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Backing or Driving forward up a hill

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-06-02          56246

The angles thread is getting pretty long. I don't know if there's enough interest in the side subject of backing or driving up but I'll start something new.

From some of the comments I understand that back flips are pretty common and that certainly is a reason from backing up a hill. I don't face extreme problems and tend to put weight in the bucket when in doubt.

I guess that overall I have the sense that my main risks are from sliding rather than flipping so I'm more concerned with traction. The soil is very sandy here and I have the sense that keeping the rear tires downhill generally gives the best traction. I also have the sense that if a slide starts then there likely is going to be little if any steering so it wouldn't make a lot of difference if I can see where I would be sliding. I suppose that looking at what's coming up might be more comforting.

I've heard that slides while going up or down hill tend to turn sideways and produce rolls. I don't know if starting out with big weight up or down hill would tend to make a tractor less likely to turn sideways.

I think that I'd probably do one thing if I thought that flipping was the greater risk and another if I thought that sliding was the main risk. I certainly agree that general safety rules probably can't substitute for operator skill and experience. I also don't know whether my particular skills and experience give me the best sense of what to do all the time.


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Backing or Driving forward up a hill

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AC5ZO
Join Date: Jul 2003
Posts: 928 Rio Rancho, NM 87144
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2003-06-02          56260

Certainly on level ground the center of gravity on a tractor is going to be more toward the rear. But if you add a FEL, it shifts forward. Add a 3PH implement and it shifts rearward. Head downhill and the action of gravity moves the apparent CG forward because of the angle.

When you drive straight up a hill, the apparent CG is shifted rearward. When the CG is very near to or directly in line with the rear axle, then a dangerous stability situation is present. The torque from driving the rear wheels and any reaction of the front wheels going over a bump all contribute to lifting the front end.

On level ground, lifting the front end off the ground is an indication of instability developing, but without the added effect of the angle of the hill, generally nothing bad happens and the operator simply corrects. When you are on a hill, you may not have the warning that you get on level ground. You may notice that the front doesn't steer well, but depending on the hill and how you are driving, you may get no such warning. Having an implement on the 3PH may give some protection to backflipping, but since the 3PH does not limit upward movement till the arms hit the stops, it is probably going to be a scary event.

The most stable configuration sliding downhill??? IF all wheels are sliding, it is probably going to be with the rear going first. This is because the CG is closer to the rear wheels and the drag of the front wheels is going to tend to act like the fletching on an arrow and follow the CG down the hill. In addition, the CG will shift rearward as I described above and that will enhance the effect.

But, it is very likely that not all the wheels are going to be sliding at the same time for the complete duration of the slide. Any wheel that gets more traction will cause the CG to pivot and head down hill from that wheel with traction. Since the rear wheels are the most likely to get some traction, the tractor may very well swing around and go sideways. This is the most unstable of configurations and will likely cause the tractor to overturn sideways if the downhill wheels hit any obstruction or get traction due to the CG being shifted toward them.

Now, if you start down a hill with the front first, the CG is going to be shifted forward of the rear wheels. Since the rears are the most likely to have traction, this is a fairly stable situation. (CG is downhill from the wheels with traction) The front wheels are so far away from the CG that it would be very unlikely to flip over the front (unless the front falls into a depression.) In this configuration, at least the driver has a chance to steer into a slide and keep the front going downhill avoiding going sideways.

As we discussed before, there are too many variables to absolutely say for sure what will happen every time. I think that you told me that you use turf tires and those are going to react differently than my R4 lugs.

There are a few things that I can say for sure. Always use 4WD if you have it. Stay off the brakes and clutch and drive to a safe place even if you are going faster than you would like. If you have HST, then try to match speeds to keep the wheels rolling and slow down when you can. Steer into a slide if heading downhill, but if pointed uphill, it is better to keep the front pointed that direction to avoid the sideways configuration at all costs. If you are traversing a hill and you start sliding downhill, you should try to get the tractor rolling downhill whether it is front first or rear first. Once it is rolling then you will have some control.

