Ground Pressure / Compaction

[John_Mc]

A friend and were discussion soil compaction during small-scale logging operations -- using a compact tractor, vs using a mini tracked skidder. The two were approximately the same weight. Obviously, the skidder exerts less PSI on the ground than the CUT, due to the skidder's hugely greater footprint.

My friend claimed that you can figure out the pounds per square inch that a tractor is putting on the ground just by looking at the PSI in the tires. The theory her is that if you run your tires at 20 PSI, then they will just deform until they have flattened out enough to match that 20 PSI onto the ground.

I can see the thinking here, but to me that would just be in ideal situations. It does not take into account the fact that the tires are not infinitely elastic. They have some stiffness, and limits to how much they will deform. In fact, R4 tires (and perhaps other, heavier duty tires) are designed to be stiff. To take it to an extreme, consider the run-flat tires on some cars. Even with a hole in the sidewall, they will barely deform (at least not until you drive it around enough to start breaking down the sidewalls). The PSI this tire exerts on the ground does not go down when it's empty. (In fact, it may go up, since almost all of the weight is on the edges of the tire. The center of the tread is probably not supporting as much of a load).

An example at the other extreme: If I go from 20 PSI up to 60 PSI in my tires, does my tractor's footprint really decrease to 1/3 of what it was at 20? That is what would be necessary for the PSI exerted on the ground to match the PSI in the tires (given constant tractor weight).

I'm interested in people's thoughts on this, and particularly if anyone can provide a link to an authoritative source with something written on the subject.

John Mc

 

[horse7]

Besides the tire carcass effects you mention, he would at least need to know the tire footprint in addition to PSI. PSI alone won't get there.

I've never investigated the effect, but offroaders use the PSI/tire footprint/vehicle weight concept, so it is probably effective to at least a first order approximation for auto tires.

 

[CurlyDave]

This may come as a complete shock to you, but there is very little relationship between the psi in a tire and the ground loading.

The air pressure stiffens the tire sidewalls, which is what holds up the vehicle.

This seems grossly counterintuitive to anyone who is not a mechanical engineer, but it is true.

 

[Youare]

The pounds per square inch force a tire or track puts on the ground is directly related to the weight of the piece of equipment and the contact area the tire or track has on the ground. On a wheeled piece of equipment the weight distribution on each wheel also is a factor. On a CUT with a bucket full of material the PSI on the front tires is much greater than the rear tires.

On a 2000 lb. tractor that had an 8" x 10" portion of the rear tire in contact with the ground and a 4" x 6" portion of the front tires in contact with the ground would have just about 10 PSI. Being that ag tires have traction lugs the pressure would be concentrated on those areas on hard ground, on soft ground it would be spread out over the whole contact area. Low tire pressure allows more tire contact with the ground.

On a tracked piece of equipment of the same weight the pressure would be spread out over the entire track which is why the ground pressure can be as low as 2 PSI or so.

 

[Soundguy]

Youare and the others nailed it.

Add up the tire contact patches, and then get the weight of the machine.. divide it up.. then you have ground #'s psi.

soundguy

 

[John_Mc]

Besides the tire carcass effects you mention, he would at least need to know the tire footprint in addition to PSI. PSI alone won't get there.

I've never investigated the effect, but offroaders use the PSI/tire footprint/vehicle weight concept, so it is probably effective to at least a first order approximation for auto tires.

Actually, my thinking was that you don't need the tire pressure at all. If you know the footprint, the only other thing that you need is the weight of the tractor. The only reason I can see that tire pressure would come into play is that changing the tire pressure can change the footprint... but again, if you know the footprint, what does the pressure matter. And, for reasons I mentioned in my original post, There is more to how the foot print changes than JUST the PSI in the tires or the weight of the tractor.

John Mc

 

[slowzuki]

There is actually more to it than that.

Due to the tire carcass stiffness, the psi is not even over the contact patch. So you can find an average ground loading that way, but not the peak which is what will compact soil.

 

[John_Mc]

This may come as a complete shock to you, but there is very little relationship between the psi in a tire and the ground loading.

The air pressure stiffens the tire sidewalls, which is what holds up the vehicle.

This seems grossly counterintuitive to anyone who is not a mechanical engineer, but it is true.

Actually, not a shock at all, since that's exactly what I was arguing to my friend with the tracked skidder.

John Mc
(Not a mechanical Engineer, nor do I play one on TV. I didn't even stay at a Holiday Inn Express last night. ... however, these references probably indicate that I've been watching too much TV, and could be suffering severe decay of brain function as a result)

 

[dirtworksequip]

I would say if you were comparing an equal weight tractor and track skidsteer the skidsteer is going to have the lesser ground pressure. Simply because the tracks have a greater foot print than a normally equiped tractor and therefore more area to spread out the weight of the machine. Unless you could get tires on the tractor that would effectively give you the same contact area as the tracks, the tracked skidsteer is going to have less ground pressure.

