If your drawing is correct, please tell me why my butt gets lite and my front tires go flat when I try to lift a too heavy load with the loader. The fulcrum would be on the front axle with a loader.
hugs, Brandi
The loader weight has the fulcrum at the front tires, yes. Increased load increases weight on the front axle and because of the lever effect it reduces load on the rear (making the butt lighter).
A counterweight has its fulcrum at the rear tires - unless the rear tires are in the air in which case it's at the front tires. Increased load here increases weight on the rear axle and reduces load on the front.
How much balast has to do with the maximum load the front axle - which is the weakest link in the system - can handle.
For a yourself tractor that weighs 5000 with loader but no cargo, and 1500# on the front axle and 3500# on the rear, and let's say the front axle max is 2500#.
If you put 1000# in the loader you're likely to overload the front axle because the load is levered out in front of it, which means that the center of mass of the entire tractor+load system is now farther forward - the rear is getting lighter and some of the previously rear weight is now being carried by the front axle in addition to that cargo weight.
This is why you need a counterweight. Filled tires aren't enough; they just make it harder to get the rear up, but they don't keep the front from getting overloaded because the weight isn't behind the rear axle (it's directly over it) so it can't reduce the front axle load.
By using a counterweight, which has its fulcrum at the rear tires, you move the center of mass rearwards and the rear axle ends up carrying more.
How much counterweight? The math is complex: it depends on
- how much cargo
- geometry of the tractor
- tractor wheelbase
- distance from the cargo to the front axle
- distance from the center of mass of the counterweight to the fulcrum (rear tires contact)
The same weight a foot farther back will balance more weight.
Ideally I suspect you want the counterweight to perfectly balance out the cargo in the loader. To make life easier though, people probably try to balance off an average to heavy load.
Best would be to have a scale; weigh at the front axle without load, then with a heavy load with counterweight, and add weight until you get back to the unloaded weight on the front - or at least a value comfortably under the axle max load.