Farm Tractor Skidding

[redpowerd]

Chocker placement on the tree is important. The chocker should be placed near the butt (wrapped if necessary). A cross piece  will make that easy.  There have been cases where for one reason or another the chocker was up the stem and the tree hooked ,stood up then  fell forward toward tractor and operator.   With smaller diameter trees this can happen very quickly.  

(Image hidden from quote, click to view.)

not sure why anyone in their right mind would hook a tree in the middle.

[Gary_C]

There is another hazard with skidding with a farm tractor. If you are skidding on any slope, the log can slide or roll sideways and the chain or cable catch on the rear tire lugs and will wrap around the tire or axle and the log will be pulled right up to the tractor real fast.

This happened to the pastor of our church where the tree rolled sideways, the chain caught in the tire lugs and was pulled right up over the wheel and hit him right in the face and took him right off the tractor. He was injured pretty badly and was lucky it did not kill him.

[Woodhog]

Your best bet is to buy a used or new winch for the tractor.

This will help with many of the safety concerns mentioned (also introduce a few more) and give you a much more productive and safer operation.

Here is an excerpt from National Agriculture Safety Board:

Hitching too high is one of the main causes of rollovers to the rear with farm tractors. Loads should only be hitched to the drawbar. If a load is hitched above the drawbar and power is applied, the tractor may revolve around the rear axle. It only takes about 1.5 seconds for a tractor to flip backwards, not enough time often for the operator to react, let alone escape.

They say 1.5 seconds to flip over backwards, we lost an operator near here this fall from back tipping, crushed under the tractor.

You should have a top canopy over the operating position also, you will eventually bump a dead tree and the top will fall right on the tractor.

Also watch for pieces of debris being caught up in the chains and being driven up near your legs and feet like spears...

Even with eyes in the back of your head you will have a lot of close calls.

The winch should have the screen to protect from anything  being dragged flipping up and hitting you from behind when the front end of the piece lodges and the whole log comes flying thru the air towards the tractor.

If you dont put on a good skid pan, radiator protector, tire valve protectors  you will eventually wreck something on the machine.

Its fun in spite of all the trouble if you can make some money doing it. Well.... thats another story...

[LorenB]

Sorry but I don't agree with any of the above.

Gary,

You seem to understand a free body diagram.  Picture the following one:

A huge, but lightweight tractor, with tires ten-feet in diameter.  This tractor weighs only 100 pounds. 

The tires on this tractor are capable of unlimited traction.  The force that the tires can exert on the earth is unlimited. 

The tractor is connected to a cable that is horizontal.  This cable is capable of producing an unlimited tension.  The cable is connected four feet above the ground, placing it one foot below the axle. 

The tractor now starts to move forward.  It will turn over.  It will do this even though the cable is attached below the axle. 

There are only two couples acting on this tractor.  The righting couple is the weight of the tractor acting downward through the center of gravity coupled with the vertical force of the earth, acting upwards at the base of the rear tire. 

The upsetting couple is the tension on the cable acting in a horizontal rearward direction coupled with the force of the earth acting on the bottom of the rear tire in a forward direction. 

When these couples are equal, there will no longer be any force between the tractor’s front tires and the earth.  When the upsetting couple becomes larger than the righting couple, the tractor will tip over backwards. 

You can sum moments about any point and get the same result, but it’s easiest to sum the moments about the rear tire’s contact point because then you don’t have to consider the forces on the rear tire. 

I hope you agree with my analysis after this “word picture”.  I found it helpful to draw it out. 

– Loren

[Gary_C]

The tractor now starts to move forward.  It will turn over.  It will do this even though the cable is attached below the axle. 

If all the conditions with your theoretical tractor remain true, It will NOT turn over. But since it is only theoretical, it doesn't matter, we are dealing with reality here. And the reality is that the best way to avoid backward tip overs is to hook below the axle and preferably as far forward of the rear axle as possible.

So no, I still do not agree in any way with your analysis. If you want to do a free body diagram, try analysing the increase in the moment arm associated with the cable as you move the hitch point forward under the rear axle.

