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

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.

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

I agree with what everyone is saying. Having done the Birdsvile and part of the Oodnadatta track recently only dropped pressures by 5 psi.

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

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.

"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.

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

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.

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.

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.

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

Suspicion isn't evidence. Where's your data?

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

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.

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.

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.

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.

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