Center of gravity??

[flwiii]

Wow! I thought for a second I was in Calculus class. This sounds more like a bunch of flight engineers discussing the roll characteristics of plane design. All this stuff has just zoomed over my head.

I just always remember Fork Lift School. The load should always be very low to the ground and up hill when on an incline.

 

[keeney]

In regards to front flips:

Even though a pure front flip is unlikely, there is the forward/side quarter roll direction to worry about. Taking a sharp turn when moving forward downhill adds a combination forward/side roll moment. Having more mass on the opposite rear wheel helps this situation dynamically even if the wider rear wheels don't help that much because the roll is partially forward.

Put the outer front wheel in a hole or the inner one on a rock in addition to the dynamics of the turn and the static down-hill angle, and it can get tippy in a hurry towards the front quarter direction.

The old narrow front "tricycle" tractor design was notorious for this roll scenario.

Also, it is possible to lft the rear off the ground statically by overloading the FEL or wedging it on something sturdy attached to the ground. Especially when pointed downhill. Once the rear end is off the ground, you lose the side-stability afforded by the wider rear wheels and a tip to the front-side quarter comes into play.

- Rick

 

[daTeacha]

I once saw my boss, from WVa, driving a skid loader and in a hurry to load some barn beams on a truck. This worthy individual, thinking himself smart, used mountain engineering to decide that if he headed down the barn bank toward the truck with a load of beams on the forks, raised the forks as he approached the truck, then slowed quickly, the beams would neatly slide off the forks and onto the truck. It worked, sort of, 'til he slowed too early while raising the forks a bit too late, tipped the thing on it's nose and put a fork into one of the truck tires. /forums/images/graemlins/blush.gif

If you work on hills with a loader, the back end gets very light going downhill. It won't go clear over because the loader will hit the ground, but you really won't enjoy the experience a whole lot with the rears up in the air.

For pulling any kind of load, connecting higher on the tractor puts the rearward force above the axle center rather than below it. Torque on the rear wheels tends to rotate the tractor around the rears, lifting the front. Watch a tractor pull sometime. Even with the weight hitched low, the front comes up in a hurry. The guys who are good actually use the up and down motion of the front to assist in getting some extra momentum to the sled. If you hitch high, you just add to the forces acting to rotate the tractor around the rear axle. With a 3 point, the lower links act as a pivot point for the resistance of the load most of the time, causing a force on the upper link in the forward direction, opposite the force of rotation caused by engine torque on the wheels, but ONLY if the load is below the lower links, as in a properly set up plow, box blade, etc. With a heavy object lifted clear of the ground, the front can come up easily. Depending on the design of the implement, it can impart a force either forward or rearward on the top link. A plow can tend to dig itself into the ground, pulling backward on the top link, but a box blade will tend to push on the top link as the load on the bottom of the blade increases.

 

[JustGary]

Good description, Rick. The bottom line is, more weight as low as you can get it is a good thing, both for traction and stability.

The forward/side rolling is called "roll-yaw coupling," and is induced by the steering force (the wheels) not lying in plane with the center of gravity. Since the wheels are below the CG, the sideways force required to turn couples into a rolling moment the opposite direction. Raising a bucket full of your favorite haul only raises the CG, which makes the problem worse by increasing the lever arm. That's why dropping the bucket is the correct thing to do when she starts to tip.

I hope we're not beating this horse too much, but I figure that knowing why something happens might just help someone save their own neck when they have it begin to happen to them. Fill the tires, tow from the drawbar, keep the loader low. How hard is that? But if you just say it that way, folks somehow think you're talking to someone else.

- Just Gary

 

[daTeacha]

.....and it only happens to someone else.

 

[California]

</font><font color="blue" class="small">( The forward/side rolling is called "roll-yaw coupling," and is induced by the steering force (the wheels) not lying in plane with the center of gravity. Since the wheels are below the CG, the sideways force required to turn couples into a rolling moment the opposite direction. )</font>

The way I visualize that to make it more real: It's easy for the CG to keep moving forward (inertia) while you turn the steering wheel and drive all four tires clear out from under the CG.

Result - the tractor is suddenly lying on its side or worse.

And a corollary I found while learning to operate in the orchard: I was making a u-turn crossing over to the next lower row turning between two closely spaced trees, turning the wheel hard and locking one brake. I hadn't really appreciated that this makes the remaining drive wheel, and therefor the u-turn, suddenly twice as fast. What had seemed simple and routine suddenly got hair-raising, due to the acceleration going down to a lower terrace in the adjacent row added to the sudden speed increase caused by raising the disk, and this brake action. No harm done, I wasn't going that fast, but leisurely suddenly became like an offroad race for a moment when I didn't expect it.

I think that sort of dynamics is more a factor of a rollover than static CG.

 

[froggy]

Excellent article. Thanks for posting the link to it.

 

[JoeL4330]

I agree, a light rear end is no fun goin' downhill ...I have posted on this elsewhere and emphasized that when this happens, you have no engine braking unless you are in 4wd, so I think 4wd downhill is mandatory ...the alternative is likened to what New Englanders used to call a Nantucket sleigh ride.

 

[RayH]

Check out this site. Some good basic info on safety and roll overs
http://www.four-h.purdue.edu/tractor%5Fsafety/

 

[bugstruck]

Mountain engineering /forums/images/graemlins/tongue.gif Cute. I agree with what you stated. The 3 point implements have the ability to impart loads either way on the top link, adding to or resisting (somewhat) the tendancy to flip backwards. Roger too on the pulling tractors. I think some don't realize that no matter how low that pulled load connection is relative to the rear axle, with enough traction and power at the ground, you can still drive the tires forward and out from under the axles...so to speak. Actually that is just what is happening, while the attached load stays put. The result is the entire tractor chassis rotates about the rear axles, assuming it doesn't have enough static weight and leverage forward to resist the dynamic loading. Otherwise, as you observed, no pulling tractor could pull the front end up.

 

[N80]

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]

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]

</font><font color="blueclass=small">( 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.... )</font>

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]

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

 

[daTeacha]

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

 

[California]

</font><font color="blue" class="small">( All this discussion truely makes me happy that I live in a giant, flat sand pile. - flwiii )</font>

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]

<font color="blue"> Funk, Ohio (northeast, but the name is fun)
</font>

Yeah it is kinda funky

 

[hayden]

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]

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]

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