Egon
Epic Contributor
Draw avector diagram of the forces involved and all will become clear.
Egon
Egon
Towing a Trailer Uphill
- Thread starter JCA
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Soundguy
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</font><font color="blue" class="small">( But you're considering only DOWNWARD pressure. If the hitch is above the wheel axels, BACKWARD )</font>
Yep Hillsider.. JCA and the others haveit right.. its a geometry problem. While the down weight is constant even if the connection is above or below the rear axle.. the forces onthe tractor change dramatically towards producing a backflip, when the load is above the rear axle.
Soundguy
Yep Hillsider.. JCA and the others haveit right.. its a geometry problem. While the down weight is constant even if the connection is above or below the rear axle.. the forces onthe tractor change dramatically towards producing a backflip, when the load is above the rear axle.
Soundguy
Phred
Veteran Member
The initial logic above appears to be that as long as the hitch point is below the axel no problem. I do not agree.
Good old RxF. Which is physics talk for torque.
The basic problem is that if you have enough torque you will rotate the tractor about the pivot point, i.e. flip it over backwards.
This torque is just the product of the force (F) and the distance at which you apply it from the pivot point (R). Conect it up high or on a very long draw bar and you just increased R. Increase the weight of the load and F goes up. The product of the two is what gets you in trouble.
Fred
Good old RxF. Which is physics talk for torque.
The basic problem is that if you have enough torque you will rotate the tractor about the pivot point, i.e. flip it over backwards.
This torque is just the product of the force (F) and the distance at which you apply it from the pivot point (R). Conect it up high or on a very long draw bar and you just increased R. Increase the weight of the load and F goes up. The product of the two is what gets you in trouble.
Fred
MikePA
Super Moderator
<font color="blue"> The initial logic above appears to be that as long as the hitch point is below the axel no problem. I do not agree. </font>
<font color="blue"> The basic problem is that if you have enough torque you will rotate the tractor about the pivot point, i.e. flip it over backwards. </font>
Help me understand, these 2 statements, Fred. /forums/images/graemlins/smile.gif How can you flip a tractor over backwards with the load attached below the pivot point? With the load connected below the axle, the rotational force is attempting to pull the front of the tractor down.
<font color="blue"> The basic problem is that if you have enough torque you will rotate the tractor about the pivot point, i.e. flip it over backwards. </font>
Help me understand, these 2 statements, Fred. /forums/images/graemlins/smile.gif How can you flip a tractor over backwards with the load attached below the pivot point? With the load connected below the axle, the rotational force is attempting to pull the front of the tractor down.
Phred wrote:
</font><font color="blue" class="small">( This torque is just the product of the force (F) and the distance at which you apply it from the pivot point (R). Conect it up high or on a very long draw bar and you just increased R. )</font>
Yes and no. It's a product, yes, but it's a "cross product" of two vectors. You don't just multiply them together. The magnitude of the torque (it has both magnitude and direction) is equal to R times F times the cosine of the angle between them. That means that if they're both--R and F--pointing in the same direction, the torque is zero. No matter how long the draw bar is, if you hook up at the same level as the pivot point, the torque is zero.
Oh, and if you hook it up BELOW the pivote point instead of ABOVE, then the torque is in the other direction; if you consider the axle to be the pivot point--and the axle's definitely the easiest point to pivot around--then the torque is forcing the front end down, not pulling it up.
It's arbitrary what you consider the "pivot point" to be. If you consider it to be the point of contact between the rear tires and the ground, then you can still tend to lighten the front end by pulling backwards. But it'll only flip, in this case, if you've got REALLY GOOD traction. Most likely the tires will slip before the front end comes up.
Jim
</font><font color="blue" class="small">( This torque is just the product of the force (F) and the distance at which you apply it from the pivot point (R). Conect it up high or on a very long draw bar and you just increased R. )</font>
Yes and no. It's a product, yes, but it's a "cross product" of two vectors. You don't just multiply them together. The magnitude of the torque (it has both magnitude and direction) is equal to R times F times the cosine of the angle between them. That means that if they're both--R and F--pointing in the same direction, the torque is zero. No matter how long the draw bar is, if you hook up at the same level as the pivot point, the torque is zero.
Oh, and if you hook it up BELOW the pivote point instead of ABOVE, then the torque is in the other direction; if you consider the axle to be the pivot point--and the axle's definitely the easiest point to pivot around--then the torque is forcing the front end down, not pulling it up.
It's arbitrary what you consider the "pivot point" to be. If you consider it to be the point of contact between the rear tires and the ground, then you can still tend to lighten the front end by pulling backwards. But it'll only flip, in this case, if you've got REALLY GOOD traction. Most likely the tires will slip before the front end comes up.
Jim
Nasty135
Veteran Member
OK everybody knows that if the hitch is above the PP (pivot point) it will want to lift front wheels given enough traction, But Think about this for a minute, Below the PP it would try to pull the front end down and if the angle of the weight being pulled is higher than the draw bar once the pulling begins the only thing it could do is lessen your traction by trying to lift the rear wheels (I'm assuming pulling something with a chain in this example)...If you have tongue weight it would add to your traction (beneficial assuming drawbar not 9ft. long)...
