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  1. #1
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    Default Hills: Forwards or Backwards?

    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


  2. #2

    Default Re: Hills: Forwards or Backwards?

    I would add it's tough to flip a tractor end for end if you have the bucket curled back a little so the cutting edge is off the ground but the "round" parts of the bucket is dragging the ground. And when I back up a steep hill, I put the backhoe outriggers down near the ground, extend the hoe so it is reaching up the hill and constantly reposition it so it's near the ground. And slow slow slow. And 4WD with my foot on the axle lock. If you spin and then suddenly get traction, it can be all she wrote. Also put something heavy in loader bucket if possible.

    I do the same with a TLB, back up the hill, head first down the hill if at all possible.

    That being said I'm probably a nervous nellie as watching "real operators" in such situations they don't seem to put much care into it. Of course those guys may "Know" their equipment better than us weekend diggers!

    If I never had to tow my tractor I'd get the rear tires loaded. Or I could buy a more macho (gas hog) truck, or transport the tractor in pieces like they do with the big stuff!

    del


  3. #3
    Epic Contributor Bird's Avatar
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    Default Re: Hills: Forwards or Backwards?

    Whew, Glenn, that's a long detailed analysis, but I think you're right and pretty well covered it all.[img]/w3tcompact/icons/smile.gif[/img] For us old simple country boys, I avoid steep slopes entirely when possible, and when it ain't possible, I go down forward and up backwards.[img]/w3tcompact/icons/smile.gif[/img]

    Bird

  4. #4

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    Ford 1710: Loader, Hoe, Snowblower, Box scrapper & 3ph Forks

    Default Re: Hills: Forwards or Backwards?

    Bird: I remember a long discussion on CTB on this subject. I recall your conclusion, based on experience, was 'down forward and up backward'. My conclusion, based on theory, was the other way around. The theory had to do with keeping the drive wheels on the downside of a hill. There is an assumption here that the downside wheels would have more traction due to weight transfer while on a steep slope.

    Don't know, but I tend to trust experience more than theory these days, so maybe I'll change my attitude. However, I'd guess that what ever feels the safest probably is. I've been able to avoid going onto steep slopes, so I may never gain experience.

    Regarding Del's comment about tractors flipping end for end. As he said, I imagine an end for end flip would be very unusual. If I understand, the main risk is not flipping end for end, but loosing traction, turning sideways and then rolling over. The way differentials work in a slide is that one wheel tends to set up as a pivot point. The tractor tends to slide, pivot around one rear wheel and then flip on its side.

    I think that maybe the idea of traction is more important than stability for going straight up and down hills. However, the idea of stability is more important when working sideways on hills.



  5. #5

    Default Re: Hills: Forwards or Backwards?

    Most compact tractors sold today have 4-wheel drive. When working on any type of slope, be sure 4WD is engaged BEFORE you get to the slope. (It doesn't engage or disengage automatically.) When descending a slope with 4WD engaged, the front wheels serve a braking function through the powertrain from the engine compression. Even if the rear wheels become so lightly loaded that they could not brake the tractor or begin to counter-rotate because of the rear differential, the powered fronts will continue to help control the downslope motion of the tractor. This is one of the most important reasons to operate in 4WD on slopes. Since I must operate on slopes almost all the time, I keep 4WD engaged all the time. If I were to disengage it on level terrain, I might sometimes forget when I drove from level ground onto a slope. Thus to help prevent an accident due to a memory lapse, I always operate in 4WD. When mowing, I avoid sharp turns to minimize scuffing the turf--this really doesn't pose a problem one you get used to it.


  6. #6
    Epic Contributor Bird's Avatar
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    Default Re: Hills: Forwards or Backwards?

    Good points, Tom. I'm sure most of us have seen motorcycles and dune buggies rear up and go over backwards because they got a run at a steep slope and had enough traction with the rear wheel(s). Not at all likely to happen with a tractor (at least not with me on it). Loss of traction, sliding sideways, and then rolling is a much greater possibility.

    Bird

  7. #7
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    Default Re: Hills: Forwards or Backwards?

    I'd like to focus the debate so far.

    Bird and TomG are debating my Scenario 1. If you have a perfectly balanced tractor, center of gravity (COG) shift is irrelevant. It is the same whether you go up or down, backwards or forwards. I say that you should neverthless ascend backwards so you have your drivewheel on the uphill side, pulling you. I think Bird agrees with this although he doesnt specify. Tom is arguing that the weight shift onto the downhill axle means that the downhill axle should be the rear (drivewheel) axle. He is say that this will increase traction on the rear wheels and help prevent side-slip. I don't buy Tom's argument. Tom, you live in Canada. You drive up steep snowy hills. Would you feel safer against side slip driving up the hill in a rear wheel car with weight shifted onto the rear axle, or in a front wheel car even though weight shifts off the fron axle. The clear answer in the car world is that the pulling dynamic of having the drivewheel uphill far outweighs the benefits of weight shift onto the rear axle of a rear drive car in preventing and recovering from sideslip.

