Tractor ballasting, weight distribution & free body diagrams

[hazmat]

A discussion about this came up in the New Holland forum. I though I'd repost it here as I spent a good chunk of the evening writing it up. I know, shame on me for double posting, but I think it deserves to stand on it's own. I'm sure Dad would like to see me putting my engineering education to good use.

OK class – today we are going to learn about free body diagrams – no this isn’t *** education it is Statics – significantly more boring.

The discussion at hand is weather adding wheel weights or filling the rear tires will lighten the load on the front axle. The short answer is that it will not lessen the magnitude of the load on the front axle, but it will change the weight distribution between the front & rear axle.

I’ve tried to use real world numbers (from my TC18). I apologize in advance for the sloppy drawings. I’m not much of an artist. The diagrams are on flat ground – no slopes taken into account.

Your FEL manual should have information on how to ballast your tractor for safe operation - rear counterweight, wheel weights, loaded tires or a combination of 2 or more. The discusion here should show you how best to lighten the load on the front axle - steering or squishing tire problems and how not every pound of ballast is created eqaul - ballast behind the rear axle counts more.

Figure 1
Baseline tractor only
Tractor weight 1,600#
Total weight 1,600#
Dist of CG from front axle 62.5% of wheel base
Front axle load 600#
Rear axle load 1,000#
Front axle distribution 37.5%
Rear axle distribution 62.5%

Figure 2
Tractor + Loader (CG of loader is 20% of wheel base in FRONT of front axle)
Tractor weight 1,600#
Loader weight 600#
Total weight 2,200#
Dist of CG from front axle 40% of wheel base
Front axle load 1,320#
Rear axle load 880#
Front axle distribution 60%
Rear axle distribution 40%

You can see that the load on the front axle has more than doubled

Figure 3
Tractor + Loader add 400# filled Rear Tires (CG of liquid is at rear axle)
Tractor weight 1,600#
Loader weight 600#
Filled tire weight 400#
Total weight 2,600#
Dist of CG from front axle 49.3% of wheel base
Front axle load 1,320#
Rear axle load 1,280#
Front axle distribution 50.7%
Rear axle distribution 49.3%

Note that while the % of weight on the front axle has decreased from 60% to 50.7% the weight is still 1,320#

Figure 4
Tractor + Loader add 800# counter weight (CG of weight is 40% of wheel base behind rear axle)
Tractor weight 1,600#
Loader weight 600#
Counter weight 800#
Total weight 3,000 #
Dist of CG from front axle 66.7% of wheel base
Front axle load 1,000#
Rear axle load 2,000 #
Front axle distribution 33.3%
Rear axle distribution 66.7%

Both % and total weight on front axle are lower

Figure 5
Tractor + Loader add 800# counter weight and fill rear tires 400#
Tractor weight 1,600#
Loader weight 600#
Counter weight 800#
Filled tire weight 400#
Total weight 3,400 #
Dist of CG from front axle 70.6% of wheel base
Front axle load 1,000#
Rear axle load 2,400 #
Front axle distribution 29.4%
Rear axle distribution 70.6%

Further reduction in % of weight on front axle, but still the same total weight as figure 4. So why bother weighting rear tires? It reduces the chance of tipping - you have to put more weight in the bucket before it'l go over. Additionally, you gain traction and it also lowers the CG to make for more stable platform (side slopes)

Figure 6
Tractor + Loader add 600# load to bucket (CG of loadt is 40% of wheel base in front of front axle) no counterweight or filled tires
Tractor weight 1,600#
Loader weight 600#
Bucket load 600#
Total weight 2,800 #
Dist of CG from front axle 23% of wheel base
Front axle load 2,160#
Rear axle load 640 #
Front axle distribution 77%
Rear axle distribution 23%

Now we have 3.6 times the original front axle load & a whopping 77% of the weight distribution on the front axle. You can see why we need to properly ballast when using the FEL.

Class dismissed – go out and get some seat time.

 

[GeneD14]

Hazmat,
Great post! A little physics refresher can be a good thing.
Because I do alot of loader work with my tractor I have filled tires with rim guard, wheel weights, and I usually have a heavy implement on back for added ballast.

 

[john_bud]

Ah yes, Statics -- the study of things that don't move. Yawn, very excitinzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

 

[whodat90]

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

 

[Egon]

Lets add the case where the bucket is placed under a very heavy load and the curl feature lifts the rear wheels off the grond??

 

[herringchoker]

Hazmat, good post. Too bad the prof didn't relate free body diagrams to a tractor when I took the course 38 years ago. I might have paid more attention.

 

[texasjohn]

Great post and diagrams.... significant contribution to ballasting discussion. BRAVO! Also, it explains my real life experience... tractor "feels" very different with and without front bucket, rear ballast, etc... drives/bounces significantly differently. Thanks for taking the time to do this detail.

 

[JoeL4330]

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

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

 

[john_bud]

Ah yes, Statics -- the study of things that don't move. Yawn, very excitinzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

After re-reading my post, I do hope that everyone understands that falling asleep is a trained Pavlovian response from having taken Statics! Not a comment on the content of the post, which is very good!

Sorry if it was taken the wrong way by anyone!!

jb

 

[texasjohn]

I sent this post to a buddy who understands physics. He says the math to do the calculations is "really ugly". However, he makes another VERY IMPORTANT point that I have not seen made before.

Under conditions where the CG is near the front wheels, if the tractor tips forward slightly the CG moves AHEAD of the front wheels and you do a nose dive. This expalins why it is VERY IMPORTANT to carry a FEL load as low as possible, keeping the CG behind the front axle.

He actually saw this demonstrated just a couple of days ago at Lowes when a driver was carrying a load out a door at waist height... all was OK on level ground but simply going over the transom cause the pallet fork truck to tip right onto its nose.

 

[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.