[LorenB]

The only reason I care about convincing anyone here is because if someone doesn't understand the forces involved he can get hurt. 

I will now stop trying to convice anyone.  You can believe me or not. 

One last parting shot:  It doesn't matter, with a horizontal cable (not real life) whether the cable is attached forward of the axle or not.  With a cable that is angled downward somewhat, as in real life, attaching the cable forward of the rear tire contact point (the same as the axle in this case) will help prevent a tipover, because the vertical component of the cable's tension is now part of the righting moment instead of part of the upsetting moment. 

-- Loren

[pineywoods]

I just gotta put my two cents in here. I use a kubota M series to do all my skidding and have never had any sign of a problem. I have pulled some logs too big for my woodmizer. However i do realize the potential for disaster is always there.
Some observations  and comments on the previous posts...
For decades here in the south, the skidder of choice for loggers and pulpwooders was an 8N ford or the close kin massey fergusons. The absolute worst choice was an old 2 cyl john deere. Just about any tricycle row crop tractor with a high center of gravity  is almost as bad as the deeres. All tractors aren't equal when it comes to skidding. Most of these old tractors did not have a 3 pt lift, therefore regardless of where you tie on, the end of the log is down in the roots and stumps, a sure invitation for disaster. Some say, hook on to the drawbar, I agree, but in case you never noticed, the front end of the drawbar is attached to the axle on most tractors. Another killer not mentioned--hand clutches. When the front end comes off the ground, you need to dump the clutch pronto. Hand clutches take too much time. By lifting the end of the log off the ground, there is another built-in safety factor. If the log hangs and there is enough traction, the front end will rise, but the draw bar/ 3pt hitch extends out behind the rear wheels. This will rotate downward, dumping the log on the ground (don't lift the log more than a couple inches) and reducing the weight on the rear wheels, usually enough to loose traction, but don't depend on it. Sorta like wheelie bars on a drag racer..Nother factor...speed..slow is good, gives you a few precious milli seconds to recover when things go wrong. I use 1st gear at idle.
I disagree with the skid pans and belly plates. They always get full of leaves, twigs, and oil, then catch fire and burn. A burned machine is usually a total loss, I can fix busted hoses etc. Watch what you run over...
Looks like most folks use chokers, I don't. My choice is skidding tongs attached to a bar across the 3 pt hitch. Dumping the 3 pt lift will usually get you out of trouble, most times the tongs will come loose from the log.

Lift the log a few inches off the ground
Drive slow with foot lightly on clutch pedal
Expect something to go wrong
Don't do nothin stoopid

[Gary_C]

  It doesn't matter, with a horizontal cable (not real life) whether the cable is attached forward of the axle or not. 

You will have a hard time convincing all the tractor design engineers that fasten the drawbars on large tractors on the belly of those tractors as far forward as possible. And you have convinced me that you do not know how to determine the length of the moment arm for a rotational torque.

I'm not even sure what it is that you are trying to convince everyone that should be done to prevent backward roll overs.  

[WH_Conley]

I am in the dark as far as what Gary-C and LorenB are talking about. I have skidded hundreds of thousands of feet of timber with a tractor, 600 series ford up a 7000 series. Safest tractor I have is a 3930 Ford with a loader on it. It will not get enough traction on dirt to pull the front end up. No such thing as perfect traction in the real world. I always hook from the hydraulic drawbar, attached to the rear end. I use a hook on the drawbar and chokers. The chokers usually rotates the log til it is on top, if the log hangs on a stump or root the front end will raise til it is level with the top of the log and tires will spin out. At some point it will break traction. Factors like chains on the tires and stumps put a completely different light on everything, if not experienced don't use them, they will kill you.

I agree with Piney, stay away from the tricycle tractors, they will kill you quick.

If you need to skid uphill get a winch.

[Gary_C]

I am in the dark as far as what Gary-C and LorenB are talking about.

That's just as well as one or both of us is in the dark too.     

You are right.
Hook as low and as far forward as possible and get as much weight on the front as you can.
FEL's and front wheel drives are good for weight on the front.
Be alert and prepared for the unexpected.

