Preventing tractor back flip

[xtn]

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'

It's not even necessary for them to skid, but that is one way it could happen.

The action of the free standing tractor tipping backwards, should the brakes be on, by itself necessitates the rear tires "rolling" backwards. Of course when the brakes are on you get a 1:1 ratio of tractor tip to tire rollback. Whether or not we have a 1:1 ratio during the pulling exercise depends on the gear ratios and the drawbar-length-from-axle to tire-radius ratio.

Either way they MUST come backwards if the tractor lifts it's nose while connected to an immovable load with any normally expected chain angle (I don't think anybody here disputes the tractor being able to raise its nose at least at first). The geometry simply requires it, and no amount of other factors can override that fact.

xtn

 

[Kyle_in_Tex]

Milkman has it right. Many of you are arguing semantics.

There is rotational torque on the tires. The weight on the front end counteracts the drawbar torque force. Lowering the drawbar gives it a much better vantage point but can still be overcome.

I've seen inertia mentioned but not yet acceleration. If you accelerate the mass of the front end up fast enough, the inertia can carry it past the point of balance even considering the tires loosing traction (friction) at some point.

There are many ways to consider this, some with forces static and some with them dynamic. But, tell an old timer who's filled his seat of his pants with crap at the point of balance that it won't tip.

PS...you Canada guys can't get it right because you spell center and tires wrong...

 

[Egon]

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.
/QUOTE]

OK, physics says it can't happen. But it does happen on the flat and level without all the drastic actions mentioned above.:thumbsup:

Maybe the Physics needs a new model to work with or maybe a new approach or maybe just a new force diagram with the actual forces involved??

 

[LD1]

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.
/QUOTE]

OK, physics says it can't happen. But it does happen on the flat and level without all the drastic actions mentioned above.:thumbsup:

Maybe the Physics needs a new model to work with or maybe a new approach or maybe just a new force diagram with the actual forces involved??

Have you ever witnessed it happen? On level ground, with a slow and steady pull, and a drawbar mounted below the axle?

 

[piller]

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.
/QUOTE]

OK, physics says it can't happen. But it does happen on the flat and level without all the drastic actions mentioned above.:thumbsup:

Maybe the Physics needs a new model to work with or maybe a new approach or maybe just a new force diagram with the actual forces involved??

I think you are right here, and by the way your post #193 made sense to me.

Do any of the theoretical physics models include resonance conditions in the system?

Do any of the theoretical physics models include the dynamics of tire flex and other forms of stored energy in the system which may build up and be rapidly released?

Do any of the theoretical physics models include the effect of a tall 350 lb operator leaning backwards rapidly while sneezing and pulling back on the steering wheel while the front end is in the air?

Do any of the theoretical physics models consider the forces from a high speed headwind gust which catches the underside of the ROPS mounted canopy and tractor belly while the front end is off the ground?

Do any of the theoretical physics models include the effect that the ground is soft and the drawbar connection point pushes into the ground as the tractor flips over backwards?

Do any of the theoretical physics models include the effect that the turbocharged tractor engine exhaust pipe opening is elevated and facing forwards and therefore producing thrust which tends to cause the tractor to flip over backwards? (assume also that the engine air intake and radiator cooling airflow further enhance the effect).

Do any of the theoretical physics models include the situation where the rear tires dig holes as the spin which lowers the rear of the tractor and moves it backwards slightly and then traction increases instantly when a couple of thousand pounds of liquid ballast in the rear tires that had been sloshing around and became momentarily unweighted suddenly became fully weighted and the tires stopped spinning and the tractor rotated and did a back flip?

Do any of the theoretical physics models include the effects of radiation pressure from forward facing high powered lights with reflectors?

Do any of the theoretical physics models consider what happens when the front tires on a 4wd tractor get coated in 500 lbs of thick heavy mud and then the front end of the tractor lifts into the air and then as the front tires are spinning rapidly in the air all of the mud is released and thrown at high speed downwards and out in front of the tractor?

Do any of the theoretical physics models consider the effects of unequal and rapid variations in traction between the two rear tires when the rear tires/wheels are set for maximum tread width?

