Truck towing capacity: Weight Vs. Drag

Discussion in 'Off-Topic' started by strantor, Dec 10, 2015.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    My truck is rated to pull about 8,000lbs. Without pulling any trailer, I can go 70mph and I'll get ~19mpg. I've pulled about 7,000lbs with it on a flatbed trailer, and the truck pulled it just fine, shifted as it should in tow mode and my mileage dropped to ~14mpg @ 70mph.

    Recently I was going to pull my company's enclosed box trailer to a job in Louisiana. I didn't think it would be a problem since the trailer was nearly empty (trailer + a few items inside = ~4,000lbs) but my truck did a terrible job of pulling it. I got 50mi. down the road and called for a hot shot driver to take it the rest of the way because I was afraid I would damage my truck. I couldn't get above 8mpg, and that was keeping the speed under 60mph. The truck would shift between 3rd and 4th gear over and over, RPM going up to 3,500 just to maintain 55mph.

    recap:
    no trailer, 0lbs, 70mph, 19mpg, perfect performance
    flatbed, 7,000lbs, 70mph, 14mpg, perfect performance
    box trailer, 4,000lbs, 55mph, 8mpg, poor/potentially destructive performance

    Obviously the problem was wind drag. Dragging the flat front of the box trailer through the air was like dragging a braking parachute same as a dragster race car does. Even though the box trailer was much lighter, it was much harder to pull through the air.

    I cannot find any reference to drag in the documentation for my truck, or online, which strikes me as odd since my experience indicates that it is more important than trailer weight. Everything I can find about towing, talks only about trailer weight. My truck is rated to pull 8,000lbs, and on a flatbed trailer, I have no doubt it could pull that; probably even double that, if the load were say a 16,000lb sphere of solid iron on a tiny trailer that didn't have any appreciable wind drag. But put that sphere inside a box trailer, and I'm dead in the water.

    If I were to install a load cell in my trailer hitch and feather my accelerator to keep the readout always below 8,000lbs*, I theorize that I could probably pull massive amounts of weight, with very slow acceleration and limited highway speed (assuming the trailer had brakes capable of stopping its own load). I theorize that on flat roads, I could probably pull even more still. I theorize that the manufacturer's towing capacity rating is just a "shoot from the hip" guesstimate and really isn't real-life applicable.

    So why is wind drag and incline excluded from manufacturer's documentation and why does nobody talk about it? Aside from retrofitting my trailer hitch with a load cell, how can I calculate the load my truck is actually able to pull, given the frontal area of the trailer?

    *EDIT: I wouldn't keep the readout below 8,000lbs, that would be much too high; like pulling an 8,000lb trailer straight up off the ground. I would need to find/calculate the proper readout that matches what the pull force is for a trailer that weighs 8,000lbs. But then we're back to "what kind of trailer?" a flatbed with no drag? a box trailer? what?
     
    Last edited: Dec 10, 2015
  2. atferrari

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    Sorry strantor; what is "hot shot"? Dictionary does not help here.
     
  3. atferrari

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    Wind drag, AFAIK is calculated basis a certain surface, which implies volume but not height or width (and shape). How could they guess on what you would pull one day or the other in the future?
     
  4. ISB123

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    May 21, 2014
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    You need a manual gearbox;).

    Pickup trucks arent exactly made to pull box shaped trailers since they dont have a proper aerodynamic shape like a semi truck has.
     
  5. alfacliff

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    Dec 13, 2013
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    the weight a trailer puts on the trailer tounge. is deetermined also by the axle position. move the axle foreward, and the wieght on the truck decreases, to the rear and it increases. there is no way to specify the wind drag of a trailer or load, since loads vary so much. the trailer load capacity of the truck is only based on the weight of the trailer plus load.
     
  6. strantor

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    It is a courier, or "delivery guy" who delivers large objects in a hurry. They typically use pickup trucks, with trailers if needed.
     
