5425 vs 4320 vs 3720 Surface Pressure

[jenkinsph]

I know what the OP was asking. Thats why I answered him the way I did. Because what you "believe" is that it dont matter what size the tire is, ground pressure is the same as tire PSI. And that is simply incorrect. If the tire were like a balloon, yes. But tires arent. They have sidewalls and belts. This keeps them from making drastic changes as the pressure changes. IE: doubling the tire pressure doesnt double its size. Therefore, while tire PSI does have an effect on ground pressure, it is far from linear.

You would have the OP believe that if he were to take the smaller tires off of the 3720, and put them on the 5425, as long as PSI is the same, ground pressure would be the same, or vice versa with the large tires on the small tractor, but they are not the same. Duallys ring a bell?? Are you saying that two dual tires @ 30PSI have just as much ground pressure as 1 @ 30PSI???



:thumbsup::laughing:

Please don't confuse what you are suggesting with what I have said.

 

[LD1]

Please don't confuse what you are suggesting with what I have said.

I am not confusing anything. Tire pressure does NOT equal ground pressure.

Here is an bit from wikipedia

Statically, the size, shape, and pressure distribution are functions of many things,[1] the most important of which are the load on the tire and the inflation pressure:
The larger the load on the tire, the larger the contact patch.
The larger the inflation pressure, the smaller the contact patch.
Unfortunately, these two properties are not linearly proportional to the area of the contact. Put another way, a 10% change in load or inflation pressure usually does not result in a 10% change in the contact patch.[2] This makes intuitive sense, because while the load or pressure of a tire can be altered freely, the contact patch size will always be limited by the tire geometry.

And here for some even better reading Fact or Fiction? Tire contact patch and air pressure.

You run your little mower at 8PSI. Great. If you doubled it to 16psi, you would NOT double your ground pressure when cutting your lawn.

 

[murphy1244]

How strong does a bridge have to be?I found that to be 3000 PSI on a bridge deck.

 

[LD1]

How strong does a bridge have to be?

That depends on how high it is.

If just a few feet off the ground, it needs to be "just" strong enough not to collapse.

If it is a thousand feet in the air crossing a canyon.....it has to be about 20x's stronger than it needs to be

 

[boggen]

Bah, can no long remember the name of the mower type. all i can remember is "gangs" blades are setup on a drum. and you just roll across the yard. they are old fashioned push mower (without an engine) the mower type reduces amount of HP needed by the tractor by a good amount compared to a rotating finishing mower. problem is they do not do so well in taller grass. *ggrr what is the name of them*

REEL type mowers. was what i was trying to remember.

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as far as deflating tires. to get a little more ground contact between tire and ground. it depends.

if you can deflate the tires to a point were side walls of the tires bend some you can get some extra contact to the ground. but major con of this, is you get to a point were there is simply not enough air / pressure in the tire to keep the tire on the rim. and one good turn or bump. and there goes the tire. and possibly needing a new one, due to tire got ripped or torn enough that yes tire will fit back on rim. but there is a good tear in the tire.

do not get me wrong, i will deflate my s-10 blazer tires some for some terrain when i know i will be working non stop on the farm and using the blazer for some better traction. vs having tires maxed at 28PSI and tires will just spin up on a rut, vs ovaling out some or perhaps even denting some, to roll up and over a rut or small log.

if i had a 4 wheeler, and had more of balloon shaped tire. were the side walls of the tire are not flat. but round as well. i might let some pressure out for some better traction, and perhaps less rutting. but again if i let the pressure out, there is higher chance, the tires will come off the rims. and for a 4 wheeler a good chance the rims will spin and the tires will not spin at all.

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traction vs rutting....

if my wheels begin to spin, or do slip some. i am making some pretty good ruts for sure.

if i can keep my tires moving along without slipping, less likely i am going to make ruts, and if i do, the ruts are not all that bad. and for most part end up compacting the dirt, or the dirt slides out from under my tires some. and need a lawn roller to kinda flatten things back out afterwards.

heavier machines do make deeper ruts (more weight to compact dirt, or cause mud to slide to the outside of the tires) but do you really need to be scalping the yard down to 1/2" height? raise the deck up some, and let the grass grow a little taller. it can help hide some rutting / low spots within the yard.

 

[couchsachraga]

To the OP - I do not know the answer to your question, but I can tell you, IMhO, how to figure it out.

1) Determine the total weight of each tractor and implements you will have on when mowing (loader, mower, you, cab?, etc...) - working weight in other words. I know without weighing the thing on a scale you won't have it exact but you should be able to get it down to within 100 lbs I would think.