:laughing::laughing:

 

[massey990]

Yes, but is there anything more memorable than listening to a P51 idleing in its unique way on the runway, and when the pilot puts the spurs to it, that heart vibrating, earthshaking sound, and to watch it lift off and climb so fast and so vertical?. It is something I will never forget. It is no jet, but just about the culmination of piston powered aircraft if you ask me.

James K0UA

I met a guy once that put an Allison V-1710 into a reconditioned PT boat, not sure how they were originally powered but the Allison did the trick

 

[bcp]

I met a guy once that put an Allison V-1710 into a reconditioned PT boat, not sure how they were originally powered but the Allison did the trick

They were powered with 3 marine versions of the V-12 Packard aircraft engine. Horsepower ranged from 1200 to 1850 each.

Bruce

 

[SPYDERLK]

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.
/QUOTE]

[[OK, physics says it can't happen. But it does happen on the flat and level without all the drastic actions mentioned above.]]:thumbsup:

Maybe the Physics needs a new model to work with or maybe a new approach or maybe just a new force diagram with the actual forces involved??

Surely there is something more documented than "It happened to Uncle Harry, God rest his soul". Then, all it would take is a look at the drawbar to see why.
larry

 

[LD1]

Surely there is something more documented than "It happened to Uncle Harry, God rest his soul". Then, all it would take is a look at the drawbar to see why.
larry

Agreed:thumbsup:

 

[RoyJackson]

I met a guy once that put an Allison V-1710 into a reconditioned PT boat, not sure how they were originally powered but the Allison did the trick

At least some had a Packard V-12 or V-16...1200 HP, IIRC

 

[The Haymaker]

23 pages? Really? Enough torque to the axle, enough traction, and it will come up. Yes, lower drawbars should theoretically lessen traction if the nose begins to lift, and yes a longer drawbar will add to that effect plus allow the nose of the tractor to work with more leverage against lift, but if the first two are perfect it will go.

The fix? Run hydrostatic so you can eliminate torque quicker.

 

[Egon]

Have you ever witnessed it happen? On level ground, with a slow and steady pull, and a drawbar mounted below the axle?/QUOTE]

There were three times I've had the front wheels in the air about four feet. Without disengaging the clutch the tractor would surely have gone over. That was when I was a teenager and was invincible.

Had the front wheels of my Kubota off the ground once. That was quickly fixed with a load of rocks in the bucket.


Back when were not many of the racing boats powered by Allison engines. I saw some racing once in Idaho or Montana back in the 60's.

 

[LD1]

There were three times I've had the front wheels in the air about four feet. Without disengaging the clutch the tractor would surely have gone over. That was when I was a teenager and was invincible.

THat is the point. The wheels WILL come up. But if properly hitched, it wont go on over unless you were jerking on it or had a heck of a running start. More than likely, If you would have stayed on it, you whould have reached the point where either

1. the object breaks free and starts moving, resulting in the front comming back down,

or 2. you would have lost traction and the front would have came down as well

 

[johnrex62]

Tmajor started another thread and included a document from Professor Dennis J Murphy who is a professor of Agricultural Engineering. Professor Murphy states definitively in his article that tractors will NOT flip when properly attached to a drawbar.

"When the front end of the tractor lifts, the rear
drawbar of the tractor will lower. This is a function of
tractor geometry. The higher the front end raises, the lower
the rear drawbar is driven. As the drawbar lowers, the
殿ngle of pull and the leverage the load has to tip a tractor
rearward is also lowered. By design, a load will always
lose its ability to tip a tractor rearward before the tractorç—´
CG reaches the rear stability baseline. As the load loses its
ability to continue to tip the tractor rearward, the front end
falls back to the ground. If the tractor operator doesn稚 stop
the pulling action, the entire process will repeat itself,
resulting in a bouncing of the tractorç—´ front end."

His illustrations indicate that he references a chassis attached drawbar and not a 3pt attached drawbar.

There you go folks! It is in writing and available on the internet both! Boo-Yeah! :laughing:

 

[johnrex62]

I was having a discussion with a friend.
Situation - a chain is attached from the bottom drawbar (several inches below the center of rotation) to an immovable object.
My contention is the front wheels are being driven into the ground. Hence, no wheelie. He says it will still wheelie the same as having the pull point several inches above the center of rotation.
Pull above the center of rotation is a no-brainer -you're going over.
Who's right?

