Where's Our Tractors' Center Of Gravity?

[Anonymous Poster]

Hi,

If I could only drive my tractor on increasingly steeper slopes until I rolled it a couple times! Then I would develop a feel for how stable it really is! But I can�ft...the cost of the thing prevents doing that, if nothing else does!

Now if I could estimate the position of the center of gravity, I could estimate the angle at which the tractor might tip over, at least in a static mode. By static mode I mean in a situation with no forward movement. This means little in the real world, but would give me some measurable reference to compare the real world to.

Since tractors are for the most part symmetrical, I figure that, when viewed from the back, the center of gravity would be close to the mid point between the wheels. It would also be somewhere between the two axles, perhaps under the steering wheel more or less. That fore/aft position doesn�ft come into play too much with respect to a side roll over.

The biggest question seems to be the height of the center of gravity. My GUESS is that it would be no higher than the top of the rear tires. And that it probably would not be lower than the top of the rear wheel rims. That is with loaded rear tires.

Do you think, in general terms, it would be fair to say that, as a �grule of thumb,�h a tractor�fs center of gravity would likely be located:

1. Between the front and rear axles
2. Close to the front-to-rear centerline, and
3. Vertically between the top of the rear wheel rims and the top of the rear tires?

I am pretty sure this is where the center of gravity is located, but I am not sure if it might be lower than the tops of the wheel rims.

Once I am pretty sure I know about where the center of gravity is, then I expect I can calculate the max/min angles within which a static roll over might occur.

Anyone want to suggest that I �gget a life?�h [img]/w3tcompact/icons/grin.gif[/img] Guess it bugs me that I don�ft have a feel for how stable/unstable my tractor really is!

So whatdayathink? Am I in the ball park? I mean as to where the center of gravity actually is...[img]/w3tcompact/icons/grin.gif[/img]

Bill in Pgh, PA

[beenthere]

Hmmmm? Maybe park your tractor between two good size trees, and attach a come-along to the trees on each side and to the roll bar. Then crank it over with one come-along while reeling out a little line at a time with the other. When the tractor balances, you will be able to measure the static angle or degrees at which it will roll over. That should be safe and not too expensive to do.

I think the center of gravity is below the top of the rear wheel rims, like more at where the seat bolts to the top of the rear end.
Seems to me on my tractor there is little or no weight (other than me, and the steering wheel) above the bottom of the seat. When that seat gets over (outside) the position of the left or right wheel where that wheel contacts the ground, then there is a chance of roll over. The wider the wheel stance, further out that seat could go. Seems the mass of the tractor is below the seat in transmission, axles, etc. Long before I am setting on the seat outside where the tire contacts the ground, I will be out of there. But I am guessing that is somewhere at a 35 degree angle.

[ScottAR]

On cars/trucks it's roughly the centerline of the crankshaft. I figure it's pretty close to that. Maybe raise it a little if the tractor has an FEL.

[mdbarb]

But whaddabout if my boxblade or backhoe is mounted and if they are up or down or left or right or if there is weight in the loader and if it is up or down or the tires are weighted or the ground slopes or I had a big lunch....

[Turfman]

Determining the center of gravity for a tractor is pretty easy, IF you can weigh each axle then raise the front or rear axle about 18 inches and weigh the grounded axle. We also need the height from the ground to the center of the axle with all tires on the ground. We do it on a regular basis for vehicles involved in crashes. I have a formula for determining the height of the center of gravity if you have the above information.

[Anonymous Poster]

Hi bill

While reading your post i recalled reading another article may years ago that many tractor rollovers were in fact back flips due to going straight up an incline. this is not due to centre of gravity but the leverage created when rear wheels drive.

[Egon]

The center of gravity on your tractor will change as the slope it is on changes. What you are looking for is the metacentric height which is a variable as the slope changes. Have fun calculating it.

Egon

[em14]

In case you don't understand Metacentric Height is about .. here is a definition www.warships1.com/W-Tech/tech-009.htm

[beenthere]

Keep your lunch low! - and do figure in all the attachments that you have hanging on the tractor. They will affect the center of gravity. The FEL has probably the biggest effect on the COV, as it can be raised high, as well as it is cantilevered out over the front wheels taking weight off the rear wheels.

[Anonymous Poster]

Hi Egon,

<font color=blue>The center of gravity on your tractor will change as the slope it is on changes. What you are looking for is the metacentric height which is a variable as the slope changes. Have fun calculating it.</font color=blue>

I don't think this is correct. As I understand it, the center of gravity of an object is determined by the mass and shape of the object.

The center of gravity does not change for the object. But the position of the center of gravity does change on a slope. When it get beyond the outermost supporting point, say the tire or maybe the backhoe stabilizer shoe, over you go.

I will read that reference you gave to metacentric height now and hopefully not have to eat crow after posting this, but that was what I learned back in the old days...[img]/w3tcompact/icons/smile.gif[/img]...and I don't think anything changed since then...

In other words, the center of gravity of an object is the same whether it is on the face of the earth or up running around with the space shuttle.

Bill in Pgh, PA