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How to figure the stress on a pulling truck frame?

  1. Jan 4, 2016 #1
    I'm in college working towards a degree in Mechanical Engineering, so far I've only made it through physics 1 and calculus 2, but I've been building race cars and pulling trucks and tractors since i could hold a wrench. I work for a guy that builds super modified four wheel drive pulling truck frames. I've recently downloaded auto-cad inventor and am slowing learning how to draw frames. So I was wondering if anyone could help me figure out how to add up the different forces, so I could run stress simulations on the frames. Any help will be greatly appreciated, thanks.
  2. jcsd
  3. Jan 5, 2016 #2


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    That's what your degree is for.

    Somewhere in your ME curriculum, there ought to be a course in statics, which will teach you about forces and moments, and following that, you should be taking a course in elementary strength of materials, where you will learn how forces and moments applied to things like beams create stresses in the material. After taking these two introductory courses, you may take more advanced courses on analyzing structures which are more complicated than simple beams.

    That's at least three semesters of work right there.
  4. Jan 5, 2016 #3
    Statics is in my schedule next fall. I know I'm probably going way beyond my math abilities right now, but if narrowing down a specific area I need to work ahead too, then that's what I'll try to do.

    Well what I was thinking is since I'm lacking in those classes, if I could find the torque force on the hitch, engine mounts, transfer case mounts, and on the front and rear ends, then I could input the amount of torque and direction on inventor and let it do the structural analysis. While it's a lot more complex and has a lot more parts, to me it would seem to fall into what we did in physics when we did torques, and rotational motion. Is this right or is that what is coming in future classes? Knowing what the engine produces, isn't there a way I could chase that torque through the drive line, knowing all gear and shaft weights, diameters, tire speeds, ground speed, etc.
  5. Jan 21, 2016 #4
    The torque on the frame is going to be your driveshaft torque, there's nothing else working to twist the frame... Engine torque isn't very useful here since we need to know what it is actually producing rather than what it is capable of producing.

    You need to know how much pull the truck is generating on the hitch first and foremost, from there, with the rear end ratio and tire size, (Lets start with a 2wd truck).. All the torque goes up to your transmission, gets multiplied or divided, and that is the torque the engine produces.. In most designs, the engine mounts take 100% of the torque, and the transmission mounts are just there to support the weight.

    So with a 32" tire, and 8000 lb hitch pull, that is a 16" or 1.25 ft radius, and the wheel torque is 4000x1.25 = 10000 ft lb. If you have a 4.56:1 axle ratio, then you have 10000/4.56 = 2200 ft lb driveline torque.

    Horsepower is a little trickier.. which number do you want? The horsepower actually used to move the sled is (hitch pull (lbs) x Speed (f/s) / 550)... Lets say 20ft/s and 8000 lbs = 160000 lb ft/s = 290 hp actually used.
    The horsepower the truck is making will be different because of wheel spin, So we've still got 8000 lbs on the hitch, but we're turning the tires at 50 mph (73fps).. so the same calculation applies, 73x8000/550 = 1062 HP (772 HP worth of heat applied to the tires and dirt.. Efficiency of 27%)

    The torque the engine is making can be figured out by the transmission ratio x driveline torque (in a 1:1 transmission it would be 2200 ft lb), and if we don't know that, we can work it backward from the horsepower.. Torque = HP*5252 /RPM... Lets say the engine was running at 4000 RPM, that means torque was 1062*5252/4000 = 1394 ft lbs

    So with 2200 ft lb driveline torque and 1394 ft lb engine torque, we can estimate transmission ratio to be about 2200/1394 = 1.6:1

    I haven't accounted for friction here, but you get the idea

    In a 4WD, to get frame twist, etc you're going to have to do some more involved math.. If the sled is pulling on the hitch level and not adding weight to the rear of the truck, the total weight of the truck will stay about same, just the weight distribution will change.
    With the previous example, lets say we still have 8000 lbs hitch pull, but this is just the portion due to the rear tires.. the truck weighs 5000 lbs in front and 3000 on the back, and has a 10 ft wheelbase.
    So we already figured out the wheel torque to be 10,000 ft lbs earlier, scaled to 10 ft is 1000 lbs of unweighting of the front end, this weight has to go to the back, so the truck is now balanced at 4000 lbs front and back.

    I can't help you much with the detailed analysis of frame twist, etc.. pay attention to which direction the forces are acting, some may cancel each other (if both front and rear driveshafts turn the same way the truck will want to roll to a side.. if they turn opposite, the truck will stay level but twist the frame harder.
    You can always go from HP->torque->force, just pick what you know and work your way around
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