Hills: Forwards or Backwards?

[glennmac]

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

 

[Anonymous Poster]

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

 

[Bird]

Whew, Glenn, that's a long detailed analysis, but I think you're right and pretty well covered it all./w3tcompact/icons/smile.gif 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./w3tcompact/icons/smile.gif

Bird

 

[TomG]

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.

 

[Anonymous Poster]

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.

 

[Bird]

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

 

[glennmac]

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

 

[gerard]

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!

 

[Kelvin]

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

 

[glennmac]

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

 

[Kelvin]

To all,
I first admit that I don't know what the real answer is, but I was under the impression the AG tires give more traction in reverse and that the design allowed the tire to shed built up dirt accumulations better while moving forward.
Kelvin

 

[gerard]

glenmac,
In your scenario 2 I would agree with you. If back heavy (maybe rear implement on, no loader) I would prefer to back up a hill, no doubt. My position was for the 50/50 weight distribution. In that case there's no particular advantage to EITHER direction until you consider the factor of tire size and traction direction. Taking these into account going up forwards would put more traction on the larger (rear) drive wheels AND allow the tires (If R1's) to roll in the direction they have the greater traction. (This applies to R1 bar type tires. It's not even close. Forwards has substantially greater traction than reverse. I can pull things forward with no problem that I can't pull backwards without the wheels spinning)

 

[glennmac]

Gerard, in the balanced situation we agree that the flip risk is unaffected by tractor direction. Therefore, the risk is a break in traction that results in a side slip. Your argument for rear wheel downhill has three components: (1) the traction-affecting directionality of the tread, (2) the greater size of the downhill rear wheel, and (3) the weight that is shifted onto the downhill wheel. Let's look at each component separately.

You are on the steep snowy hill going up to the Carrier Dome. Your car is evenly weitht balanced, has all the tires the same size with non-directional tread. Your car has fwd, rwd and 4wd. To avoid sideslipping traction risks do you go up the hill in fwd or rwd. I say fwd. If traction breaks on the front drive wheels there may be some back slipping and side jigglling, but the traction will probably catch again and the front-driving wheels will pull you back up on a straight course. If you are in rwd and traction breaks on the rear wheels, you will never recover onto a straight course even if the rear wheels catch again. I think this argument holds true, even though more weight is on the downhill axle than the uphill axle. Thus I do not agree that your weitht shift component for traction-saving is valid.

What about the tread component. Well, so far we have differnt views on whether ags have more traction going forward. (I'm not saying your wrong). But if we were on turfs, you would have to concede this component, too. And even if ags have more traction going forward, how much is the differential? Does it outweith the uphill pulling benefit? What about R4's.

Your third component argument would assume you car was now outfitted with larger tires on the rear (nodirectional and evenly balanced). Do you go up in rwd over fwd? Maybe the chance of breaking traction on the larger wheel is less than on the smaller wheel, but even if so, once it breaks on the rear wheel, you will not recover and are a goner. Plus, I'm not sure that there would be any traction differece if all the tires are filled with snow or mud. Wouldnt it be better to go up the hill backwards in four wheel drive. Your big diameter wheels will be pulling up hill and your front wheels, albeit smaller, will also be driving with a weight shipt on them.

I think your arguments have force, but I still dont think the combined effects outweith the clear anti-side sliding value of driving backwards uphill in 4wd.

By the way, if you are right and we should go up frontwards, do we agree that you should have rear wheel differential lock on to minimize side slip?

 

[gerard]

I definitely agree about having the diff lock on and using 4wd, no argument there. In a 50/50 weight example I just don't see that there's really that much advantage either way, but then I'm basing this only on practical "seat of the pants" experience. Ive done it in both directions and have not felt one superior over the other. Trying to compare the tractor situation to a car is a little apples and oranges. Tractor tires turn slowly and don't have the same impact when slipping that a car would have. You also have the different size tire factor in a tractor that you don't have in a car. With regards to R4's tires the traction is similar to that of an R1. (unidirectional) but not as drastic, at least not on the R4's I've seen. Using your scenario of turfs, I could agree that going up the hill backwards might be safer than frontwards, especially in a "pop the clutch" situation. Going forwards I could see the front end coming up or at least getting light. If you were in reverse backing up the hill this would not happen. (See you're logic is overwhelming me but then you keep taking away the conditions on which I based my conclusions!

