Wheel weights affect rear bearings?

[Matt G.]

Just to clarify-wheel weights will not increase the load on the rear axle bearings because the weight is on the wheels and not the tractor frame.

A few years ago I had started doing calculations to figure out how much weight could be added to the rear without destroying the bearings; I can't remember what I did with those calculations, but what I was close to arriving at was that more than ~250# or so on the frame may drastically shorten the life of the bearings. You do NOT want to have those bearings wear out, because they will take the axles with them. And on the higher hp tractors, too much weight on either the wheels, frame, or both may make you liable to twist off an axle. A little bit of wheel slippage can save your drivetrain from damage.

[CADplans]

"Just to clarify-wheel weights will not increase the load on the rear axle bearings because the weight is on the wheels and not the tractor frame. "


Matt, your statement is perfectly true, IF you leave the tractor parked in the garage for the rest of its life. Your statement is true in a static situation.

Add the dynamics to the situation (driving over rough terrain), suddenly the calculations change.

Without wheel weights, the impact load is a small load (wheel, tire, and axle) impacting through the bearing to the heavy transmission and frame weight.

Add weight to the wheel, the dynamic force between the two goes up.

Hit your fist on the table.

Add 50 pounds to your fist then hit the table again.

Which hurts more?

Add 50 pounds to the table and hit it with your empty fist. That will hurt less than a full fist.

"Proper balance in life is the key to success!!".

[Matt G.]

CADplans-

Your load path is backwards. Let's say you are driving along and hit a bump that temporarily stops your tractor's forward movement. Assuming you didn't high-center the tractor or hit something with the frame to stop it, the wheels have stopped moving, and the tractor's inertia is trying to carry it in the direction it was originally traveling. Consequently, the deceleration of the tractor's mass is the force applied to the wheel bearings.

Now if if you were instead plowing snow, for instance, and stopped the tractor through the frame and not the wheels, the wheels would want to continue moving and then what you are saying would be correct, except that for the loads to be the same or worse than what I explained in the previous paragraph, each rear wheel would have to weigh at least half of what the tractor/operator does. And if that was the case, you'd likely change the failure mode to the axle instead of the bearing. The other thing is that the additional weight on the frame is ALWAYS increasing the load on the axle bearings.

[CADplans]

Originally Posted by Matt G.

CADplans-

Your load path is backwards. Let's say you are driving along and hit a bump that temporarily stops your tractor's forward movement. Assuming you didn't high-center the tractor or hit something with the frame to stop it, the wheels have stopped moving, and the tractor's inertia is trying to carry it in the direction it was originally traveling. Consequently, the deceleration of the tractor's mass is the force applied to the wheel bearings.

Now if if you were instead plowing snow, for instance, and stopped the tractor through the frame and not the wheels, the wheels would want to continue moving and then what you are saying would be correct, except that for the loads to be the same or worse than what I explained in the previous paragraph, each rear wheel would have to weigh at least half of what the tractor/operator does. And if that was the case, you'd likely change the failure mode to the axle instead of the bearing. The other thing is that the additional weight on the frame is ALWAYS increasing the load on the axle bearings.

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.

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.

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.
Yea, using static calculations adding wheel weights sound good.

There are factors you are not considering.

[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.

[rrager]

CADPlans,

I'm going to have to side with Matt on this one.

You are correct in saying that the reaction forces and subsequent bearing wear will be different when hitting an obstruction with increased weight of the wheels. But it would be worse to have that weight added to the frame and hit the same obstruction because of gravity. I don't know your background, but I'm pretty sure that Matt is familiar with free body diagrams and when you add up all the forces on the bearings in the different weighting options, having more weight on the frame is going to be worse more often than not. I'm sure there are situations where more weight on the wheels would be worse for the bearings, but these are the exception, not the rule.

Lets say you're driving along and you hit a ground hog hole:
1) If you have addition weight on the wheels the entire tractor would accelerate (or "fall") down into the hole faster, pulling the frame/chassis down faster, putting a higher impact load on the bearings than if no weight had been added to the wheels

2) Hit the same ground hog hole with that weight added the the frame and the tractor will still accelerate more quickly into the hole, but now along with the increased downward acceleration of the frame/chassis loading the bearings, you also have the acceleration of the additional frame weight - end result, worse for the bearings.

If you hit a rock instead of a hole, the details are different, but the end result is similar.

Later,
Ross

[rrager]

Haha, I was trying to defend you Matt, but you beat me to it.

[Matt G.]

Ross-



I'm going to correct one slight flaw in your post, though. A heavier tractor will not fall/accelerate faster than a lighter one. Remember, a bowling ball and a marble dropped from the same height at the same time will hit the ground at the same time. The acceleration is the same regardless of mass; the force greater because the mass is greater.

[rrager]

AH, the bowling ball and feather, how could I forget

Thanks for the correction Matt, I'm glad you knew what I meant.
Ross

[CADplans]

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.

I was trying to get you to understand YOUR explanation.


In my example the hand is the weak point.

In your example the bearing is the weak point.

If it "is in no way equivalent to what we are discussing" ,

your hand will not fail

your bearing will not fail.


If is equivalent to what we are discussing

your hand will be subjected to greater forces

your bearing will be subjected to greater forces.


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