Center of gravity??

N80 · Dec 12, 2005

I had an interesting event yesterday on my Kubota L4400 w/ FEL. I was digging out a large stump and had dug down around the base of it pretty widely. When I approached the stump the front wheels would go down into the hole but the back wheels didn't - so the front is angled downward. Several times when I hooked the edge of the bucket under the stump and lifted the boom, the front wheels pushed down into the mud and the back wheels started to come up off the ground and the stump wasn't budging. That was a very weird and unexpected reaction. There was no real risk of a roll-over, but I didn't like it and I'm sure it isn't good for the front axle. It was basically pivoting on the front axle!

The rear of the tractor has no ballast and back tires are not loaded, yet. I'm going to load them real soon.

I did get the stump out. It was huge! The root system was so big I couldn't get it in the loader. Had to chain it to the bucket in order to haul it off.

Will post pics in a separate thread later.

bugstruck · Dec 12, 2005

Anytime the wheels come off the ground and it's not intended you get a very,very uncomfortable feeling I'm sure. /forums/images/graemlins/shocked.gif I haven't had that happen on mine yet and I hope it never does. But I have done some other not too smart things that got my attention and held it until I got things back to normal. Not fully engaging Low range and having it pop into neutral on a very steep slope sticks in my mind. I didn't think those brakes were worth much but they saved me that day. /forums/images/graemlins/smile.gif

daTeacha · Dec 12, 2005

( Several times ..... the back wheels started to come up off the ground ..... That was a very weird and unexpected reaction. There was no real risk of a roll-over.... )

That's what happens to me sometimes going downhill with a load in the bucket, but I have ballast and loaded tires. The only way I can feel more stable is with the bush hog (too heavy for my 3 pt. to pick up) hanging out the rear, but that makes the tractor/loader/hog pretty awkward to maneuver around in the woods. If you're moving, however slowly, it feels a lot worse. Throw in a little bit of angle away from straight up and down the hill, the thing will not only lift the rear, but pivot around the longitudinal axis of the front end, too.

Now you understand why I want a bit more tractor. I can't help but think my 48 inch wheelbase is not helping me at all, nor is my light weight. I'm figuring another 30% or so added to the lever arm plus an increase of 100 to 150% in overall weight will make things more stable. I know I'll still need rear weight and careful driving, but I think I'll be better off.

flwiii · Dec 14, 2005

All this discussion truely makes me happy that I live in a giant, flat sand pile.

daTeacha · Dec 14, 2005

Just watch out for those soft spots! /forums/images/graemlins/laugh.gif

California · Dec 15, 2005

( All this discussion truely makes me happy that I live in a giant, flat sand pile. - flwiii )

I don't think level ground will help.

The first time tried to pull a fencepost I lifted a rear tire off the ground - just before this photo. Even with water in the tires and the 4 ft rotary mower attached. Starting from level didn't prevent it.

To pull the next post I hitched to the center of the bucket and lower but got the same result. Plenty of lifting force but insufficient ballast.

Somewhere on TBN there is a picture of the ballast I then added temporarily, a pier block into the mower's tailwheel assembly plus a couple more on its deck. Then the posts pulled out easily.

After that I made the equivalent of a ballast box, a deck on the 5 ft BB for 200 lbs of concrete pier blocks.

Recently for hauling gravel all day I temporarily added another 80 lbs, a cast iron pipe hanging off the back to get it as far back, and low, as possible.

That felt rock steady and had good traction even backing up a wet grassy slope, with a full bucket of gravel, each time I turned around to go deliver a load. It simply felt like a larger tractor.

I'm learning that ballast is one of the subtle elements of proper operation and I encourage others to experiment.

LBrown59 · Jan 10, 2006

Funk, Ohio (northeast, but the name is fun)


Yeah it is kinda funky

hayden · Jan 11, 2006

Any progress on the spreadsheet? I think it's well worth doing, even showing just the static analysis. As someone else said, if it's more stable statically, it will be more stable dynamically.

I agree that a perfect model is very hard, but I think an imperfect model still provides a lot of value in doing comparative analysis. For example, I've considered adding wheel spacers to get a wider track and hence more stability (I have very hilly terrain, so this topic is near and dear to my heart). I'd like to understand how much difference the spacers will make. Does it change my roll angle from 20deg to 21deg, which would hardly be worth it, or does it go from 20deg to 25deg, which woudl be well worth it. In both cases I can still tip the tractor over, but it would be harder.

The same comparison could be done to evaluate the relative effect of washer fluid as balast, versus Calcuim cloride or foam filled tires. You coudl look at 50% full versus filling to the valve stems.

You could also look at the sensitivity of your assumptions around the tractor's CG height. What if it's 8" above the axel, or 12" above the axel, not 10".

One other random thought on figuring out the CG for the tractor. What if you broke it into a small number of individual masses, then calculated the resultant CG? I think you could pretty accurately estimate the CG of the following components.

- Wheels
- Trans
- Engine
- Tractor body

The break the tractor's weight into the component weights and calculate from there. Wheel weights are easy to figure out. The engine and trans may be more difficult and might have to be estimated.

Another, perhaps simpler method is to break it into only two components, the wheels, and the rest of the tractor. The rest of the tractor's CG will run pretty much down the middle of the engine/trans.

daTeacha · Jan 11, 2006

Yeah, but it's on the map, which is more than I can say for the community of Cow Run, Ohio. I haven't been able to find that one for a few years. It's somewhere in northwest Ohio, or used to be.

Paddy · Jan 11, 2006

Hayden,

Quick reply.

No, I have not progressed on a universal calculation yet. A simple and quick method ca be done with graph paper and clear sheet.

Assume;

View from rear
Use 3 shapes/mass, tractor body, L and R wheels. Place a dot where the CG is of each of the 3 masses.

Draw to scale of graph paper but on clear sheet.
Place thumb tac as pivot on one outside corner of wheel.
High lite vertical line of pivot.
Rotate clear sketch to desired angle and tape to hold. Distance is defined as the horizontal distance from your point on each mass to the highlited vertical line of the pivot point.

You will have 3 masses and the 3 distances they are to the highlited vertical line. Note as the angle is changed, the distances change. Place positive values for one side of the line and neg for the other. Make your calculation. Example;

M1=L wheel, M2=Tractor body and M3=R wheel
M1x42" + M2x16" -M3x6"

When the tractor body is in line with the pivot;

M1x21" + M2x0" - M3x 12"

Once M2 crosses the pivot things happen quick due to the big mass.

When value goes neg, your rolling!

Patrick