    Bird concurs with my conclusions for Scenarios 1 and 2. But I think Scenario 3 (front heavy tractor) is the controversial one. I am saying that COG shifting risks say you should drive the hill the other way: ascend forward and descend backward. I am further saying that the traction principle--as I see it, not as Tom sees it--is in conflict with this result. Bird, would you ascend the hill backward with a full bucked and nothing on back, risking a downhill tip?

    We also have suggestions the 4wd be on for both ascent and descent. Also differential lock. Do we all agree with that? It certainly makes sense to me to ascend in 4wd for the same reason I would ascent a snowy hill in 4ws over front 2wd. Descending I'm not so sure. If I am descending frontwards, might I not want the front wheels to freewheel, so they dont cause any directional changes?

    Glenn


  8. #8
    Veteran Member gerard's Avatar
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    Default Re: Hills: Forwards or Backwards?

    I would tend to agree with most of the analysis except with going up a steep slope backwards, especially if you have R1 tires which are directional and have MUCH better traction forward than in reverse. Also going up frontwards puts more down pressure on the rear tire which are bigger and impart greater forward motion due to size to begin with. The risk of flipping over backwards is pretty minimal if going slow and the analysis of the front wheel car isn't totally true. (A front wheel car has superior traction to a rear wheel primarily due to the weight of the engine directly over the front axel and, in snow, the effect of the tire "digging" its way through the snow instead of being "pushed" through it as in a rear wheel drive car.) Other than that I've been up and down some pretty steep slopes and as long as I'm straight up and down and not "tilted" sideways, (o deg on the tiltmeter) I feel totally comfortable and stable. Throw in even 5 deg of side to side tilt and the blood pressure goes up and all bets are off!


  9. #9
    Platinum Member
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    Default Re: Hills: Forwards or Backwards?

    To all:
    I'll pass along these definitions since I was embarrassing slow to learn these simple definitions in college (these are from memory since I don't feel like digging out my "Static & Dynamics" textbook:
    STATIC: Constant or unchanging, as in sitting still, or a constant velocity (without acceleration).
    DYNAMIC: Under a state of change (such as acceleration), moving a FEL up and down (changing the center of gravity).

    Glenmac,
    You "static" analysis is particularly applicable to 2wd tractors. I'll add some "dynamic" additions to your analysis. In addition to the weight vectors pointing down the hill, a 2wd tractor driving forward up a hill has the dangerous application of the law? "every action has an equal and opposite reaction"; that is, when torque is applied to turn the rear wheels, there is the tendancy for the front of the tractor to rise and rotate around the rear axle (the same concept behind a "wheelie" on a motorcycle). I believe that there have been a few deaths attributed to 2wd tractor "end-over-end" turn overs, and that most of these occurred as a result of say "popping the clutch" while pointed uphill or pulling a heavy load with a chain attached above the center of gravity.

    4wd minimizes this "wheelie" effect by distributing the drive torque to the front and rear axles.

    4wd doesn't necessarily mean that all 4 wheels are pulling. Most smaller tractors have drive power going to one front wheel and one rear wheel; when differential lock is actuated the other rear wheel is engaged.

    As mentioned in another post, 4wd adds the significant feature of front axle (engine) braking, which is critical on slopes.

    I have some moderate slopes that I easily go up and down with my 4wd truck (sometimes successful in 2wd mode when dry). I have a B7100 4wd with mid-mower and a B2150 4wd with FEL and rear 5' finish mower (both tractors reasonably balanced front-to-rear). I have mowed the hills with both tractors in both directions without any noticable difference (as long as 4wd was engaged).

    The only problem that I have ever had while in 4wd mode was a hill that I had dirt hauled in to give vehicle access to a lower section of my property. While SLOWLY driving foward down the DIRT hill one WET day in 4wd mode the front wheels were performing their braking action very well and the rear wheels decided they were in a bigger hurry so the rear end decided to fish-tail. I used "slick road automobile driving" techniques by steering into the skid and simply ended up at a different point at the bottom of the hill than I had planned. I dread what would have happened had I panicked.

    Kelvin



  10. #10
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    Default Re: Hills: Forwards or Backwards?

    Gerard is essentially agreeing with TomG and disagreeing with me and Bird in scenarios 1 and 2. He says not to worry about COG shift flipping and that it's more important to have weight on the downhill rear axle.

    Gerard, what about my scenario 2. You are severely back heavy. Nothing on the front. You really want to point the front of your tractor up this steep hill and risk a flip or tilt siutation just to get traction on a downhill rearwheel. Woulnt an uphill rear wheel with 4wd on a downhill front wheel, with no risk of flip, be an overall safer situation.

    Gerard is also making an assumption that I think deserves other opinions. He is saying that ag tires have much more traction going forwards that backward. Is that true? There could be other reasons for the unidirectionality, inlcuding that they have more traction when driven backwards. I dont know.

    Glenn


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