Preventing tractor back flip

[crash325]

All the bla, bla, hook up point, length of chain. on & on. None of this BS matters.

Simple fact. if there is enough horse power & traction, a tractor cam or will go over back wards.

 

[xtn]

Here is an overlay of the two conditions. Please note that the physical pivot point of the whole tractor as a unit has remained within the space of the yellow circle throughout the movement so far. It is NOT at the axle.

 

[LD1]

IF the tractor has enough power to go past the equillibrium point I illustrated, AND the tires move backwards as you suggest, How exactally do they move backwards????

Are they slidding? (loss of traction) which whould result in the front comming down.

Or are they rolling backwards? IF so, How? with the clutch engaged, arent you trying to drive them forward? How do they go backwards? Lack of power?Clutch slippage?

Once you pass the equillibrium point, the forces trying to set the front down are een greater. Just the shear weight of the front in the air, plus the chain pulling on the drawbar.

The tractor DOES rotate around the axle, up to the equillibrium point. Because up to that point, the tractor is NOT moving backwards. If you care, do your little drawing again, with the chain and drawbar in a straight line (what I am calling equillibrium) and WITHOUT the rear tire moving backwards, and see where the point of rotation is???

Using the rear axle as a pivot, the whole tractor becomes like a lever. with the front wheels in the air, a rearward pulling force on the drawbar is going to try to set them back down.

 

[xtn]

IF the tractor has enough power to go past the equillibrium point I illustrated, AND the tires move backwards as you suggest, How exactally do they move backwards????

Are they slidding? (loss of traction) which whould result in the front comming down.

Or are they rolling backwards? IF so, How? with the clutch engaged, arent you trying to drive them forward? How do they go backwards? Lack of power?Clutch slippage?

Once you pass the equillibrium point, the forces trying to set the front down are een greater. Just the shear weight of the front in the air, plus the chain pulling on the drawbar.

The tractor DOES rotate around the axle, up to the equillibrium point. Because up to that point, the tractor is NOT moving backwards. If you care, do your little drawing again, with the chain and drawbar in a straight line (what I am calling equillibrium) and WITHOUT the rear tire moving backwards, and see where the point of rotation is???

Using the rear axle as a pivot, the whole tractor becomes like a lever. with the front wheels in the air, a rearward pulling force on the drawbar is going to try to set them back down.

Not sure I can explain how/why it goes backwards. But it does. Think of it like this: If there is enough power/traction to move the system at all when it's absolutely restrained - which since we've seen tractors pull wheelies from pulling loads we know at least some of them do - then the torque available from the engine is enough to make the pinion gear start climbing the stationary ring gear. This brings the front of the tractor up, takes the drawbar end down, and by necessity starts wrapping the tractor backwards. So now the ring gear itself is slowly turning backwards, but no matter, there is plenty of torque for the pinion to keep climbing it.

Your right that the further it wraps itself, because of the design/location of the drawbar end, it becomes harder and harder to do it. Perhaps most tractors are designed such that their power/weight/geometry create a limit such that the engine will stall before it gets far enough back to flip. But assuming enough power and traction, which is what I've said all along, it will continue to wrap itself over. The only geometric condition that can prevent it is having the load vector pointing below the contact patch of the tires. Which is consistent with what I've been saying all along: The only things that prevent the flip are lack of power, lack of traction, or the resisting load vector pointing below the contact patch of the tires.

Here is my drawing angled such that the chain and drawbar create a straight line - ie. the longest possible scenario - please note that the tractor itself has backed up even to get to this point. I did not change the length of the chain at all. It can't be denied that the tractor must move backwards assuming any typically manufactured geometry.

 

[RoyJackson]

Your mistake is suggesting that this component will somehow outweigh all the other component forces in action during the pull and prevent the front of the tractor lifting. It just won't, unless the overall load vector points below the contact patch of the tires as I've described already. This fact is true no matter the exact length or height of the drawbar or the connection details of how and where it mounts to the frame of the tractor.

xtn

Although I think we'll have to agree to disagree...what other components are you discussing? Traction? Weight? Pulling speed? Others?
Although I don't think any of these would overcome the forces I described in my previous post, my assumptions were (for sake of discussion) a straight pull on level ground. I can't make any assumptions about traction since soil conditions vary too much and I don't know enough about tractor tire traction on any given soil type. But I do believe the tires would slip first.
(EDIT: Reading your response to LD1, you assume the tires don't slip. That would likely be an erroneous assumption. And, your comment about "The only things that prevent the flip are lack of power, lack of traction, or the resisting load vector pointing below the contact patch of the tires" describes a theoretical situation more so then real world scenarios.]

As I wrote early on, I've never heard or read of any rear roll overs when pulling a load using the draw bar on level ground...never even read of a tractor rearing up...not on level ground. Closest one I'm familar with was a guy skidding a tree trunk upslope. It was a local guy (didn't know him personally) operating an old Ford or something like that.
Other then that one...no others that I'm aware of...do you have any examples you can share or link to?

 

[RoyJackson]

All the bla, bla, hook up point, length of chain. on & on. None of this BS matters.

Simple fact. if there is enough horse power & traction, a tractor cam or will go over back wards.

But do you know of any stock tractors that meet those two criteria?
BTW, another thing to complicate this discussion is front ballast...keeping it simple, front suitcase weights. BTW, I'm talking stock tractors...not machines designed for tractor pulls.

I'm sure a trained technician could show all kinds of scenarios using a 3D CAD system that can animate the situations. And, in theory, a rear roll over is possible with the right variables factored in...probably the same variables that tractor designers factor in to design preventative measures.

 

[xtn]

Okay here is an overlay of all three conditions. Note that the drawbar itself HAS moved forwards at your "equilibrium" point, because like you said the straight line allows the longest distance. But also note that since the axle is above the drawbar, it and the wheels attached to it have moved backwards. Of course how much it does so depends on the geometry of the bar in relation to the axle. I just eyeballed my etchasketch tractor here, but I think it's fair to say it represents typical geometries reasonably enough.

So we know that the tractor can begin to wind itself backwards. And we know that passing your "equilibrium" point will definitely increase the effort required. But if there is enough power to overcome that increased effort, the tractor can keep winding itself up.

xtn

 

[xtn]

But do you know of any stock tractors that meet those two criteria?

You weren't asking me, but I would like to answer that question for myself just so we're all clear...

I don't know one way or the other. I've never said a tractor will flip. I've only said what I think are the conditions that will prevent it from happening, defended my thoughts, and debated conflicting information when I could.

xtn

PS - I also want to say that I hope nobody finds my arguing to be rude. It's simply interesting debate as far as I'm concerned. I hope some may find me to be consistent, clear, intelligent, and able to back up my points. Am I always right? No, of course not. But I am always stubborn!

 

[RoyJackson]

You weren't asking me, but I would like to answer that question for myself just so we're all clear...

I don't know one way or the other. I've never said a tractor will flip. I've only said what I think are the conditions that will prevent it from happening, defended my thoughts, and debated conflicting information when I could.

xtn

PS - I also want to say that I hope nobody finds my arguing to be rude. It's simply interesting debate as far as I'm concerned. I hope some may find me to be consistent, clear, intelligent, and able to back up my points. Am I always right? No, of course not. But I am always stubborn!

Although I try to support my post through logic and clear thinking...sometimes I use a big stick to get another's attention. And sometimes even logical discussions get heated. That's when the players have to step back and agree to disagree.

 

[xtn]

Although I think we'll have to agree to disagree...what other components are you discussing? Traction? Weight? Pulling speed? Others?

Sorry, when I said components I was talking about component forces; the shear load on a particular bolt, for example. You brought up the downward forces that might be measured where the drawbar attaches to the frame. I was trying to point out that none of many component forces matter; they all are part of the net sum forces anyway.

Although I don't think any of these would overcome the forces I described in my previous post, my assumptions were (for sake of discussion) a straight pull on level ground. I can't make any assumptions about traction since soil conditions vary too much and I don't know enough about tractor tire traction on any given soil type. But I do believe the tires would slip first.

(EDIT: Reading your response to LD1, you assume the tires don't slip. That would likely be an erroneous assumption. And, your comment about "The only things that prevent the flip are lack of power, lack of traction, or the resisting load vector pointing below the contact patch of the tires" describes a theoretical situation more so then real world scenarios.]

Yup I believe tires are likely to slip first too. That satisfies my condition #2, lack of traction. That's real world, isn't it? You said you believe the tires would slip first, then accuse my conditions, which include that possibility, of being more theoretical than real world? I don't get the logic there. You say you think the tractor won't flip because the tire will slip first in the real world. I said the tractor can flip unless - among other limits - there isn't enough traction. So we've said the same thing but you're real world and I'm not?

In my post to LD1 I assumed the tires don't slip only so we can explore what happens further. If the tires slip it's a no-go exercise, so for purposes of seeing what happens I make the tires non-slipping. I said IF there is enough power/traction... THEN xyz will happen. That doesn't mean I've gone back on my original statement of 3 conditions that prevent the flip.

xtn