD. And I am a fan of lowering tyre pressures but also take heed of all the other factors. And I'd rather damage a tyre than damage the vehicle. Everything is a compromise.

But its a free world. Anyone who travels with me can do whatever they like. If they have a problem then I am happy to make a suggestion.
AnswerID: 613525

Reply By: gbc - Monday, Sep 04, 2017 at 21:33

Monday, Sep 04, 2017 at 21:33
On graded rock roads when I've been too lazy to let my tyres down enough I've always been rewarded with chipped tyres.
When we are completely off road and I am too lazy to let my tyres down I am always rewarded with lost traction and boggings.
Maybe in your part of the country staking is an issue, but everywhere I go it is about the least of my concerns when setting up pressures for off roading.
AnswerID: 613529

Follow Up By: Baz - The Landy - Monday, Sep 04, 2017 at 22:23

Monday, Sep 04, 2017 at 22:23
Hi GBC

Just to highlight, I did say...

"Tyres are about providing traction with the surface you are driving on, and if you have traction there would seem little need to deflate tyres. There are circumstances where deflation may assist and is most likely needed"


Cheers, Baz - The Landy
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Reply By: Member - Blue M - Tuesday, Sep 05, 2017 at 00:24

Tuesday, Sep 05, 2017 at 00:24
Tyre Pressures do my head in trying to understand what is right and what I have done in the past and has worked for me.
One that gets me is the 4/6lb rule. I am now running on Toyo Cross Country AT11 tyres and previously Bridgestone Duelers at 250kpa (36 psi) in the front and 250 kpa (36 psi) or 350kpa (50 psi) in the rear depending whether loaded or light..
The rear tyres are wearing down more in the middle which indicates they are over inflated, but the pressure rises to 60/65 depending on the bitumen temperature.
If I pump them up to 64 then they rise by the 4-5 psi which tells me from what I have read is correct.
Running at this pressure will only cause the centers to wear more because of over inflation, and compress my back bone in the process.

Now to the off road side of things. I am still having a trouble determining what
"OFF ROAD" driving is, but coming to the understanding that if it's not bitumen, it is Off Road.
Before I gave up work I used to be a Trackside Safety Officer for the railway. I drove approx 300 klm's a day up and down the side of a railway line over ballast and ungraded tracks. Worse driving conditions than any gazetted dirt road I have driven on.
I did this for 11 years and mainly had Toyota Hilux's. In my tyres on those vehicles I always run 28 in the front and 35 in the back. That covered me for both on the bitumen and along the track. I never had a compressor nor the time to adjust tyre pressures from the rough to the smooth driving conditions even if I wanted to.
The amount of punctures I got was minimal, the chipping was horrible, and the mileage was atrocious. If I got 20,000k out of the rear and 30,000k out of the front tyres I was doing well, but I put that down to the ballast I was continually driving on.
In my own vehicle which was a Hilux for 7 years, I ran the same pressures and rarely got a puncture, the wear was ok. Naturally chips out of the tread was what I would expect for the road surfaces I drove on.
I now own a cruiser ute which is heavier and needs a bit more air than the Hilux. Since I have been on this site and others like it with all the talk about adjusting tyre pressures to suit road conditions, I thought I have been doing the wrong thing all my life on dirt roads. So I started lowering pressures for the rough, stoney, rocky and other road conditions accordingly, then pumping back up when getting on a better surface.
In the past 2 years since I have been adjusting pressures, I have had more punctures than I have had in a life time of driving.
I can see the need for a TPMS when driving on these lowered pressures, for it would be hard to tell if you have if your tyre is deflating because you are already driving on a half flat tyre.
So the jury is still out for me with the low pressures that some run on.

What I have said in the above does not relate to driving in the sand as that is a whole different story.
Every one will have a different experience and what is right for one may not be right for others.

If what I have written sounds like BS please don't hold back, let me know as I am always willing to learn.
As my Dad used to say, "you are never to old to learn if you aren't to ignorant to listen".
Cheers
AnswerID: 613531

Follow Up By: Baz - The Landy - Tuesday, Sep 05, 2017 at 07:04

Tuesday, Sep 05, 2017 at 07:04
Hi Blue,

Our experiences are similar.

I posted a response to Sigmund (below) and that gives some more insight to my rationale...

Cheers, Baz
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Reply By: Sigmund - Tuesday, Sep 05, 2017 at 05:39

Tuesday, Sep 05, 2017 at 05:39
Suspicion isn't evidence. Where's your data?

As for 'conventional wisdom' don't Cooper's recommendations qualify?
AnswerID: 613532

Follow Up By: Baz - The Landy - Tuesday, Sep 05, 2017 at 06:55

Tuesday, Sep 05, 2017 at 06:55
Hi Sigmund

Fair question...

Data: My wallet. Rest assured, under-inflation of tyres has led to side-wall damage and a draining of my finances in the past...

And perhaps I could have been clearer in my initial post - I define under-inflation as something below what the pressure load tables for your vehicle and weight require...

My point is tyres should be inflated according to pressure load tables for the weight being carried on "your vehicle". Not an arbitrary number picked out from somewhere based on certain road conditions (conventional wisdom) as this usually leads to under-inflation.

Conversations are often had on this forum and elsewhere relating to tyre pressures and what people should "run" without any discussion or reference to the load being carried.

Adding, on the data side, sometime ago I discussed with the technical people at Toyo Tyres (I use Toyo M55F - 265/75/16) and we have formulated a tyre pressure chart using the ETRTO formula. It is based on the weights of my vehicle, referencing weight being carried and individual axle loadings.

At any given weight I know what pressures I should be running.

This is not a precise science as I do not calculate individual axle load constantly, but I do have a good understanding of what a change in weight means for each axle as most of the load change occurs in the rear of the vehicle, either through fuel burn (I can carry 250 litres in tanks) or the addition of a TVAN camper trailer.

I have a baseline for off-road, based on the weight of the vehicle, which I know at any given time as I run a spreadsheet that details what load is being carried, and additionally I can adjust for fuel used based on an average fuel use.

I add a safety margin for higher speed on-road use (increase 20% front; and 15% rear).

Cheers, Baz - The Landy
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FollowupID: 884027

Follow Up By: gbc - Tuesday, Sep 05, 2017 at 07:20

Tuesday, Sep 05, 2017 at 07:20
I think you need to look deeper at your equation.

Just because you damaged your tyres off road on lower pressures, doesn't automatically mean that higher pressures must be better. Unless the same parameters are repeated it degenerates quickly into a forum chat - which is the highest level of my expertise anyway ;)

The bloke in the thread damaged his at high pressure which kind of works against your theory?

For an example of coping at lower pressures I'll outline my last trip in June.

Ford ranger, loaded to somewhere near GVM towing a rear fold camper coming in somewhere between 1500 k.g. and 2000 k.g. on any given day.

265/70/17 LT Yokohama geo12 tyres on the car 265/75/16 Goodride LT MT on the trailer.

Tyres were lowered to 26 psi cold all round - car and trailer somewhere near Eulo in QLD and spend 3000 k.ms at that pressure through Hungerford,cameron corner, merty, strez,oodna track, highway to Flinders, around flinders, (aired up for the barrier highway to broken hill) then back to 26 from broken hill to Bourke up the darling run.
Zero chips/stakes/blowouts, and that is completely in line with all of my touring experience.
One of our touring companions is of the 'higher pressure persuasion' and he chipped out his Pirelli scorpions on his cruiser pretty badly.

I'm not saying you are wrong and I am right, just outlining why I would need a whole lot more evidence to make me think about not lowering pressures unless I was roo shooting in a paddock which had been ripped or running through mallee/gidgee off track - that is why those guys run MRF steel tyres like the OKA referenced above - when they can get hold of them - the war in the middle east has seen demand for MRF's increase greatly if you believe the scuttlebutt. There are some places where a radial tyre simply cannot survive no matter what the pressure.


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FollowupID: 884030

Follow Up By: Sigmund - Tuesday, Sep 05, 2017 at 07:40

Tuesday, Sep 05, 2017 at 07:40
The late Adam Plate suggested a 30% reduction in the recommended pressure for those SA desert country roads and I've followed that rule of thumb and found no reason to change it. For sand driving of course it goes down further.

I've done two Simpson crossings with PC tyres at 14-16 and several days at Robe at 12. Popped the beads once with that. Otherwise lots of outback corrugated routes at 30% down. So when the vehicle or tyre maker says to run n psi or n+4 psi when loaded, the reduction of course differs.

I'd take as assessment criteria wear rate, fuel consumption and puncture rate.
So far no punctures.
Wear rate on the PC tyres was faster than others report for that model but then it's not a controlled comparison.
Fuel consumption - I don't keep figures and only watch what the computer shows as average consumption. Unsurprisingly it goes up when aired down and goes up noticeably more when the surface is softer. But again there's a range of factors influencing consumption and I'm not doing controlled comparisons.
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FollowupID: 884032

Follow Up By: Michael H9 - Tuesday, Sep 05, 2017 at 12:06

Tuesday, Sep 05, 2017 at 12:06
I follow Adam Plate's recommendations too ever since I stopped in at Oodnadatta many years ago to get some tyres fixed that had too high pressure in them. I haven't had any failures since.
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FollowupID: 884040

Follow Up By: Member - Phil G (SA) - Tuesday, Sep 05, 2017 at 21:03

Tuesday, Sep 05, 2017 at 21:03
Adam Plate spoke from lots of experience on how to drive the coarse gravel roads in outback SA. His advice to me dates from the 1990s when we did the odd family trip up the Oodnadatta track in a 47 seriesTroopie. I've lowered tyre pressures since and his advice has done me well. He was an awesome human being.
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FollowupID: 884060

Follow Up By: Crackles - Monday, Sep 25, 2017 at 22:05

Monday, Sep 25, 2017 at 22:05
http://www.l8ter.com/wp-content/uploads/2015/08/tyre-pressure-pdf.jpg

This from the man who fixed more flat tyres in the outback than most have had hot dinners. The reason for punctures was an obsession with Adam observing what cars people drove, the tyre type, brand, loading asking drivers what speed they were doing at the time.
The benefits of reducing tyre pressure off-road far out-way keeping them high & I struggle with people trying to justify otherwise.
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FollowupID: 884517

Reply By: Sigmund - Tuesday, Sep 05, 2017 at 09:14

Tuesday, Sep 05, 2017 at 09:14
Just some opinions ...

Tyres are part of the suspension. I see no problem in adjusting them for the conditions in this respect. They cope better than standard shocks with corrugations and when aired down reduce wear and tear on everything above them. On the north end of the Tanami recently (north of the crater turn off which can be atrocious) I aired down to 24 cp recommended 40. That's lower than my 30% rule and the few psi difference made a big improvement to the ride. There are costs to this of course, most obviously in fuel consumption.

Re grip, I see it as a matter of degree, rather than a dichotomy. Airing down on gravel I find increases traction noticeably.

A TPMS is instructive as regards testing assumptions about tyre behaviour. In a set of 5 BFG T/A KOs for example one tyre performed differently to the others, getting warmer and more pressured from the start of the day's drive. Go figure. A batch effect? A one-off QC failure?
Whatever the case, a TPMS can amuse you on long solitary legs!


A TPMS with temp measure is instructive.
AnswerID: 613534

Follow Up By: Member - Andrew & Jen - Tuesday, Sep 05, 2017 at 10:08

Tuesday, Sep 05, 2017 at 10:08
Sigmund
As a matter of interest, was the tyre with higher temps near the exhaust?
Cheers
Andrew
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Follow Up By: Member - Phil G (SA) - Tuesday, Sep 05, 2017 at 21:07

Tuesday, Sep 05, 2017 at 21:07
Tyres with higher temps are usually on the rear and on the side facing the sun. I see it every trip!
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Follow Up By: Sigmund - Wednesday, Sep 06, 2017 at 05:23

Wednesday, Sep 06, 2017 at 05:23
Good question.
Was at the rear but I can't recall which side. Will look out for it next time.
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FollowupID: 884068

Reply By: Ron N - Tuesday, Sep 05, 2017 at 12:47

Tuesday, Sep 05, 2017 at 12:47
There's no simple equation to define satisfactory tyre operating pressures.

In the earthmoving, construction and mining industry, we use calculated tyre engineering data to determine any particular tyres ability to do a specified job.

You'll find a Goodyear OTR Tyre (Tire) Engineering Data webpage online.
The calculations to determine a tyres ability to carry out any particular job are extensive, with multiple inputs to be considered.
Those inputs are pressure, speed, type of working conditions (operating surfaces), weight carried, and distance involved.

The formula used is called Ton-Mile-Per-Hour. OTR tyres are marked with their TMPH rating.
Interestingly enough, TMPH calculations are limited to a maximum of 20 miles (32km) of job distance - over that distance, there's a whole new set of calculations.

