Broke one of the most preached rules

[tallyho8]

Refer to the 2 images attached:
In image A the tractor is trying to go forward with the wheels spinning forward and is 6 feet from the immovable object.
In image B the tractors wheels are still spinning forward but the tractor is now only 5 feet from the immovable object where it would have to be if the chain were a consistent length and the front end was now lifted.

So your theory is that if the tractor produced enough torque going forward, the wheels would slip and go backward as the front end lifts?

 

[California]

your theory is that if the tractor produced enough torque going forward, the wheels would slip and go backward as the front end lifts?

Not slip. Roll back. Kinda like how the horse's rump moves back as he rears up.

 

[Bill_C]

Folks, if the drawbar is below the rear axle, the force from the drawbar will not pull the tractor backwards. Above the axle, it pulls the tractor backwards.

Here's a worst-case scenario, forgive me if somebody already pointed this out. Suppose the rear wheels can't slip, or even rotate, they are literally frozen to the ground. The tractor will flip if there's no drawbar pull. Attach a drawbar below the axle. As the tractor tries to flip (rotate backwards), the end of the drawbar moves down--and forward. If the drawbar were attached to an immovable object, with a strong enough cable, the tractor could not flip, because the drawbar eye is constrained from moving forward (by the cable attached to the immovable object). To draw this out, notice that the end of cable moves in an arc, the center of that arc is fixed to the immovable object and doesn't move. The drawbar eye also follows an arc, the center of this arc is the tractor's axle.

Okay, suppose the tires aren't frozen. The cable still moves in an arc, attached to the immovable object. The tractor's pivot point is now actually the eye of the drawbar, not the tractor's axle or the tire's contact point with the ground. This pivot point moves, incidentally.

However, from an engineering analysis, we can figure out if the tractor flips by using the tractor's axle as a frame of reference. We sum the moments (engineering word for torque) around that particular point. This point is convenient because it represents the point where the engine is transferring its torque to the wheels. So we can easily equate the engine torque to the force applied by the tire to the ground...then another force of gravity, finally the drawbar pull. Even though this isn't the pivot point, using statics we can use that point to simplify our math. It's late and I don't have time to draw it out and show this on the computer, though...

 

[Reg]

It is clear at tractor drags; the winners DO wheelie and the chain shackle just about touches the ground.

Here's another "don't";
When pulling out shrubs it can be tempting to just lasso them with a chain and lift with the loader's bucket hook.
This can work well according to how shallow rooted the shrub is, how sandy the soil is, how much lift capacity you have, whether your rear tires are loaded, etc.
If you are in the seat it can also be tempting to back the tractor at the same time to add drag to the lift.
If you do this (and I suggest/recommend that you don't) try to throw the clutch as soon as one rear wheel lifts.
It can't happen in 2WD, but in 4WD mine gets a bit nasty, even with loaded tires and my fat butt on the seat NOT holding things down.
I disagree with the O/P; when a wheel gets off the ground reflexes can get wick'd sharp (-:

The "mechanics" of it (simple version) is that the loader's lift gets translated to down pressure on the front tires, so you get some SERIOUS traction AND the drag is a horizontal force component holding the bucket at the end of a fairly long lever, maybe several feet above the ground.
So the tractor does the front end wheelie in reverse; probably cute on video, probably goes to some sort of a side spin if the other rear wheel gets light enough.

 

[Reg]

Folks, if the drawbar is below the rear axle, the force from the drawbar will not pull the tractor backwards. Above the axle, it pulls the tractor backwards.

Here's a worst-case scenario, forgive me if somebody already pointed this out. Suppose the rear wheels can't slip, or even rotate, they are literally frozen to the ground. The tractor will flip if there's no drawbar pull. Attach a drawbar below the axle. As the tractor tries to flip (rotate backwards), the end of the drawbar moves down--and forward. If the drawbar were attached to an immovable object, with a strong enough cable, the tractor could not flip, because the drawbar eye is constrained from moving forward (by the cable attached to the immovable object). To draw this out, notice that the end of cable moves in an arc, the center of that arc is fixed to the immovable object and doesn't move. The drawbar eye also follows an arc, the center of this arc is the tractor's axle.

Okay, suppose the tires aren't frozen. The cable still moves in an arc, attached to the immovable object. The tractor's pivot point is now actually the eye of the drawbar, not the tractor's axle or the tire's contact point with the ground. This pivot point moves, incidentally.

However, from an engineering analysis, we can figure out if the tractor flips by using the tractor's axle as a frame of reference. We sum the moments (engineering word for torque) around that particular point. This point is convenient because it represents the point where the engine is transferring its torque to the wheels. So we can easily equate the engine torque to the force applied by the tire to the ground...then another force of gravity, finally the drawbar pull. Even though this isn't the pivot point, using statics we can use that point to simplify our math. It's late and I don't have time to draw it out and show this on the computer, though...

As I read the beginning of this I was reminded that a long time ago I was told to always park tractors facing the wall, not to back them in.
a) It shelters them from the freezing wind, making early morning starts a bit easier.
b) They CAN get frozen to the ground and being forced to start in reverse could save your life.

I think you were going to do that; "In the limit, as the shackle's vertical distance from the axle's center line approaches the wheel's radius..." thing.

Yes ?

 

[Bill_C]

Actually, I was approaching a limit...of being brain dead...

Tractors or dragsters wheelie because the force accelerating the vehicle forward (the tires biting into the ground), is below the vehicle's CG. When the couple (force times distance) of the forward accelerating force is greater than the couple of the vehicle's weight, the front tires rise. The vehicle is trying to rotate around its CG.

 

[Bill_C]

Alright, I've got to amend my post #33 to add a constraint: IF the wheels can spin, then having the drawbar below the axle shouldn't allow enough overturning moment to flip the tractor.

If the wheels can't spin (coefficient of friction much greater than 1) then the load could pull the tractor backwards...I think. My "Mechanics of Vehicles" book doesn't cover that case. But here's how to simulate it...hook the drawbar up. Now put one end of a come-along on the bottom of the rear wheels, the other end to a tree in front of the tractor, and crank on the come-along, pulling the tractor forward. If the object being towed weighs enough, the front of the tractor will rise. This is a very special case though...I think...

 

[tallyho8]

But here's how to simulate it...hook the drawbar up. Now put one end of a come-along on the bottom of the rear wheels, the other end to a tree in front of the tractor, and crank on the come-along, pulling the tractor forward. If the object being towed weighs enough, the front of the tractor will rise. This is a very special case though...I think...

But if you are pulling the bottom of the wheel toward the front of the tractor, the wheel will rotate backward sliding on the ground while the tractor remains still until the wheel turns 90 degrees and the come-along attachment is on the front of the wheel, and then that's when things get interesting.

either:
a. the come-along will break.
b. the wheel will break.
c. the tree will fall down.
d. the drawbar or chain will break.
e. the immovable object will move.


I think.....

 

[California]

How about a crawler?

Seems to me it will walk right around inside its tracks and flop over backward if nothing on the back of it, drawbar or structure, meets the ground first.

And a rubber-tire tractor would have the same physics.

 

[California]

...
either:
...
...
e. the immovable object will move.

But what if it's on a conveyor belt?