Why 4 Wheel Drive

[JasG]

I am not unwilling to explain and the above posts go a ways toward it. In the interest of brevity [and laziness] they were written assuming a reader with a good grasp of the fundamentals. Even without that a question about my statement giving you trouble would help me address any issues in a directed manner. Instead were getting arguments that are based on links that the quoter has not fully understood. Sometimes a link will be correct but still mislead. Sometimes they are wrong in part or whole. Even worse there is the posturing, exaggeration, and twisting the flavor of my posts.
larry

See here is the problem, with me and others you are holding us to one standard and your self to another. You can use generality and we have to be in absolutes. Your terminology is incorrect, yet you nit pick and cherry pick mine. You say we are twisting the flavor of the quotes, yet you are the one using brackets to cherry pick what you want.

This all started with a statement I made that a drawbar by design help to hold the front end down and at times push down on the front end, I never said all the time. The drawbar does not need to be longer than the radius of the wheel. As another poster put about the 3 pt hitch, if the front end comes up it would have come up much sooner than it would have other wise.

I will break down my understanding of pull again as plain as I can. Lets start with the most basic pull which is from the ground. As your tractor moves forward you have opposing forces and these are dynamic forces, that will change. I'm not going to go into every little detail but basically as the tractor goes forward the load is going to resist this in 2 ways pulling back and down. This downward force will being to load the rear axle of the tractor. This gives us 2 forces trying to lift the front end, the torque of the axle and the downward pull of the load on the drawbar. 2 forces are trying to resist this, one is gravity pulling on the front end, the 2nd is the drawbars location being below the axle and usually connected forward. Lets say just to get to the good part the load does not move. This due to the down force and the rear force are greater than the forward force, but the tractor has HP and traction enough to move the load. Now the front end begins to lift, once it passes the point of alignment of the attachment points now the drawbar leverage has shifted. Now instead of loading the rear end the drawbar is unloading the rear end and will continue to do so until traction is lost and the front end goes down. This is why a modern farm tractor under normal conditions will not flip.
Now most of the time we don't pull from the ground, we pull from a neutral or even a slightly elevated position. Neutral being one that is straight and does not load the rear. Look at wagon gear, or a manure spreader usually the drawbar is slightly higher than the tractor. These are the conditions that the drawbar leverage work to help keep the tractors front on the ground.

As far as the connection point needing to be below the ground, its all about what force in total is greater. Take 2 bars one bar 1 meter long, the other 2 meters long, apply 100 lbs of force to the end of the 1 meter and 10 lbs to the 2, which one is going to exert the most force all other things being equal? You keep saying that the drawbar would need to be below the ground, again it depend on the force. If the tractor is pulling a 200 lbs load and the operator pops the clutch, I would bet the front end on a large tractor is going to lift if it already has a light weighted front.

Here is a link to artical about a demo that they actually connected a tractor at different points to show just that.

Tractor roll-over demonstrations show severity, reality of accident | Farm and Dairy - The Auction Guide and Rural Marketplace

 

[SPYDERLK]

See here is the problem, with me and others you are holding us to one standard and your self to another. You can use generality and we have to be in absolutes. Your terminology is incorrect, yet you nit pick and cherry pick mine. You say we are twisting the flavor of the quotes, yet you are the one using brackets to cherry pick what you want.

This all started with a statement I made that a drawbar by design help to hold the front end down and at times push down on the front end, I never said all the time. The drawbar does not need to be longer than the radius of the wheel. As another poster put about the 3 pt hitch, if the front end comes up it would have come up much sooner than it would have other wise.

