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  1. #1

    Default Tractor Stability

    Tractor stability has always been a popular topic, with lots of opinions expressed. Usually these opinions are just gut feelings. I decided to see if I could add a little bit more to the topic. Many things affect stability and the effect will be different on different tractors but you may find the following interesting.

    I was curious what effect loaded tires, wheel weights and a canopy would have on the center of gravity (CG) of a compact tractor. I took some measurements on my B2400 and made some calculations. The NASD guide stated a tractor CG is typically 10 above the rear axial. I assumed 8 since this is a compact tractor. The axial is 16 off the ground and that would make the CG 24 above the ground. I used 80 lbs. for the weight of the canopy. The Curtis fiberglass canopy weighs 80 lbs. and looks like it is very similar to the kubota canopy. Some of the steel or heavy-duty aluminum ones may be even heavier and would affect stability even more. For these calculations I used a B2400 without any implements attached and without an operator in the seat. For filled tires I used 110 lbs. per tire. This would be 75% filled with water, calcium would be heavier and alcohol would be lighter.

    I had suspected that a canopy would take away more stability from a tractor than loaded tires would add, and this proved to be true. The other interesting thing was that the lower CG of 75%-loaded tires compared to wheel weights was offset by the limited volume (weight capacity) of the tires.

    What I calculated was that adding the canopy raises the tractor CG 2.8 from the base height of 24. Loaded tires lower the CG by 1.6 from the base; the effect of both the loaded tires and the canopy is to raise the CG by 1. (You can not just subtract the 1.6 from the 2.8 because the mass is different for each of the calculations) It is interesting that the tractor with both canopy and loaded tires has less sideways stability than a tractor without loaded tires and without a canopy.

    The other interesting result came from comparing loaded tires with wheel weights. The loaded tires have a lower CG than the weights since the top 25% of the tire is left unfilled. As mentioned above they lowered the tractor CG 1.6. The wheel weights have a CG that is the same height as the axle. My wheel weights at 165 lbs each side, are heavier than loaded tires. The effect was to lower the tractor CG the same amount as the loaded tires did, although there would be an advantage with the weights for traction and counteracting loader weight.

    The last thing I compared was some operator weights. I looked at 3 different operators (or one that has been dieting) weighing 160 lbs, 200 lbs and 250 lbs. with their CG 10 above the seat. Compared to no operator in the seat, the tractor CG was raised by 2.3 for the 160 lb operator, 2.8 for the 200 lb operator and 3.4 for the 250 lb operator. A 250 lb operator with loaded tires has a 0.3 higher tractor CG than a 160 lb operator without loaded tires.

    Andy


  2. #2
    Elite Member
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    Mar 2000
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    Eastern Virginia
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    EarthForce EF-5 mini-TLB (2001)

    Default Re: Tractor Stability

    Interesting info, Andy. I think I'll go ahead and start that diet. Of course, I had also planned to start working out, so I'll have to cancel that plan, since muscle weighs more than fat. Maybe I'll take up marathon running instead and go for that emaciated look. Of course, that reminds me of Jim Fixx, which brings up the question: "Do I want to die from a rollover or a heart attack?" Decisions, decisions...

    I wonder how much wheel offset and tire width factors into rollover considerations, compared to CG changes. Any ideas?

    By the way, while we're on the subject, here's something for the "how stupid can the world be" file? I won't mention any names, since I don't have permission, but I know for a fact that the PR folks from a very large tractor manufacturer vetoed the idea of putting inclinometers on their tractors because it would send a message to the public that they were having trouble with rollovers for their brand vs. the other brands. They said the only way they would do it is if the other major brands did it at the same time. Interesting theory: "Never mind how many of our customers we lose to death by rollover, let's just make sure we don't lose any sales." Reminds me of the situation when ROPS were first being developed. When the same manufacturer was approached by an engineer who had designed a ROPS, the PR folks vetoed putting them on their tractors for the very same reason: it might send a message to the consumers that their product was inferior. Then the government got involved and mandated them, which put everybody in the same boat, and made the issue a moot point, but the rollover deaths that occurred during a several year period were solely due to the fact that the PR folks were worried about their image. It didn't seem to occur to them that they might be able to pitch it the same way Mercedes has pitched their pioneering in airbags, unibody construction, etc. It just reminds me of the saying: "The major difference between genius and stupidity is that genius has its limits."

    Mark


  3. #3

    Join Date
    Apr 2000
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    1,490
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    Hico, Texas
    Tractor
    Kubota M6800SD/LA1002 Loader Kubota RTV900

    Default Re: Tractor Stability

    Hmmm....