[woodmills1]

here is a link that says the ground is the rotation point when the roll is due to pulling

http://www.nstmop.psu.edu/tasksheets/4.12%20Tractor%20Stability.pdf

[woodmills1]

also check this link as to why hooking lower is better


http://www.farmanddairy.com/news/tractor-roll-over-demonstrations-show-severity-reality-of-accident/1027.html

[Gary_C]

here is a link that says the ground is the rotation point when the roll is due to pulling

http://www.nstmop.psu.edu/tasksheets/4.12%20Tractor%20Stability.pdf

That is a somewhat confusing task sheet as written. This is what the written part says:

Engaging the clutch of a tractor
results in a twisting force, called
torque, to the rear axle. Under
normal circumstances, the rear axle
(and tires) should rotate and the
tractor will move ahead. If this
occurs, the rear axle is said to be
rotating about the tractor chassis.
If the rear axle cannot rotate, then
the tractor chassis rotates about the
axle. This reverse action results in
the front end of the tractor lifting
off the ground until the tractor’s
CG passes the rear stability
baseline. At this point, the tractor
will continue rearward from its
own weight until the tractor
crashes into the ground or other
obstacle.

No matter what the pictures and descriptions show in other places, that is the real story. It is not physically possible for the wheel to stop rotating and yet having the center of rotation of the tractor at the bottom of the wheel. If the bottom of the tire were actually the center of rotation, then the rear part of the tire should be disappearing into the ground.

This part is where the problem begins:

When a two-wheel drive tractor is
pulling a load, the rear tires push
against the ground. At the same
time, the load attached to the
tractor is pulling back and down
against the forward movement of
the tractor. The load is described
as pulling down because the load
is resting on the earth’s surface.
This backward and downward
pull results in the rear tires
becoming a pivot point, with the
load acting as a force trying to tip
the tractor rearward. An “angle
of pull” is created between the
ground’s surface and the point of
attachment on the tractor.

The part in bold is where they go wrong. According to that description and the drawing that goes with it, it does not matter where the load is hitched, if the load is resting on the ground it "results in the rear tires becoming a pivot point" which is not true or possible.

So even though their technical description is flawed, they give some good pointers on how to avoid rollovers.

[LorenB]

here is a link that says the ground is the rotation point when the roll is due to pulling

http://www.nstmop.psu.edu/tasksheets/4.12%20Tractor%20Stability.pdf

woodmills1,

I appreciate the link stating that the tractor rotates around the ground contact point, but there are too many errors in that document for me to use it as a reference.  Apparently Gary and I agree on that issue at least, since he doesn’t think much of it either. 

That document states that going around a corner too fast will cause the CG of the tractor to move.  That is nonsense, of course.  The only thing that will move a CG on a tractor is changing the configuration of the tractor, such as lifting a load in a loader bucket, or adding weight to the front end. 

Another glaring error in the document is the drawing on page three that shows the angle of pull with the tractor attached to a stump with a cable.  The angle of pull is not directly down the cable. 

Whenever you are pulling something with a cable, chain, rope, etc., the angle of pull is always directly down the cable.  A cable, chain, etc. cannot support a bending moment, so therefore it can be loaded in direct tension only.  Therefore the force created by a cable is directly in line with the cable. 

I’m afraid that this document was written by agriculture specialists, not by engineers who understand how forces work in nature.  It may be useful in explaining tractor tipping to a farmer who doesn’t want to study statics and free body diagrams (second year engineering), but it’s basically not correct. 

– Loren

[LorenB]

No matter what the pictures and descriptions show in other places, that is the real story. It is not physically possible for the wheel to stop rotating and yet having the center of rotation of the tractor at the bottom of the wheel. If the bottom of the tire were actually the center of rotation, then the rear part of the tire should be disappearing into the ground.

Gary,

Okay, I will comment one more time on this, then I promise I’ll shut up. 

I think I now see what you are saying.  I submit that the tractor does rotate around the bottom of the rear tire, but I recognize that that point is moving forward as the rotation is happening. 

