Tire inflation question

[Reg]

There is a basic law of physics that states " for every action there is an EQUAL and opposite reaction". This would mean that if there is 10 psi of pressure in the tire then there would have to be 10psi of reaction (ground pressure). This would be all well and good if your tires were perfectly flexible like a balloon, but they are not.

In a semi rigid structure like a tire, the tire structure will transfer load internally. That means at the center contact point of the tire it will be more than 10 psi and at the periphery it will be less than 10 psi.

The bottom line is that the total contact area will AVERAGE 10psi, but you may actually get 15psi of compaction.

I guess it has been a while ?
If you hurry you MIGHT be able to re-take Fizzzix 101 in summer skool (-:
{deliberate spelingz}

 

[SPYDERLK]

There is a basic law of physics that states " for every action there is an EQUAL and opposite reaction". This would mean that if there is 10 psi of pressure in the tire then there would have to be 10psi of reaction (ground pressure). This would be all well and good if your tires were perfectly flexible like a balloon, but they are not.

In a semi rigid structure like a tire, the tire structure will transfer load internally. That means at the center contact point of the tire it will be more than 10 psi and at the periphery it will be less than 10 psi.

The bottom line is that the total contact area will AVERAGE 10psi, but you may actually get 15psi of compaction.

[[[I guess it has been a while ?
If you hurry you MIGHT be able to re-take Fizzzix 101 in summer skool ]]](-:
{deliberate spelingz}

Hey Reg. You did that here too in Post #18 of this thread. http://www.tractorbynet.com/forums/buying-pricing-comparisons/145738-conversion-factor-3-point-hitch.html
Instead why dont you state specifically what your point of disagreement is?
larry

 

[Heavyduty]

Thanks for ALL the info on tire footprints and pressure. Those dang conservation people are about to give use all an heart attack over it.:thumbsup:

 

[RoyJackson]

Yes. The 300sqin contact patch would have to come from tires that were approx 8000/300 psi. [Not 12.] If 12 then Area= ~8000/12 = 667sqin. The point is that all it takes to know ground pressure is to know the pressure in the tire. Area of contact naturally follows from that.
larry

Not sure I agree with you, Larry...and the way I read that Penn State .pdf file on soil compaction doesn't follow your logic either (why would they suggest using flotation tires or duals to reduce compaction (by distributing the weight) unless the tractor's weight had a bearing?). It seems you're ignoring the weight on the tractor...am I mistaken there?
Here's a couple links that may be of interest: http://www.ars.usda.gov/SP2UserFiles/Place/64200500/csr/ProjectReports/PR04.pdf and http://www.no-tillfarmer.com/pages/News---High-Flotation-Tires-May-Make-Sense.php

One thing I glean from both is by going to a lower pressure or wider tire (flotation tire), the load (weight) is spread over a greater area, thus reducing the pressure per square inch on the soil.
What I could not find is anything to support the "tire pressure PSI equates to ground pressure" statements.

So, how about a link to provide a bit of objective evidence to your theory?

 

[orezok]

I guess it has been a while ?
If you hurry you MIGHT be able to re-take Fizzzix 101 in summer skool (-:
{deliberate spelingz}

WOW! You're creating new laws ...huh?

 

[paul314159]

Here's a link from Boeing that supports the contact area = psi * weight per tire.
calctirecontactarea.pdf


Paul

 

[RoyJackson]

Here's a link from Boeing that supports the contact area = psi * weight per tire.
calctirecontactarea.pdf


Paul

Very interesting...thanks for posting!

 

[xtn]

Tire internal air pressure should equal contact patch pressure IF the tire is very pliable and smooth like a slick. Imagine if you were driving around on tubes only. You would get an oval shaped contact patch that would increase or decrease in size to exert pressure against the ground almost exactly equal to the internal air pressure.

Put lugs on the tire though, and the point loading goes up. A turf tire would most closely follow the ideal condition, but still wouldn't do it anywhere close to 100%. An R1, for an example of the other extreme, will have significantly higher point loads where the tread touches the ground. Significantly. As in probably at least an order of magnitude, or ten times as much. In other words a 20psi R1 might exert 200psi on the ground at the lugs' contact areas. Or maybe twice that; I'm just guessing at those numbers. But trust me they are significantly high. I mean duh, that's why R1s will dig down into dirt so easily after all.

xtn

 

[SPYDERLK]

Good info. It doesnt differ from what I said regarding inflation and contact pressure tho.
larry

: Decrease contact pressure by using flotation tires,
doubles, or tracks. Topsoil compaction is caused by
high contact pressure. To reduce contact pressure, a
load needs to be spread out over a larger area. This
can be done by reducing inflation pressure. A rule of
thumb is that tire pressure is the same as contact
pressure. Tires inflated to 100 psi such as truck road
tires should be kept out of the field. To be able to
carry a load at low inflation pressure, bigger or
multiple tires are needed, hence the need for flotation
tires and doubles. Large-diameter tires also help
to increase the tire footprint. Tracks help to spread
the load over a large area, but having multiple axles
under the tracks is necessary to avoid high spikes of
pressure. Tracks have the advantage over doubles of
reducing contact pressure without increasing the
area of the field trafficked.

Not sure I agree with you, Larry...and the way I read that Penn State .pdf file on soil compaction doesn't follow your logic either (why would they suggest using flotation tires or duals to reduce compaction (by distributing the weight) unless the tractor's weight had a bearing?). It seems you're ignoring the weight on the tractor...am I mistaken there?
Here's a couple links that may be of interest: http://www.ars.usda.gov/SP2UserFiles/Place/64200500/csr/ProjectReports/PR04.pdf and No-Till Farmer - High-Flotation Tires May Make Sense

One thing I glean from both is by going to a lower pressure or wider tire (flotation tire), the load (weight) is spread over a greater area, thus reducing the pressure per square inch on the soil.
What I could not find is anything to support the "tire pressure PSI equates to ground pressure" statements.
So, how about a link to provide a bit of objective evidence to your theory?

See above color hilited from your link.
larry

 

[SPYDERLK]

Yes, that is pretty much right. If a tire were like an inner tube it would be dead right, but carcass stiffness makes a difference as tires deflect. With a smooth tire its still pretty darn close. With an AG of course, the tread bars give intense pressure until they penetrate fully. ... But averaged over the full area of tire that is "flat on the bottom" the ground pressure is only slightly above the tires internal pressure.
larry

Tire internal air pressure should equal contact patch pressure IF the tire is very pliable and smooth like a slick. Imagine if you were driving around on tubes only. You would get an oval shaped contact patch that would increase or decrease in size to exert pressure against the ground almost exactly equal to the internal air pressure.

Put lugs on the tire though, and the point loading goes up. A turf tire would most closely follow the ideal condition, but still wouldn't do it anywhere close to 100%. An R1, for an example of the other extreme, will have significantly higher point loads where the tread touches the ground. Significantly. As in probably at least an order of magnitude, or ten times as much. In other words a 20psi R1 might exert 200psi on the ground at the lugs' contact areas. Or maybe twice that; I'm just guessing at those numbers. But trust me they are significantly high. I mean duh, that's why R1s will dig down into dirt so easily after all.

xtn

:thumbsup: :thumbsup: