Preventing tractor back flip

[Reg]

Not trying to be argumentative or anything, just trying to provide info to tractor users.

But with that said, if the drawbar is attached to the tractor, isn't that the real hitch point?
I understand that the drawbar can be made 3 feet long, and below the axle, but the main problem seems to be that the tractor can gain traction, thus spinning the rear wheels on their axis............causing a flipover.
The only thing I can see is that a long drawbar..........may act as a 'wheelie bar' of sorts, and could prevent a backwards flip, but could in turn cause a sideways flip when the drawbar hits the ground............couldn't it?

No, it isn't a wheelie bar per se, it is a lever that acts to achieve a state of equilibrium.

No, the draw bar is rigidly attached to the tractor, i.e. it is an extension of the tractor.
The point where the chain attaches to the draw bar is the point of interest with respect to forces.

There is an interesting "balance" of skills and risks in tractor pulls.
If you notice the chain usually goes to the boat very low, but the tractor draw bar isn't HUGELY long, i.e. the attachment point remains relatively high.
By NOT acting as a long lever and getting way down to ground level there is SOME amount of lift afforded the front end of the boat.
In this respect it isn't JUST a straight "PULL", but a partial lift and partial pull.

To understand why this is the way to do it; figure you might be able to carry a large boulder through a field in a pick up truck, the mass is pretty much ON the drive wheels.
You might not be able to tow the same boulder if it was on a trailer behind the same pick up truck, i.e. with the mass not bearing on the drive wheels.
Then you might be able to tow it if you moved it forwards enough on the trailer to get enough tongue weight - and therefore load on the drive wheels.

Bearing in mind that Pulls are Pulls, i.e. they are not working conditions and the drivers anticipate the front end rising - they are READY for it, so this balancing act is OK.
I wouldn't volunteer myself for same/similar balancing acts in "working conditions", i.e. I have sensibly long draw bars

There is SOME advantage to getting the tractor's center of mass ON the rear axle too, so yes they want the front up ~~SOMEWHAT~~

 

[JoeL4330]

for the tractor pull video, take a close look at 2:30 onward ...notice the wheelie bars ? No dummies, these guys.

 

[RoyJackson]

If you tie the bumper of a drag car to a post with a chain, it's going to wheelie even worse.

xtn

But the drawbar is below the axle, thus "pulling" the front end down. The rear tires should slip before a rear roll over.

I, personally, have never heard or read of a tractor flipping over backwards when using the drawbar as a hitch.

I'm sure a tractor could roll (perhaps, technically not a rear rollover) if one of the rear tires suddenly lost traction and pulled to one side...or, if the pulling was done across a slope.

As far as those tractors used in tractor pulls...to me, these aren't good for comparison as the power to weight ratio is far above the typical ag, utility or compact tractor.

 

[JoeL4330]

never saw a backflip in the flesh, myself, but did notice:

Backflip With Tractor - YouTube just beyond 0.50 seems convincing

 

[xtn]

But the drawbar is below the axle, thus "pulling" the front end down. The rear tires should slip before a rear roll over.

Wrong. The drawbar would need to be below the tire-to-ground contact patch to pull the front end down.

Or the angle of the pull must be such that the chain, when drawn tight, traces an imaginary line that would continue under the tire-to-ground contact patch. In other words you would have to connect to the load up high. Of course that would tend to lift the rear tires, causing a loss of traction. But if we assume perfect traction no matter what, that is a geometry that would pull the front end down.

The pivot point at the axle controls the arc the tractor rotates through, but it does NOT effect the moments that cause the rotation. If the tractor has an immovable load, the pinion gear WILL try to climb the ring gear. The ONLY condition that will prevent the nose coming up are:

1. Lack of power,
2. Lack of traction, or
3. Pull geometry resulting in moments that cause the front end to be held down despite power and traction. The only geometries that do this, assuming typical drawbar setups, are as I've described above.

 

[Short Game]

I continue to find food for thought scattered around this thread. What's got hold of my brain cell now is the draw bar. Moving a low hookup point farther aft is important. The first link of chain is a hinge. If that hinge is under the tractor, that point of leverage, in relation to the rear tire traction patch, will put lift on the front. If a long draw bar is used, that point significantly changes the triangular force diagram that was posted so many pages back. The chain will be pulling upwards on the draw bar end as soon as the draw bar tries to go down below of line of the chain.

