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.