Kevin,
Wheel weights or loaded tires put NO load through the axle. The load goes through the tire side walls only. You could fill your tires with lead (not worth it) and you would never break your rear axle. You may shear it off in torque trying to move it, but you would never snap the axle in bending.
I would get rid of the duals ASAP and look at any options that Kubota has for wheel spacers. Kubota may have none. The other option is to custom design either a steel or cement weight that attaches to the rim. You could make a custom cement weight for the rim buy taking the wheel off of the tractor, lining the inside of the rim with heavy plastic (.010 mil PE sheeting) and a 1.5” wood spacer where the lugs nuts sit. Fill the inside cavity of the rim that you covered with plastic with the cement. Add three or four through holes for mounting to the rim. I bet you could add 80-100#’s of cement to each wheel this way.
To remove your tire in the future, you would need to remove the weight from the rim first.
Email me if you do not understand.
And, be careful with the other axle. It could fail at some later time with out warning. The part integrity has been compromised by the use of the dual wheels.
Design