    My tractor weighs a tad less than 8000#. 1500# of that is water/antifreeze (non-toxic). I would be willing to go on a diet if it would help the stability of the tractor. I have had a long debate about a bigger straw hat or a steel canopy. [img]/w3tcompact/icons/laugh.gif[/img]

    Best I recall the calculations I made, reducing the tire centers by using 24 inch instead of the stock 30 inch wheels did exactly the same for stability as spreading the rear wheels (and fronts) out by 2 inches a side. So I did that instead. The weight of a steel canopy is considerably offset by ballast in the tires. Just buying cast wheel centers on the M6800 would add another 600# to the tractor weight plus wheel weights if I needed even more ballast, but it is heavy enough now.

    I also have my front tires filled and they were nearly 200# a tire in ballast weight. If your tires only hold 100# of water (12 gallons) each then they are not very big tires so extra wheel weights would certainly be beneficial for you.

    I suspect there is one gross error in your calculations. The CG is pretty low on most compact tractors and if you stand back and look where 8 inches above the axle would be, that seems way too high for many compacts. Probably ok for a big Massey, though. Guess you calculated stability the same way I did, with the tractor on a side slope and you tilt it until the cg falls in line with the center line of the downhill tire. I think it would be more representative to calculate the difference in critical angle with and without a canopy - and that should be a pretty small angle. Another way would be to show how much the rear tires have to be spread to account for the weight of the canopy.

    Many of the things that help stability include wide rear tires and large front tires as they don't fall in small holes and cause the tractor to upset. 4WD gives you a lot of control when you are stuck in a ditch and need the traction to get out without increasing the angle of the tractor any more with the front wheels unable to get out of the ditch on a 2WD tractor.


  4. #4

    Default Re: Tractor Stability

    Yes but you should have mentioned the important fact that if you buy an old 35,000 lb track loader you can still have your twinkies because your weight will only effect the CG .0001% [img]/w3tcompact/icons/smile.gif[/img]


  5. #5

    Join Date
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    Kubota M6800SD/LA1002 Loader Kubota RTV900

    Default Re: Tractor Stability

    Yeah, I started out to buy an old Case 310 tracked loader and a tractor. The small case only weighs about 7000#. Finally decided to compromised on a M6800 tractor to use for both the tractor jobs and the loader jobs. It ain't no dozer, but have done ok on most jobs. The loaders that are removable just don't look as strong as those that are not removable (like the L35).

    Still need to get in and clean out my tank. It is 135 foot in diameter and not sure whether I can do as well as just getting a dozer in to get all the silt out of the tank and shape up the walls again. It is only 9 feet deep and that is set on solid rock. Next I have to put a couple of truckloads of clay in the bottom to seal it. It has never held water very well and is about dry again.


  6. #6
    Elite Member
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    EarthForce EF-5 mini-TLB (2001)

    Default Re: Tractor Stability

    Look Del, if you're going to contribute to technically challenging discussions like this one, you need to get your facts straight: the correct figure is .00009 %, assuming you were talking about the minimum number of twinkies that it takes to qualify as plural, two. And, also assuming the trackloader does indeed qualify as "old" with between 50% and 60% U/C.

    Mark


  7. #7

    Default Re: Tractor Stability

    Wen,

    The comments I made were relative to the small compact tractors that many people here have. While they may be relevant for B-series owners, the additional weight of the larger L and M-series makes the canopy and operator weight a much smaller factor in the equation.

    I still think the tractor CG location I used is as good a guess as any. At that height, the mass of the engine, part of the wheels and transmission, ROPS, sheet metal, etc. is above the CG and the frame, driveline, rest of the wheels and transmission, etc. is below.

    I calculated center of gravity not tip over angle as you did. While center of gravity is directly related to stability, there are even more variables in the stability equation. For example the NASD guide draws a line from the outside of the front wheel to the outside of the back wheel. This is the stability baseline they say that the tractor will pivot on, and when the center of gravity gets vertically past this baseline the tractor will tip over. According to NASD, this line for a tricycle tractor is much closer to the centerline of the tractor than on a wide front-end tractor. (The tricycle tractor baselines form a triangle and the wide front-end tractor baselines form a rectangle.) I am not sure I agree with that assessment, because the front axle, at least on my tractor, pivots side to side. The axle pivots so the wheels can follow the ground contour. The effect of the pivoting front axle on stability would be the same as the having a tricycle tractor until the front axle pivot reaches the maximum travel. When the axle pivot reaches the maximum, the baseline would shift further out, but the tractor may already have gained enough momentum to tip over any way.