Sincerely, Dirt

 

[daTeacha]

Lugs on the tires and cleats on the tracks will also make a difference. The pressure from an Ag tire lug will be different from the interlug area of the same tire. The soil type and moisture content will affect things, too. Some soils will deflect sideways more than others.

 

[Soundguy]

For the generic tractor compaction 'argument' I think 'average' PSI should be fine..

soundguy

 

[John_Mc]

Found a good research article on the subject at this link the source is a professor at Penn State's College of Agricultural Sciences.

The shocker to me was the statement that "With most farm tires, surface contact pressure is about 1 to 2 psi higher than tire pressure due to stiffness in the tire." However, it did go on to say "The best way to determine contact pressure is to calculate the load in pounds per wheel and divide it by the area of the tire that touches the soil (in square inches). This will give you the average contact pressure under that tire in psi."

Another interesting wrinkle: compaction of Topsoil (0-12" depth) is dependent on "contact pressure" (basically weight of equipment divided by the footprint). Compaction in the upper subsoil (12-20" depth) is a combination of contact pressure and Axle Load. Below 20", soil compaction is entirely a function of axle load.

Axle load is the total load supported by one axle, usually expressed in tons or pounds. Farm equipment with high axle loads will cause compaction in the topsoil and subsoil, whereas low axle loads will cause
compaction in the topsoil and the upper part of the subsoil only

topsoil compaction will genreally go away in about 5 years. Upper subsoil in about 10 years. Lower subsoil compaction is essentially permanent - even in northern lattitudes where the freeze/thaw cycle reaches 40-50 inches deep.

Got my soil sciences lesson today.

John Mc

 

[Rara Avis]

In Vermont...our freeze-thaw cycle goes a little deeper...
Frost Penetration Prediction Using Simulation With GIS

 

[Nuru]

Ok, so, where can one find out the load per wheel? Or what is the easiest method to calculate it?

 

[RalphVa]

Where you get compaction is from the treads on a skidder. They'll put a huge amount of pressure on the soil. Same with Ag treads on a tired tractor. This is the benefit of using turf treads. They likely put less pressure on the soil than the heal of your shoe while you're walking.

Ralph

 

[John_Mc]

In Vermont...our freeze-thaw cycle goes a little deeper...

Interesting article. I live less than 10 miles from one of their test points (New Haven). I wouldn't dispute that the frost here can extend below the 40-50" cited in the article (even here in Vermont's "banana belt"), especially under pavement -- it tends to freeze quicker and deeper than soil. I'd guess this is at least in part due to the fact that the pavement is kept clear of snow, which would act as an insulator if left on.

The article I cited indicated that the soil compaction at least in the "lower subsoil" (which they defined as below 20" depth) was not affected by the freeze-thaw cycle. It was essentially permanent... even in places where the freeze-thaw cycle goes to 40 or 50 inches.

 

[Rara Avis]

Well, when I ran a waterline out to the barn...I was done by a lifelong Vermonter...he dug it 7 feet deep...as he said just in case we have one of those really cold winters...

 

[Z-Michigan]

Well, when I ran a waterline out to the barn...I was done by a lifelong Vermonter...he dug it 7 feet deep...as he said just in case we have one of those really cold winters...

I have to visit Vermont sometime... sounds weird. Michigan gets pretty cold and snowy too. In my area frost depth for building code is 52". While it is slightly less in the southern tip of the state, as you would expect, strangely it is significantly less in the northern lower peninsula - which is both colder and snowier. Apparently in many areas up there the snow is reliable enough to insulate the ground and reduce frost depth, despite low temps that may be 10+ degrees lower than where I'm at, and much more consistent freezes. (My area often thaws every 1-2 weeks even in January.)

 

[Rara Avis]

I have to visit Vermont sometime... sounds weird. Michigan gets pretty cold and snowy too. In my area frost depth for building code is 52". While it is slightly less in the southern tip of the state, as you would expect, strangely it is significantly less in the northern lower peninsula - which is both colder and snowier. Apparently in many areas up there the snow is reliable enough to insulate the ground and reduce frost depth, despite low temps that may be 10+ degrees lower than where I'm at, and much more consistent freezes. (My area often thaws every 1-2 weeks even in January.)

What happens up here is as the thaw happens on the surface ground...the frost is driven deeper...

 

[daTeacha]

Getting back to the original topic, there was an article I read somewhere in the past -- sorry, I'm no librarian so I can't tell you where -- about the different ways compaction is calculated in Europe and on this side of the pond. The European method takes into account some different factors and I believe measures compaction to a greater depth as well to either side of the tire. I know someone here will have the thing at their fingertips. I think it might have been on a tire manufacturer's website or something like that.

Along those lines, an energy efficiency article in the latest 'Ohio Farmer' states that most farmers run their tractor tires at too high a pressure, increasing slippage. It also emphasized the importance of the correct balance or front-rear weight distribution for maximum traction and efficiency.