If anyone is interested, I can explain how to measure and determine where the CG is of your tractor. But as I mentioned, things change when you add implements or slopes. The main advantage of calculating it is that the absolute point when a roll over occurs is when the CG gets directly above the axle or in the case of a side roll over, when the CG is directly over the line between tire traction patchs. These may be nice numbers to know, but you still have to rely on your common sense and instincts. ....

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Backing or Driving forward up a hill

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-06-03          56423

From the comments, I'm going to revise my thinking and back up hills in marginal situations. The comment about rear wheel torque got me thinking. On a level it takes about a half revolution of the rear wheels to flip a tractor and would happen in under a second. It'd take far less when going up a steep hill and also would be more likely to happen with light front end.

I think there'd be some reduction in traction due to weight-transfer toward the downhill smaller front tires and that might increase the risk of a slide. However, backing uphill now seems a little better as a general rule to me, especially when there's a lot of weight on the 3ph.

The situation when going downhill isn't clear in my mind. Sliding would be pretty hairy and I keep in mind that ground speed increases, there's little steering or braking since there's already little if any traction and differentials counter-rotate. Many people say that if a slide starts it's usually a wild ride to the bottom unless the tractor pivots and rolls first. There's not a lot an operator can do once a slide starts.

Going down hills I might think the best thing to do is concentrate on minimizing the risk of sliding. Using weight transfer to downhill rear wheels may minimize the risk of sliding but that's still just my impression. A pro highway equipment operator quite awhile ago did say here that they back down hills but his experience might have been equipment rather than tractors.
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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2003-06-03          56441

I can't believe that the C. of G. of any CUT is not WAY forward of the mid point of the unit, the engine, front axle and tires are certainly going to outweight the rear axle and tires. If this is the case then driveing uphill would have the effect of centering the C. of G. by shifting it back, it would also shift more weight onto the rear wheels. A loader would make it even more so.

Also, bear in mind that if the implement on the 3pth is a bush hog, then only 50% of it's weight would be on the 3pth, the balance would be riding on it's tail wheel.

I do agree however with the notion of reversing from the point that IF you were to slide, at least you would be in a more natural driving position to attempt some sort of steering input to regain at least directional control.

With equipment such as a dozer, you always back down and push up the hill when possible, mostly to keep the heavy engine on the uphill side, and the weight is then better centered to improve stability and more importantly traction. The theory being if you run out of steam with a blade full of dirt half way up the hill you can always back downhill away from the pile and the machie is not then stuck on the hill.

Around here the ski hills are cut with the same groomers they use in the winter, instead of a dozer blade in front they have a self-powered flail mower out front, and they always cut bottom to top. When they reach the top they drive down an access trail off to the side then start cutting again from the bottom.

Best of luck. ....

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slowrev
Join Date: Jul 2003
Posts: 231 Winchester , KY
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2003-06-03          56468

Murf,
With dozers the center of gravity is far forward that of a CUT.
They are also not prone to flip over backwards as with a CUT where the front rear balance point is about right under your feet. You will be suprised how quickly a tractor can flip over backwards. I have never heard of a tractor flipping over forward going down a hill, unless it is pretty much vertical.
....

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AC5ZO
Join Date: Jul 2003
Posts: 928 Rio Rancho, NM 87144
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2003-06-03          56476

I might add that most loaders are going to shift the CG forward especially when carrying a load. That is why you have to add rear ballast. They also tend to add weight up high and would raise the CG making a sideways rollover easier. Most 3PH implements are going to have the weight down low, but they can only make the front end lighter. In a back flip situation the 3PH may or may not hit the ram stops and arrest the flip. A back flip that might be made more likely because of 3PH weight might also arrest when a tail wheel hits the dirt and takes some of the weight of the mower or other implement away from the tractor. ....

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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2003-06-03          56480

I'm going to have to disagree with you on the C. of G. point of a dozer, they are designed to be as equally balanced across the entire length of the tracks as possible so as to maximize BOTH floatation and traction.

As for back-flips, I don't think I would be surprised at all, as a ninth generation farmer I know all too well how easy it happens, and as an PE I know why it happens. It is a result of the rotating tractive force exceeding the dead weight of the tractor forward of the fulcrum. When you put a tractor on a hill you shift the balance point, you also change the traction, dramatically. However since a 'standard' 4wd CUT should have a basic weight distribution of 55% of the weight on the front wheels and 45% on the back wheels, and correct ballasting should restore this after a FEL is added for example, then you have to be on a pretty severe slope before you overcome all that extra weight up front.

Of course, safety first, if in doubt, DON'T !!!

Best of luck. ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-06-04          56557

It's good for me to have gotten the ideas about driving or backing uphill clearer. When I've driven uphill in situations that take some thought I usually have a heavy bucket load and my turf tires lessen the torque issue. These factors combined with the risks of sliding due to the sandy soil here and maybe my instincts to drive up aren't all that bad. Everybody here is saying that these things are complicated and general rules can't substitute for good operator sense.

I recall the equipment operator's comment about backing down hills was in the context of tractors and run away slides. I didn't think he was talking about dozers but he might have been. Even so, the reasons for backing down may have been something like Murf mentioned rather than a balance or traction problem. Irrespective of the reasons, those dozers sure don't compact the soil much so I've got to believe the weight is fairly evenly distributed even given that the tracks are much larger than tire bottoms.

Several people in past discussions have been pretty definite that in a run away slide there isn't much steering and whether a person can see where they're going may not be too relevant. To date I've kept myself out of trouble so I'll have to take them at their word. However, the old discussions did mention that engaging the diff lock before starting up or down a hill may help keep a tractor from turning sideways and rolling. However, that would also eliminate any steering that was present. Guess I'm still more slide than tip sensitive and maybe I should think more about balance.




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AC5ZO
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Posts: 928 Rio Rancho, NM 87144
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2003-06-04          56582

I have driven caterpillars before and I agree with comments that the weight is evenly distributed on the track and that they have lower pressure in pounds per square foot of track area. The real traction comes from the cleats that are high pressure and force their way into the soil. Having some experience in this area I also agree with Murf about cat operation. But there are also fundamental differences between track and wheel tractors and they way they respond.

As far as qualifications are concerned, My family has farmed in Central Missouri as far back as the records go. Other than the experience that I gained from growing up there, I don't feel that the history qualifies me to analyze sliding tractors but being an engineer for several decades probably does. I have done a lot of interesting things over the years including designing and fielding oil drilling equipment and off-road racing. I learned a lot about weight balance and traction running in the Baja 500, Baja 1000, and a variety of other venues.

I will reserve any specifics about the weight balance of my tractor until I measure it. On my CUT the engine is entirely behind the front axle and there is a considerable amount of cast iron right around the rear axle. My experienced eye tells me that the balance point is probably going to be very close to the front of the operator platform. (That would put most of the weight on the rear axle) Another data point about having more weight on the rear axle is the very low air pressure run in the small front tires. I have been wrong before and I may be wrong again. But, it does not make sense to me that a tractor would be designed with 55% of the weight on the very small front tires. But, there are a variety of things to consider including the attachment of implements to the 3PH and front end loaders.

My tractor has calcium ballast like many others with FELs, but my educated guess is that the calcium pretty much balances the forward shift in CG that a FEL would cause. I will make some measurements and report back. This is not an easy measurement since I do not have a platform scale, but I will get it done some time this summer. It is also simply an academic exercise since my tractor is not likely to be configured exactly the same as most others.

Your only hope of getting some control in a runaway slide is to get the front wheels rolling so that you can steer them. The slide could very well be over and you may never get the chance to intercede in one of these. I don't know of anyone with a lot of experience with uncontrolled slides on tractors. I imagine the life expectancy for that job is not long. When a slide starts, you are just going to have to let instinct and some training take control and hope for the best outcome.

Instincts are what they are, and you can't do much about them, but you can train for slides and skids. I have done this in the Baja vehicles. We use the front end steering and independent rear wheel steering brakes to bring the vehicles under control. I have intentionally put the vehicles into skids at speeds up to 60 MPH to learn how to recover. You do this on a dry lake bed where the ground is flat and sandy and there is little chance to turn the vehicle over. But, sometimes they roll over anyway.

I don't know how you would ever practice this safely with a tractor, but the actions and results from the training are the same. The training will kick in before you have time to analyze and think about what is happening. Basically training boils down to experiencing the event and implementing a reasonable action to bring things under control and surviving the whole ordeal. ....