Another way to look at this is that the cable stops moving and the tractor keeps on driving.  If the tractor keeps on driving, the rear tires will move forward, flipping the tractor onto its back. 

– Loren

[woodmills1]

I have really been thinking here and I would say all rear rolls not pulling from behind are pivoting around the axle.  I would also say most rolls pulling will be actually because of the motor turning the tractor around the rear axle.  However I think the second there is a large rearward(and downward) pull from say a snag on the load, the point of rotation is at the ground contact,  however once the tractor stops going forward, for example the log is caught, a huge force rearward and downward is created causing the tractor to lift around the tires and because the connecting point is moving forward due to rotation this will continue to pull, but with a low connection the downward componet of force on the tires will decrease and the tires should spin stopping the rotation.  If the tires don't spin then it is possible that the rear roll would continue but at that point around the axle as long as the motor is still running and the clutch still engaged.  Any high connection is just bad weather or not the first rolling is at the ground or the axle.  As far a going up hill just remember that the tractor is already partly "rolled" backwards due to the angle so the pulling force component to roll is larger and the hold down componet of the tractors weight and length is reduced.

all this said I still think any actuall rear roll over will be ultimately around the axel.  If you think about it, I could use a come along attached to the top of my roll bar and tip my tractor around the tires or the axle, but attached to the draw bar it would move towards me way before it would roll and even if I got the tires to lift with the comealong attached to the draw bar how far would the front wheels be off the ground before the draw bar was so close to the ground that the wire or chain pulling would be horizontal thereby reducing the downward force causing the rotation to be zero

[woodmills1]

the center of gravity of anything is where it is, but that is the location where any centripedal force is theoretically applied during rotation and he explains quite well how that c of g changes its position relative to the ground during movement on slopes

he does list turning too fast under how the cofg changes but goes on to explain correctly how forces occur in turning, though he calls them centrifugal and they are really centripedal.

I used to put a cup full of water on a small wooden disk I had attached to a string set up and whirl it around in a circle sideways and up and down way back teaching.

[thecfarm]

I worked with my Father with a NAA Ford,just about the same size as an 8 or 9N.We never had a skid pan or anything to protect the radiator.We bought a 2120 40hp Ford to really go at it,still no protection.But we only cut on our land.All the trials are brush free.I cut trees going to any trees I want.Yes,it takes time,but these trails are used over and over again.Before we got the 3 pt winch it was a real bother to cut trails.But we use them for years to come.Now that My father is gone I still do the same thing,but now I have to stop to tractor to throw a piece of brush out of the trail.Yes,it takes time,but it's on my land so I might just as well keep it clean.The only trees I ever do run over and those are no more than 2 feet tall are fir trees.In places they grow real thick and they don't grow good on my land.I don't remember any scary momets on a tractor in the woods.My father was real careful on a tractor and passed it on to me.Things can happen even if you do know what can happen.But some people have no idea or think it will never happen to them or get in a hurry.

[LorenB]

the center of gravity of anything is where it is, but that is the location where any centripedal force is theoretically applied during rotation

I can't seem to find where the article says the cofg moves on a turn.  I see it says as the tractor moves the cofg location changes with respect to the ground, and it talks quit correctly about the forces in turning though

James (sorry, I didn't see your signature before),

You're absolutely right about the CG being the place where all inertial forces (including centripetal) act. 

The article has a table in the top left of page two that lists "Reasons the CG moves around".  Only the second one is absolutely correct.  The first one is more a description of how the position of the CG changes relative to the "stability baseline".  The last three are just wrong. 

-- Loren

[Magicman]

I always hook my rear grader blade to the tractor before skidding logs.  True, this adds weight to the rear, but it also gives me a rear contact point when the front end comes off of the ground.  And I said when.  I've driven 1/4 of a mile with the front tires 6" off of the ground and steering with the rear brakes.  The higher I have the blade, the higher and easier it is for the front end to come up.  I always raise it until I have the toe of the log off of the ground.  Using the blade on the rear, gives me "rear rollover protection" plus being able to control the height of the log being skidded.