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