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Chief
Join Date: Jul 2003
Posts: 4297 Southwest MiddleTennessee
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2003-06-04          56659

Tom I tried experimenting today backing up hill and driveing forward up hill. I found that I didn't have nearly as much traction backing up as I did going up forward. I could back up fine in 4WD but not everyone has that. I have a 430 loaders and fluid filled R-4's which may influence the center of gravity. Unless the hill was exceedingly steep, I would drive up frontward rather than back up. ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-06-05          56674

Chief: That always has been my sense too. The best traction is with the big tires downhill. I started out with the attitude 'back down and drive up.' Then the comment about light front ends and tire torque was made. My solution for light front-ends has been more weight in the bucket but I can now see that's there's a risk of catching some traction and the torque flipping a light front-end up and over. I believe I can see that in many situations a person might not get up a hill backing up but they'd be safer trying to. I think I'd still feel more secure backing down most times, but maybe there are things here too that haven't dawned on me.

AC: I can't agree more. When tips and slides happen the reaction is all instinct and we have to hope that we have good ones. I also believe that it's possible to train instinct and that's what's good about these discussions. First we have to get the right ideas in mind and then try them out in safe situations. Eventually they become what we do in a crisis. I do have a few more things in mind to work on now.

I'll add there are some things about tractor slides I've heard that may bear on comparisons to dirt car racing. When a tractor slide on a hill starts speed is likely to increase, which should further reduce control. There's really no way to reduce speed. Brakes or engine braking won't be effective since there's already insufficient traction--diesels have little engine braking anyway. I'm not sure that controlled power slides used in some racing would work with the limited rpm range of diesels. I don't know; like you I've never been in a slide so it's just theory with me. Irrespective of any differences, the comparison remains a good think to think about.
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AC5ZO
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2003-06-05          56714

Chief, what you were experiencing is the forward shift of the apparent CG taking weight off your rear tires reducing the traction. This is an indication of how high the CG is off the dirt surface and will certainly reduce traction as you say. Certainly driving straight up a hill will shift the CG rearward and increase traction but it happens at the cost of front traction and steering control. You should find that you have better traction backing up a hill with an implement on the 3PH which will tend to keep the overall CG closer to the rear axle.

The comments about 4WD and not everyone having it are also right on target.

Tom, what I meant by instinct would be the human software programmed into your DNA. You get certain internal warnings that make your adrenaline flow even if you don't know what they are. Things such as a growl from a cave, unstable footing or the sound of a 12 ga pump shotgun action will get just about anybody's attention without training. You just know that there could be trouble.

Training such as driver education, pilot training, race car driving and so forth condition responses so that you do indeed respond as with an instinct. You don't think about and analyze what is happening. You simply sense and act.
I was just distinguishing between innate and learned behavior.
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slowrev
Join Date: Jul 2003
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2003-06-05          56719

My philosophy on this subject is. If the hill is to steep to back up because of lost traction then I sure do not need to go up it forward. Of course your tractor traction might vary from mine.
Also I have located an anti-roll bar for my tractor. Not just a ROPS, but anti roll. Tractor will fall on its side but not roll over unless on EXTREME hillside. It extends out to the top outside of the rear wheels. I know they work because of reports from friends in the Army Corps of Engineers that have used them. I procured a surplus one from them on a bid. It is made for one of my tractors, which was also purchased from them several years ago.
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AC5ZO
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2003-06-05          56721

Slow, is the bar just a taller roll bar? I would be interested in seeing a picture if that is possible. ....

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slowrev
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2003-06-05          56747

I will do a link to a picture on a new thread when I get it mounted. It is about the same height as a standard ROPS but wider, it extends out as wide as the outside of my rear tires. It also has 2 bars that go down and connect onto the front of my loader frame, but I don't think I will put those on. I guess with those on all you could damage if you roll over is the exhaust pipe.

....

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AC5ZO
Join Date: Jul 2003
Posts: 928 Rio Rancho, NM 87144
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2003-06-06          56805

I get the idea. It is more like a supported roll cage than a single roll bar. ....

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