Preventing tractor back flip

[RoyJackson]

CAll me hard headed, but I still dont see the tractor tires rolling backwards as the chain tenses up...

Yeah, the whole thread has gone to H3LL...too much theory vs real world. This one is way out in La La Land.

 

[LD1]

SpyderLK...Larry

This is a quote from your post #37 in the link you gave in your first post on this thread

Yaay! The frozen tire thing is a bit different tho, as in that case the tire cannot move and the tractor just rotates around it. However, if a pullchain were attached to the rigid towbar in that case it would still stop backtip in just the way you describe - by going to ground. It occurs to me that the drawbar motion is forward and down relative to the axle and the axle serving a frozen tire is fixed. So the chain trys to force the tractor to move backward as it tips. This leads me to posit that, with just the right initial chain slack, and the tire remaining glued, you could end up with a tractor rared up beyond its balance point and sitting there stably.
larry

This is exactally what I am saying can happen if the tires/chassis do NOT move backwards. Becase the unlimited traction/HP trying to drive forward ARE preventing the tractor from doing just that.

Are you still in agreement with what you said then?

 

[Farmwithjunk]

Yeah, the whole thread has gone to H3LL...too much theory vs real world. This one is way out in La La Land.

Well Roy....It could be worse...Just think. Would the tractor flip backwards if it were pulling something on a moving runway?

Yeah. We gotta look at reality every now and then. (A foreign concept to some) The fact is, tractors do occasionally get "back flipped" while dragging something heavy/immovable by the belly mounted draw bar. No matter how rare that is, it's hard to sell a theory when reality tells us otherwise.

 

[Egon]

1. In your post, #193 above, I have no idea what point you're trying to make with regard to the current discussion, and I'll be surprised if anybody else reading this has any idea./QUOTE]

What, you have never heard of Empirical data gained from

"As it happens Tractor Operation"

 

[BotaBigot]

Love these discussions...inertia...a component of human behavior as well as physics...

 

[johnrex62]

Yeah. We gotta look at reality every now and then. (A foreign concept to some) The fact is, tractors do occasionally get "back flipped" while dragging something heavy/immovable by the belly mounted draw bar. No matter how rare that is, it's hard to sell a theory when reality tells us otherwise.

It would seem to me that reality includes lots of factors and variables excluded in our theoretical scenarios to prove or disprove this phenomenum. Wouldn't it be crazy if the truth is that without those external factors tractors cannot flip?

I have seen many tractors sitting in the yard....none of them flip on their own. I have also seen many working hard pulling things around, both above and below the axel, and I have never seen any of them flip although I have seen a few wobble.

I have seen pictures of flipped tractors and they all had something external to the equation. Usually a lot of momentum, unlevel ground, or unbalanced ballast. Each of those things change center of gravity and exert unnatural forces against the tractor balance. Include those types of forces and any naturally stable object can become unstable.

I propose it IS possbile to flip a tractor despite never seeing it firsthand. I also suggest that with proper care and attention to the enviroment it is possible to do the same task that results in the flip without flipping by eliminating the "stupid". I don't exactly mean that the operator did something stupid as much as there was some inattention, oversight or carelessness involved that allowed the "stupid" to occur.

In a perfect world we never miss anything and only operate our tractors in a controlled and careful manner. So in a perfect world I suggest tractor flipping is something only children with toy tractors could achieve.

 

[LD1]

Yeah. We gotta look at reality every now and then. (A foreign concept to some) The fact is, tractors do occasionally get "back flipped" while dragging something heavy/immovable by the belly mounted draw bar. No matter how rare that is, it's hard to sell a theory when reality tells us otherwise.

Yes, reality tells us that it can ocassionally happen. VERY RARE though.

And that is why I mentioned situations when it "can" happen.

But the physics show, with a slow steady pull, that it cannot happen. When you throw clutchpopping, jerking, running starts, and hills into the mix, its a crap-shoot. Anything can happen. And wether the tractor can attain enough momentum to go over depends on how hard you jerk, how hard you pop the clutch, how fast the running start is, and how steep the hill is.

But the fact still remains, that a slow and steady pull from the drawbar on level ground will not flip the tractor without one of the above mentioned scenerios.

