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 sex 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.