Moving cut tree branches & logs

[JWR]

Wouldn't it be a curious arrangement to throw a chain around the FRONT tractor axle, dive over the chain and use that to pull a load behind the rear wheels.

Careful if you try, there is stuff under most tractors that don't take well to being thrashed with a tight chain ;-)

Well I hope you drive rather than dive (Ha! just kidding your typos!) but that is foolish and just asking for many kinds of trouble and damage. No reason on the face of the earth to even try that. You surely know that a drawbar was put on a tractor for some purpose don't you think? (And of course the drawbar is purposely located BELOW the center of the rear axle so that rotational forces help hold the tractor down, not buck up the front end.)

 

[JWR]

How do you explain a tractor flipping backward when the rear tires are frozen in mud?

Pulling a load that refuses to move creates similar forces, no? I don't think the small downward force created by that lower drawbar would overcome the extreme torque applied to the tires.

Anybody have a convincing vector diagram?

To answer your question, NO.

Draw yourself a diagram from the side view. Rear tire circle, ground below, showing center of axle and which direction is the front of the tractor. Leave out the mud and buried tires as that is not relevant. Pulling a load that refuses to move (or whether it moves or not) DOES NOT create "similar forces" so forget that for the moment.

In your simple diagram lets make the front of the tractor to the right. To represent forces use arrows. One arrow is down at the drawbar BELOW the center of the rear axle, to the left , the rear. One more arrow is the towing force exerted on the entire tractor at the center of the rear axle to the right (forward.) That simple diagram should illustrate that rotation of the tractor caused by those two force arrows is CLOCKWISE which means helping to keep the front end on the ground, actually slightly adding weight on the front wheels while trying to rotate the front of the tractor downward when both forces are in play. Right? End of story. [Note: you are certainly correct that the downward force on the front tires while pulling correctly, using the lower drawbar, is a small downward force because of the lever lengths. BUT THE POINT IS direction, not amount! The force being downward on the front tires means it cannot raise the front end of the tractor but in fact contributes a little to keeping it down on the ground where it belongs.

Now to digress to the "tires frozen in mud" circumstance. First off, that is not an issue here and irrelevant but I'll comment on it anyway: If the rear tires are frozen in mud (or in the extreme, lets say frozen solid in frozen below zero mud and been there for a month...) what happens when you put the tractor in gear while running and try to move forward? The ONLY relevant force then is rotational and cannot be represented by straight line force arrows. In that case the entire tractor would attempt to rotate counterclockwise with NO FORWARD FORCE exerted by the rear tires. Now put it in reverse and try to back up. In that case the entire tractor would attempt to rotate clockwise with NO FORWARD FORCE exerted by the rear tires. I do not know how to say this more simply -- it is totally irrelevant to the issues being discussed.

 

[California]

Now put it in reverse and try to back up. In that case the entire tractor would attempt to rotate clockwise with NO FORWARD FORCE exerted by the rear tires. I do not know how to say this more simply -- it is totally irrelevant to the issues being discussed.

Ok, you're beginning to convince me. I'm still thinking if the drawbar is well above ground level and below but behind the axle, then there are forces that can raise the nose. As the axle tries to walk out from under that rearward pull. Maybe I'm overthinking this. A friend told me how his Dad became disabled, was a small crawler came over backward on him when he didn't release the brakes on a big equipment trailer.

But your second case is highly relevant. We've seen real-life examples where pulling a fencepost lashed to the back of the rear tire and driving forward can flip the tractor. However with the post lashed to the front of the tire and driving backward, that risk doesn't exist.

 

[CalG]

Well I hope you drive rather than dive (Ha! just kidding your typos!) but that is foolish and just asking for many kinds of trouble and damage. No reason on the face of the earth to even try that. You surely know that a drawbar was put on a tractor for some purpose don't you think? (And of course the drawbar is purposely located BELOW the center of the rear axle so that rotational forces help hold the tractor down, not buck up the front end.)

And of course the example was to illustrate forces and not to encourage anyone to do silly things,

Like pulling a large log with a front end loader.

Jeesh! Is there no common sense at all?

 

[civilian]

Maybe irrevalent to the proper connection point on the rear of the tractor, I have used a chain around my backhoe bucket pivot/connection to its support arm. Not sure right now if that point is above the rear axle center or below it. I have dragged 3 butts of ash, sometimes maple that are 12" dia up to maybe 18" dia on the butt. Lengths, maybe 12ft up to 24ft long. With the backhoe as close to full closed/travel position tends to raise butts off the ground. Hate dragging logs out full length in dirt, hard on sawmill or chain saws. I run the brakes disconnected from each other. Empty loader bucket on the front, maybe empty forks. Paths not wide enough to carry out on forks. Front end will either be on ground or dancing in the air depending which brake is applied to help steer the front end between trees. Ground speed is slowest available. Even looking backward, twisiting the backhoe side to side if necessary,I can feel the front end lift up and I can release hydrostat pedal and dropped the front end. Never have been going fast enough with my B26TLB to feel that the tractor would go completely over. I pay attention to the situation at hand, ground speed, and maybe my tractor doesn't have enough power/speed to flip it over backwards.

Also when I was working for a logger and was running a John Deere 648 skidder, I would have to break free frozen down bundles of full length trees, I could always feel when the front wheels wanted to lift off the ground. Now the skidder use an arch to connect to the grapple. Not sure where the grapple is in height compared to center of rear axle center.