And I am NOT trying to imply that it is all operator error either. Not trying to take any credit away for any seasoned operators that have been kille by a flip. There are things like skidding a large log from the drawbar. Hooked up right and safe. While skidding that log out on 100% flat ground at ~5mph, all of the sudden the log catches a low cut-unseen stump. Well...that is like a running start. Or maybe like when the chain slips and grabs again befor you have time to react. Jerking?

 

[xtn]

CAll me hard headed, but I still dont see the tractor tires rolling backwards as the chain tenses up.

Stop and ask yourself what causes the front to climb up anyway? isnt it the the "equal and opposite" reaction force to the tires turning forward??

So if the tires roll backwards wouldnt the front set back down?

And dont we have the clutch engaged and infinite power trying to turn the tires forward?? yet, somehow that infinite power and traction tightens up the chain in a way the tractor rolls backwards??

And to open up another can of worms here, I think I an in agreement with Roy Jackson, in that a straightr pull, with the chain STRAIGH and LEVEL with the drawbar, if the drawbar is below the axle, it does pull down on the front.

XYZ: and others involved, I know it may sound childish, but I encourage you to do this. Get out one of your 1:16 scale model trators. Set this very expirement up. Attach a string to the drawbar and then to something immovable on a bit of an angle like your diagram. Then with your hands, try to drive the back tires. You can get a good bit of weight/traction just with your hands. You may be suprised at what happens. I did this yesterday before I even posted.

Hard headed? Hey, it's all good. I'm hard headed too as you can see!

Okay, I'll try another approach.

Yes, it is the equal and opposite reaction going on. You agree the tractor car raise its nose if the tires are locked still. So although the forces are equal and opposite, the resulting motion does not have to be. That is to say that if you lock one side of the system, the other side will be moved with all the force. That's what's happening if the tractor raises its nose while the wheels are locked in place, right?

So if you agree that the motion does not have to be equal and opposite, even though the forces are, then you agree that the tires are not necessarily forced to turn forwards because the motion can be elsewhere, ie raising the nose of the tractor instead of turning the tires. Now if you force the rear wheels to turn backwards, the equal and opposite force results in the motion of the nose rising that much faster.

Imagine the rear axle of a tractor chained down onto a conveyor belt or a dyno drum. Imagine that the wheels are forced to rotate slowly backwards no matter what. Now engage the clutch and give it some throttle. The nose will rise at the expected rate PLUS the extra induced rate caused by rotating the tires backwards. Make sense?

In our scenario of pulling against an immovable load, the geometry requires that the tires are forced to roll backwards. This is no different than if they are forced to do so by conveyor belt, or dyno drum, or anything else. If you restrain the drawbar with a chain, and give the tires enough traction, the only work left for the engine to do is to raise the nose of the tractor.

Now we seem to have agreed that as the nose rises the leverage to overcome increases, but never reaches zero as long as the drawbar does not extend past the tire radius. So as the nose rises and the effort required increases, we only have to ask ourselves these questions:

1. Do the tires have enough traction so that the work the engine does is forced to raise the nose? If so, the tractor can flip.
2. Does the engine have enough power to keep raising the nose even as the effort required increases because of drawbar geometry and leverage? If so, the tractor can flip.
3. Does the geometry of the drawbar and angle of the chain fail to ever cause the vector of the restraining force to point below the contact patch of the tires? If so, the tractor can flip.
4. Even if the geometry of the drawbar and angle of the chain do ever cause the vector of the restraining force to point below the contact patch of the tires, can the nose be raised with enough momentum to carry it up and over despite such an equilibrium point being reached? If so, the tractor can flip.

I propose it IS possbile to flip a tractor despite never seeing it firsthand. I also suggest that with proper care and attention to the enviroment it is possible to do the same task that results in the flip without flipping by eliminating the "stupid". I don't exactly mean that the operator did something stupid as much as there was some inattention, oversight or carelessness involved that allowed the "stupid" to occur.

I agree with that wholeheartedly. I believe the answers to the four questions I asked above are all "not likely." But that answer leaves room for possibility.

xtn

 

[LD1]

I follow what you you are saying, but I think we are going to have to "agree to disagree" on this "theory" that really is a meaningless argument anyway.

