Preventing tractor back flip

[beppington]

If you ask me, this question was settled on page two. If not there, then on page four when he bought the guy a case of beer.

I think absent video of a tractor actually flipping over backward, even though his load is hitched below the rear axle, we will still have skeptics. Heck, I'm not a skeptic, but I'd still like to see such a video ... not a guy getting hurt, just proof that it can happen. The tractor in my sample video would of course have a ROPS, & the driver would be belted in :thumbsup:

 

[Short Game]

Teevee has ruined our brains.

 

[Reg]

Ever notice the lousy tread and amount of wheel spin these guys rely on to keep from flipping? They want the front wheels off the ground so that all the weight is on the rear driving wheels. But they do not those rear wheels to to grip so good that the engine torque that is twisting the axle will flip them over backwards.

If the back wheels grabbed while the engine's full torque was being applied to turning the rear axle, the only thing left that could rotate would be the tractor.

Again, you can remove the traction issue by regarding it as a cogged railroad track.

The point where the chain attaches to the draw bar does NOT move forwards - unless the chain breaks, or is a lot more elastic than what most of us understand to be "chain".
The rear axle does NOT move backwards.
An equilibrium is reached.

I don't see why folk are not seeing this ~~~

 

[Baby Grand]

While the theories are bouncing around....It may be worth mentioning that there have been hundreds, if not thousands of cases where tractors HAVE flipped backwards when pulling from a low mounted draw bar. The fact that it DOES happen with some minor degree of regularity SHOULD influence the possible v impossible voting.

Why muddy the waters with experience?

 

[xtn]

Some of you keep mentioning a relationship of the axle height to the drawbar. Not important.

You've got force #1, the contact patch of the rear tires on the ground trying to push forward. And you've got force #2, the attachment at the drawbar trying to pull backwards. Since those two forces are not colinear, they result in a torque. Since force #2 is above force #1, that torque acts to rotate the tractor up and over backwards.

The only things that will prevent a backflip are the lack of either enough power or enough traction to complete the job.

Force 2 acts at the axle. At the ground point, you have the tire lever arm pushing backwards.

No force #2 does not act at the axle. It acts where it happens. In this case it acts where the chain is attached to the draw bar.

Let's get rid of the whole tractor and just imagine there is a rigid steel triangle running from the drawbar attach point down to the contact patch of the two tires. Three points; drawbar hole, right contact patch, left contact patch. Rigid steel triangle made of angle iron or something. That's what's being acted upon by the two forces. The top point is leaning against a flag pole and loosely chained to it. You grab the lower two points and move them further away from the pole. What happens? As the lower two points are pulled further from the pole, the triangle must incline further back against the pole and the upper chained point must slide a bit down the pole.

Now it so happens that as that triangle gets inclined further back, so does everything else that happens to be rigidly attached to it. Which basically means the whole rest of the tractor, and you too if you're sitting in the seat.

So sure, it has an effect at the axle, just as it has an effect everywhere else on the tractor. That effect can be calculated/predicted at the axle just like it can be for any other point. And the result of any such calculation, for any point on the tractor, is that the forces will tend to make any chosen point rotate backwards. But none of that means the force is acting AT the axle. I mean there ARE forces going on at the axle of course, but not either of the two simple vectors needed to deal with the question asked by this thread.

xtn

 

[Short Game]

I think we are forgetting that there might be an alternate universe where it can't flip over if hooked up below the axle. Just click the heels of your ruby slippers together to get there.

I started reading here with the notion that my tractor was pulling from the rear axle and a lower than the axle hookup would keep me down. I'd never given it any real thought. I used to think that racecars with the center of gravity below the axles didn't flip, though I saw them do it from time to time. All that low center of gravity did was change the traction distribution, but when something stops the tires from sliding, the center of gravity is above the contact patches and it's quite likely going to go over.

 

[xtn]

Again, you can remove the traction issue by regarding it as a cogged railroad track.

The point where the chain attaches to the draw bar does NOT move forwards - unless the chain breaks, or is a lot more elastic than what most of us understand to be "chain".
The rear axle does NOT move backwards.
An equilibrium is reached.

I don't see why folk are not seeing this ~~~

Yes of course you're right. Depending upon the height of the draw bar, how far back it is from the rear wheel center line, and the angle the chain is at when pulled tight, there is some point at which an equilibrium must be reached. So technically the tractor won't flip over on it's back completely.

That is... unless you start figuring in momentum and materials elasticity. I mean what happens if the equalibrium point is reached while 2000 lbs of tractor front end is rotating up with some speed?

xtn

 

[skspurling]

Yes of course you're right. Depending upon the height of the draw bar, how far back it is from the rear wheel center line, and the angle the chain is at when pulled tight, there is some point at which an equilibrium must be reached. So technically the tractor won't flip over on it's back completely.

That is... unless you start figuring in momentum and materials elasticity. I mean what happens if the equalibrium point is reached while 2000 lbs of tractor front end is rotating up with some speed?

xtn

Okay, you know, I have had many many classes in this stuff and done quite well. Some of these "scenarios" you guys are coming up with are quite mind boggling. NO, a tractor does not flip by pure nature. You have to hook the thing up wrong to get it to flip. Most normally the flipping occurs because the pulling causes the heavy object being pulled to lift and exert a reverse torque downward acting at the draw bar. No rolling action on the part of the tire is going to cause the front to lift. If your tractor rares up because the tires are frozen to the ground, you would have to be backing up. Otherwise the torque pushes the front into the ground resulting in the tractor moving forwards. The triangle illustration given earlier is backwards.

Page 2 on the following document gives a free body diagram:
http://www.bae.ncsu.edu/people/faculty/mrburche/BAE200_10/Week9_TK_Ellington/BAE200Traction.pdf

If you observe the forces on that diagram Rmr is the actual moving force, and Rtr is the reacting traction force. Rtr is NEVER larger than Rmr. They are at most equal, which means the tractor will not be slipping. That diagram was drawn to illustrate traction forces, but they did add something to it wich shows why tractors flip. If you look at the very back there is a vector called the line of pull. Notice that it is at an angle? That is what I was talking about above. If you want to look at the situation where the nose lifts, you set the origin to the contact point of the ground and tire. When you do that, there is no reason for the nose to lift due to forward tire rotation, unless you have additional forces reacting to the forward motion of the tractor. In that case, you have to have a backwards torque acting around your ground contact point that exerts a torque greater than your forward driving force x the height of your axle + the mass of your tractor x the horizontal distance from the ground contact point to the center of mass. Now, the reason that it appears that the tractor flips it's self over is that the torque is usually created as a resultant force of the load pulling back against the tractor frame. Add that to the fact that the horizontal distance from the ground contact point to the center of mass shrinks very quickly once the front rises off the ground, and you can see why it looks like they flip easily.

Confused?:confused2::confused2:

 

[dwhite]

I am more confused now than when I started this blasted thread!

 

[beppington]

I am more confused now than when I started this blasted thread!

Man you are REALLY confused - You didn't start this thread! :laughing: