Hills: Forwards or Backwards?

There was a thread on the CTB forum that discussed this called "Flipping Tractors". There was discussion of centers of gravity, traction, back-over-front flip potential, front-over-back flip potential, sideways sliding potential, brake recovery, steering brake recovery, clutch recovery, bucket stab recovery. It seemed rather confusing and, in the end, inconclusive.

This would seem to be the commonest of situations, and one that should have developed a clear answer in the first week tractors were invented. Perhaps you all can help develop clear answers for the safety of us all.

I do not have practical experience but I can suggest an analytical model, based on my dim recollections of simple physics, at which you then can take experienced shots.

I think some of the CTB confusion can be cleared up by considering three scenarios: (1) a tractor that is evenly weight balanced front to rear; (2) a tractor that is back heavy; and (3) a tractor that is front heavy.

To picture my analytical model, visualize a tractor as a rectangle with circles for wheels at each end. If the tractor is evenly balanced front-to-back, the center of gravity (COG) can be represented by a dot in the middle of the rectangle. If the tractor is at rest on level ground, the force of gravity on the COG can be represented by an arrow pointing straight down from the dot. I will call this arrow the center of gravity vector (COGV). A tractor will flip only when the COGV moves past the front or rear axle, which are the pivot points. What would make the COGV move toward the axle? A slope. If either end of the tractor points up a hill, the COGV will move further toward the downhill end of the tractor. If the COGV moves past the downhill axle, the tractor will flip. Thus I derive what I will call the "COGV Pinciple", which states: to avoid flipping on a slope, point the tractor in the direction that minimizes the chance that the COGV moves past an axle.

Let's try to understand and apply the COGV Principle by considering the three scenarios. These scenarios assume that the rear wheels are the drive wheels and are also the braking wheels. This is important, because I will develop another principle I call the "Drivewheel/Brakewheel Principle".

1. Scenario 1: Perfectly balanced tractor front-to-back. Since the COGV starts out exactly in the center of the tractor, the potential for it to move past an axle point is exactly the same no matter whether the tractor is pointed uphill or downhill on either ascent or descent. Hence, the COGV Principle doesnt care how the tractor points in either situation. However, the COGV Principle is not the only consideration. I would say that the tractor should be backed up the hill so that the drive wheels are on the uphill side. Being pulled up the hill by the drive wheels reduces the chance of side-skidding as compared to having the drive wheels pushing the tractor up the hill from the downhill side. This is the common experience that a front wheel drive car is much easier to drive up a slippery hill that a rear wheel drive car. The same logic would apply to driving down the hill: keep the drive wheels, which are the braking wheels, uphill to minimize side-sliding potential. I will call this the Drivewheel/brakewheel Principle, which states: to minimize side-sliding, on either ascent or descent, keep the rear wheels (the drivewheels/brakewheels) uphill. So the answer in this scenario under both the COGV Principle and the Drivewheel/brakewheel Principle would seem to be: ascend the hill backwards and descend frontwards. (Would 4wd affect this analysis? I dont know how tractor 4wd works, but I doubt it. The COGV Principle does not change. The Drivewheel/brakewheel Principle probably doesnt change either. If 4wd only kicks in when the rear wheels slip, then the rear wheels remain the primary drive wheels and the analysis stays the same. Even if 4wd is distributed evenly somehow to all 4 wheels all the time, I would think that the rear wheels remain the dominant drive wheels because of their greater diameter, but I'm not clear on this.)

2. Scenario 2: Back heavy tractor. The COGV on this tractor is almost at the rear axle. This means that if the tractor's front lifts up even a little, the COGV will move downhill (rearward) past the axle, and flip. What's the strategy under the COGV Principle? Back up the hill, so the COGV moves toward the downhill front axle. The case for backing up in this situation is more compelling, and for the same reasons, as with the perfectly balanced tractor. The Drivewheel/brakewheel Principle, to avoid side-sliding, also says back up the hill. The answer is the same for descending the hill under both of the principles. You descend frontward to move the COGV downhill and to keep the brakewheels uphill. Therefore, for both a balanced tractor and a rear heavy tractor, both the COGV Principle and the Drivewheel/brakewheel Principle say that you ascend the hill backwards and descend going forwards. (Same 4wd analysis?)

3. Scenario 3: Front heavy tractor. This is the confusing one because the COGV Principle clashes with the Drivewheel/brakewheel Principle. Here, the COGV is right near the front axle. If the front tips downhill, that will move the COGV in the dangerous direction, and flip. Hence, the COGV Principle says: ascend the hill frontwards and descend the hill backwards (to move the COGV to the rear). But here's the confusion. The Drivewheel/brakewheel Principle is now being violated. Both ascending and descending, the rear wheel is now downhill, thereby increasing the side-sliding risk. What do you do? It would seem that experience must guide you on weighing flip potential against side-sliding potential. From an analytical standpoint, if the COGV is right near the front axle (a really heavy bucket with no rear

OK, here are the theoretical final answers to safe hill driving:

1. Unless you are front heavy, alway ascend steep hills backwards and descend them frontwards.

2. If you are front heavy, BE VERY CAREFUL. If you have a good traction situation, ascend frontwards and descend backwards. If you have a slippery traction situation, don't attempt it unless you have enough experience to weigh the flipping risk against the side-sliding risk.

Well, all that is theory. Take experienced shots.

