Outer Limits 4x4 Board

The tyre pressure is not the ground pressure.

Only effect tyre pressure has is to change the shape of the tyre where it touches the ground.

Contact pressure increases with vehicle weight.

It decreases as contact area gets larger, assuming no weight change.

christover

christover1 wrote:The tyre pressure is not the ground pressure.

Only effect tyre pressure has is to change the shape of the tyre where it touches the ground.

Contact pressure increases with vehicle weight.

It decreases as contact area gets larger, assuming no weight change.

christover

the pressure in the tyre is what transfers the weight(a force) of the load(the vehicle) to the ground. The contact area is a function of the tyres internal pressure and the load applied to the tyre.

Shadow wrote:

christover1 wrote:The tyre pressure is not the ground pressure.

Only effect tyre pressure has is to change the shape of the tyre where it touches the ground.

Contact pressure increases with vehicle weight.

It decreases as contact area gets larger, assuming no weight change.

christover

the pressure in the tyre is what transfers the weight(a force) of the load(the vehicle) to the ground. The contact area is a function of the tyres internal pressure and the load applied to the tyre.

yes, that's what I said, I believe.

4WD Stuff wrote:Yes it will. Divide 750kg 1/4 of your 4wd by the contact patch area.

Whats the contact patch area?

The only way I know to set the contact patch area is by changing your tyres pressure. And the reason this changes the contact patch is because you are directly changing the surface pressure or contact pressure, whatever you want to call it.

christover1 wrote:

Shadow wrote:

christover1 wrote:The tyre pressure is not the ground pressure.

Only effect tyre pressure has is to change the shape of the tyre where it touches the ground.

Contact pressure increases with vehicle weight.

It decreases as contact area gets larger, assuming no weight change.

christover

the pressure in the tyre is what transfers the weight(a force) of the load(the vehicle) to the ground. The contact area is a function of the tyres internal pressure and the load applied to the tyre.

yes, that's what I said, I believe.

Well you said the tyre pressure is not the ground pressure, but it is.

You said contact pressure increases with vehicle weight. It does (very slightly and can in most cases be ignored) but only because your tyre must squash down more to increae the contact area to balance the extra weight. (decreasing the volume of the tyre, and since theres the same mass of air in a smaller volume the tyre pressure does increase).

You said contact pressure decreases as contact area increases. This is absolutely correct. But the tyre pressure(which is the contact pressure) is actually what changes.


tyre pressure = ground pressur, surface pressure, whatever you want to call it.

Shadow wrote:

christover1 wrote:

Shadow wrote:

christover1 wrote: Well you said the tyre pressure is not the ground pressure, but it is.

You said contact pressure increases with vehicle weight. It does (very slightly and can in most cases be ignored) but only because your tyre must squash down more to increae the contact area to balance the extra weight. (decreasing the volume of the tyre, and since theres the same mass of air in a smaller volume the tyre pressure does increase).

You said contact pressure decreases as contact area increases. This is absolutely correct. But the tyre pressure(which is the contact pressure) is actually what changes.


tyre pressure = ground pressur, surface pressure, whatever you want to call it.

The tyre pressure affects the ground pressure, but it isn't the ground pressure. They would effect each other with some quantifiable formula, but they ain't each other.

Contact pressure is weight over contact area, and thus any change in either variable must therefore change the other variable. This includes the change in contact area changed as a result of internal pressure (which can change as a cause or result)


tyre pressure is related to ground pressure, and both are related to weight, but they are not the same entities.

christover

sounds like something out of monty python

I think there might be confusion. Christover is talking about tyre pressure in a sence of how much air there is in the tyre. I think shadow might be talking about tyre pressure, ie the pressure that the tyre is exerting on the ground.

muppet_man67 wrote:sounds like something out of monty python

I think there might be confusion. Christover is talking about tyre pressure in a sence of how much air there is in the tyre. I think shadow might be talking about tyre pressure, ie the pressure that the tyre is exerting on the ground.

I live in a constant state of confusion, that's what keeps me sane

they are the same :S

give me a formula to calculate ground pressure.

Shadow, you are arguing that air pressure is the only thing that governs the size of the contact patch (on a given vehicle - mass constant) - and is independant of tyre size.

If all tyres had the same side wall stiffness (or sidewall stiffness was insignificant) then I would agree with you. However this is definitely not the case.

Shadow wrote:they are the same :S

give me a formula to calculate ground pressure.

weight or force over area

as in KG per meter squared

or lbs per square inch

ISUZUROVER wrote:Shadow, you are arguing that air pressure is the only thing that governs the size of the contact patch (on a given vehicle - mass constant) - and is independant of tyre size.

If all tyres had the same side wall stiffness (or sidewall stiffness was insignificant) then I would agree with you. However this is definitely not the case.

sorry i did say in several of my posts negating side wall load, but in my last 10 attempts to make my point i forgot to point this out.

the sidewall is always going to play a part, but going from 32psi to say 20psi the sidewall does not contribute a whole lot. Going down to 2 psi the sidewall stiffness certainly does.

Shadow wrote:

ISUZUROVER wrote:Shadow, you are arguing that air pressure is the only thing that governs the size of the contact patch (on a given vehicle - mass constant) - and is independant of tyre size.

If all tyres had the same side wall stiffness (or sidewall stiffness was insignificant) then I would agree with you. However this is definitely not the case.

sorry i did say in several of my posts negating side wall load, but in my last 10 attempts to make my point i forgot to point this out.

I saw where you mentioned it, but there is no point arguing a theory that is so oversimplified it is incorrect.