This was the orginal question and it was not specifically about flip, but rather about front end wheelie. Sorry, retiredmgn, it will wheelie but you are right that it will only flip if attached above the drawbar.

 

[tmajor]

Tmajor started another thread and included a document from Professor Dennis J Murphy who is a professor of Agricultural Engineering. Professor Murphy states definitively in his article that tractors will NOT flip when properly attached to a drawbar.:

I believe, he may have qualified that statement, by saying the situation was somewhat different on an incline, as the tractor's CG was closer to the rear stability baseline.

 

[johnrex62]

I believe, he may have qualified that statement, by saying the situation was somewhat different on an incline, as the tractor's CG was closer to the rear stability baseline.

He only qualified it for a specific scenario, moving uphill too quickly when the load digs generating a swift change in load momentum. The inclusion of the "too fast" condition kinda violates the safe operation principal and introduces operator error into the equation.

"It is also possible to flip a tractor
rearward when the load is properly hitched to the drawbar.
This may happen when several factors occur. If the
tractor is headed up an incline at too fast a speed and the
load, such as a large log, suddenly digs into the ground, the
rearward pull may be so quick and strong that the momentum
generated by the rearward lift may result in a rear overturn."

 

[LD1]

Tmajor started another thread and included a document from Professor Dennis J Murphy who is a professor of Agricultural Engineering. Professor Murphy states definitively in his article that tractors will NOT flip when properly attached to a drawbar.

"When the front end of the tractor lifts, the rear
drawbar of the tractor will lower. This is a function of
tractor geometry. The higher the front end raises, the lower
the rear drawbar is driven. As the drawbar lowers, the
殿ngle of pull and the leverage the load has to tip a tractor
rearward is also lowered. By design, a load will always
lose its ability to tip a tractor rearward before the tractor痴
CG reaches the rear stability baseline. As the load loses its
ability to continue to tip the tractor rearward, the front end
falls back to the ground. If the tractor operator doesn稚 stop
the pulling action, the entire process will repeat itself,
resulting in a bouncing of the tractor痴 front end."

His illustrations indicate that he references a chassis attached drawbar and not a 3pt attached drawbar.

There you go folks! It is in writing and available on the internet both! Boo-Yeah! :laughing:

He only qualified it for a specific scenario, moving uphill too quickly when the load digs generating a swift change in load momentum. The inclusion of the "too fast" condition kinda violates the safe operation principal and introduces operator error into the equation.

"It is also possible to flip a tractor
rearward when the load is properly hitched to the drawbar.
This may happen when several factors occur. If the
tractor is headed up an incline at too fast a speed and the
load, such as a large log, suddenly digs into the ground, the
rearward pull may be so quick and strong that the momentum
generated by the rearward lift may result in a rear overturn."

Sound exactally like what I have been preaching. Do you have a link?

 

[tmajor]

The post with link is here.

 

[xtn]

"As the drawbar lowers, the
angle of pull and the leverage the load has to tip a tractor
rearward is also lowered. By design, a load will always
lose its ability to tip a tractor rearwards."

The dear professor generalizes. The load's ability to tip the tractor rearward does NOT drop all the way to zero in every possible geometry. There is a pretty good range of connection angles and drawbar positions that have that tendency drop pretty low, but always remain positive.

Now there may be no real world tractors that can force the issue with enough power and traction to flip themselves under level and steady conditions. And that's fine; I've not argued that. But it is not accurate to imply that the load's ability to tip the tractor always reaches zero at some point.

LD1 you have me confused. Did you not tell me you think the tires and axles cannot move rearwards? And don't you agree that the geometry requires them to move backwards if the nose comes up? So you must then believe that there is a locked condition in which the tires must spin, the engine must die, or something must fail.

But now I've seen you mention the fact that the nose DOES start to go up. Then by geometric necessity you must agree the rear tires and axles do start to move backwards.

xtn

PS - I just checked my own little tractor, and found that the drawbar sticks 8" out past the tires. Guess I'm relatively safe.