I follow what you are saying, but I dont think it works out if traction AND power are BOTH unlimited as well as the object being immovable.

Let me ask you this question. And this questions is the only thing I woudl like you to answer. You dont have to waste your time trying to change my mine, and I wont waste time trying to change yours:thumbsup:

This is kinda based on the qoute of SpyderLK's I posted a few posts ago.

Lets supposed the tires are IMMOVABLE. Frozen down, set in concrete, superglued down, etc. They absolutly cannot go forward or backwards.

Now lets attach an UNBREAKABLE chain to an IMMOVABLE object and to the drawbar so that it is tight and no slak.

Now lets fire up out tractor with absolutly UNLIMITED power/torque.

Can this equation be solved? If yes, What will happen?

This is all I want you (XYZ) to answer for now. I think we may actually agree on the outcome of this, but we'll see.

 

[xtn]

RoyJackson...

Your second link explains it pretty well in the section titled Drawbar Leverage. And it fits perfectly with what I've said.

Typical geometries tend to raise the front of the tractor... until the drawbar reaches a low enough point that the vector of the restraining force points below the contact patch of the tires.

Nowhere does either of you links say that as long as the drawbar is below the axle the front of the tractor will stay down, as you have claimed. You are the one that is wrong.

No such thing as 100% safety. Too many variables (such as topography).

Anyway, the drawbar, as designed below the rear differential, is the lowest part of the tractor....and the safest place to hitch for pulling.
There is no way for the restraining force can be below the contact patch of the tires. Physically impossible...we're discussing real life considerations, not theory, correct?

I only now notice this communication error. I never said the restraining force can be below the contact patch. I said the ANGLE OF the restraining force must be such that it's VECTOR POINTS below the contact patch.

Here is a picture of what I'm talking about. If your drawbar is long enough and your chain connected high enough on the immovable load, you may eventually reach THIS condition, which DOES in fact pull the nose down as you attempt to drive forward. It can be any combination of chain angle and drawbar length or height that results in the vector of the restraining load pointing below the contact patch of the tires. Only THIS condition results in the nose being pulled down as you attempt to drive forward.

So for those looking for any configuration that would guarantee no back flip, here you go.

 

[LD1]

So for those looking for any configuration that would guarantee no back flip, here you go.

Any solution to the question I posed?

 

[xtn]

Lets supposed the tires are IMMOVABLE. Frozen down, set in concrete, superglued down, etc. They absolutly cannot go forward or backwards.

Now lets attach an UNBREAKABLE chain to an IMMOVABLE object and to the drawbar so that it is tight and no slak.

Now lets fire up out tractor with absolutly UNLIMITED power/torque.

Can this equation be solved? If yes, What will happen?

Something must bend or brake.

The geometry REQUIRES that the tires roll backwards as the nose comes up. If they are not allowed to roll backwards, the nose cannot come up. If the nose cannot come up, something in the drive train between the unlimited source of power and the frozen contact patch, or something in the linkages controlling the geometry (chain, drawbar, case, axle housing, wheel rim, etc.) must fail.

xtn

 

[hawkeye08]

.....

 

[AGGIE00]

I can add some experience to this one. I stuck a deere 450 lgp dozer clearing on my property. All I had to try and get it out was my old deere 2640. My wife came over to help and drove the tractor. I didn't have a pin to attach the chain to the drawbar so like an idiot that knew better I attached it to the centerlink bracket and to the centerof the dozer blade from the bottom. I told my wife to go slow and if she felt it begin to raise up in the front push in the clutch. Of course the front raised up and in her panic she couldn't push in the clutch and the tractor went to a 45 degree angle when the tires started to slip alternately causing the tractor to bounce from side to side. I got off the dozer with it still in gear and ran to the tractor and pulled the kill knob. It was a moment of panic and having known better made it so much worse. The next attempt involved rigging the chain to the drawbar and attaching the other end to the corner of the dozer blade. I let my wife pull forward until the tractor dug down to anchor itself and rotated the blade which worked 2 feet at a time until the dozer was out. From the drawbar the tractor never raised up.

