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2 square tubing

  1. Apr 4, 2007 #1
    2" square tubing

    Im not sure you all do this but I need some help from some smart people. Im want to build a rack on the back of my motor home to carry my motorcycles and ATV, I wil be carring a Honda Goldwing weighing about 800 lbs. I plan on using 2" square tubing with 1/4" thick wall and a 1 1/2" square 1/4" wall pressed inside the 2" this will extend beyond the reciever hitch about 5' and a 4'x 78" retangular frame with a 5/8" plywood deck. The reciever is a 6000# class hitch that I intend to add some more gussets and conections to the frame rails. In laymen terms what kind of forces will this 800# exsurt and do you think the 2' and 1 1/2'' tubing will carry the load? It wiil also have 2 more 1 1/2 square tubing conected to the reciever to make a 3 point conection. Thank you, Bill Sosebee
     
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  3. Apr 4, 2007 #2

    FredGarvin

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    Bill,
    Is there any way for you to provide a drawing of the layout? I think I know what you are planning, but I want to make sure.

    The one thing that comes to mind is not the bending moments in the 2x2 box tubing but the inserted material. That configuration tends to tear the ends of the box tubing (I have done this a few times). This depends on a few things, but just keep that in mind.

    If we can see a drawing that would really be a big help.
     
  4. Apr 4, 2007 #3
    Thank you Fred, I dont know how to send a drawing over the internet but I could fax one or could give you a call if i had you #'s Thank you bill
     
  5. Apr 4, 2007 #4

    Q_Goest

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    Hi Bill,
    You can attach a sketch directly to your post. Just look below the box where you write your message, and you'll find a button reading "Manage Attachments". Click on that, it'll open a browser window, and you simply attach the sketch to your post just like you were writing an email.

    I'd agree it would be easier to see what you're doing with a sketch, but I presume you're making a platform which is cantilevered off your trailer hitch. If that's what you're thinking, the quick answer is, it'll break. :cry: (too much weight)

    Next question would be how you could make anything cantilevered off the frame of your RV. You need a minimum of two bolt locations on each frame rail, similar to how the hitch is connected. They resist the moment of this cantilevered beam and also carry the weight. The farther apart they are (along the axis of the frame rail) the better at carrying the moment. Measure the distance between those holes and include it in your diagram. Also measure another point that might be used as a potential hole that's as far from those two as possible. The method of attaching this platform will be important.
     
  6. Apr 4, 2007 #5

    Danger

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    This is irrelevant given that it's a motorhome with a hitch rated at more than your anticipated weight, but others contemplating such an addition to different types of vehicles should also make sure that the torque about the rear axle won't lighten the front end too much for proper braking and steering.
     
  7. Apr 4, 2007 #6

    Q_Goest

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    Danger makes a very good point. The front wheels will get lighter and the back ones will need to support the weight not just of the motorcycle and platform, but also the additional weight that came off the front tires.

    To determine the weight removed from the front tires, you can determine moments around the rear axle. For example, if the CG of the bike/platform is 9 feet behind the rear axle, and weighs 1000 pounds, that's a moment of 9000 foot pounds. The amount of weight that must come off the front tires to compensate for this must then be equal to 9000 foot pounds. If for example, the front tires are 8 feet in front of the rear wheel, then 9000 foot pounds divided by 8 feet gives you 1125 pounds. That's how much weight will come off the front tires.

    Now the rear wheels must support not only the 1000 pound motorcycle/platform, they must also support the additional 1125 pounds that came off the front tires. Total added weight to the rear axle in this case is 2125 pounds. You should determine if the vehicle can handle that much added weight to the back axle. Both the tires and suspension need to be able to handle it. You may also notice the vehicle doesn't handle quite the same with the reduced weight on the front tires.

    Good catch, Danger! :smile:
     
  8. Apr 4, 2007 #7

    Danger

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    Thanks, Q. Thinking outside of the box pays off once in a while. :biggrin:
     
  9. Apr 4, 2007 #8
    Thanks for the help guy's. Ill measure the wheel base and the CG of the bike and its position behind the rear axel. With the 2" 1/4''wall square tube and the 1 1/2" 1/4" wall tube sleeved inside it and the CG of the platform 3' from the receiver what do you think my maxium load might be. I will also have a 1 1/2" tube 18" from center on each side of the main 2'' tube to help stablelize the platform these tubes will mate with their on 2'' square receiver . thanks bill
     
