Center of gravity??

Paddy · Dec 8, 2005

Bob,

You are correct, CG will not change by moving your wheels out unless you move the left more than the right /forums/images/graemlins/cool.gif

Ray,

You are correct too. Static problems do work into dynamic but...if you roll down that hill a cut hard to the ...right, your going to roll. But a tractor that is more stable in a static.. issue than another will also be more stable in a dynamic situation. the math for dynamic is quite difficult. Static is realy quite easy. (mass x Distance from pivot). We enginerds call this teeter toder math. A 100 lb guy sits 1' from the pivot will sit level with the 10lb baby ten feet away.

In summary, a carefull driven top heavy tractor is far safer than a wide foot print tractor driven by Mario Andreti?

Patrick

JustGary · Dec 8, 2005

The mass X distance business is called lever arm. It does increase with wheel width, but so does the rotational inertia, which makes the tractor resist tipping even more by slowing the tip rate. This is particularly true with filled tires, since the rotational inertia is greatest in objects with their mass furthest from the CG.

In other words, filled tires increase side-tipping inertia by moving mass away from the CG. The fact that the CG moves lower is good, too.

Like everything, inertia is a two-way street. Once you get that rotation going, more inertia makes it harder to stop!

- Just Gary

Paddy · Dec 8, 2005

Wide and with weight! Easy to demo with a 10' tele pole. Rolls easy down the hill but, very diff to flip end over end down!

Get the flywheel going and it's diff to stop.

bugstruck · Dec 8, 2005

Bingo. That is exactly what is reducing the tendency to roll. I'm no engineer, but those of us who have worked materials and methods for any length of time can see it and feel it. Just can't calculate it. At least not past the real easy stuff. In lay terms the mass was moved further and equally in both directions from the the vertical center of mass. So the added leverage of the outermost weight resists the tendency to rotate that same mass about the pivot point/fulcrum. In this case the downhill tires.

Take a 4' x 8' x 1" steel plate, Stand it on edge. One person could easily tip it over. Wouldn't even want to approach it actually. /forums/images/graemlins/shocked.gif Now weld some 3 x 3 x 1/4" square tubing out to the sides 3' each way near the ends, but so that they are barely contacting the ground. Try and roll it. You could argue the center of gravity was little changed, but it takes considerably more energy to tip it over. The lever location resists the roll. I know it's not gusseted and the welds may fail /forums/images/graemlins/wink.gif. You get the point.

Slamfire · Dec 8, 2005

I believe I have solved the roll over problem with another set of rear wheels.
My tractor has reduction gears ala the Volkswagen bus, how does this arrangement affect the end to end flip?
Pulling from the 3ph seems much better than from a static drawbar, as some load is transferred to the top links and pushes forward on the upper part of the differential. /forums/images/graemlins/wink.gif

RayH · Dec 9, 2005

( I don't believe widening the tires changes the actual CG. It gives the tractor a larger foot print changing the angle of the dangle required to tip it over side to side only. )

On second thought, I believe you are right and Im wrong. I'll have to rethink that. I do remember reading long ago that a tight rope wlaker can move his CG below the rope by using the horizontal pole and there have been racecars with a CG was actually below ground. That may have been a dynamic CG, I know that can move all around depending on the forces applied.

California · Dec 9, 2005

( there have been racecars with a CG was actually below ground. )

I think thats 'roll center' below ground. CG would be impossible.

bugstruck · Dec 9, 2005

Initially I bet the engineers say it doesn't effect it, as their first thought may be a flip over backward. Old wood folk (good engineers too) like us almost immediately see the ability of things to move in any potential direction. Because we've seen the unlikely, less obvious, movements many times before. They can kill us if we don't anticipate them or at least acknowledge/understand the possibility. Even when an engineer would find it most improbable, not worthy of consideration, we're working it for good reason.

So the flip over backward question landed and stuck for about 1.5 seconds and the flip over frontward question hit, almost immediately followed by the flip forwards or backwards, rotating about just one tire. The last is a little outside the general direction of this discussion so I'll pass. Plus it will make my head hurt longer.

Now in tractor parlance, the flipping over forward is incredibly unlikely, unless one could find a low wall while coming down a steep incline...with real velocity. But yes, those rear tires greatly reduce that tendancy for a front roll. /forums/images/graemlins/tongue.gif

The backwards flip? No static difference in my view and little dynamic difference assuming traction (it isn't), gearing, and engine speed are the same in both configurations. Same static tendancy as one tire each side.

TMcD_in_MI · Dec 9, 2005

( I do remember reading long ago that a tight rope wlaker can move his CG below the rope by using the horizontal pole ... )

Not if it's straight. You may be thinking of something like a tight-rope walking toy that has a pole that bends downward on each side of the rope, and has a weight on each end. If the pole bends below the level of the rope and has enough weight on it, then the CG of the toy and pole together will be below the rope.

I think the main reason for the pole in tight-rope walking is to increase the walker's rotational inertia.

Tom

JustGary · Dec 9, 2005

Slamfire,

I don't think that tire width will change any tendency toward back flip. For that matter, rear tire loading will not either. The reason is that during the flip the rear tires stay still and the tractor rotates around them. The best way to avoid a back flip is loaded front tires and maybe suitcase weights on the front bumper (or whatever else you can hang out front, like a handy FEL).

Like a previous poster mentioned, a front flip can happen, but probably not under the direct force of the tractor because most of that force will cease when the rear tires lift off the ground. They'll spin before they lift off anyway.

As for pulling with the 3PH, it all depends on what you are pulling and how you do it. For a single point pull the drawbar is safest because 1) it is below the axle and GC of the tractor, and 2) it will not change its position under load like the 3PH can. This has seen much discussion on TBN according to my short history as a member.

- Just Gary