Towing a Trailer Uphill

[JCA]

Hi folks,

Please help me think this through. If you have a trailer attached to the drawbar BELOW the level of the rear-wheel axle, it OUGHT to IMPROVE the stability of the tractor, making a rear flip LESS likely than WITHOUT the trailer.

If you hook the trailer up ABOVE the level of the axle, the tractor would tend to rotate about the axle, and could flip backwards. This could happen fairly easily, since you only have to lift the front end of the tractor

But 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.

My conclusion is that towing a properly attached trailer up a hill--with the attachment made below the level of the rear-wheel axel--ought to be at least as safe as, and maybe even safer than, driving the tractor up the hill WITHOUT the trailer.

Make sense?
Jim

 

[JeffRey30747]

Jim,
That makes sense as long as you maintain the proper tongue weight on your trailer. An area where most folks get into trouble towing a trailer is by not loading it properly which can even result in a negative tongue weight, reducing traction and causing stability to go out the window at highway speeds.

Jeff

 

[JCA]

Jeff,

I don't know the terminology. But from what you said, what makes sense to me is that you just need to make sure you don't have too much weight pushing down on the drawbar/hitch, which could tend to lift the front of the tractor--or too much weight pushing up (because the load is behind the wheels on a single-axle trailer), which might tend to lift the rear wheels and reduce traction. Do I have it right?

 

[hillslider]

This does not make sense to me. The downward pressure is going to be the same no matter at what height you have the trailer. Now.....having the trailer loaded properly is key to how things will work out. To much tounge weight and you have a light front end....to little and you have no traction.
Now....having the hitch high or low will not change the downward pressure so it does not matter for stablity front to rear. but this will effect side to side stability. My kubota has a hitch bar and this is what I would use because it is stationary. If The front end did come up it would only come up until the trailer and hitch hit the ground. If you have it a quick hitch on the 3pt.....be careful beacuse most tractors do not have down pressure so if the front end would come up the hitch will hit the ground but the front end would continue until the 3pt went up all the way. This would be bad. Make sense?

Later and be carefull, I have used a trailer as a wheely bar and it is not fun.

Rob

 

[Egon]

It all might depend on how far the hitch extends behind the tractor.

Egon

 

[JCA]

Hillslider (with a monicker like that I'm supposed to listen to you? /forums/images/graemlins/grin.gif )

You wrote : </font><font color="blue" class="small">( Now....having the hitch high or low will not change the downward pressure so it does not matter for stablity front to rear. but this will effect side to side stability. )</font>

But you're considering only DOWNWARD pressure. If the hitch is above the wheel axels, BACKWARD pressure will also make the front end light, because the tractor can rotate on its axle. Right?

Jim

 

[keeney]

The other area you can get into trouble is just plain having enough traction to control the trailer. On a hill, a 10,000 lb trailer can easily drag a 4,500 lb tractor down or up, or sideways, or whatever way it wants to go.

One thing about losing traction in such a situation is that you can go from a safe, comfortable portion of the hill, perhaps sideways to a steeper part of the hill or maybe towards a ditch, etc.

Going down hill can be bad too - the trailer can push the tractor in directions you don't want to go.

In general, as a rule of thumb, I would limit towing to trailers that weigh no more than the ballasted tractor.

- Rick

 

[Laminarman]

<font color="blue"> </font> reducing traction and causing stability to go out the window at highway speeds

I haven't been at highway speed on my tractor in at least a week.

 

[Slamfire]

While towing a two wheel cart loaded with firewood, I found the trailer actually lifting the drawbar. Not much caring for that I went up to my shop and got the bars that run from the lift arms to the top link connector. That made the lifting force push the front end of the tractor down, like a 3 point hitch ought to. You've seen them they come in pairs which bolt together through slots, to make them adjustable and have offsets with holes in the ends to allow hookup. I think they were standard equipment with N series Fords. At least that's the first place I saw them. I use them now whenever I use the drawbar, even skidding logs. /forums/images/graemlins/grin.gif

 

[hillslider]

You wrote "If the hitch is above the wheel axels, BACKWARD pressure will also make the front end light, because the tractor can rotate on its axle. Right?"

Well kind of. The hitch if mounted to the frame of the tractor above the axle would make the front end lighter than if mounted below the axle. But if you are talking about the 3 pt lifting high or low it would not because the attachment point to the trator does not change.

All you can do is make sure you keep the weight low to the ground in case things do get light your wheels on the front will not get to close to the sky, load the trailer properly, go slow, rather make one big load make several smaller ones. And of course expect the unexpected!! Oh yeah.....using the tractor is fun and relaxing right. /forums/images/graemlins/grin.gif

 

[Egon]

Draw avector diagram of the forces involved and all will become clear.

Egon

 

[Soundguy]

</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

 

[Phred]

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

 

[MikePA]

<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.

 

[JCA]

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

 

[Nasty135]

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

 

[Phred]

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

 

[MossRoad]

<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.

 

[Soundguy]

And proper 3pt hitch geometry and hookup will help prevent a backflip via the soild toplink.

Soundguy

 

[California]

</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.