2) Look up the contact patch size for each tire you are considering for each vehicle (this may be more difficult to get of course). It will be vastly different from R1s to R3s for the same diameter ( and pressure ) tire due to tire width and tread pattern.

Computer the TRACTORS working PSI. (weight / total contact patch area in inches). Be sure to always use the same units for each of your calculations. This will allow you to compare between tractors as well as different tire combinations (remember to account for all four).

While I haven't really done this for my tractor I do this extensively where it matters far more - snow grooming. When comparing what groomer to get (or use among the fleet) ground pressure matters immensely. Take an ATV with tires - no go in most cases. Toss some tracks on it and WOW you have all the floatation you can ask for. As to TRACTION, and moving forward instead of downward that is up to tread design (on a tractor or groomer) and we'll leave that for another debate

If you do find the info please post your results here - I'll be curious to see them!

 

[buickanddeere]

Thats not my point.

My point is the soft ground CAN support my 10 and 30 psi tires MUCH better when the tractor weighs 2600 lbs. Throw on my bushhog and FEL and double its weight.......and it dont support it worth a hoot.

Call it what you want, I dont care. But for the application that 99% of TBN'ers are askign about this for.....tire pressure dont matter. Questions like "what tractor will rut my lawn worse"? And using my above example......If I take my Tractor with loader and bushhog and lower the pressures, it is still going to make a rutted up mess. Likewise, I could UP the pressure of the bare tractor, and still not make nearly the mess.

The soil has to bear the same psi per square inch as the tire above it. If the soil deforms at 5psi then the wheel will sink into the soil.
Granted the tire psi, contact patch size and weight relationships are not absolutely linear but.......the the relationship can not be ignored. Laws of physics can't be bent either.
Quoting the Firestone Farm Tire Data Book circa July 2000 using two different 9.5L-15 tires one 6ply 32 psi and other 12ply 64 psi .

6ply, 32psi tire pressure and 2040 lb load. 58 sq in flat plate tread contact area with 35 psi average contact pressure. 118 sq in contact area with 3" penetration 17lbs psi average contact pressure.

12ply, 64 psi tire pressure and 3150 lbs load.58 sq in flat plate tread contact area with 54 psi average contact pressure. 118 sq in contact area with 3" penetration has 27 psi average contact pressure.

Keeping mind as Mark Twain said quote "there are lies, d*mmed lies and statistics" unquote.

The belts in a heavy tire do stress and redistribute the size and shape of the tire patch vs. the reaction of a soft latex party balloon or an ultra low pressure ATV flotation tire.
The psi applied to the ground surface does vary if the ground deforms or stays solid as shown with the flat plate and 3" penetration readings.
Too bad there was't a test with the 64psi tire and 2040lbs load and a 32 psi tire and 3150lbs load.

 

[LD1]

6ply, 32psi tire pressure and 2040 lb load. 58 sq in flat plate tread contact area with 35 psi average contact pressure. 118 sq in contact area with 3" penetration 17lbs psi average contact pressure.

12ply, 64 psi tire pressure and 3150 lbs load.58 sq in flat plate tread contact area with 54 psi average contact pressure. 118 sq in contact area with 3" penetration has 27 psi average contact pressure.

That proves my point right there:thumbsup:

Tire with 32psi had 17psi ground pressure.

Different tire but same size and 64psi....only 27psi ground pressure.

Now how can that be???? shouldnt the 32psi tire have 32psi ground pressure. And shouldnt the 64psi tire have 64psi ground pressure???

 

[buickanddeere]

That proves my point right there:thumbsup:

Tire with 32psi had 17psi ground pressure.

Different tire but same size and 64psi....only 27psi ground pressure.

Now how can that be???? shouldnt the 32psi tire have 32psi ground pressure. And shouldnt the 64psi tire have 64psi ground pressure???

Keep in mind the 64 psi tire is carrying 50% more weight
Also the belt construction of a bias tire .
Plus lets compare the flat plate pressure vs. 3" penetration pressure.
One can easily start comparing apples to oranges.

 

[couchsachraga]

And really all you are talking about is the change in the size of the contact area due to either tire or ground deformation.... hence airing down off road tires - lower PSI allows the tire to spread out, more contact area, lower PSI to the ground. As stated above definitely not a linear relationship though.

Once the tire pressure has exceeded what it takes to deform the ground (and it doesn't take much) higher PSI isn't going to make much of a difference - be it 20 or 120psi, if the ground is soft it is going to deform around the tire first.

And if it is nice soupy mud lower the PSI all you want... you'll still be stuck up to your axle if it is deep enough mud