Why 4 Wheel Drive

[SPYDERLK]

Second: It MAY be not be true for all tractors, and for all geometries, but some tractors can pull down on the front end. If you look at the details of the "Ferguson system," when it was introduced, they made that claim.
Many older tractors had a tremendous coil spring behind the bracket that the top hitch attached to. That spring had to be compressed to get draught control (up) action. If you look at the geometry of the situation, it would appear that tractor can apply down pressure on the front end in some cases.

I will dare say that any tractor pulling a three point hitch load where the pulling force is more than the weight of the implement, it will create a down force on the front end. The attachment points of the lift arm are below the axle level, therefore that creates down force (under the circumstances I described.) Also, the top link is well above the axle, therefore that will also create down force on the front end. Bear in mind, that even if the front end comes up off the ground, this is STILL true because the front end would have come up earlier and further than if it had not had the system.
In the words of my logic professor "Now, go home and convince yourself this is true."

Yesterday's Tractors - Ferguson System Principal and Theory of Operation

Note that the description is regarding ground engaging implements. All of their resistance to motion is at or below ground level. So yes in that instance the front end is pushed down. The distance below ground the force is applied times the force applied by the tires at ground level is the net torque trying to rotate the tractor forward [front tip]. If the resisting force is at ground level there is no net torque. If above ground there is net torque trying to back tip the tractor. In each case these torques are proportional to the vertical distance from the level at which the tractor exerts horizontal force to the level at which the load exerts horizontal force.
larry

 

[JasG]

Spyderlk

I think you are mistaking the source of the force vs the point it is applied. The source of the force and pivot point on a tractor is the rear axle. It is exerted at the ground but the torque trying to lift the front is from the center of the axle. So pulling below that point will try and counter the force trying to tip the tractor over. If this were not the case tractors would not be able to pull the loads they do without massive counter weights on the front of the machines. Think about a tractor starting off pulling a load up an incline. If what you are explaining is true, it would almost not be possible.

 

[/pine]

When you get a 4 wheel drive stuck...it's really stuck...!

 

[SPYDERLK]

Spyderlk

I think you are mistaking the source of the force vs the point it is applied. The source of the force and pivot point on a tractor is the rear axle. It is exerted at the ground but the torque trying to lift the front is from the center of the axle. So pulling below that point will try and counter the force trying to tip the tractor over. If this were not the case tractors would not be able to pull the loads they do without massive counter weights on the front of the machines. [[Think about a tractor starting off pulling a load up an incline. If what you are explaining is true, it would almost not be possible.]]

[[How do you figure that?]].

Nothing I said prevents a tractor pulling a load up a hill. The hill just lightens the front a bit more.

The tires apply force rearward and the axles apply the same force forward into the tractor body. Force resisting motion of the tractor [for acceleration or pulling a load] has effect based on where relative to the ground that force is applied. That resisting force is applied at the tractor Center of Mass [pretty high]for acceleration or when climbing hills. It is applied at the level of the hitch point when pulling a load at steady speed. Accel and/or a hill will still increase the tendency to tip back regardless of the hitch level because of the high COM.
larry

 

[jagyzf]

[[How do you figure that?]].



The tires apply force rearward and the axles apply the same force forward into the tractor body. .
larry

Your thinking is incorrect. Axles only apply rotational force, it is neither forward or backward force. In the case of going forward the axle is trying to rotate the tires in one direction, the frame in the opposite. The force of gravity pulls down on the front end forcing the tires to rotate and move forward. The drawbar being below the axle is trying to counter this force of lift. I have had this covered in several farm safety courses I have taken over the years. It is why it is recommened to always when pulling, other than a 3 point impliment, connect to the drawbar. Also when possible not to pull from the ground because the load wants to line up with the point of attachment to the tractor to create a straight line of pull.

The link below explains drawbar leverage and how as the front end of the tractor raises the drawbar counter acts it when pulling straight. If the load is below the the point of pull it helps lift the front until a straight line is formed.

NASD: Tractor Overturn Hazards

 

[JasG]

EE_Bota and Jagyzf your links seems to explain it as it was explained to me many years ago on 3 point and drawbars. One farmer I worked for would put weights on the front of the tractor for plowing due to it was a semi mounted unit and then take them off for disking. He always said that it wasn't needed for anything but plowing due to disks and drags are connected to the drawbar and it was a straight pull. The plows were using only 2 points of the 3 point hitch with an upward pull so it could come up under heavy load. He didn't want to be fighting the none driven front wheels any more than needed over plowed ground for seconday tillage so we would alway pull them off when done and leave them off until time to plow again. I also remember some old timers when I was a teenager discussing how tractors built in and after the 1950's were so much safer than the years before. One of the things they discussed was the drawbar being moved from just attached to the rear axle anyway possible to under and ahead of the axle. Back in the day, by the way they talked it was not uncommon for a tractor to flip over backwards due to different methods of attachment of impliments.