ISUZUROVER wrote:

Shadow wrote:

ISUZUROVER wrote:Shadow, you are arguing that air pressure is the only thing that governs the size of the contact patch (on a given vehicle - mass constant) - and is independant of tyre size.

If all tyres had the same side wall stiffness (or sidewall stiffness was insignificant) then I would agree with you. However this is definitely not the case.

sorry i did say in several of my posts negating side wall load, but in my last 10 attempts to make my point i forgot to point this out.

I saw where you mentioned it, but there is no point arguing a theory that is so oversimplified it is incorrect.

ok well i have read several webpages talking about the ground pressures of 6x6 and 8x8 vehicles and they claim the sidewall or tyre carcass pressure correlates to around 1PSI on those vehicles with the tyres inflated to only 8PSI giving a ground pressure of 9PSI.

So are you saying that a tyre pumped up to 32 psi exerts 32 psi onto the ground also, independant of all other variables?

christover1 wrote:So are you saying that a tyre pumped up to 32 psi exerts 32 psi onto the ground also, independant of all other variables?

no, i did mention earlier that the tyre wall also takes some of the load

but pumped to 32PSI i think the sidewall load would be very insignificant.

there are probably several variables that effect the actual ground pressure but i believe they are largely insignificant.

so yes it would be very very close to 32PSI.

so what exactly are you saying?

christover1 wrote:so what exactly are you saying?

im saying the ground pressure of a fatter tyre is the same as a skinny tyre (very close)

Then are you saying ground pressure would be equal to the tyre pressure? roughly
This could be true, never done the maths.

But they are not the same thing.

Shadow wrote:

christover1 wrote:so what exactly are you saying?

im saying the ground pressure of a fatter tyre is the same as a skinny tyre (very close)

So if both are at 32psi and both are same ground pressure, then contact area must be the same?

weight over area?

christover1 wrote:Then are you saying ground pressure would be equal to the tyre pressure? roughly
This could be true, never done the maths.

But they are not the same thing.

tyre pressure will be smaller, the amount it is smaller depends on tyre wall stifness and other things taking the load. If the tyre had no side wall strength (like a round wheelbarrow tyre) the tyre pressure would be extremely close. The thicker the sidewall and the lower the tyre pressure the more load will be taken by the sidewall.

Shadow wrote:

christover1 wrote:The tyre pressure is not the ground pressure.

Only effect tyre pressure has is to change the shape of the tyre where it touches the ground.

Contact pressure increases with vehicle weight.

It decreases as contact area gets larger, assuming no weight change.

christover

the pressure in the tyre is what transfers the weight(a force) of the load(the vehicle) to the ground. The contact area is a function of the tyres internal pressure and the load applied to the tyre.

So let me check this: are you truly saying if there is no tyre pressure, then there is no force on the ground? Holy sh1t batman, we have a hovercraft.

Or if the tyre is not pumped up it's a flat blob of rubber on the ground?


What part of this are you struggling with?

The thing that puts pressure on the ground is not how much presure is in the tyre, it's the weight of the whole vehicle.

The things that stop the tyre from collapsing under that weight are the carcass of the tyre and the tyre pressure.


The vehicle will still be putting 2 or 3 ton of force on the ground whether or not it's sitting on a tyre or on the rims. After all we still put force on the ground and we dont have tyres with air pressure......

Beastmavster wrote:

sudso wrote: But dont ya want the tyres to bite down to the harder stuff rather than float?

Depends on how deep down the hard stuff is

exactly

So what you mean is: at 32 psi a theoretical skinny tyre would balloon out to the same contact area on the ground as the fat tyre does, thus exerting the same pressure per square inch on the ground..ignoring realistic variables?

From what I think you are meaning, the ground pressure may be a similar pressure to the tyre pressure, but they both have two different meanings?

So ground pressure is still not tyre pressure, though they are similar numbers

Beastmavster wrote: After all we still put force on the ground and we dont have tyres with air pressure......

Well, not totally true, I got a few spare tyres, but I guess they don't touch the ground yet, so maybe ya right

F*** this, I'm gonna chuck a 31x10.5x15 on one wheel this weekend and then scan that and my 285x75x16.

Both tyres will be identical pressure. Someone can do the tape measure work.

I will two lots of tread prints ..... one at 32 psi and one at 16psi.

Now it wont be exactly equal because the construction of the carcass will be different, and the car will be leaning on a slight angle cos I'm not changing all 4 tyres, but at least we will see some mythbusting here.

christover1 wrote:

Beastmavster wrote: After all we still put force on the ground and we dont have tyres with air pressure......

Well, not totally true, I got a few spare tyres, but I guess they don't touch the ground yet, so maybe ya right

Are they full of air pressure, or carbon dioxide pressure from beer and baked beans.

Beastmavster wrote:Are they full of air pressure, or carbon dioxide pressure from beer and baked beans.

Damn, now my secret is out, everybody just assumed Frog had NOS
But this is a tech thread, so I'll stop the wize cracks now

Example only not accurate.

Bicycle Tyre pumped up to 32PSI has a Ground contact area of 2 inches wide x 20 inches long which = 40 square inches of Contact area.

750kg/40Sq Inches = 18.75kg per square inch

4wd Tyre pumped up to 32 PSI is 12.5 inches wide x 10 inches long = 125 square inches of contact area

750kg/125Sq Inches = 6kg per square inch.

Does this help

4WD Stuff wrote: Does this help

Well, sort of, but they are real tyres.
The theoretical tyres with no sidewall strength would balloon out till the internal pressure was same as outside air pressure, and would thus give no support, and truck would fall onto the rims , but this depends what structural parameters you endow the theoretical tyres with

this thread, and my late night, is all your fault