Frame Design Help

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Frame Design Help

Postby RunnerDuck » Tue Sep 15, 2015 7:51 pm

I'm back from vacation and going to get serious about designing and building my TD. It's going to be a 5' X 10' classic design.

First step I would like to build the frame. Here is what I have so far.

I'm planning on using 2" X 2" 3/16" steel tube for the cross members and the angle members in the front. I think I'll use 2" X 2" 1/8" for the side rails and the tongue.

Here are two views I'm working on. One is obviously the trailer and the other is the frame with dimensions.

Any comments would be greatly appreciated.

Thanks,

KennyB
Chasis Perspective early on.jpg
Chasis Perspective early on.jpg (44.8 KiB) Viewed 670 times

Frame Perspective Dimensions.jpg
Frame Perspective Dimensions.jpg (35.63 KiB) Viewed 670 times
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Re: Frame Design Help

Postby angib » Wed Sep 16, 2015 9:07 am

The first two cross-members, that the tongue attach to, are important structurally so using tube for those is good. The other three are mainly there to hold the side rails in place while you are building and do very little (assuming you are not going to host a barn dance for short people inside the trailer), so angle is fine for those - but tube won't hurt.

You do have some overlap between the angled tongue members and the side rails, but not a lot. Going for at least 10" of overlap would be better, to reduce the stress on those welds.

2x2x3/16 is a bit stronger than 2x2x1/8 but not by a lot - 20% extra strength for 50% extra weight (and probably cost) is a poor deal. If you need more strength than 2x2x1/8, and I'm not convinced you do, then 3x3x1/8 is a better step up.
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Re: Frame Design Help

Postby RunnerDuck » Wed Sep 16, 2015 9:38 am

Thank you for those comments Angib, I appreciate them.

I would really like to say with 2" X 2" for the tongue. What would you think about 2" X 2" X 1/4"? I'm thinking about putting a group 27 battery and small propane tank on the tongue.

Do you think the extra thickness on the side rails is necessary or could I get away with 1/8"?

Thanks,

KennyB
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Re: Frame Design Help

Postby angib » Thu Sep 17, 2015 7:17 am

Aaargh! 2x2x1/4 is a useless tube for bending loads, which is what tongues experience. (I'm sure there are plenty of uses for it in trusses, carrying mainly axial loads.) Men do have this faith in thick steel but while it may sound manly, it ain't clever.

If you need something just a bit stronger than 2x2x1/8, then 2x2x3/16 may be useful, even if it is inefficient for reasons of weight and cost.

But really you need to work out what tongue strength you need and then choose tongue tubes that provide that strength. See my tongue strength page in the 'Design Library'.
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Re: Frame Design Help

Postby RunnerDuck » Thu Sep 17, 2015 9:28 am

Thank you, it's looks like 2" X 2" x 1/8" will work.

Would you please explain one thing for me. Why does "Braked" have a lower capacity than "Unbraked"? It seems to me that if your trailer has brakes it would relieve some of the load when breaking and have a higher capacity number.

Thanks,

KennyB
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Re: Frame Design Help

Postby angib » Thu Sep 17, 2015 9:42 am

The braked/unbraked distinction is just copied from the International Standard ISO 7641 that I based this spreadsheet on. I think the logic is that a braked trailer will try to pitch nose down when the brakes are applied, putting a bending load on the tongue, whereas an unbraked trailer will just push forward on the tow vehicle which doesn't put any great bending load on the tongue.

The reason that the hitch eccentricity is required as an input is becasue if the coupler is above the tongue, even an unbraked trailer will try to bend its tongue during braking, becasue the push and the reaction are not in line.
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