I am designing a 4'x12' trailer frame. 8' of it will support weight, the balance will be the tongue. My area of concern is bending at the ends of the long supports. There will be a wheel assembly located 4' from the frames end that will support all but the tongue weight of 100 -120 lbs. I plan to use commercial steel tubing so the moment of inertia ($I$) and Section Modulus ($Z$) are known quantities.

What I need is a formula that is suited to this design - ie. something that will tell me how much the frame might bend under load. I have some knowledge of simple design as well as an AISC and Machinery's handbooks

Its going to be a 4' x 8' with a 4' tongue, the tongue weight should be 50 to 80 lbs. I'm designing it to keep the weight below 900 lbs. I want it to have 15" wheels. I'm using components for a design weight of 2000 lbs. and plan on using axle-less wheel hubs. There will be a close to a uniform load spaced over the wheels. The wheel assembly will be off center to improve tracking.

I'm looking for a suitable "Beam Diagram" to ensure I stay within a reasonable deflection. I have seen them designed with 2" x 2", and 2" x 3" tubing.

  • 2
    $\begingroup$ Do a google search for trailer plans and find one that best fits your needs. No need to completely design something yourself when plenty of others have already done it. $\endgroup$
    – diceless
    Commented May 1, 2015 at 17:17
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    $\begingroup$ Can you add a sketch of your basic design? It doesn't need to be perfect, but it will help to give users a better sense of the dimensions and design. $\endgroup$ Commented May 1, 2015 at 17:49
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    $\begingroup$ Also try Autodesk Force Effect (free) to solve various static problems. $\endgroup$ Commented Apr 15, 2016 at 14:12
  • $\begingroup$ Can you give us a diagram of the locations and magnitudes of the moments, forces, and supports? $\endgroup$ Commented Apr 10, 2017 at 21:20

1 Answer 1


Had a similar design need. Found a free software tool called Beamboy2.2 With the engineering data you have this simple piece of software can be a great testbed for your solutions.

The software accommodates cantilever beam setup so you can test specifically for the deflection of the steel based on your specific loading conditions.

The link I downloaded from is http://www.geocities.ws/richgetze/beamb22.zip

Basically I set up a beam model that goes from hitch to end of frame. Representing one support rail so halve your loads. I design at 2.5 times actual loads to accommodate the effects of bumps and dips. 2.5 is what the better commercial shops use.

I use $\ell/300$ as my deflection limit measured from the support at the axle to the end of the frame.

I suppose for liability I should wave and scream that after you use this to see what works and what you want to build you must take it to a real live engineer. I am not one. Nor is this software. But the math seems accurate and I'm sure you'll find it useful.

  • $\begingroup$ Thanks for this! I wish that I had this program when I was going through mechanics of materials! $\endgroup$ Commented Apr 10, 2017 at 21:18

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