I will break down my understanding of pull again as plain as I can. Lets start with the most basic pull which is from the ground. As your tractor moves forward you have opposing forces and these are dynamic forces, that will change. I'm not going to go into every little detail but basically as the tractor goes forward the load is going to resist this in 2 ways pulling back and down. This downward force will being to load the rear axle of the tractor. This gives us 2 forces trying to lift the front end, the torque of the axle and the downward pull of the load on the drawbar. 2 forces are trying to resist this, one is gravity pulling on the front end, the 2nd is the drawbars location being below the axle and usually connected forward. Lets say just to get to the good part the load does not move. This due to the down force and the rear force are greater than the forward force, but the tractor has HP and traction enough to move the load. Now the front end begins to lift, once it passes the point of alignment of the attachment points now the drawbar leverage has shifted. Now instead of loading the rear end the drawbar is unloading the rear end and will continue to do so until traction is lost and the front end goes down. This is why a modern farm tractor under normal conditions will not flip.
Now most of the time we don't pull from the ground, we pull from a neutral or even a slightly elevated position. Neutral being one that is straight and does not load the rear. Look at wagon gear, or a manure spreader usually the drawbar is slightly higher than the tractor. These are the conditions that the drawbar leverage work to help keep the tractors front on the ground.

As far as the connection point needing to be below the ground, its all about what force in total is greater. Take 2 bars one bar 1 meter long, the other 2 meters long, apply 100 lbs of force to the end of the 1 meter and 10 lbs to the 2, which one is going to exert the most force all other things being equal? You keep saying that the drawbar would need to be below the ground, again it depend on the force. [[If the tractor is pulling a 200 lbs load and the operator pops the clutch, I would bet the front end on a large tractor is going to lift if it already has a light weighted front.]]

Here is a link to artical about a demo that they actually connected a tractor at different points to show just that.

Tractor roll-over demonstrations show severity, reality of accident | Farm and Dairy - The Auction Guide and Rural Marketplace

I use brackets to highlight something that I have something to say about in a quoted post. I dont know why you would call that cherry picking.

Im not sure what you refer to as incorrect terminology. The only thing I can think of that would affect what were talking about is "push down". I use it literally, as increasing the downforce the tractor front has in static conditions or at least non tractive situations. You may be using it as offsetting the tendency to rise exhibited by a less optimum setup.

[This is not part of the drawbar problem. Its COM and acceleration - I mentioned this idea earlier].

I said the pull point would have to be below ground to add more down force to the front. That makes that lever longer than the tire radius lever. Equal force on that longer lever tries to tip the tractor forward at the rear tire contacts . That would put more downforce on the front.

It looks as tho you are saying that a downward drawbar angle causes a lift force on the back of the tractor. What the downgoing drawbar does is decrease the pull point to at or near ground level where there is a physics enforced balance between tractor and load torque forces. It is then impossible for the tractor to turn itself over unless it has built significant rotational momentum andgets past the point of no return by inertia. Thats why a long drawbar is safer - the enforced balance comes earlier.... -Its possible you meant a downward angle of the chain from the load caused by the drawbar dropping causes the rear to lighten. That is right, but often the chain or other link angle is quite small because the link is long.
larry

 

[gpflepsen]

I want to say these discussions are a bit interesting. My background is Mechanical Engineering and Physics.

(Spyder, I see you just responded again. You strike me as having a handle on these things. That other thread about the 3PH "lift" capacity proved that to me in the end. What's your background?)​

Let me throw this out there to try to help with understanding... This can be broken down to a system of balanced forces and looking at the critical cases of impending movement.

The first illustration is of the only forces acting upon the tractor. Any question of the pulling action can be broke down into these components.

These forces are:

1. Driving force on the tires, front and rear acting to propel forward, is a result of the axle torque of the machine.
2. Drawbar force acting to oppose the driving force
3. Gravity acting upon the CG of the tractor
4. Support force on the tires acting to oppose the gravity force

That's it, nothing more. Talking about the torque of the axle just confuses the topic in that the only function of the torque is to produce the driving force on the tractors tires.

Now, look at the forces as they act about the pivot point of the rear tire contact patch (FXd). Sum the moments of these forces and that will tell you if you could get rotation. If the drawbar force is large enough, or more correctly the moment, the tractor will come to a point of rotation about the tire contact patch. Every force (it's moment) is trying to rotate the tractor counter-clockwise, except the drawbar force. Look at the drawbar moment as a function of the lever arm. The lever arm is always acting to rotate in a clockwise direction. Always. If the tractor is rotating, the lever arm is getting shorter and reducing the tipping moment.