    You can see that calculating the tip over angle would be much more complicated than determining the center of gravity. The tip over angle would be dependent on the location of the CG front to back as well as side to side. Implements and the location of the implements, wheel spacing and many other factors come into play. If a tip over angle was quoted, someone could get hurt thinking it was the correct angle for their situation. Tip over angle is for a tractor that is not moving and the movement of the tractor, implement and operator can change things significantly. I chose to compare CG because it can tell the relative stability between two different configurations, with all other variables equal, without giving anyone a false sense of security.

    Andy



  8. #8

    Join Date
    Apr 2000
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    1,490
    Location
    Hico, Texas
    Tractor
    Kubota M6800SD/LA1002 Loader Kubota RTV900

    Default Re: Tractor Stability

    Yeah,

    I don't disagree with your analysis. I did the cg thing, too, but calculated the tip over angle because every time I have wondered if the tip over angle had been reached, I was stopped or very close to it with a wheel burried in the dirt or ditch. Using the tip over angle gives you a much better appreciation for spreading out the wheels of the tractor and using as large of diameter tires as you can. You can usually gain much more for stability in spreading out the wheels than you will decrease it by raiseing the cg with an operator or canopy.

    The crankshaft is usually not too far from the centerline of the rear axle and has a lot of weight below it. The oilpan on my Massey weighed over 120#. The front axle casting was over 100#. Somehow most tractor manufacturers have fortunately put a lot of the tractor weight at or below the axle. On my Massey, you straddled the transmission. On my kubota, you set a little higher and it is really nice.

    The main thing that determines cg of the tractor is the need for ground clearance and large wheels. Most tractors that have 24 inch wheels or smaller, have pretty low cg and pretty low ground clearance.

    A tracked loader is a really neat compact tractor without a 3 point hitch! It is quite stable and has a low cg. [img]/w3tcompact/icons/smile.gif[/img]

    There is a reason they no longer sell 3 wheel Honda off-road cycles!


  9. #9

    Default Re: Tractor Stability

    This is all fascinating, but does anyone have a clue what the maximum angle is that
    I can take without killing myself or my beloved L3600?

    The answer to this question has been bugging me ever since I bought a killer finish mower (Bush Hog ATH900 (1100+lbs) and was cutting several sloped areas on my property. I have an L3600 4wd with a LA680 loader, Canopy, and the mower. The tires are water filled AG tires and are the 14.9-24sz. They are fixed at their widest stance, which my manual says is 60.8" The front tires (no water) are the 8.3-16sz and are factory fixed at 45.5". My well fed carcass adds another 230lb.

    I have been puzzling over the effect of the loader (carried low) on the CG and whether I could further lower the CG by raising the 3point a little to add the low-slung weight of the mower to the tractor
    weight (rather than letting the mower roll on its 4 wheels). I am not smart enough to know intuitively where the weight is acting with respect to the CG: Is it at the height of the support/lift points, or at its true location? Am I better off removing the loader or leaving it in place and carrying it low?

    Any insight, rules of them, or flat out guesses, simple tests, etc, you can offer are welcome. Also, does anyone know of any static test data (mfr data?) that might be available? I understand real life is not
    static, but it would be a start.

    Confused on the hillside,

    Don


  10. #10

    Join Date
    Apr 2000
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    Kubota M6800SD/LA1002 Loader Kubota RTV900

    Default Re: Tractor Stability

    Anything that you carry on the 3 point that is below the CG (and stays below the cg), and that is probably everything you carry on the 3 point, makes the tractor more stable in most conditions. Stability is determined differently if you are going up the hill, down the hill or across the hill. Across the hill is NORMALLY the worse of the three. The mower is VERY heavy with respect to the tractor weight and is probably heavier than kubota recommends for your tractor (this is listed in the owners manual recommended implement specifications.) Going down the hill, the mower must not be allowed to jack knife. Going up the hill, the mower tends to make the tractor tippy unless the loader balances the mower and the loader must be kept low to the ground.

    My experience is that I get very concerned about the stability before there is any problem. Always wear your seatbelt and keep the ROPS locked up and go very slow in any condition that you think might be a problem. Search this site and the archives for stabilty and for tiltmeter and you will find some numbers that others have determined for their tractors, including a picture of one that tumbled down a long hill.

    You have done the right things so far. Your tractor cg is pretty low with the 24 inch tires, they are loaded with water, and you have the tires set to the maximum width. However; I think your mower is too heavy for your tractor when used on steep slopes.


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