 

[glennmac]

Another factor to add to the mix in the evenly balanced situation.

If I'm backing up the hill and traction does break, I am now going forward (more or less)and I have familiar options: steering, braking, clutching. If I am going forward up the hill, traction breaks and I start going downhill backwards or sideways, I'm really in unfamiliar panicky territory. In other word, I'm in a more comfortable recovery situation if I'm backing up when the worst happens....Glenn

 

[gordon]

I'm with you Bird, keep it simple down forward and up backwards. Sure does take the fun out of the thread though.

Not only using four wheel drive on a steep hill but different types of tires and tire pressure can add so many different combinations alone to the hill subject without even taking into consideration of the heavy front end and so on.
Gordon

 

[glennmac]

Gordon,

I'm in favor of simple safety rules if they are correct.

Let me ask you the same simple question I ask of Bird: If you have a very heavy bucket and no ballast on the back, are you going to go up a steep slope backward?

Glenn

 

[Bird]

Glenn, I don't know about Gordon, but a very heavy bucket and no ballast on the back? I don't intend to do that in the first place, and in the second place if I did, yes, I'd go up backwards with the bucket practically dragging the ground, and if I cain't get there that-a-way, I ain't goin'./w3tcompact/icons/laugh.gif

Bird

 

[glennmac]

Bird, that's a reasonable response. Agreed, none of are going to put ourselves in these hypothetical situations that I am posing. But I am trying to flush out a real safety rule by continuing to box people in.

I will continue with my hypothetical.

You MUST go up the mountain, and the Grand Canyon is at the base of the slope if you slip down. You had taken all the ballast off your tractor except your bucket because you were repairing things. Just then, you are asked to take 750 lbs of radium to the hospital on the top of the mountain to some cancer patients who critically need it. Your bucket barely lifts it. Your tractor is level on the level ground but you know you are extrmely front heavy. You are committed to save the patients.

The slope is steep and you suspect it gets even steeper a quarter mile up. You start up backwards. As your rear rises up the slope the COGV shifts even closer to the front axle. You take comfort in knowing that even if you pitch over the downhill front wheel your lowered bucket will keep you from pitching over. It happens. The slope steepens and you are balanced on your front wheels and the bucket one quarter mile above the Grand Canyon. What are you going to do now?

Suppose you had gone up frontward. There are two things I think you know for certain. First, you will never pitch over backward (downhill) because of your super heavy front bucket. But you also know that you cannot pitch forward into the hill. Why? Because you did not pitch forward on level ground. That means the steeper it gets, the LESS likely it is that you can pitch forward and go out of control. Therefore, by going up forward you eliminate BOTH the possibility of pitching forward or backward.

You save the patients. No?

Glenn

 

[gordon]

Once again I'm with Bird---up a hill and no rear ballast of any kind---I might be alittle crazy but I'm not stupid! Ballast is key to safe operation not only on hills but on flatland as well. I've got weighted tires liquid in the rear and when using the loader I usually have the boxblade or gradeblade on the back as well. The steadier the tractor the more work you can get done with it. Don't have to worry about lifting a wheel or being light in the rear.

Now to address your last post Glenn---If I was working on my tractor in my yard I've got enough heavy steel that I could put on for ballast to hang on the rear in a five minute time frame without using the three-point hitch. Lets say for a hypothetical situation that I was working on the three point and it was out of action.

So in essence I would make it up the second part of the hill not only safer but could do it faster because I have ag tires and they grip better going up the steeper part of the hill.---Did I save the patients and do it in a safe manner so I will be able to see the sunrise tomorrow---YES

There is one thing that you have to always take into a hypothetical situation---handlin yes thats it handlin. You see Glenn I have been to handlin school and I sure can handle the quest that has been put to me. One other thing alot of hours in the seat of a tractor thats something that you can't take away---plain old experience that will always help to get you through the tough times./w3tcompact/icons/smile.gif

Just remember that going up a steep hill with no rear ballast and a loaded bucket you might not flip over but you will be spinning some rear tires so you better be sure to have that tractor in four wheel drive before you start the hill. So at least the front wheels on your tractor would be gripping the ground between you and the Grand Canyon./w3tcompact/icons/smile.gif

Gordon