This is the safety feature of a properly located drawbar; a longer and lower drawbar is quicker acting to remove the lever arm, and thus the moment.

You can move the "drawbar" force to a point below the ground to make a moment that wants to rotate the tractor onto the front tires. This is accomplished by a rigid attachment (3ph of appropriate geometry) effectively extending the machine to a point below the ground surface.



*Now the disclamer- this is not accounting for any theoretical dynamic transient situation where an axle torque could result in popping a wheelie, or a tire frozen to the ground

 

[carpenter383]

This whole drawbar torque and 3ph pushing down on the front end sounds like the geometry of "instantanious center" Basically the center of gravity changes via angles of attaching points to a member. In drag racing you can change your car's instantanious center to create lift of the rearend which increases load on the rear tires, and also less wasted energy from "squatting" the rearend (using energy to compress the springs) By using geometry in the connecting points from the rearend to the car's frame you can utilize axle torque to lift the wieght of the rear of the car. In a "stock" car the factory design will squat under hard acceleration. A factory setup may put the instantanious center in front of the car while a "traction tuned" setup may put the instantanious center in the middle of the car. The trade off is you get more frontend "dive under braking.

 

[The Dougster]

IMHO, this thread has gotten away from the original question and has gotten tedious...... very tedious...

 

[LBrown59]

Y-Knot

 

[LD1]

This thread has gotten real interesting and I always enjoy these types of debates

I agree with most of what spyderlk said and even some of what Jasg said.

After following this thread for a few days, here is my understanding of what everyone is trying to say.

Jasg: I think you are trying to emphasize that a tractor CANNOT flip backwards if hooked to the drawbar. I'll agree with this because it is true that the rears will break traction before a flip can happen, BUT...the fronts can come up a foot or so, as explained in one of the links with the remote controlled tractors. Which leads me to Spyderlk's position.

Spyderlk: Correct me if I am wrong but I don't think he's arguing the flipping of the tractor, rather the comment made about hooking to the drawbar putting downward force on the front end.

My understanding of this is that a drawbar, below the axle line, and a straight pull rearward, does indeed help to counteract the axle torque, but not stop it completely. It's about leverage, the drawbar is only about a foot below the CL of the axle. The tires are contacting the ground about 2' or more below the axle cl. The wheels have more mechanical advantage than the drawbar, therefor the possibility is still present to raise the front of the tractor. How far it raises is dependent on several other variables like length of drawbar, weight over front, traction, etc. In all reality, everyone knows that a tractor will break traction first, if hitched properly, but it DOES NOT add more weight to the front end, even if pulling in a straight line.

My conclusion is that traction, or lack of, is the only thing that keeps the front planted, not the hitch adding weight.

For sake of argument, take traction out of the equation. Say that the tires absolutly could not slip no matter what (like frozen, or poured in concrete), hook tractor to something with the drawbar, and put scales under front tires and see what happens. (This is just a hypothetical situation, I hope no one is dumb enought to do this)

 

[Reg]

I want to say these discussions are a bit interesting. My background is Mechanical Engineering and Physics.

(Spyder, I see you just responded again. You strike me as having a handle on these things. That other thread about the 3PH "lift" capacity proved that to me in the end. What's your background?)​

Let me throw this out there to try to help with understanding... This can be broken down to a system of balanced forces and looking at the critical cases of impending movement.

The first illustration is of the only forces acting upon the tractor. Any question of the pulling action can be broke down into these components.

These forces are:

1. Driving force on the tires, front and rear acting to propel forward, is a result of the axle torque of the machine.
2. Drawbar force acting to oppose the driving force
3. Gravity acting upon the CG of the tractor
4. Support force on the tires acting to oppose the gravity force

That's it, nothing more. Talking about the torque of the axle just confuses the topic in that the only function of the torque is to produce the driving force on the tractors tires.