 

[LD1]

Something must bend or brake.

The geometry REQUIRES that the tires roll backwards as the nose comes up. If they are not allowed to roll backwards, [the nose cannot come up.] If the nose cannot come up, something in the drive train between the unlimited source of power and the frozen contact patch, or something in the linkages controlling the geometry (chain, drawbar, case, axle housing, wheel rim, etc.) must fail.

xtn

We agree:thumbsup:

The situation I posed CANNOT be solved. IF everything is unbreakable, and IF you hav unlimited power, and IF the tires cannot move, there is no solution because

1. unlimited power says the tractor will flip, but...
2. the unbreakable chain sats it stays put:confused2:

If the tires cannot rotate backwards the front CANT raise []

and THAT is MY argument as to why the tractor cannot flip. Because I dont believe the tires move backwards when under load and trying to drive forward regardless of all these scenerios we have posed.

So to sum it up, YOU believe the tires roll backwards and THAT is how it can flip. Thats your belief and you are entitled to it.

MY belief is that the tires do NOT go backwards and that (above scenerio that you agree with) is why it cannot flip.

Again, I think we can agree to disagree and part ways on this matter. From one hard-headed-stubborn-SOB to another:thumbsup:

 

[AGGIE00]

In order for the tractor to flip on flat ground the point from which you pull would have to originate above the center of the axle.

 

[xtn]

If the tires cannot rotate backwards the front CANT raise []

and THAT is MY argument as to why the tractor cannot flip. Because I dont believe the tires move backwards when under load and trying to drive forward regardless of all these scenerios we have posed.

So to sum it up, YOU believe the tires roll backwards and THAT is how it can flip. Thats your belief and you are entitled to it.

MY belief is that the tires do NOT go backwards and that (above scenerio that you agree with) is why it cannot flip.

Yup that does seem to be our point of separation. Guess I'm just gonna have to fab up a model and make a video of it for you. Actually I don't know if my stubbornness surpasses my laziness enough to do that.

 

[LD1]

Actually I don't know if my stubbornness surpasses my laziness enough to do that.

:laughing::laughing::laughing:

 

[SPYDERLK]

LD1, yes I believe what I wrote previously about frozen tires. But its like changing the subject to go there. We are not talking about tires that cannot move, but instead tires that will not slip. The tire and chassis can move by rolling back as they have to to allow for the substantial chain takeup that occurs while the tractor rears up an amt allowed by a short drawbar [a long one wont pull in much because back tip is limited as it approaches touchdown]. In either case much of the takeup caused by the bar swinging down and under is fed back by the tractor axle moving back and with the tire roll allowed by the lifting front end. [Lets keep the chain long to avoid a needless complexity.] Now if your drawbar is short .. say lacking 2" of reaching the ground at the closest in the arc it would swing in a flip, then at the very least point you will have 1/6' x traction force net backtip in your "dueling" lever system. You have gravity acting on the forward located tractor COM that can give a little help to the disadvantaged short drawbar, but with unlimited force available at the ground its going over no matter how much the COM can offset. Cross up some different length wrenches. I think itll help give you the idea.

... Oh yeah -- also back tips are not favored by a running start since the COM is way above the drawbar hitch point, and the sudden deceleration at the end of the tether would thus tend to nose the tractor down. Back tips are pure grunt work.
larry

 

[JoeL4330]

We agree:thumbsup:

The situation I posed CANNOT be solved. IF everything is unbreakable, and IF you hav unlimited power, and IF the tires cannot move, there is no solution because

1. unlimited power says the tractor will flip, but...
2. the unbreakable chain sats it stays put:confused2:

If the tires cannot rotate backwards the front CANT raise []

and THAT is MY argument as to why the tractor cannot flip. Because I dont believe the tires move backwards when under load and trying to drive forward regardless of all these scenerios we have posed.

Hmmm...I wonder if the tires can move backward but not (not) rotate backwards? Easy to think of situations where tires move but do not rotate: get your pre-anti-lock-brake vehicle up to speed, lock the brakes...skid--each tire (indeed, the whole vehicle) moves but none of the tires rotate ...just sayin'