Tractor ballasting, weight distribution & free body diagrams

[N80]

At least my general experience mirrors the math, whether it is ugly or not. My rear tires were factory filled but with a full bucket things felt just a little squirrely when I hit a bump or travelled at speed. Then I got a box blade. I think it weighs 600+ pounds and it made a _huge_ difference as a counterweight when using the loader. Box blades are pretty unobtrusive as far as implements go, so it stays on when I'm using the loader.

 

[whodat90]

Does the loader CG stay constant (fore-aft) as it moves?

And, w.r.t. the "sprung/unsprung weight" comment ...whose tractor has springs?

Mine sure doesn't but that's still the best term to describe the physics. Your tires sit on the ground. The tractor hangs from the axle between them. Adding weight to the tires doesn't add direct stress to the tractor; i.e. it doesn't place additional weight on the axles. Adding weight to your 3pt hitch adds weight and stress on the axles which is then transferred through the tires to the ground.

 

[schmism]

AH im always a sucker for a good FBD

but way to much sketchy stuff in there... should just be distances, weights and arrows.


and to be picky....

if you note the CG for the loader above the CG for the tractor...
the combined CG is between the 2 (ie higher off the ground)

by adding liquid ballast to the bottom half of the tires you can lower the CG back down. Same goes for the height of your counter weight off the rear.

 

[California]

... if the tractor tips forward slightly the CG moves AHEAD of the front wheels and you do a nose dive. ... going over the transom caused the pallet fork truck to tip right onto its nose.

And since a tractor doesn't have front springs there is nothing to control it to tip straight forward. An off-center CG might put the tractor on its side.

 

[dfkrug]

An off-center CG might put the tractor on its side.

This is also a big issue with fluid-filled tires. The CG of the tires
moves as the tractor tips sideways, increasing the rollover potential.

WHODAT90 beat me to it, but it is also true that adding fluid or
wheel weights adds NO static rear axle load. Even if the CG of
the whole tractor is affected.

 

[whodat90]

Another thing pointed out (obliquely) in the 4x4 thread is that filling your tires makes the rear of the tractor heavier. Adding weight to the 3pt makes the rear of your tractor heavier and the front lighter. Big difference when you don't have a load in the bucket.

 

[daTeacha]

And any weight added ahead of or behind the axles increases the polar moment of inertia more than the same weight added to or between the axles, making the tractor less prone to rotating on the vertical axis -- yaw or pitch. Adding weight outside the longitudinal centerline makes it less prone to roll, or rotation along the longitudinal axis (Front to Rear). Call it the dumbell effect -- a weight lifting bar with the weights in the center is much easier to rotate than the same bar and weight combination with the weights on the ends of the bar.

Both these statements are from a stationary POV. Once the tractor starts rotating in any of the aforementioned manners, the same weight that made it resistant to start rotating will also act to make it continue. Hence the rationale for doing things slowly when you have the slightest doubt about stability.

As has been said many times here, what you really want is a good balance between power, traction and weight. Too much weight steals power and also makes the tractor handle sluggishly. Not enough weight keeps you worried about falling over or results in too little traction.

 

[Egon]

From what I read we should be looking at Dynamic Statics.

 

[jimmysisson]

Excellent post which brings up an interesting point about counterweighting to keep weight (traction, braking) on the rear tires. What about putting, say, an 800lb load in that loader and running the numbers again? Also, an important point about why to add weight to the rear tires: Counterweights on the 3pt hitch hang from the tractor. The tractor hangs from filled tires. You get all the weight at almost no stress to the tractor. Sprung weight vs. unsprung weight.
whodat

************
Unsprung weight is a term used in auto sports, refers to the wheels, tires, and part of the suspension: the parts not supported/controlled by the springs and shocks. Less unsprung = more control by the car/driver, and is desirable. More unsprung = bad. Tractors = all unsprung = bad [as far as racing is concerned]. Springs on a tractor = bad for every other reason + use. Tractor racing, now I don't know....
Jim

 

[mike194]

i am a a mere Industrial Engineer but am thoughly impressed with your knowledge of freebody diagrams and statics and dynamics.