Broke one of the most preached rules

[jimmyj]

I did not know that CurlyDave, thanks for the detailed comments.

 

[Willl]

Actually, my 'huh ?' was for the statement "drawbar end hits the ground".

Most of the draw bars I've seen on tractors are not long enough to come close to touching the ground regardless of how far the front comes up.

What we need are 'dragster' like wheelie bars.

 

[CurlyDave]

Willl:

Take a look at this this movie http://www.mne.psu.edu/sommer/projects/tractor/tractorFY1/videos/rear2.mpg , which is from the Safety Forum on TBN, and then look at the two frame grabs from the movie I have attached.

Just above the pointy finger you can see the shiny metal of the drawbar in one of the grabs, and in the next, it has buried itself in the ground. If the chain had been hitched to the end of the drawbar with a clevis, somewhere between those two frames, the overturning torque on the tractor would go to zero, and the incident would be entirely survivable. You can see from the angle of the tractor that it would just sit itself back down on all four wheels if the chain were properly attached.

I have to agree, it doesn't look like this is going to happen if I just eyeball the drawbar on the tractor before the test, but this is one of the reasons why manufacturers hire engineers.

 

[ragkar]

No one has mentioned loading the front bucket, Of course I'm thinking that if the tractor does go front over back, the stones and boulders tumbling out of the bucket may sting a bit.

 

[Willl]

Connected correctly to the draw bar you'd be lucky to get the front wheels to leave the ground at all.

Correct hitching to drawbar (onboard camera) - Movie

 

[2manyrocks]

Thanks CurlyDave for that explanation.

 

[Richard]

When the tip of the drawbar is on the ground, the height difference between the attachment point and the bottom of the rear wheels is zero, which means there is no overturning torque. The operator will get the scare of his life, but good mechanical design will have saved him.

You could have just said, the drawbar works as a wheelie bar!

 

[hallbax]

I guess I had always thought of the pivot point for flipping to be the rear axle of the tractor, not the contact of the tires with the ground. If you assume that the tractor flips around the axle, then connecting below the level of the axle would create a force that would force the front end of the tractor down into the ground, and connecting to a point above the level of the axle would tend to create a force that would pull the front end of the tractor up.

If the pivot point is the contact of the tires with the ground, then any connection point is going to be above that pivot, and would tend to pull the front end up.

Does that sound right?

 

[California]

I don't think there is an attachment point, no matter how low, that can prevent a backflip if you are chained to an immovable object.

So long as the rear wheels are still turning and you have perfect traction the nose will keep rising and come back over on you. Engine torque or traction have to fail before the backflip is prevented.

It takes more torque to lift the nose if you are attached lower, but that lower attachment doesn't reverse the forces involved.

As CurlyDave said, a drawbar designed like a wheelie bar that breaks the traction (or a wimpy engine) are the only tractor design features that can negate the effect.

 

[CurlyDave]

Does that sound right?

That is exactly right.

And, if I had a nickel for every drawing I have seen where someone who should know better has called the axle the pivot point, I could probably buy a box or two of .308 (If I could find any).

Sure, the wheels go around and pivot on the axle, and it looks like that is right, but is isn't.

Now I am surely showing my age, but what in world is a "wheelie bar" ? Right up until I came back to this thread, I would have had visions of a drinking establishment, probably with girls my mother warned me about hanging out.