Tire Selection Calculating ground contact and PSI

[s219]

For a simple tire with smooth tread, the psi inside the tire is approximately the same as the psi on the ground. If you can measure the contact patch area and know the weight on the tire, weight/area will be within a couple percent of the psi inside the tire (or reverse the math to get weight from psi and contact area). This rule is used in designing and sizing aircraft landing gear and tires, and I remember doing a lab on this decades ago in engineering school. Also works on simple car tires (makes a great science project for kids).

This won't work too well for tires that are so stiff that they hold shape without air -- needs to be a tire that completely goes flat. Will work for complex tread shapes and lugged tires, but it is so complicated to measure the contact patch area that a lot of error is introduced.

 

[otlski]

Maybe you can find a good fit here.
http://www.yournexttire.com/content/Tirecatalog2017FirestoneAgb.pdf

For a 2WD tractor (no load in the bucket) assume 30% of mass biased to the front,70% to the rear.

 

[thoner7]

Maybe you can find a good fit here.
http://www.yournexttire.com/content/Tirecatalog2017FirestoneAgb.pdf

For a 2WD tractor (no load in the bucket) assume 30% of mass biased to the front,70% to the rear.

Wow, 30/70 seems off to me. At least with a loader. I feel like the balance point would be further in the middle. Like 40/60 or something.

Maybe I can try that psi trick.

 

[3Ts]

Yes, I'm basically guessing at the weight distribution at the moment. I don't have any way of really measuring that. Especially with the loader added in over the front, makes it more difficult.

The loader acts kinda like a see-saw or a lever with the pivot over the front axle. You have to many unknowns to actually calculate anything, but the loader calculations are done with rotational torque equations (force times distance). The weight in the bucket tends to raise the rear wheels which puts more weight on the pivot point (front axle) which increases the psi on the footprint of the front tires.