In a pulling competition, one would want to combine pull and lift at the front of the load. Lifting the front of the drag load reduces the load's traction patch.

 

[Short Game]

Considering an immovable load, an unbendable draw bar, and an unbreakable chain, combined with unlimited torque, horsepower, and perfect traction, I believe what would happen would spin us up a freaking singularity and the birth of a new universe. Maybe we're messing with things that should be sorted out by the various deities.

 

[RoyJackson]

Wrong. The drawbar would need to be below the tire-to-ground contact patch to pull the front end down.

I believe you'll find most experts will disagree with your statement and thought processes on this subject.

Read these:
http://www.wellnessproposals.com/safety/handouts/stptractorrol.pdf (specifically the section labelled "Hitching")
and a very good paper:
http://agecon.uwyo.edu/RiskMgt/humanrisk/TractorOverturnhazardspdf.pdf

So, your blanket statement "Wrong" is in error...as is your math.

 

[SPYDERLK]

Wrong. The drawbar would need to be below the tire-to-ground contact patch to pull the front end down.

Or the angle of the pull must be such that the chain, when drawn tight, traces an imaginary line that would continue under the tire-to-ground contact patch. In other words you would have to connect to the load up high. Of course that would tend to lift the rear tires, causing a loss of traction. But if we assume perfect traction no matter what, that is a geometry that would pull the front end down.

The pivot point at the axle controls the arc the tractor rotates through, but it does NOT effect the moments that cause the rotation. If the tractor has an immovable load, the pinion gear WILL try to climb the ring gear. The ONLY condition that will prevent the nose coming up are:

1. Lack of power,
2. Lack of traction, or
3. Pull geometry resulting in moments that cause the front end to be held down despite power and traction. The only geometries that do this, assuming typical drawbar setups, are as I've described above.

[[I believe you'll find most experts will disagree with your statement and thought processes on this subject.]]

Read these:
http://www.wellnessproposals.com/safety/handouts/stptractorrol.pdf (specifically the section labelled "Hitching")
and a very good paper:
http://agecon.uwyo.edu/RiskMgt/humanrisk/TractorOverturnhazardspdf.pdf

So, your blanket statement "Wrong" is in error...as is your math.

... But they themselves would be wrong. This was well hashed over on TBN in the thread linked in my previous post and in links on that thread. We need to build on previous effort rather than repeat from 0 every time it comes up.
larry

 

[SPYDERLK]

No, it isn't a wheelie bar per se, it is a lever that acts to achieve a state of equilibrium.

No, the draw bar is rigidly attached to the tractor, i.e. it is an extension of the tractor.
The point where the chain attaches to the draw bar is the point of interest with respect to forces.

There is an interesting "balance" of skills and risks in tractor pulls.
If you notice the chain usually goes to the boat very low, but the tractor draw bar isn't HUGELY long, i.e. the attachment point remains relatively high.
By NOT acting as a long lever and getting way down to ground level there is SOME amount of lift afforded the front end of the boat.
In this respect it isn't JUST a straight "PULL", but a partial lift and partial pull.

To understand why this is the way to do it; figure you might be able to carry a large boulder through a field in a pick up truck, the mass is pretty much ON the drive wheels.
You might not be able to tow the same boulder if it was on a trailer behind the same pick up truck, i.e. with the mass not bearing on the drive wheels.
Then you might be able to tow it if you moved it forwards enough on the trailer to get enough tongue weight - and therefore load on the drive wheels.

Bearing in mind that Pulls are Pulls, i.e. they are not working conditions and the drivers anticipate the front end rising - they are READY for it, so this balancing act is OK.
I wouldn't volunteer myself for same/similar balancing acts in "working conditions", i.e. I have sensibly long draw bars

There is SOME advantage to getting the tractor's center of mass ON the rear axle too, so yes they want the front up ~~SOMEWHAT~~

Thanks Reg.
larry

 

[xtn]

I believe you'll find most experts will disagree with your statement and thought processes on this subject.

So, your blanket statement "Wrong" is in error...as is your math.

It wasn't a blanket statement. It was conditional based on typical drawbar configurations, and even allowed for an alternative condition when I said, "OR."

The fact is we have a tractive force at the contact patch trying to move the whole assembly forward. Any resistive force in the other direction that has a vector passing above that tractive force point will tend to tilt the whole assembly back towards the resistive load as the the tires try to move themselves forward.