NOW for the real world experience..I have a boom pole and chain and when pulling stumps the tractor will pull the front wheels up to the point at which the three point hitch is triangulated and balanced with the weight of the front end against the the pull of the stump which in turn is pulling against the PP on the tractor limiting my traction to the precise amount needed to keep the front end up on tractor ,tires spinning low gear at idle you can watch the stump for signs of surrender..push in clutch hold brakes keeping front end up or ease off brakes back to ground try different direction...Have done this on asphalt as well idling low gear tire spinning(must have good chain)It is not at all scarry as you might think,ZERO pucker factor ..BTW I don't live on the san andreas fault line either...I must add that This tractor a MF135 has low CG and I wouldn't try this with my newer 1433v It just doesn't feel the same not as stable /forums/images/graemlins/shocked.gif
Sorry for the long winded explanation ,Now I must go soak my phalanges....Mark
NOW for the real world experience..I have a boom pole and chain and when pulling stumps the tractor will pull the front wheels up to the point at which the three point hitch is triangulated and balanced with the weight of the front end against the the pull of the stump which in turn is pulling against the PP on the tractor limiting my traction to the precise amount needed to keep the front end up on tractor ,tires spinning low gear at idle you can watch the stump for signs of surrender..push in clutch hold brakes keeping front end up or ease off brakes back to ground try different direction...Have done this on asphalt as well idling low gear tire spinning(must have good chain)It is not at all scarry as you might think,ZERO pucker factor ..BTW I don't live on the san andreas fault line either...I must add that This tractor a MF135 has low CG and I wouldn't try this with my newer 1433v It just doesn't feel the same not as stable /forums/images/graemlins/shocked.gif
Sorry for the long winded explanation ,Now I must go soak my phalanges....Mark
Phred
Veteran Member
Guys,
Well I guess that's what happens when I stay up past my bed time and attempt to talk in a quasi intelligent way /forums/images/graemlins/blush.gif
Of course your all right that its a cross product so you need to figure the direction with the right hand rule.
To clarify this issue of the hitch point. If your pulling with a chain then yes your right. Mounting the tow point below the axel will pull the front end down. This is due to the fact that the force is completely to the rear of the tractor.
Now if the problem is tongue wieght then it's not so simple. Mount the tow point low and you still get a torque which pulls the front end up since the force is downward. Make the draw bar longer and the problem gets worse. Now all this may be balanced by the backward weight of the trailer which is a force to the rear.
What I was attempting to bring up in the last post is the tongue weight issue. And in that case a longer draw bar makes it worse.
I get a first hand dose of this when I switch between my rear blade and box blade. Both weigh about the same but you don;t notice the box blade. Lift that rear blade up height and the front end of the tractor starts to get a little light. This is due to the fact that the box blades weight is closer to the tractor than the rear blade, and in this case the force is down not to the rear of the tractor.
Fred
Well I guess that's what happens when I stay up past my bed time and attempt to talk in a quasi intelligent way /forums/images/graemlins/blush.gif
Of course your all right that its a cross product so you need to figure the direction with the right hand rule.
To clarify this issue of the hitch point. If your pulling with a chain then yes your right. Mounting the tow point below the axel will pull the front end down. This is due to the fact that the force is completely to the rear of the tractor.
Now if the problem is tongue wieght then it's not so simple. Mount the tow point low and you still get a torque which pulls the front end up since the force is downward. Make the draw bar longer and the problem gets worse. Now all this may be balanced by the backward weight of the trailer which is a force to the rear.
What I was attempting to bring up in the last post is the tongue weight issue. And in that case a longer draw bar makes it worse.
I get a first hand dose of this when I switch between my rear blade and box blade. Both weigh about the same but you don;t notice the box blade. Lift that rear blade up height and the front end of the tractor starts to get a little light. This is due to the fact that the box blades weight is closer to the tractor than the rear blade, and in this case the force is down not to the rear of the tractor.
Fred
MossRoad
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<font color="blue">What I was attempting to bring up in the last post is the tongue weight issue. And in that case a longer draw bar makes it worse. </font>
True, but don't confuse tongue weight on the drawbar with weight on the 3pt hitch. Aren't the two different? Both will act like a lever... with enough weight on either it will tip the front wheels skyward. My drawbar does not stick out anywhere near the length of my 3pt hitch arms.
True, but don't confuse tongue weight on the drawbar with weight on the 3pt hitch. Aren't the two different? Both will act like a lever... with enough weight on either it will tip the front wheels skyward. My drawbar does not stick out anywhere near the length of my 3pt hitch arms.
Soundguy
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And proper 3pt hitch geometry and hookup will help prevent a backflip via the soild toplink.
Soundguy
Soundguy
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</font><font color="blue" class="small">( if you're hooked up BELOW the axle the only point about which the tractor can rotate is the point of contact between the tire and the ground...and THAT would require very good traction and a very heavy trailer, and isn't likely to happen. )</font>
But it did happen to the father of a friend. Friend told me how his father became disabled was that a Cat crawler somehow couldn't move a heavy loaded trailer due to a trailer brake malfunction. The Cat simply walked a half revolution backward inside its treads, which never lost traction.
This isn't directly applicable here because a CUT should lose traction first, but it illustrates the physics when traction is sufficient and everything goes wrong.
But it did happen to the father of a friend. Friend told me how his father became disabled was that a Cat crawler somehow couldn't move a heavy loaded trailer due to a trailer brake malfunction. The Cat simply walked a half revolution backward inside its treads, which never lost traction.
This isn't directly applicable here because a CUT should lose traction first, but it illustrates the physics when traction is sufficient and everything goes wrong.