The other items I remember them discussing was the switch away from the narrow front end and a "standard" PTO.

I didn't think about until I read the link on the drawbars, 4wd due to the torque is divided between 2 axles this also makes the tractor less likely to lift the front end. That seems to make some sense. I only throught about the added weight. So that would be another plus for 4wd.

 

[jagyzf]

I think in the future more front mount attachments for tractors of all sizes from Compact all the way up through large farm machines will be made. I don't see many attachments made for 2wd other than loaders. I think the added level of control will make this and even more important option. The newer tractors with the ride controls that can only be purchased on the 4wd.

As Grrr said it's getting to a point that 2wd just doesn't look right anymore. The newer tractors are built to be 4wd 1st 2wd 2nd. Notice in his pictures the contours of the hoods, that looks like to me its for extra clearance for a 4wd to turn sharp.

 

[SPYDERLK]

Your thinking is incorrect. Axles only apply rotational force, it is neither forward or backward force. In the case of going forward the axle is trying to rotate the tires in one direction, the frame in the opposite. The force of gravity pulls down on the front end forcing the tires to rotate and move forward. The drawbar being below the axle is trying to counter this force of lift. I have had this covered in several farm safety courses I have taken over the years. It is why it is recommened to always when pulling, other than a 3 point impliment, connect to the drawbar. Also when possible not to pull from the ground because the load wants to line up with the point of attachment to the tractor to create a straight line of pull.

The link below explains drawbar leverage and how as the front end of the tractor raises the drawbar counter acts it when pulling straight. If the load is below the the point of pull it helps lift the front until a straight line is formed.

NASD: Tractor Overturn Hazards

Obviously no, since the axles are the only connection of the tractor to the wheel, whose traction applies the push. Yes, the tractor tries to rotate backward about the axle. A pull point below the axle resists this and a pull point that is below the axle an amt = to the wheel radius will resist it completely. The drawbar not being this low does not counter it completely until the tractor rears up enuf to bring the pull point [bar end] down to ground level - long bars are pretty safe.
While the link has good info, the pull angle diagram on the stump is totally bogus and discussed in a previous thread.

There is a lot of misinfo out there. The most effective weapon is an inquiring nature and an understanding of physics.
larry

 

[EE_Bota]

JasG, jagyzf, and SPYDERLK: I appreciate the discussion you guys have been having. It is as good a discussion as I have seen yet on TractorbyNet.

I think JasG and jagyzf did such a good job explaining it, that I am free to play with the problem mentally like a little kid instead of trying to explain. (I can't explain worth a darn anyway.)

SPYDERLK said:
"The tires apply force rearward and the axles apply the same force forward into the tractor body. Force resisting motion of the tractor [for acceleration or pulling a load] has effect based on where relative to the ground that force is applied. That resisting force is applied at the tractor Center of Mass [pretty high]for acceleration or when climbing hills. It is applied at the level of the hitch point when pulling a load at steady speed. Accel and/or a hill will still increase the tendency to tip back regardless of the hitch level because of the high COM."


Let's make the tractor simpler. A David Bradley tractor is actually a good model for the hitch attachment question.
This simplifies the COM issue considerably, and I hope it helps clarify the issue a bit. The center of mass issue is mostly gone, but the implement attachment issue still remains.

 

[JasG]

Obviously
There is a lot of misinfo out there. The most effective weapon is an inquiring nature an an understanding of physics.
larry

OK so you have a better understanding of physics that Penn State's Ag Department which is putting out safety misinformation? You also seem to know more than probably every farmer I've ever known.

You know it's not about being right or wrong, I come here to learn and share knowledge if I can. If you want to say others are wrong and don't understand, why don't you show us something other than "your understanding of physics". Others here have been good enough to put links to information that supports there point. All you have done is put up at times gobbldygook, and now you say something that does not agree with you is misinformation. No one with a real understanding of physics would say a rotating shaft of any kind, axle or not puts out a forward force, shaft are for torque. Not only did I take some physics, but I have a job that I actually apply physics in on a regular basis.

You have an inquiring nature, why don't you connect a tractor to something and see for yourself. Or at the very least show us where you are getting your information from.

Look up the Nebraska test data on some tractors. How could a tractor have a drawbar pull almost equal to it's weight if the drawbar didn't keep it from going over? That's pounds of pulling force, not pulling a load on wheels that once moving only takes a few HP to keep the load moving. Going by what you claim the only force counter acting the rotation is the weight over the front axle which on most tractors is 40% or less, any weight on the rear axle would be trying to lift the front end.


EE_Bota - Thank you for the compliment.