I don't recommend dancing the front end, but have done it. Mostly on level gound. Jon

 

[JWR]

And of course the example was to illustrate forces and not to encourage anyone to do silly things,

Like pulling a large log with a front end loader.

Jeesh! Is there no common sense at all?

Not to nit pick you, but running a chain from the front axle to some rear load to be pulled is approximately the craziest suggestion I've heard in years. Far sillier than using the front end loader to move heavy objects around. Of course "pulling large logs" is a broad generality and we are out here in the peanut gallery visualizing the 3ft diameter oak logs the OP mentions without seeing the entire situation. Weight of course depends on how long the pieces are, the size and capacity of the tractor, etc. Early this year I had occasion to pick up a 14ft long solid cherry log that measured 29" in diameter with the grapple on the FEL of my MF 2660 tractor. Not skidding it along the ground but picking it up and placing it in a trailer. It would have been much less of a challenge to drag that large log than to pick it up, that's for sure. No problem to pull it on the ground or push it with my FEL.

It is "possible" to flip a tractor while pulling from the draw bar, but it's much harder.

No, I disagree CalG. There is no physical way for you to "flip" a tractor pulling anything from the properly located and installed standard drawbar below the level of the rear axle center. I assume "flip" means to raise the front of the tractor and tip it over backwards which is the context of this discussion. Can't happen. Physically impossible.

 

[JWR]

But your second case is highly relevant. We've seen real-life examples where pulling a fencepost lashed to the back of the rear tire and driving forward can flip the tractor. However with the post lashed to the front of the tire and driving backward, that risk doesn't exist.

Very accurate description of events when pulling a fencepost by wrapping a chain over top of a rear tractor tire to pull q fence post. That's a new one on me; never heard of that being done but why not? That said, it has nothing to do with pulling logs and that's why I said it was/is irrelevant.

 

[CalG]

Not to nit pick you,.....

No, I disagree CalG. There is no physical way for you to "flip" a tractor pulling anything from the properly located and installed standard drawbar below the level of the rear axle center. I assume "flip" means to raise the front of the tractor and tip it over backwards which is the context of this discussion. Can't happen. Physically impossible.

We have run into an impass of understanding.

What does the term "MOMENT" mean to you?

If I may elaborate....

I like to consider thought problems of mechanics in the most extreme conditions and then walk back to real world possibles.

What if the conditions were:
Infinite engine power to turn the wheels.
A perfectly non- slipping clutch between engine and rear tractor axle.
Unlimited grip of the tires at the ground engagement point.
All members are unbreakable. and pivot or slide without friction.
The load object is UNMOVABLE.

What happens when the clutch is let out and the load is secured by way of "the properly located and installed standard drawbar below the level of the rear axle center." ?

 

[JWR]

Sorry we are not communicating accurately or with satisfaction. Not sure why you ask. No matter what the term 'moment' means to you or me, one cannot flip any commonly available farm tractor by pulling forward using a tow bar below the level of the center of the rear axle. Period. The End.
Ask the University of Nebraska tractor testing lab if for some reason my conclusion is in question.

Since moment of inertia or some similar term has come up, that has to do with rotational inertia. Dynamics, not statics. There is no rotational inertia involved in this (originally very simple) discussion. Since it came up, there is a very small rotational movement of the tractor around the axis of the rear axle once one starts to pull the load. VERY tiny and lasting a very short time compared to the pulling force. Amounts to the front tires squashing downward and bulging a miniscule amount. Dies out to zero quickly (from an originally small number) as soon as motion forward has begun. The moment of inertia created by the pulling is so small there is zero chance of it creating some sort of far-fetched rebound from the added weight on the front tires bouncing on the ground (if that is where you were headed.) Or maybe I do not understand the question.

Backing up, I should ask -- why do you ask?

 

[CalG]

Sorry we are not communicating accurately or with satisfaction. Not sure why you ask. No matter what the term 'moment' means to you or me, one cannot flip any commonly available farm tractor by pulling forward using a tow bar below the level of the center of the rear axle. Period. The End.
Ask the University of Nebraska tractor testing lab if for some reason my conclusion is in question.

Since moment of inertia or some similar term has come up, that has to do with rotational inertia. Dynamics, not statics. There is no rotational inertia involved in this (originally very simple) discussion. Since it came up, there is a very small rotational movement of the tractor around the axis of the rear axle once one starts to pull the load. VERY tiny and lasting a very short time compared to the pulling force. Amounts to the front tires squashing downward and bulging a miniscule amount. Dies out to zero quickly (from an originally small number) as soon as motion forward has begun. The moment of inertia created by the pulling is so small there is zero chance of it creating some sort of far-fetched rebound from the added weight on the front tires bouncing on the ground (if that is where you were headed.) Or maybe I do not understand the question.

Backing up, I should ask -- why do you ask?

Because you do not seem to grasp the concept.

Inertia is not relevant to this situation.(though certainly present in a dynamic situation. But let us agree to keep the velocities to a snail's pace. ) MV^2 whee haa!

Moments are relevant!

A seesaw, a teeter-totter, a lever arm, a fulcrum.
"I can move the earth....."


The wheel diameter is a moment arm. (Static)
The draw bar is a moment arm (force), again static
The pinion gear is a moment force
The Bull gear (ring) is a moment force
The load is a force vector. static
The wheel "traction" is a force vector that is the result of a moment.
Do the math.. The tractor can flip over backwards by "climbing the differential gear". as they say in the drag racing world. And not move forward an inch. Static!
Even if it means digging a furrow with the draw bar. With the tractor winding it's self BACKWARDS as it does so. Like a capstan cable winch.