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Thread: Need help on figuring out weight distribution when adding a third axle to truck.

  1. #1 Need help on figuring out weight distribution when adding a third axle to truck. 
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    We have a truck that weighs 20,000 lbs, 8,100 on the front and 11,900 on the rear. The wheel base from the front axle to the rear axle is 272". The center of gravity is 161.84" which we found by taking 272" * 11,900 / 20,000 = 161.84".

    What would the load be on each axle if we add a third axle that weighs nothing (for the sake of keeping it simple) at 220" that is an 'independent non-load sharing axle'? Meaning its just like the original rear axle and not like a normal tandem setup that shares (splits) a load evenly between the two.

    Any insight into this is much appreciated,

    Thanks.



    p.s.

    To get more into it what would the loads be on each axle when we add a payload of 26,000 lbs at 200"?


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  3. #2  
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    I don't think there is a single answer to the question. If the springs on each axle were equally stiff, then the truck would tend to pitch forward, because there are two axles lifting the back, and only one in the front. If you made it sit level by putting lighter springs on the rear, there are still many combinations that would result in different loads on each axle. The load distribution would also change as you are driving. If you drive over a dip in the road, that will put more load on the front and rear axles and less on the middle. If you drive over a hump. it would tend to lift the rear axle off the road.


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  4. #3  
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    Hi thanks for inquiring. There's got to be a way to answer this question. I'm not trying to figure what happens to the axle loads while driving i'm just talking about sitting still and level on a scale what would each axles load be. I don't think the springs have anything to do with the load exerted on an axle I think springs are more for making the ride smoother, but maybe im wrong its kind of hard for me to think about. However if that was the case lets say a truck had no springs and it was just a box sitting on top of two axles. Then we add the third, there ought to be a way to calculate how much load each axle would now take on.
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    Okay, let's take moments (i.e., torques) about the front axle. Imagine that the front axle is on the left, and the rear axle is on the right. The weight at the center of gravity is 20,000 lb, and the center of gravity is at 161.84 inches. That's a moment of 20,000*161.84=3,236,800 inch-pounds in a clockwise direction about the front axle. Opposing this we have two moments, Wm*220 and Wr*272, where Wm is the weight on the mid axle and Wr is the weight on the rear axle.
    Wm*220+Wr*272 = 3,236,800

    That's one equation with two unknown variables. There is not a unique solution.
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  6. #5  
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    I see where you're coming from, but I can't fully wrap my mind around this I don't know why. But let me redirect here.. What if three men were carrying a boat over there shoulders, guy #2 is 220" away from guy #1, and guy #3 is 272" away. Same setup, wouldn't guy #2 be holding more of the weight since he is closest to the center of gravity?
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  7. #6  
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    Quote Originally Posted by rlpjr View Post
    I see where you're coming from, but I can't fully wrap my mind around this I don't know why. But let me redirect here.. What if three men were carrying a boat over there shoulders, guy #2 is 220" away from guy #1, and guy #3 is 272" away. Same setup, wouldn't guy #2 be holding more of the weight since he is closest to the center of gravity?
    That's a good example to illustrate the problem. Guy 2 can slouch a little and carry no weight at all, or he can put forth some effort and carry more than his fair share.

    Going back to the situation with the truck, you could have the mid axle just touching the frame of the truck, but not supporting any weight. Then you could put a shim in between the axle and frame, and that would put more weight on that axle. You could keep shimming it up as much as you want, until you lifted the rear axle off the ground.
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  8. #7  
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    When I worked for Sears, my co-worker Rudy and I often had to carry a long, heavy extension ladder out of the building, and set it up outside. The first couple of times it worked: I grabbed one end and told him to carry close to the middle, it would go around the corners easier that way!

    Tried the same idea on my wife: bad move!

    Off thread. Sorry! joc
    Last edited by jocular; August 28th, 2013 at 12:21 AM.
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