Does overly biased tandem weight add drag?

Discussion in 'Experienced Truckers' Advice' started by Pmracing, Nov 10, 2017.

  1. Pmracing

    Pmracing Road Train Member

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    Hello,
    It's been a while.

    A driver claimed that heavy tandems compared to balanced tandems/drives or heavier drives makes it easier to climb hills, better on fuel mileage. Being a guy interested in the physics of things I am curious how this could be? Below is his post. What do you think?


    Lately I've picked up some loads from terminals and a few from swaps. It seems some people may not know that pulling a load with more weight on the trailer uses more fuel and doesn't climb as well. Some have been 11560 steers, 30400 drives and 33100 trailer. If you were pulling your kids little red wagon up a hill and it weighed more than you, you would expend more energy (fuel), because you (power unit) weigh less than the towed unit (trailer). If you (power unit) weighs more than the wagon (trailer) you would use less energy (fuel). Also, you could pull it up a steep hill easier/faster. So.. when possible the trailer tandems should weigh between 400 and 600 less than the drive axles. This may not always be possible due to bridge laws (ie Cal, FLA etc), however if you pick up a load in Georgia going to LA, you don't have to be at 40' til you get to California. Why drag 2000# extra on the trailer for 2000 miles? Stop at the state line and change it.

    Mikeeee
     
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  3. x1Heavy

    x1Heavy Road Train Member

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    It matters not.

    Gross weight versus a mountain, you have 4 forces. Horsepower, torque for forward versus total vehicle weight modified by wind conditions, traction problems if any and the specifications of the tractor. There are thousands of combinations of ratios etc. (God help us all when they begin to make variable infinite transmissions big enough to take class 8 duties...)

    And there is the drag that is created by the passage of the vehicle through the earth's atmosphere, severity of the upgrade and the tires contact with the pavement and so on.

    It does not matter where you put the weight.

    I say this...

    I absolutely hated the 20 foot seacan with 60K in the box. The #### thing will lay down on a simple 35 mph curve if you let it.

    The Ravens would allow 40000 pounds on the back spread axles. It still meets bridge law. It ran the same as it would at 34 and 34 etc on tandems. In fact if you piled too much on the tractor drives more than what they are built for total weight at the factory then you will hurt the tractor.

    Anything else is down to behavior on the road which varies in small ways not worth mentioning here.
     
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  4. TripleSix

    TripleSix God of Roads

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    I agree with him. I prefer the drives to be heavier than the trailer. Not because of fuel savings, but because of handling issues. It may not mean much on dry road, but on wet and slick roads, you don’t want the tail wagging the dog. And traction is much better when you have weight on your drives.
     
  5. Pmracing

    Pmracing Road Train Member

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    Ya, I only have 6 years driving, only once in the big mountains out west. A few times in the east ranges before getting my loco Chicago gig. Balanced weight makes a smoother ride but I don't see how it could change fuel mileage.

    Mikeeee
     
  6. Ffx95

    Ffx95 Road Train Member

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    I'm no expert but I would think having more weight on the drives on slick conditions would allow less slippage when using the jakes. That's my guess again I'm no expert.
     
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  7. Pmracing

    Pmracing Road Train Member

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    The question is, "Does heavy tandems cause lower fuel mileage."


    Mikeeee
     
  8. ReeferOhio

    ReeferOhio Medium Load Member

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    I think with a closed tandem trailer more weight on the drives feels like the truck pulls better, don't know about fuel mileage. I will say with my spread I prefer it to be 2-3K heaver than the tandems (great ride) but I do feel as though the truck won't pull as strong. I used to pull a lot of local van loads of stamped steel in baskets and they'd always be loaded a little different every time, I always knew when to much weight was back of the trailer truck did not pull as strong. So I tend to agree it does effect how the truck pulls. I honestly never gave a thought to fuel mileage.
     
  9. Ffx95

    Ffx95 Road Train Member

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    Well the analogy you wrote had some flaws. You said something about the dad pulling the kids wagon. Thing is if dad was heavier he probably got stronger from holding his weight but the lighter person pulling heavier kids is weaker. In the case of the truck power is the same regardless because an engine can only put out a certain amount of force.

    Fuel pushes the pistons which turn the crank goes to the tranny then diff, amount of fuel used depends on the amount of force needed to pull. Amount of force needed depends on weight, resistance, velocity, and traction. I think the only thing that being heavier on one side does is affect stability.
     
  10. spyder7723

    spyder7723 Road Train Member

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    The physics its sound. Basically its easier to carry weight than it is to pull it. Just don't take it to extremes and load the truck axles to max while the trailer axles have very little load on them.
     
    Last edited: Nov 10, 2017
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  11. ReeferOhio

    ReeferOhio Medium Load Member

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    But if a truck puts that much torque out, is it loosing efficiency when the suspension expands due to light weight on drives compared to what it's pulling? Is that energy expanding into the suspension and not the driveline? IDK if that makes any sense, it kinda does in my head.
     
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