  7. strantor

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    The manufacturer recommends 10-15% tongue load. Are you saying that the towing capacity rating of the truck is based on the weight placed on the truck suspension by the trailer?
    If so, that would make sense; 10-15% of an 8,000lb load is 800-1200lbs; same as the cargo rating for a 1/2ton truck.
     
  8. strantor

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    My reading in the past left me with the conclusion that it's based on cross-sectional area; this is why aerodynamic things like planes and rockets are long and slender, to minimize cross sectional area.
    They couldn't that's why I question how they arrive at this "towing capacity" rating. I tried pulling a trailer that was well below the rated capacity, and it was way too much for the truck.
     
  9. wayneh

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    Sep 9, 2010
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    I think drag and incline are not talked about much for one simple reason - it's too complicated. When a mfg says you can tow 5,000lbs, I'm pretty sure they mean within certain reasonable ranges of speed, incline, braking distance and so on. They don't want to specify all those caveats or disclose a derating table. After all, you could tow thousands of tons if you don't expect to stop within X feet, accelerate at X g's and so forth. Like in those truck commercials where they tow a jumbo jet or a train.
     
  10. BR-549

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    Sep 22, 2013
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    Those numbers are curious to me. I would first check the bearings and brakes on the box trailer.

    Then preform another test to verify that yours.....wasn’t a fluke.

    Were you driving against a weather front?
     
  11. Brownout

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    Jan 10, 2012
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    I agree. This is very suspect.
     
  12. Brownout

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    That's part of it. There are a number of specifications that must be satisfied when determining the maximum trailering capacity. What you are referring to above is determined by the payload, which is in turn determined by the vehicle's GVWR. The payload is the GVWR - curb weight of the vehicle (and minus passengers, fuel and such). But there's also a figure that is hardly discussed: the GCWR. This is the total weight of the vehicle, trailer, passengers, fuel, cargo, etc. This is more determined by the engine power and breaking ability of the vehicle. The maximum trailer weight is chosen as to not exceed either of these figures.

    When sizing vehicles for towing, the GVWR is always available. The GCWR is much harder to find, in my experience.
     
  13. strantor

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    I don't remember the weather. The bearings had been replaced before I took it out (perhaps a botched job?). I asked the hot shot driver what kind of gas mileage he got pulling that trailer to LA and he said it was typical, maybe 1mpg less than what he gets not pulling a trailer; he was driving a huge lifted Dodge 3500 dually that probably blocked 90% of the wind hitting the trailer.
     
  14. Brownout

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    As you can see, I have a pretty big profile going into the wind, but wind resistance has never been a factor.
    [​IMG]
     
  15. strantor

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    Your van is an e350 Diesel IIRC?
    Diesels are a good choice for applications demanding high torque. Typically they have lower gear ratios, too.
    As I said the hot shot driver drove a diesel; that trailer didn't put a hamper on him like it did me.
    My truck has decent horsepower (345hp 5.3L vortec) but it spins twice as fast as a comparable HP diesel, which would mean that it produces half as much torque to achieve the same HP.
    Couple that with the fact that my truck has highway gears (I have gotten up to 27mpg in it, tank average over 500mi) and it adds up to a truck not really suited to applications demanding a lot of torque.
    It probably had to exert itself much much harder than the diesel. (well, obviously)
     
  16. wayneh

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    This may be useful.

    towing-gas-mileage-2.jpg
     
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  17. Brownout

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    I have the 5.3L Vortec, but rated at 240HP.
     
  18. Brownout

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    That was my previous rig.
     
  19. Papabravo

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    Feb 24, 2006
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    There is more than one type of drag. The one you are interested in is called "form drag" because it is proportional to the area of the object moving through the flow. There is also "skin friction drag" which is included in the equation for form drag. For a lifting surface, there is another kind called "induced drag". Since the trailer is not producing lift, you don't need to worry about it. The components of the drag equation are summarized here:

    https://en.wikipedia.org/wiki/Drag_equation
     
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  20. Lestraveled

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    May 19, 2014
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    The trailer could be getting ready to loose a wheel bearing. Touching the wheel hubs will tell the tale, warm is OK, very hot is not.
     
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