# Thread: Trailer Trashed

1. ## Trailer Trashed

In addition to the multi-purpose crane in my other thread, I'm building a trailer.

I know how much I'd like it carry, but don't have a clue about the dynamic(?) weight loads (how much the weight increases with each bump), or how to use what I think is the proper calculator.

"Calculation Supported on Both Ends Loaded Two equal Loads"

http://www.engineersedge.com/beam_be...flection_4.htm

This sorta looks like an axle with two places for leaf springs to push down on it.

A 3" round tube (has to be round) with .25" walls gives me an MoI of 2.

A load (W) of 3,000# sitting 4" (a) from the axle anchor point on a 78" (l) long tube with a neutral plane of 1.5" gives me a stress at all points of 9,000# (units?).

The above is based on (x) being 4" (same as (a)) and (v) being 39" (half of (l)). Mind you, I have no clue what "x" and "v" are, but the calculator drawing shows them to be about equal to "a" and 1/2 "l", respectively.

So, assuming I'm using the right calc, and assuming my inputs are correct, do I simply take the 9,000# stress amount and divide that by the number of square inches of mated surface between the spring plates and the axle tube, and compare that to the yield PSI of the choice metal?

Thanks,

Dave

2. "I know how much I'd like it carry, but don't have a clue....."

You either need an engineer or a 4 year course in engineering.

This forum is not a consulting engineering service.

3. "You either need an engineer or a 4 year course in engineering."

Actually, considering that my question was specifically about the output of the calculator, it turns out that I needed a 1st year community college physics student.

In fact, I only asked the question here because this is where the calculator is located.

The funny part is that the answer to my question was shorter than you writing to tell me that you won't answer it.

4. OK, my last answer

9000 psi

period.

You should spend some time in trying to understand how this works rather than blindly taking answers from graphs.

5. Originally Posted by zeke
OK, my last answer

9000 psi

period.

You should spend some time in trying to understand how this works rather than blindly taking answers from graphs.
Thanks: That matches the answer the 1st year gave me last night.

As for spending time trying understand this stuff, I actually do: Unfortunately, only 1st year physics students seem to being willing to teach me anything.

6. You’re obviously no dummy. You seem to be quick witted and bright, not to mention resourceful. Given this… you can surely see why this would be the case.

And of course you know that you can pick up a catalog and buy rated trailer axles in any imaginable configuration very reasonably priced… So I’ve got to ask you this… What’s the exciting twist to an axle you have planned for this next great adventure?

7. Unfortunately, I can't buy a 10" drop axle. Believe me that I would if I could. The largest anyone makes is a 4", because that is the "standard" for a 12" rim (a standard trailer rim). I could have one custom made, but if it's going to be custom made, I can do that myself for a fraction of the price.

The whole design is worked out, and 90 percent of the components are readily available, including the tube for the axle. The design is based on existing products, so it's not like I'm trying reinvent the wheel: Pretty much the only question left to answer is what size axle tube I need to be able to take the weight. I know the weight, I know I want a 3:1 safety margin, I just need help working with the (generously supplied) online calculators and understanding a few of the principals.

As Zeke points out, I could hire an engineer, but then I'd be no smarter then I was before the project. I could go to engineering school myself, but that would result in my being FAR smarter than necessary for the projects I design (not to mention 8 years older). If somebody wants to tackle their own car tune-ups, they really don't need to go to mechanic's school, they just need an afternoon of instruction. I will assume that my engineering questions are similar, since the lion's share of the design work was already done by an engineer, I'm just making some modifications.

Similarly with the engine crane: It's a tried and true design, I just want a longer arm, and would prefer to use actual math to determine the designs' new capacity, rather than just build one and see when it fails.

I asked the questions in this forum because, unlike someone saying "Here's what I want to do, design me something that will do it.", I don't believe that my questions would prove particularly taxing for engineers: The design work is basically done, I just need help with the math and a few concepts.

8. I like your approach wsdave, and I commend you for trying to round up the science behind your design. Well done also for your well tempered response to some not so constructive responses. 'Noblesse Oblige' - should be a bare minimum. I bet you've shared your knowledge and skills with others along the way, always a rewarding lifestyle.

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