[Matt G.]

Originally Posted by CADplans

I am saying nothing about forward motion, I am talking about hitting a bump while traveling. Having a heavy wheel assembly will have a greater influence on the bearing than having a light wheel assembly.

You are missing the point...

Let's start with the tractor just sitting somewhere. The tractor's weight is reacted at each of the 4 wheels. Only the weight of the tractor, sans wheels, is on the bearings because the weight of the wheels is being reacted by the ground. The weight of the rest of the tractor must also be reacted by the ground, since no part of the tractor except the wheels is touching the ground; the weight of the rest of the tractor is applying a radial force to the axle through the bearing. Since the axle is connected to the wheel, the load goes through the wheel and is reacted by the ground. This same situation still exists when the tractor is moving over smooth ground.

It does not matter what direction the tractor is moving. If you hit a bump while driving forwards (or backwards, it doesn't matter) the wheels are touching the ground, the entire tractor must move, assuming that the bump is not completely absorbed by the tire. To keep this simple enough, we'll assume that the tire does not absorb any of the impact with the bump. As the tractor goes over the bump, the wheel moves up, but it does not impart a force on the tractor through the axle bearings. Newton's 3rd law states that every action has an equal and opposite reaction. The bump is trying to push the tractor up; since no part of the tractor wants to move up with the bump (Newton's 1st law) the force exerted by the tractor on the ground increases because the mass of the tractor is accelerating towards the ground, opposite the direction of the bump so that the sum of the forces acting on the tractor remains zero. Since the force exerted on the ground by the tractor increases, and the tractor is contacting the ground only by its wheels, the reactions at the ground increase. There are two components to the increased ground reaction force at each wheel: 1) the force from the downward acceleration of the wheel itself (which once, again, is only reacted by the ground) and 2) the force from the downward acceleration of the tractor, which first goes through the bearings, increasing the radial load on the bearings, and then through the wheel where it is balanced by the reaction force on the ground. If you increase the mass of the tractor by adding weights to the rearend/frame, the magnitude of the increase in the force applied by the tractor to the wheels and eventually the ground through the axle bearings will also increase. Adding wheel weights does not change the load on the axle bearings because their weight is reacted by the ground.

Originally Posted by CADplans

Hold a 4# hammer head in your hand. Go to a 150# anvil and hit the back of your hand on the anvil, let me know when the pain stops.

The hammer is way lighter than the anvil.

This hypothetical situation you continue to propose is in no way equivalent to what we are discussing.

Originally Posted by CADplans

Go to any performance car website. Tell them you want to make the wheel assembly heavier to improve performance, see what happens.

Yea, I know a car is different.

So why bring it up? Just another apples-to-oranges comparison...

Originally Posted by CADplans

Yea, using static calculations adding wheel weights sound good.

There are factors you are not considering.

I am not using static calculations. In fact, there are no calculations here, this is just a conceptual application of Newton's laws, no numbers required. What I have just described to you above is the DYNAMIC situation that you proposed, and I have now explained it to you twice. There are factors you are not considering/do not understand, such as the concept of load paths.