Tire inflation question

[Heavyduty]

There are limits at each end, i.e. once it is "rock hard" another few psi won't change the contact area significantly, unless it blows it off.
Similarly, if it is so low that the rim is pinching the sidewall to the ground a couple of psi won't reduce the contact area by much.

Within the useful range pressure is close to weight/area.

Anyway, second guessing that we all are MASTERS at,,,; it still makes more sense to get the load/inflation tables, weigh each axle, do the table look up, adjust for special factors, then you KNOW you've done it RIGHT.

What about this line of thinking? :laughing:

I'm looking at some work that requires my tractor to have a low ground impact, around 10 psi.

I looked on the Firestone ag tire website and my 20.8 R 34 rear tires have a tire footprint of 360 sq inches each and the front 11 x 16 have a tire footprint of 70 sq inches.

Can I divide my tractor weight of 8600 lbs by the total tire footprint of 860 sq inches? 8600lbs/860sq in=10psi Would this hold up if a conservation person asked about the low ground impact?

 

[RoyJackson]

Can I divide my tractor weight of 8600 lbs by the total tire footprint of 860 sq inches? 8600lbs/860sq in=10psi Would this holp up if a conservation person asked about the low ground impact?

I think that wouldn't be totally accurate...

Here's a bit of light reading:
http://pubs.cas.psu.edu/freepubs/pdfs/uc188.pdf

Also, use this phrase to search the web (I used Google):
How to calculate tractor weight for soil compaction

You'll get a number of hits...

 

[paul314159]

Only if your tractor weight is evenly distributed on each tire.

Paul

 

[Reg]

What about this line of thinking? :laughing:

I'm looking at some work that requires my tractor to have a low ground impact, around 10 psi.

I looked on the Firestone ag tire website and my 20.8 R 34 rear tires have a tire footprint of 360 sq inches each and the front 11 x 16 have a tire footprint of 70 sq inches.

Can I divide my tractor weight of 8600 lbs by the total tire footprint of 860 sq inches? 8600lbs/860sq in=10psi Would this holp up if a conservation person asked about the low ground impact?

No.
You can weigh it axle by axle - divide each by two, etc.
That goes to all to heck once you put the loader back on - and/or pick up a load IN the bucket, but it MIGHT get you through whatever quiz the conservation dude puts to you (-:

Plan B;
Air your tires to 9.9 psi and see how they look.
Then put the loader back on, load the bucket, etc.
Only do the job if they don't look like they're about to roll off the rims.

Yeah, I think I would go with Plan B first.
At least you could present a defensible method to the Cons dude if reqd.

 

[SPYDERLK]

I was wondering if you run your tires at 12 psi, would the actual weight from the tractor have a ground pressure of 12 psi? :confused2:

I've always been told that, by farmers when talking about soil compaction.

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

 

[RoyJackson]

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

Larry, take a look at that link I posted in #22, specifically "MANAGING SOIL COMPACTION" on page 11 of 12.

 

[SPYDERLK]

Larry, take a look at that link I posted in #22, specifically "MANAGING SOIL COMPACTION" on page 11 of 12.

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.

 

[RoyJackson]

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

Maybe I'm misunderstanding what you wrote in your first response about ground pressure.
If the tires put 12 PSI and there is a total of (example) 300 sq in contact patch (I'm disregarding weight distribution and assuming all four tires have the same patch size), that would result in a total of 3600 pounds. That's likely just a percentage of the tractor's weight.
So, assume an 8000 lb tractor with a 300 sq in total contact patch...still think it's 12 PSI?

 

[orezok]

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.

 

[SPYDERLK]

Maybe I'm misunderstanding what you wrote in your first response about ground pressure.
If the tires put 12 PSI and there is a total of (example) 300 sq in contact patch (I'm disregarding weight distribution and assuming all four tires have the same patch size), that would result in a total of 3600 pounds. That's likely just a percentage of the tractor's weight.
So, assume an 8000 lb tractor with a 300 sq in total contact patch...still think it's 12 PSI?

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