When you have a full bucket, weight transfers from the large, traction-producing rear wheels/tires forward, reducing traction.
The solution is to mount counterbalance weight on the Three Point Hitch.
Or only fill the bucket 50%.
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Bare tractors are designed with a standard weight distribution of 40% front, 60% rear.
Add a Front End Loader and the weight distribution becomes 50% front, 50% rear. (+/-)
Fill the bucket and the weight moves forward, leveraged by low-forward position of a laden bucket.
Could be 60% front, 40% rear. (Tractor wheelbase affects weight distribution.)
Traction is reduced because weight has transferred away from the big rear wheels.
(Hence the need for rear axle
ballast.)
Rear wheels may or may not lift from the ground at this moment, but they will certainly be "light" if bucket payload is wet.
Green tree trunk sections and laden pallets on pallet forks are often heavier than wet bucket loads. Laden pallets protrude forward. Weight distribution could be 70% front, 30% rear. Rear wheels will lift.
Suffecient Three Point Hitch counterbalance restores weight distribution to 40% front, 60% rear. (+/-)
Too much rear weight and the tractor front will lift when you move up a slope with the bucket empty.