What's your dune climbing technique?

[the evil twin]

snip...
The weight of a laden ute does affect the footprint, & recommended pressures are, I suspect mainly for wagons & possibly less weight.

Yes and No... the issue is one of density of the medium over which you wish to travel

18 psi is 18 psi IE that is the pressure inside the tyre 'pushing' on the inside surface and the ground is pushing back with a similar pressure on the outside surface.
Yes there are other factors like sidewall stiffness etc but they are much less a factor so for the purposes of the discussion lets ignore them for the moment...

If the pressure is raised on the inside of the tyre it will 'push away' and decrease the amount of tyre surface in contact with the ground... if the pressure inside is lowered the tyre surface in contact with the ground will increase until the two forces get back in equilibrium.

Yes, the more you load a vehicle (wagon or ute, doesn't matter) the more tyre surface ends up in contact with the ground but the key is the pressure exerted per square inch has increased but the density of the sand has not so neither has its ability to support the extra pressure/weight.

The medium (sand in this case) has a certain density and will react pretty much similar to a fluid so is analogous to a boat in a way (except a boat cannot change its footpringt).
The more weight in a boat the further it sinks until equilibrium IE the deeper in the water it rides and the more power it takes to displace.

The 4WD reacts in a similar way in sand.
Lets assume the sand/mud/snow density etc will support a surface pressure of 18 psi before you sink to a point where your 4X no longer have sufficient power to proceed.
Then 18 psi is the max pressure you will be able to exert onto the surface and get through.
A tyre with say 24 psi will exert a higher surface pressure than can be supported and will bog down or mire.

Sorry for the ramble so a quick Summary maybe in order?
If two vehicles of different weight but similar power availability and drive fitted with identical tyres attempt to cross a surface the tyre pressure at which point they will succeed or fail depends on the ground pressure exerted and is independant of the actual mass of the vehicle.
If a 3 ton Patrol fails at say 24 psi but gets thru at 18 psi then a 15 ton truck will fail at 24 but get thru at 18 psi also (all other things being equal)

[PeeBee]

If two vehicles of different weight but similar power availability and drive fitted with identical tyres attempt to cross a surface the tyre pressure at which point they will succeed or fail depends on the ground pressure exerted and is independant of the actual mass of the vehicle.
If a 3 ton Patrol fails at say 24 psi but gets thru at 18 psi then a 15 ton truck will fail at 24 but get thru at 18 psi also (all other things being equal)

ET, I am not sure you have this correct. Negating the internal pressure of the tyre, which in your example states the tyres and pressures are the same, if you raise the weight of the vehicle by 500% your unit pressure is raised on the sand also, ie if say the 3t vehicle running at 18psi, has at each wheel 100 sq inches of ground contact, so 400 sq inches across 4 wheels. 3000kg = 6600lbs/400=16.5lb/inch2 unit pressure. If the load carrying capacity of the sand is 18psi, then yes it would float across the top. Now, next vehicle, same tyres/pressure, 33000lbs/400= 82.5psi, hence it will sink if the load carry capacity of the ground is 18psi, this is assuming the actual footprint of both vehicles is the same. So mass/footprint = ground pressure, nothing to do with internal pressure.

What you need to achieve is a 'footprint pressure that is less than the resistive pressure of the ground'. What cannot be determined is what the footprint of the tyre would be at 18psi on a 3t v 15T vehicle and I think this is more the issue than the internal pressure. I admit that logically the tyres on the truck would bag out more than at 3T, at 18psi, but I think the principle lies in actual footprint pressure than internal pressure of the tyre.

In conclusion, if we put 20 tyres on the 15T truck the ground pressure would be equal to the 3T one, being 33000/2000inch2 = 16.5 psi ground pressure, assuming the footprint is the same. Does this make sense?

[Touses]

My head is spinning! Ummm......sandshoes?

[the evil twin]

snip...

So mass/footprint = ground pressure, nothing to do with internal pressure.

What you need to achieve is a 'footprint pressure that is less than the resistive pressure of the ground'. What cannot be determined is what the footprint of the tyre would be at 18psi on a 3t v 15T vehicle and I think this is more the issue than the internal pressure. I admit that logically the tyres on the truck would bag out more than at 3T, at 18psi, but I think the principle lies in actual footprint pressure than internal pressure of the tyre.

In conclusion, if we put 20 tyres on the 15T truck the ground pressure would be equal to the 3T one, being 33000/2000inch2 = 16.5 psi ground pressure, assuming the footprint is the same. Does this make sense?

We are saying the same thing pretty much but in different ways... except for the internal pressure bit.
Totally agree on footprint which is what I am referring to also but perhaps should have used different numbers

The internal pressure (excluding other factors prev mentioned, construction, compressability etc) has a direct relationship to the ground pressure.
Yes if you used the same tyres as the Patrol on the truck you would need more or the truck would still be sitting on the rims but adding more tyres
for the same footprint and the pressure (18 PSI) in each one to achieve the same ground pressure remains the same.