Glenn

From a safety perspective, when ascending and descending steep hills is it safer to face uphill or downhill?There was a thread on the CTB forum that discussed this called "Flipping Tractors". There was discussion of centers of gravity, traction, back-over-front flip potential, front-over-back flip potential, sideways sliding potential, brake recovery, steering brake recovery, clutch recovery, bucket stab recovery. It seemed rather confusing and, in the end, inconclusive.This would seem to be the commonest of situations, and one that should have developed a clear answer in the first week tractors were invented. Perhaps you all can help develop clear answers for the safety of us all.I do not have practical experience but I can suggest an analytical model, based on my dim recollections of simple physics, at which you then can take experienced shots.I think some of the CTB confusion can be cleared up by considering three scenarios: (1) a tractor that is evenly weight balanced front to rear; (2) a tractor that is back heavy; and (3) a tractor that is front heavy.To picture my analytical model, visualize a tractor as a rectangle with circles for wheels at each end. If the tractor is evenly balanced front-to-back, the center of gravity (COG) can be represented by a dot in the middle of the rectangle. If the tractor is at rest on level ground, the force of gravity on the COG can be represented by an arrow pointing straight down from the dot. I will call this arrow the center of gravity vector (COGV). A tractor will flip only when the COGV moves past the front or rear axle, which are the pivot points. What would make the COGV move toward the axle? A slope. If either end of the tractor points up a hill, the COGV will move further toward the downhill end of the tractor. If the COGV moves past the downhill axle, the tractor will flip. Thus I derive what I will call the "COGV Pinciple", which states: to avoid flipping on a slope, point the tractor in the direction that minimizes the chance that the COGV moves past an axle.Let's try to understand and apply the COGV Principle by considering the three scenarios. These scenarios assume that the rear wheels are the drive wheels and are also the braking wheels. This is important, because I will develop another principle I call the "Drivewheel/Brakewheel Principle".1. Scenario 1: Perfectly balanced tractor front-to-back. Since the COGV starts out exactly in the center of the tractor, the potential for it to move past an axle point is exactly the same no matter whether the tractor is pointed uphill or downhill on either ascent or descent. Hence, the COGV Principle doesnt care how the tractor points in either situation. However, the COGV Principle is not the only consideration. I would say that the tractor should be backed up the hill so that the drive wheels are on the uphill side. Being pulled up the hill by the drive wheels reduces the chance of side-skidding as compared to having the drive wheels pushing the tractor up the hill from the downhill side. This is the common experience that a front wheel drive car is much easier to drive up a slippery hill that a rear wheel drive car. The same logic would apply to driving down the hill: keep the drive wheels, which are the braking wheels, uphill to minimize side-sliding potential. I will call this the Drivewheel/brakewheel Principle, which states: to minimize side-sliding, on either ascent or descent, keep the rear wheels (the drivewheels/brakewheels) uphill. So the answer in this scenario under both the COGV Principle and the Drivewheel/brakewheel Principle would seem to be: ascend the hill backwards and descend frontwards. (Would 4wd affect this analysis? I dont know how tractor 4wd works, but I doubt it. The COGV Principle does not change. The Drivewheel/brakewheel Principle probably doesnt change either. If 4wd only kicks in when the rear wheels slip, then the rear wheels remain the primary drive wheels and the analysis stays the same. Even if 4wd is distributed evenly somehow to all 4 wheels all the time, I would think that the rear wheels remain the dominant drive wheels because of their greater diameter, but I'm not clear on this.)2. Scenario 2: Back heavy tractor. The COGV on this tractor is almost at the rear axle. This means that if the tractor's front lifts up even a little, the COGV will move downhill (rearward) past the axle, and flip. What's the strategy under the COGV Principle? Back up the hill, so the COGV moves toward the downhill front axle. The case for backing up in this situation is more compelling, and for the same reasons, as with the perfectly balanced tractor. The Drivewheel/brakewheel Principle, to avoid side-sliding, also says back up the hill. The answer is the same for descending the hill under both of the principles. You descend frontward to move the COGV downhill and to keep the brakewheels uphill. Therefore, for both a balanced tractor and a rear heavy tractor, both the COGV Principle and the Drivewheel/brakewheel Principle say that you ascend the hill backwards and descend going forwards. (Same 4wd analysis?)3. Scenario 3: Front heavy tractor. This is the confusing one because the COGV Principle clashes with the Drivewheel/brakewheel Principle. Here, the COGV is right near the front axle. If the front tips downhill, that will move the COGV in the dangerous direction, and flip. Hence, the COGV Principle says: ascend the hill frontwards and descend the hill backwards (to move the COGV to the rear). But here's the confusion. The Drivewheel/brakewheel Principle is now being violated. Both ascending and descending, the rear wheel is now downhill, thereby increasing the side-sliding risk. What do you do? It would seem that experience must guide you on weighing flip potential against side-sliding potential. From an analytical standpoint, if the COGV is right near the front axle (a really heavy bucket with no rear ballast ) you really have no choice: any further movement of the COGV toward the front WILL flip the tractor. Hence you must risk side-sliding by keeping the front uphill both on ascent and descent. On the other hand, if the front heaviness is not that great, but the hill is obviously slippery, you might conclude that the side-sliding risk outweighs the COGV risk, and hence keep the rear uphill on both ascent and descent.OK, here are the theoretical final answers to safe hill driving:1. Unless you are front heavy, alway ascend steep hills backwards and descend them frontwards.2. If you are front heavy, BE VERY CAREFUL. If you have a good traction situation, ascend frontwards and descend backwards. If you have a slippery traction situation, don't attempt it unless you have enough experience to weigh the flipping risk against the side-sliding risk.Well, all that is theory. Take experienced shots.Glenn

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