Slopes and tractor tilt

[Reg]

I was grinding some stumps for someone last week and at one point this thread came to mind.
I wasn't having a close call, but it just occurred to me that I was on compound slopes, I had a bucket on the front, I was in a very a "dippy" area and my mind was much more on what I was doing (lining up for the next stump) and where I was going. My mind was NOT on C of G and slope angles.

For MOST of this thread I have been thinking about mowing across a simple single plane, NOT backing around the sort of complex curves of the "dippy" area that I was working in with its stumps, surface roots and soft fill spots that I was in the process of creating.

It ain't text book geometry and applied math - well, it is, but the number of variables is LARGE and their values are essentially UNKNOWN ! so Ya just can't do the arithmetic.

It IS about watching where you are going (sometimes backwards and attempting precision), about feeling unstable and anticipating whether it will become more or less stable as you continue.

IOW it is a QUALitative thing more than a QUANTitative thing.
We probably DO the math, but it is a hugely analogue computation, not numeric.

BTW, diesel exhaust and 2 stroke chain saw exhaust does NOT chase away New Hampshire bugs (-:

 

[dennis52]

I have been reading different posts about slopes, stability and rollover for about 8 weeks now and have decided to experiment with my new tractor slashing on some steep slopes with FEL on front and 5t slasher (Brush Hog) on rear

First I have had it on narrowest setting and no loaded tyres; The tractor has been surprisingly much more stable than I expected. Even carrying bucket full of rock across slopes (slowly)

I then widened the rear tyres out and took it up same slopes. It has felt a little more stable but not as much as I expected.

Next I am going to load the rear tyres and and see if another 900lb (on top of the 650lb slasher at rear and 800lb or so Fel at front) really makes a significant difference.

What I still do not fully understand is that when you fully load tyres, half the weight is always above the axle and the COG.

Wouldn't it be better to half fill them and keep the weight below the axle.

I have been climbing and slashing these slopes for 15 years with fully loaded tyres but now have my doubts.

What I do know is to trust my backside.

 

[Reg]

I have been reading different posts about slopes, stability and rollover for about 8 weeks now and have decided to experiment with my new tractor slashing on some steep slopes with FEL on front and 5t slasher (Brush Hog) on rear

First I have had it on narrowest setting and no loaded tyres; The tractor has been surprisingly much more stable than I expected. Even carrying bucket full of rock across slopes (slowly)

I then widened the rear tyres out and took it up same slopes. It has felt a little more stable but not as much as I expected.

Next I am going to load the rear tyres and and see if another 900lb (on top of the 650lb slasher at rear and 800lb or so Fel at front) really makes a significant difference.

What I still do not fully understand is that when you fully load tyres, half the weight is always above the axle and the COG.

Wouldn't it be better to half fill them and keep the weight below the axle.

I have been climbing and slashing these slopes for 15 years with fully loaded tyres but now have my doubts.

What I do know is to trust my backside.

As long as the C of G is high (and it is always ABOVE the axle center-line) adding ballast below it will lower it.
Yes, even adding ballast ABOVE the axle center-line, since that line is below (WELL below) the C of G.

Of course, adding ballast above the C of G will raise the C of G, but we would always try to not do that.

 

[dennis52]

As long as the C of G is high (and it is always ABOVE the axle center-line) adding ballast below it will lower it.
Yes, even adding ballast ABOVE the axle center-line, since that line is below (WELL below) the C of G.

Of course, adding ballast above the C of G will raise the C of G, but we would always try to not do that.

So where, generally speaking, would the COG line be on a CUT if it isn't the rear axle?

 

[North Country]

So where, generally speaking, would the COG line be on a CUT if it isn't the rear axle?

It has to be higher. I see air below my axle and engine above - it's higher.

Loading the tires isn't just about raising or lowering cg, it's about adding weight far away from the rollover axis. When (if?) you roll, you're rolling about an axis that goes from the ground contact point of the downhill tire to the front axle pivot point. The loaded uphill tire is added weight about as far away from that axis as you can get - it raises your moment of inertia about that axis and makes it harder to roll. The downhill tire is very close to the axis and the weight doesn't affect the MoI much.

Try this thought experiment - park sideways on a slope. Think about rolling the tractor by lifting the uphill rear tire. Now fill both tires, to any level you want. Now think about lifting the uphill tire again. It just got harder, didn't it? Just because some of that liquid is above the axle doesn't mean it can't help prevent rollovers.

 

[Reg]

So where, generally speaking, would the COG line be on a CUT if it isn't the rear axle?

Well,,,, above that (-:
There is a lot of STUFF above that center line, just about all the tractor is built above there.
At least in THEORY you could add mass below the axle center line, e.g. some sort of belly tanks or big slabs of lead hung underneath, but just about anything like that would reduce ground clearance, which is generally not a good thing.
It is true that more than 1/2 the fluid ballast in tires is below that line.

One theme here has been the loader and its sub frame have their centers of mass above the axle center line - and in all probability above the base tractor's center of mass, hence adding the loader raises the total mass and raises the center of gravity(/mass).

Think of this kinda/sorta like averages; anything added that is above the existing average will raise the average to a new average - anything below when added will lower the average.
In this case we are just totaling the weights and averaging their (weight times distance)s from the ground up.
(From the low side tire's contact point up.)