[the evil twin]

My head is spinning! Ummm......sandshoes?

ROFL... nope, Snow shoes mate not sandshoes.

Try to cross snow drift in sandshoes you won't make it.
Put on snowshoes (lower ground pressure) you will
A skinny bloke can use smaller shoes than a fat bloke but as they exert the same ground pressure ( lets say 18 psi) they both make it.
If they swap snow shoes the skinny bloke will do it even easier but now the fat bloke won't make it even tho he is wearing snow shoes.

A tyre distributes weight mainly by air pressure rather than direct pressure therefore the air pressure in the tyre is related to the pressure seen by the ground.

[Touses]

ROFL... nope, Snow shoes mate not sandshoes.

Try to cross snow drift in sandshoes you won't make it.
Put on snowshoes (lower ground pressure) you will
A skinny bloke can use smaller shoes than a fat bloke but as they exert the same ground pressure ( lets say 18 psi) they both make it.
If they swap snow shoes the skinny bloke will do it even easier but now the fat bloke won't make it even tho he is wearing snow shoes.

A tyre distributes weight mainly by air pressure rather than direct pressure therefore the air pressure in the tyre is related to the pressure seen by the ground.

Oh I'm so much the wiser now! Off to trade in the maxtrax for snowshoes!

[the evil twin]

Oh I'm so much the wiser now! Off to trade in the maxtrax for snowshoes!

ROFLMAO... we don't get snow here (same as you) but keep the Maxies they'll still work... as much as I'd hate too i guess we'll need to ask the Vics or the Tassie Frozen Ones to be sure tho

[Bulbous]

Think of driving on sand as requiring a triangle or trinity of parameters, and yes they are listed in order of importance

1. Flotation.
2. Momentum.
3. Power.

If all three are present, then your triangle is good and you'll get through.
If any one of them is missing, then it's still possible to drive on sand, but you're compensating for the missing parameter. Sooner or later something will change and you will lose the second parameter also. See below.
If two of them are missing, then it's inevitable that you will get stuck.

Earlier I mentioned having a couple of PSI more in the front tires than the rear - this is because the forces applied to the front (steering) wheels are greater than the rear wheels, so if any tire is going to pop off the rim it will be a front one. Hence the slightly greater pressure.

[Cuppa]

In Tibooburra, first time we have reception since Birdsville. Did Walkers crossing track to Innamincka, up to Coongie Lakes, before coming south to Tibooburra via Old Strzelecki Track & Camerons Corner.

.............Anyway, whilst up at Coongie Lakes we had 41 degrees & a howling gale which did a pretty good job of 'fluffing up' the dunes there around the back of the lake, so I had a chance to experiment with tyre pressures. It's one thing being told lower pressures make all the difference, & another thing to experience it yourself. Tried initially without the Tvan hitched up. 18psi in the fronts & 24 psi in the rears. Purred over the dunes in 2nd or 3rd low doing between 1700 & 2000 rpm. Amazing, & as Plassy said earlier it has really helped to build confidence. Pulling the Tvan over the same dunes, with same pressures was almost as easy, generally requiring 1 gear lower.

On Walkers crossing track, once past the initial rocky gibber section & into the white sand I dropped the pressures by around 30% & had a ball driving what was a soft sand switchback for a couple of hundred kilometres.

Thanks all, especially ET for your insights.

Cuppa

[GQtdauto]

In Tibooburra, first time we have reception since Birdsville. Did Walkers crossing track to Innamincka, up to Coongie Lakes, before coming south to Tibooburra via Old Strzelecki Track & Camerons Corner.

.............Anyway, whilst up at Coongie Lakes we had 41 degrees & a howling gale which did a pretty good job of 'fluffing up' the dunes there around the back of the lake, so I had a chance to experiment with tyre pressures. It's one thing being told lower pressures make all the difference, & another thing to experience it yourself. Tried initially without the Tvan hitched up. 18psi in the fronts & 24 psi in the rears. Purred over the dunes in 2nd or 3rd low doing between 1700 & 2000 rpm. Amazing, & as Plassy said earlier it has really helped to build confidence. Pulling the Tvan over the same dunes, with same pressures was almost as easy, generally requiring 1 gear lower.

On Walkers crossing track, once past the initial rocky gibber section & into the white sand I dropped the pressures by around 30% & had a ball driving what was a soft sand switchback for a couple of hundred kilometres.

Thanks all, especially ET for your insights.

Cuppa

Only trouble we had with that track coming from the opposite direction was the gate at the Birdsville track was bogged up badly but apart from that no issue and tyres were 24 all round .
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