I have to laugh because every time I read this statement my head starts to spin!
At the end of the day, I suppose it is all a matter of what one defines as being "the tractor". How about the tires the ballast is in? Do they add weight to the tractor? Oil in the engine, fuel in the fuel tank...do these things add weight to the tractor?
Well - lets see if I can stop the spin ...........
WEIGHTS on the tractor are physically placing weight on the steel of the machine and then through the tires in the end.......... and all the fluids you mentioned do the same thing.......... THEY add weight to the tractor's frame and then THAT WEIGHT is transferred to the tires - the tires MUST carry the load of all that ..........
WEIGHT in the tires are not doing any of that ---- yes the tires are on the tractor, but the tractor does NOT carry ANY of the weight ..... that is UNTIL the tire begins to lift off the ground - then the tire acts on the tractor as added weight. Now - the CG is lowered, because most of the weight is below the axle thus when on a hillside the CG is closer to the ground than tires without ballast.
The weight in tires ONLY add stability WHEN a force is supplied on the opposite side thus causing the weighted side to lift!!! DOES that make any sense?
LET me see if I can simplify this ...........
you are on ice skates ........ you weigh 200 pounds and I place 50 pounds on your shoulders .......... you weigh 250 lbs right????
Now -I place the 50 lbs on your feet, and you are not picking them up, 50 pound skates....... you still weigh 250 .......... but YOU are not carrying the extra 50 pounds....... still spinning?
weighted tires and weights hanging are different but .,...... can be the same .....depending