Now, look at the forces as they act about the pivot point of the rear tire contact patch (FXd). Sum the moments of these forces and that will tell you if you could get rotation. If the drawbar force is large enough, or more correctly the moment, the tractor will come to a point of rotation about the tire contact patch. Every force (it's moment) is trying to rotate the tractor counter-clockwise, except the drawbar force. Look at the drawbar moment as a function of the lever arm. The lever arm is always acting to rotate in a clockwise direction. Always. If the tractor is rotating, the lever arm is getting shorter and reducing the tipping moment.

This is the safety feature of a properly located drawbar; a longer and lower drawbar is quicker acting to remove the lever arm, and thus the moment.

You can move the "drawbar" force to a point below the ground to make a moment that wants to rotate the tractor onto the front tires. This is accomplished by a rigid attachment (3ph of appropriate geometry) effectively extending the machine to a point below the ground surface.



*Now the disclamer- this is not accounting for any theoretical dynamic transient situation where an axle torque could result in popping a wheelie, or a tire frozen to the ground

So, is your point that a plow buried (say 8 inches down) represents a force below ground level that tends to "load" the tractor's front wheels ?
If so.... Well, Yeah, kinda, iffy/maybe.

But (Butt, but, butt) the empirical evidence suggests that the plow ALSO "pulls down" on the 3PH (mostly via tension in the top link) as it lifts soil to turn it - - and that down pull certainly overwhelms any below grade drag.
This is typically "solved" by adding front weights (suitcase) to hold the front end down.
Now I just have to b'leeve that smart old farmers know a thing or three...
They would LOVE to shed those weights off the front bumper and I don't mean just to replace it with a loader.
So given all this time and country experience I think they would have figured out how to keep the front end down by now without using weights, i.e. JUST by mucking with lift arm and implement geometry if it were practical.
{NOTE: "possible" .NE. "practical"}

 

[RobertN]

smaller tractors don't have individual rear brakes. having the front wheels pulling gives you a lot more steering in difficult situations similar to what you can get by riding the left or right brakes on a rwd larger tractor.

My Kubota has individual brakes. The issue, is it is about impossible use the brakes to steer in conjunction with the hyrdostat pedal. I have done it, and turned that tractor on a dime. It was an exercise in contortions though Trying to get a heel on the hydro pedal, and a toe on the right or left brake pedal reminded me of playing that "Twister" game when I was a kid.

I have seen my tractor with a gear tranny. It would be pretty easy to use the breaks for steering. Not with the HST though...

Of other note is power steering. A large percentage of CUT's have PS now days. For the most part, with PS you don't need steering brakes. Sure, they are a benefit for tillage, when you want to make a sharp u-turn to start your next row.

One thing of note, after making some small use of steering brakes on my Dad's 9N and my old Farmall-A, It would seem to be more of and advantage on a tricycle tractor than one with a wide front. With the 9N and the Farmall-A, it sure drug the front tires.

Having my Kubota for 10 years, there have only been once or twice where steering with brakes would have been handy.

 

[Grrrr]

My Kubota has individual brakes. The issue, is it is about impossible use the brakes to steer in conjunction with the hyrdostat pedal. I have done it, and turned that tractor on a dime. It was an exercise in contortions though Trying to get a heel on the hydro pedal, and a toe on the right or left brake pedal reminded me of playing that "Twister" game when I was a kid.

I have seen my tractor with a gear tranny. It would be pretty easy to use the breaks for steering. Not with the HST though...

Of other note is power steering. A large percentage of CUT's have PS now days. For the most part, with PS you don't need steering brakes. Sure, they are a benefit for tillage, when you want to make a sharp u-turn to start your next row.

One thing of note, after making some small use of steering brakes on my Dad's 9N and my old Farmall-A, It would seem to be more of and advantage on a tricycle tractor than one with a wide front. With the 9N and the Farmall-A, it sure drug the front tires.

Having my Kubota for 10 years, there have only been once or twice where steering with brakes would have been handy.

I find I barely ever use separate wheel breaks on a 4WD tractor, they are always left locked together.

2WD tractor, they are always left unlocked but I still don't think I use them very much.
Only time I can think of using them in earnest is when pulling a heavy load up a hill or something heavy on the linkage with a 2WD and then front wheels are off the ground. Then you use them to steer.