Tire Selection Calculating ground contact and PSI

[thoner7]

I'm trying to calculate out the PSI of pressure on the ground. So by taking the tire width, and multiple it by the length of the tire that actually hits the ground. What should I use for that dimension?

Like a 14.9-24 rear would be 14.9" wide x ??? =

Or a 6-16 from would be 6" wide x ??? =

 

[oosik]

Drive onto any hard surface. Make a scratch mark where the tire makes contact - front & rear. That will give you the "length" of the patch.

 

[CobyRupert]

Yikes! This can get complicated.

Are we talking about a smooth tire (or something like a turf tire) or a tire with lugs?
Are we talking about just the downward PSI if tractor is not moving? Or if pulling a load?
Lug tire PSI will be a lot less uniform.

The length that hits surface will depend on tires internal PSI and what surface material is. Concrete, soft dirt, mud up to the axle?
Just think about the extremes: Concrete and mud to axle.
Looking at mud to axle: Calculating the (downward) PSI at any point on the tire's arc that's in contact gets complicated. ...but I guess you're just looking for what the PSI is at the maximum point.

There's probably a rule of thumb (% of circumference) if you're just spit-ballin'. :confused3:

 

[Chris616]

How much tire area is in contact with the ground will depend on inflation pressure and how soft the ground is, but for an average, use this resource:
http://www.ntstractortires.com/assets/uploads/pdf/tires/MichelinData.pdf

Example:



When I did a "sanity check" of the 212 sq. in. number shown in that example it works out to a contact patch rectangle 16" wide by 13" long, which is longer than I would have expected. More explanation of "flat plate area" here:
Flat Plate & Flotation | AGTireTalk

I couldn't find a 6-16 tire in that Michelin resource, so here's another one that does have that tire size:
http://www.ntstractortires.com/assets/uploads/pdf/tires/GoodyearData.pdf



Chris

 

[LD1]

Agreed that this isnt a simple and straight forward calculation

 

[thoner7]

Well, let me explain why I am asking, maybe that will simply.....

My light to-20 never gets stuck in mud. My heavy tractor with loader always gets stuck in mud.

I'm looking for a lighter loader tractor, and would like to fine a comparable one with a similar psi on the tires/ground as a to-20.

 

[LD1]

IS the loader tractor 2wd?

Takes weight off drive tires? Adds mass amounts of weight over SMALL and SKINNY non-drive tires?

Answer is simple, get a 4wd loader tractor.

 

[thoner7]

How would a loader(empty) overhanging the front end affect the weight carried on the front tires?

My small tractor has 19 sq inches of contact per tire. A 12.5l-15 has almost 100 sq in each. On a heavy tractor like a Ford 4400 those front tires should help keep me from sinking in, right?!?!

 

[thoner7]

IS the loader tractor 2wd?

Takes weight off drive tires? Adds mass amounts of weight over SMALL and SKINNY non-drive tires?

Answer is simple, get a 4wd loader tractor.

Yes it's 2wd. 4wd is too expensive, don't have the money. Plus my issue isn't 2wheel drive, it's sinking into the mud. I've never been stuck with my light tractor because I never sink in

 

[LD1]

You honestly cannot see why a loader (even empty) overhanging the front axle effects the weight carried by the front?

Tractors of all sizes and all makes are scaled fairly proportional. Sure there are standouts and some are the exception to the rule....

But an "average weight" 20 hp tractor, 40hp tractor, and 60hp tractor....will all share a similar ground pressure, and a similar tendency or resistance to get stuck in the mud. Because as size/weigh go up....so does tire size in a fairly proportional manner.

Meaning that if you have an average 30hp tractor with loader that has a tendency to get stuck in the mud.....going bigger or smaller isnt going to change anything.

You need to change ANOTHER variable. IE: REAR BALLAST if 2wd, going to R1 tires instead of R4, or going to a 4wd machine instead of a 2wd.