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]

Originally Posted by 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.

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]

Originally Posted by 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.

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.