What does tyre pressure have to do with ground pressure?

[stablegirl]

Other than altering a tyres footprint?

On a few threads i've read different opinions on tyres size and what effect it has on the ground, some people discussing the volume of air in a tyre making the difference? Another thread saying the psi in a tyre is the psi a tractor exerts on the ground?

If Pressure (Ground) = Force(weight on axle) / Area (Tyres footprint)

Has the volume of air or tyre pressure got anything to do with it apart from the effect on a tyres footprint?

 

[davidroberts30]

Other than altering a tyres footprint?

On a few threads i've read different opinions on tyres size and what effect it has on the ground, some people discussing the volume of air in a tyre making the difference? Another thread saying the psi in a tyre is the psi a tractor exerts on the ground?

If Pressure (Ground) = Force(weight on axle) / Area (Tyres footprint)

Has the volume of air or tyre pressure got anything to do with it apart from the effect on a tyres footprint?

Cant see that
A 7t tractor still weighs the same if the tyres are flat or 15psi
Let your tyres down flat and drive across a wet field and it will still make a mess....

 

[stablegirl]

I think the original thread was about new fastracs like of tyre options, maybe @Clive mentioned volume of air?

 

[Clive]

threads like theses are always interesting if only to usually highlight how poor GCSE physics is taught in schools

 

[Courier]

To state the obvious - Air is a mixture of gasses at NTP - Normal Temperature and Pressure - is defined as air at 20oC (293.15 K, 68oF) and 1 atm (101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 29.92 in Hg, 407 in H2O, 760 torr).

The volume of air within a vessel (tyre) will not vary significantly at a different pressure but the mass will.

The lower the pressure within a tyre then the lower the pressure exerted on the ground (Plus weight transferred by virtue of the stiffness within the tyre walls.)

 

[puppet]

Other than altering a tyres footprint?

On a few threads i've read different opinions on tyres size and what effect it has on the ground, some people discussing the volume of air in a tyre making the difference? Another thread saying the psi in a tyre is the psi a tractor exerts on the ground?

If Pressure (Ground) = Force(weight on axle) / Area (Tyres footprint)

Has the volume of air or tyre pressure got anything to do with it apart from the effect on a tyres footprint?

A force must be met by an equal and opposite force otherwise there is movement. So if your tyre is 30 psi it will sink until the soil footprint is equal to 30 psi either by a bigger contact area or firmer subsoil.
It is all about compaction not just weight.
What would you prefer standing on your chest, an 8 stone lady on one stiletto heel or a 12 stone bloke on a 6x2 board.

Obviously there may be other reasons than physics as to how some will answer this.

 

[yellowbelly]

Every force has an equal and opposite force.
No idea if 20psi in your tyre = 20psi on the ground though

What I do know is that a terra tyre with 5psi in it will go where an ordinary tyre with 20psi in it won't.

..a similar tractor on terra tyres went down that tramline 5 minutes before this ordinary tyred machine tried.

 

[vantage]

What would you prefer standing on your chest, an 8 stone lady on one stiletto heel or a 12 stone bloke on a 6x2 board.

A lady for me , but each to their own!

 

[stablegirl]

I understand each force must have an equal and opposite force.

But the psi in a tyre acts on the tyre itself, in an elastic manner.

Just thinking about it, if you put a car on a lift and put 30psi in the tyres it clearly has 30psi in the tyre but exerts 0 pressure on the ground or air around it.

 

[Dave W]

Well unless I'm totally missing the point. You alter the amount of air in the tire to change its shape and thus area in contact with ground.
The tractor will weigh the same but the footprint varies with the amount of air. Bigger footprint =less pressure exerted on the ground.
the fact that ground pressure and volume of air in the is expressed in the same way is purely coincidental.

 

[ridger]

I understand each force must have an equal and opposite force.

But the psi in a tyre acts on the tyre itself, in an elastic manner.

Just thinking about it, if you put a car on a lift and put 30psi in the tyres it clearly has 30psi in the tyre but exerts 0 pressure on the ground or air around it.

It wouldnt move far anyway that's for sure unless you have the lift on a trailer

 

[stablegirl]

Still doesn't answer the question.

We shall just have to wait for @Clive to explain to us.

 

[Clive]

Still doesn't answer the question.

We shall just have to wait for @Clive to explain to us.

im staying out of this ! got bored last time listening people tell me a 30t quadtrac was in fact lighter than a fergie 135 !

google the difference between mass and weight would be a good starting point

 

[curly]

The lower pressure in a tyre makes the footprint bigger so there is less pressure on the ground. The actual tyre pressure does not mean the actual pressure on the ground, try a small tyre at 20 psi for the marks it leaves in the soil & then a big one at 30 psi with the same weight on top & then compare the depth of marks. If a tyre goes flat to 0 psi it still leaves marks!

 

[stablegirl]

All these replies with the original post that tyre psi is only relevant to footprint.

And ground pressure is only effected by the footprint and axle weight.

 

[colhonk]

A 1 foot wide tyre at 30psi will exert a hell of a lot more ground pressure per square inch than a 10 ft wide tyre at the same pressure bearing the same weight. My older brother would insist that a tyre exerted the same pressure per square inch as the tyre pressur, poppycock.
As for the lady with the syleto`s on,depends if I had the right glasses on

 

[Skimmer]

what you need to know is at what ground pressure damage is being done otherwise fitting wider tyres may just damage more ground.

 

[betweenthelines]

No air pressure in tracks, is that why they're used on heavy machinery?

 

[Phil P]

Tyre pressure it the pressure exerted on the inside of the tyre and is completely different to ground pressure!
All those that keep saying let the tyre down to 0psi and it will still leave a mark, of course it will The tyre side walls and the carcasses will support the weight to an extent, or the rim will just put more force through the tyre where it contacts the carcass.

Tyre press will affect ground pressure but it isn’t equal to it.


Now, let’s here @Clive ’s bridge theory

 

[mo!]

Ground pressure is the weight (mass x g (9.8)) divided by the contact area. Depending on the soil conditions that pressure may or may not cause compaction. Ground pressure on earth can only be changed by altering the weight or the contact area. The contact area can be changed by using different ground engaging equipment, typically either tyres or tracks, or by changing the pressure in those tyres.