If the contact patch moves forward, and any fixed point above that contact patch doesn't move forward, then the assembly must tilt up and back. Pretty simple no matter what anybody else says. The tires are trying to claw themselves out from under everything else that is restrained. (The above includes only typical drawbar configurations and relatively straightforward chain angles. As detailed in my prior post there are possible setups that would actually lock the front end of the tractor down.)

 

[Short Game]

We need to build on previous effort rather than repeat from 0 every time it comes up.

Well, I'm not going to search it all out and read it. It's good that I get it in nearly real time here. I want to get it, and figure it out, and correct my wrong guesses. I know it's good for me.

With all the smoke that has come out of my ears, I'm glad I have a winch to pull with. I've uprooted some pretty sizable trees by hanging a block 12 feet up. With the front of the tractor tied to another tree at near ground level, the only thing I've worried about is snapping a cable. The only work the tractor did was charge the winch battery. That strikes me as funny.

 

[JoeL4330]

sorry to repeat; check out 50 seconds into Backflip With Tractor - YouTube

 

[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.

 

[SPYDERLK]

This was well hashed over on TBN in the thread linked in my previous post and in links on that thread. We need to build on previous effort rather than repeat from 0 every time it comes up.
larry

Well, I'm not going to search it all out and read it. It's good that I get it in nearly real time here. I want to get it, and figure it out, and correct my wrong guesses. I know it's good for me.

With all the smoke that has come out of my ears, I'm glad I have a winch to pull with. I've uprooted some pretty sizable trees by hanging a block 12 feet up. With the front of the tractor tied to another tree at near ground level, the only thing I've worried about is snapping a cable. The only work the tractor did was charge the winch battery. That strikes me as funny.

I see that you have posted several times on this thread. Yet the time it takes to follow a pertinent and resolved link, given to you, is too much.
http://www.tractorbynet.com/forums/buying-pricing-comparisons/146173-why-4-wheel-drive-2.html#post1691951
larry

 

[Short Game]

I see that you have posted several times on this thread. Yet the time it takes to follow a pertinent and resolved link, given to you, is too much.

Well, Larry, the time I'm goofing off here right now is probably too much. I should be in the seat finishing my mow, as there is possible rain tonight. I'm not dissing your effort. It sounded like volumes had come before and I'm lysdexic and a slow reader, That idea scared me off. This thread has kept my interest and made me think. That's why I'm here and enjoying it. But now, I must get to work in meat-space.

 

[RoyJackson]

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?

 

[JoeL4330]

Note, in the video Backflip With Tractor - YouTube that the tractor that flips has a drawbar ...and, given the momentum generated, it simply digs in lifts the rear wheels a little but doesn't stop the flip.

 

[RoyJackson]

... But they themselves would be wrong. This was well hashed over on TBN in the thread linked in my previous post and in links on that thread. We need to build on previous effort rather than repeat from 0 every time it comes up.
larry

Larry...do you design tractors? Do you...other then theory, know anything about tractor design and the dynamics involved?
Although I'm in Quality Assurance (not design...automotive and (currently) aerospace, but not tractor specifically), I did a fair amount of study on safe usage of a tractor as I didn't want to become an unfortunate statistic. Does that make me an expert on tractor design...nope...but it does make me a very knowledgeable owner of tractors.

As far as your previous threads...once they're off the first page of a forum...I doubt anyone reads or has knowledge of them (BTW, that applies to all posters, not just you). This topic has gone on (now and then) for at least 10 years...sort of like the "HST vs. Gear" debates.

 

[RoyJackson]

Note, in the video Backflip With Tractor - YouTube that the tractor that flips has a drawbar ...and, given the momentum generated, it simply digs in lifts the rear wheels a little but doesn't stop the flip.

I think you're missing something here, Bro...the discussion is concerning pulling with a drawbar to reduce the possibility of a back flip. The video doesn't really have much to do with that.
First rear rollover I ever heard of (on TBN, IIRC) was a Nebraska farmer who had left his tractor out in a field overnight. Next morning he started his tractor, put it in gear and the machine rear rolled on him. Tires were frozen to the ground. Drawbar wasn't part of the incident.
In that video, how the draw bar is used (isn't used, actually) has no effect on that roll over...stupidity (along with some liquor, I'd wager) does.