  10. Apr 4, 2007 #9

    Q_Goest

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    Hi Bill,
    - There are probably laws and industry standards for building what you're inquireing about. I don't know if there are in fact, but knowing how everything is carefully controlled, I'd be a bit surprised if there weren't. If the unit fails and the bike drops off the back and kills someone, you'd be at much greater risk with your handmade unit than if it were built by a corporation that did it to some standard. I'd suggest not building this yourself.
    - Regarding the trailer hitch, is that a class III hitch that's good for towing up to 6000 pounds? That hitch is good for a given verticle load, and it may even be rated high enough for the load you're planning on putting on it, but I promise you it's not rated for the moment you're about to put on it. It is rated for the maximum load times the moment arm which is only the distance out a conventional trailer hitch would extend, which is on the order of 6 inches, give or take. You'll need to look at the entire hitch, not just a platform.
    - The 2" with 1.5" tube inside won't provide any additional strength to speak of over the 2" tube alone unless the 'transverse shear stresses' can be resisted. To do that, you just need to weld the ends of the tubes together, assuming they are tight to begin with.
    - You need to consider what loads the cantilevered beam would see. The bike alone at 800 pounds plus 100 pounds of platform isn't enough. Add the 100 extra pounds, then consider verticle g forces. I'd suggest it will normally see up to 2 g's. I'd suggest then adding at least a safety factor of 1.5, and since you're doing this yourself, add 2.0 to yield.
    - If you can keep the CG of the bike/platform 3 feet from the hitch (verticle) your moment is:
    M=2*2*(800+100+100)lb *3 ft = 12000 foot pounds.
    That's the moment the 2" tube must resist without yielding.
    That doesn't consider fatigue which will increase the foot pounds of moment this unit needs to resist. Without doing any analysis of it, I'd suggest using another factor of 1.5. (18,000 foot pounds)
    - Stress is Mc/I.
    In this case, I = 1/12*(D^4-d^4) = 0.9114 in^4
    c = 1in
    So stress = 119,000 psi.
    You'ld need a very high grade of steel for that, and your trailer hitch still couldn't handle it.

    I'd suggest checking on laws governing this kind of platform. Check with your local police station first, maybe they could help, or a DMV in your area if there are any might be best.

    But even if there aren't any laws, it doesn't seem like it's physically possible to do what you want to do with any degree of safety.

    The only safe way to do this is to mount it directly to the frame.
     
    Last edited: Apr 4, 2007
  11. Apr 4, 2007 #10
    Q, Thanks for your time and the advise it is well taken. To eliminate the "transverse shear stress" i had planed to do some 5/8" holes in the 2" and button weld them 12" oc staggered to the 1.5" what is you thoughts on that?
     
  12. Apr 5, 2007 #11

    Danger

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    If Q's supposition of your hitch rating is correct, then I misinterpreted your original post. Where I live, that 6,000# rating would indicate tongue weight, not pulling capacity.
    You must also factor in shock loads, wherein the 'bouncing' effect under an oscillating situation seriously adds to the apparent weight of the load.





















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  13. Apr 5, 2007 #12

    Q_Goest

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    Hi Bill,
    Can you provide drawings of what you intend to do?
     
  14. Apr 5, 2007 #13

    FredGarvin

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    Sorry I took so long to get back. Thanks for picking this up Q and Danger. All are very good points. I immediately made the assumption that this was going to the frame. I have seen what Bill is trying to do but on much smaller scales. 800 is going to be way too much.

    Honestly from what has been mentioned here, the best way to transport the motorcycle would be an existing tow trailer made for bikes. I have seen them behind RVs quite a bit. Even easier and cheaper would be a single flat bed trailer. Around here you can find them for next to nothing. You would spend more than that in your time and revisions to designs, etc... Something to consider.
     
  15. Apr 5, 2007 #14

    Q_Goest

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    I gotta agree with Fred. The idea is entertaining, but it seems technically unsound and doesn't have any economic advantage over a trailer.
     
  16. Apr 5, 2007 #15

    Danger

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    Agreed. I think that the only way to support all of that weight would be to put wheels under the platform. If you're doing that, you pretty much need a pivoting attachment to the RV... leaving you with a trailer. The original idea is really nice in principle, but not physically practical.
     
  17. Apr 16, 2007 #16
    I didn't see it in the posts but I'd be more worried about the torsional stress. I think the square tube would hold the load static but I doubt a single point will last long before it twists like a corkscrew.
    I saw a mount on a jeep a few months back that came out either side of the 3" hitch cross piece. I did a quick drawing of it. The side pieces bolted to the tray. It didn't look like he had done anything to the hitch, just plugged it in.
     

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  18. Apr 16, 2007 #17
    Thank you for your reply, I think you telling me that you are concerened about the main tube twisting between the two flanges that attatch to the frame? I dont know how to send you a drawing of what I plan, but my thoughts are to extend the receiver tube on the same plane about 3 feet forward then attatch another cross member from frame rail to frame rail, also gussetts will be added to the existing cross tube to the flanges.
     
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