The bottom line is, the following are primary rules for tyre use calculations.

1. Heat build-up is the killer of tyres, and the primary factor to be considered in calculations and use.

2. Tyre heat build-up is a function of the amount of work the tyre is doing.

3. The road surface conditions are the second most important consideration.

On a smooth, sandy surface, tyres will cope satisfactorily with lower pressures, as compared to stone-and-gibber riddled road surfaces, where sidewall damage is a near-certainty with substantially lowered pressures.

On rough bush tracks, stick and stakes are a vastly-increased threat to sidewalls, and an increase in the bulge in sidewalls only adds to the risk of stakes.
When you are running substantially-reduced pressures to cope with soft footing, you can only counter that increased stake risk, by reducing speed and keeping a sharp eye open for potential stakes.

If you reduce your tyre pressures substantially below manufacturers highway recommendations (say, more than 15% below), on roads in poor condition, you are well-advised to keep your speed below 80kmh, as tyre heat buildup accelerates at an exponential rate with every 10kmh added to operating speed.

Cheers, Ron.
AnswerID: 613539

Reply By: Member - Paul B (WA) - Tuesday, Sep 05, 2017 at 17:03

Tuesday, Sep 05, 2017 at 17:03
There seems to be something wrong - we just about all agree! Or at least the points of disagreement are largely minor.

What usually brings me undone is after I've deflated appropriately for the rough/corrougated/sandy/etc conditions, I encounter good conditions and without thinking, the speed goes up to match said good conditions but I forget about my poor old tyres until wobble wobble rattle rattle and goodbye to another $350.

And every bloody time I kick myself, but still do it again every few years! And I reckon therein lies the reason most people don't adjust tyre pressures much.
Paul B Kalgoorlie

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AnswerID: 613546

Reply By: Sigmund - Wednesday, Sep 06, 2017 at 05:29

Wednesday, Sep 06, 2017 at 05:29
IME another influence on puncture rate (tread, not sidewall) is tread depth. All my punctures, on bitumen or dirt, have come when it's getting low.
AnswerID: 613558

Reply By: The Bantam - Wednesday, Sep 06, 2017 at 21:48

Wednesday, Sep 06, 2017 at 21:48
All the various tyre pressure comments I have always found interesting.

So much of it is just hokus pokus and simply defies any engineering logic.

One thing that can not be avoided is that if you are operating at highway speed that is anything over 80KPH you must be running the minimum pressures from the load V pressure tables.
NO matter the road conditions or the surface.

Some people don't even know what load V pressure tables are.

The 6pis / 3psi or what ever rule that is espoused by cooper simply is nor reliable, it simply can not be consistent from tyre to tyre and application to application.
I have proven it simply does not work AT ALL on lightly loaded tyres.


without knowing the appropriate pressure for the load carried at highway speeds...... all other discussions are nonsense.

From all that I have seen a 20% reduction in pressure requires a 20% reduction in speed .... so this is from around 32 to 35 down to about 25 and nothing over 80Kmh.

Which for me with the tyre I run seems to achieve an effective improvement in traction and a modest improvment in flotation.

It is in my opinion to run a 50% reduction in pressure and a 50% reduction in speed ... so on sand I run 18Psi and and stay under 50 or 60 Kmh.

I have 2 sets of stauns calibtarated for those pressures.


yes the above is rough and ready ... but close enough.
Running off road I travel farly light and I have very close to 50:50 weight distro.

Here is another issue .... so many people are running vehicles close to or past their on road loading limits .... which does not make anything any easier

In the military, vehicles are load reduced for off road work.

Back when I was driving tippers, we used to run 75-80PSI all round , where the recommended fully loaded pressures where 110 front and 90 rear.
The thinking was if we where running fast we where unloaded or light loaded, if we where fully loaded we where traveing slow .... and the owner seemed to be happy with his tyre wear across the fleet ..... yeh and it did help with off road traction

just some thaughts.
cheers
AnswerID: 613582

Follow Up By: Alloy c/t - Friday, Sep 08, 2017 at 10:21

Friday, Sep 08, 2017 at 10:21
A big difference is loadings ,a military '10' ton truck never carries more than '5' ton whereas in civi street a '10' ton truck carries '10' ton....or more.
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FollowupID: 884098

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