Fluid tire ballast in tires is not only LOW it is also about as far OUT from the tractor's front/rear center line as practical.

 

[dennis52]

Well,,,, above that (-:
There is a lot of STUFF above that center line, just about all the tractor is built above there.
At least in THEORY you could add mass below the axle center line, e.g. some sort of belly tanks or big slabs of lead hung underneath, but just about anything like that would reduce ground clearance, which is generally not a good thing.
It is true that more than 1/2 the fluid ballast in tires is below that line.

One theme here has been the loader and its sub frame have their centers of mass above the axle center line - and in all probability above the base tractor's center of mass, hence adding the loader raises the total mass and raises the center of gravity(/mass).

Think of this kinda/sorta like averages; anything added that is above the existing average will raise the average to a new average - anything below when added will lower the average.
In this case we are just totaling the weights and averaging their (weight times distance)s from the ground up.
(From the low side tire's contact point up.)

Fluid tire ballast in tires is not only LOW it is also about as far OUT from the tractor's front/rear center line as practical.

Thanks to Reg and North Country for providing me with the engineering explanations I've been after.

Just a final question: if the mass of the fel increases the COG of the tractor wouldn't a 650lb brush hog at the rear and carried 3" off the ground counteract the fel mass and lower the COG.

To me, my tractor feels nicely balanced and stable with both implements always on. I travel slow and am constantly adjusting height of both implements according to degree of incline I am traversing.

 

[North Country]

Just a final question: if the mass of the fel increases the COG of the tractor wouldn't a 650lb brush hog at the rear and carried 3" off the ground counteract the fel mass and lower the COG.

More importantly, it moves it to the rear.

I think the most important point about FELs that most people miss is not how high or low it moves the cg - it's how far forward it moves the cg. Remember - you're driving a triangle. Your 3 points are the two rear tires and your front axle pivot. Putting that FEL on - and even worse, adding weight in the bucket - moves the cg forward towards the narrow "tippy" part of the triangle. Adding a rear mower moves it back towards the wider, stable part of the tractor.

If you mention "center of gravity" and "front end loader" in the same sentence, you should also mention "forward." If you're only talking about "higher" or "lower", you're missing the biggest reason* that a FEL makes a tractor more likely to roll.

It's a nice day - time to get outside and get a few sidehill pictures.



* I'm assuming that the operator is carrying the bucket low to the ground. Obviously, raising the bucket raises the CG significantly.

 

[RoyJackson]

Thanks to Reg and North Country for providing me with the engineering explanations I've been after.

Just a final question: if the mass of the fel increases the COG of the tractor wouldn't a 650lb brush hog at the rear and carried 3" off the ground counteract the fel mass and lower the COG.

To me, my tractor feels nicely balanced and stable with both implements always on. I travel slow and am constantly adjusting height of both implements according to degree of incline I am traversing.

Yes it would lower the CofG. As far as counteracting the weight of the FEL...depends.
The problem with the cutter on a slope is that it also has a tendency to move downslope (that tail wheel) and can make the tractor unstable too.
There's another current thread in which it seems the cutter may have caused the tractor (a Kubota BX series) to start a roll.

If you've ever felt that sudden lurch when the cutter tail wheel rolls a bit to the side, you'll know what I mean by instability.

The math and the theory make for nice and interesting discussions, but too often they don't really apply in a real word scenario as they don't take in account uneven topography and other variables (such as how much your cutter moves side to side (due to the uneven ground) as you traverse the slope.

North Country: Pretty day here in PA. I should get those pictures posted by this evening.

 

[SPYDERLK]

Thanks to Reg and North Country for providing me with the engineering explanations I've been after.

Just a final question: if the mass of the fel increases the COG of the tractor wouldn't a 650lb brush hog at the rear and carried 3" off the ground counteract the fel mass and lower the COG.

To me, my tractor feels nicely balanced and stable with both implements always on. I travel slow and am constantly adjusting height of both implements according to degree of incline I am traversing.

A bushog does a real good job of partially or near fully counteracting the effect of the FEL as you say. Keeping it low [tailwheel just skimming] and well controlled so that it stays on tractor centerline are paramount. A bushog that can move very much off center can be pretty major in destabilizing a small tractor on a slope.

More importantly, it moves it to the rear.

I think the most important point about FELs that most people miss is not how high or low it moves the cg - it's how far forward it moves the cg. Remember - you're driving a triangle. Your 3 points are the two rear tires and your front axle pivot. Putting that FEL on - and even worse, adding weight in the bucket - moves the cg forward towards the narrow "tippy" part of the triangle. Adding a rear mower moves it back towards the wider, stable part of the tractor. ... :thumbsup:

If you mention "center of gravity" and "front end loader" in the same sentence, you should also mention "forward." If you're only talking about "higher" or "lower", you're missing the biggest reason* that a FEL makes a tractor more likely to roll. ... :thumbsup:

It's a nice day - time to get outside and get a few sidehill pictures.

* I'm assuming that the operator is carrying the bucket low to the ground. Obviously, raising the bucket raises the CG significantly.

Good asterisk.

I have heard that ballpark CG height is 10" below the seat ... we know that its forward of the rear axle as well ... and this, in particular, changes in composite with addition of end effectors/implements.
larry