Frame help

[droid_ca]

So I might be getting my frame welded up in the next few days (might of sold my boat) So that brings me to the question of is there some sort of formula to designing a frame chassis as I'd like to have an A frame front end on it and I'd like to go 7 feet wide by 12 feet long...now I'm stuck...please help

[CARS]

http://www.teardroptrailers.us/Trailer.html Has answered every question I've had. And any detailed explanation I needed I just asked on the forum.

[Bogo]

He might also need help on choosing the sizes of the frame tubes, bolts, etc.. That is where I'm at at this point and the tutorial doesn't cover that.

[droid_ca]

He might also need help on choosing the sizes of the frame tubes, bolts, etc.. That is where I'm at at this point and the tutorial doesn't cover that.

You are correct Bogo I'd like to build my frame out of aluminum as well so the wall thickness to upgrade too I know has to be thicker then steel but I'd still like the overall frame to be lighter then steel

[Shadow Catcher]

Our 10X6 is all aluminum, wall tubing is 1.5 X 1 X .060 tubing main frame 1.5 X 3 and the tongue should have been heavier (I have had it reinforced). It was MIG welded and should have been TIG You can see its construction in the album. Have you decided on axles yet?

[droid_ca]

Were you thinking of going with channel or square tube?

I'm thinking of going with square tubing for the main part of the frame and probably get some gussets welded in for extra protection then I'd like to use come channel for a few of the cross members so I could use it as a base for some of the wood to prevent it from sliding around then on the outer edge I'd like to use some angle stuf....
Clear as mud hey

[droid_ca]

Our 10X6 is all aluminum, wall tubing is 1.5 X 1 X .060 tubing main frame 1.5 X 3 and the tongue should have been heavier (I have had it reinforced). It was MIG welded and should have been TIG You can see its construction in the album. Have you decided on axles yet?

I was thinking possibly going 1.5 x 1.5 x .080 if possible and then do the 1.5 x 3 and get it all tig welded as for the axles still doing some research on them as I'd like to not pay too much but I do understand that you get what you pay for....I just need to get some of the weights for the material so I can start doing some more calculations

[Bogo]

From here: http://www.angib.pwp.blueyonder.co.uk/t ... tear00.htm
I found: http://www.angib.pwp.blueyonder.co.uk/t ... tear84.htm < good read for tongue strength needed.
written by: memberlist.php?mode=viewprofile&u=66 a guy on this site.

From that and a page on various aluminum alloys I think I've figured out that I should use 6061-T6 aluminum alloy for the tongue beams to get similar strength to a mild steel tongue with the same size and thickness. The 6061-T6 has a similar yield strength to the mild steel mentioned in the tongue strength page. I need to learn more about how to calculate stress loads, what the gotchas are, etc.. Then I could look at other aluminum allows and possibly use a larger beam with a thinner wall thickness. That is where the weight savings comes in. I just need to find the right book and I could teach myself in a few weeks. I actually think I know the general equations and data I need to use, but I'm not sure. For the main beams in the frame I think I can go to a less costly aluminum alloy or use smaller sections. The tiny travel trailer I'm designing is a bit larger, mostly longer, than a standard TD. I'm wanting to make a hard sided popup that the lifting top is very well streamlined.

[angib]

The 6061-T6 has a similar yield strength to the mild steel mentioned in the tongue strength page.

Beware. The problem with designing an aluminium frame is not its ultimate strength but its fatigue strength. Steel, especially mild steel, has the wonderful property of a 'fatigue limit'

For the techs amongst you, here is a graph from Wikipedia comparing steel and typical aluminiums - except it calls is an 'endurance limit'.



Typically for steels, the fatigue limit (where the blue line levels out) is around 2/3rds its yield stress or around 1/2 its ultimate stress. What this means is that as long as you don't continually stress the steel beyond that percentage of its maximum strength, it will last forever without cracking.

For aluminium there is usually no fatigue limit and so eventually even quite low stresses will cause cracking. This means that even really light loads, like the 'thrumming' vibration you get driving on ribbed concrete, are eating away at the lifetime of an aluminium frame.

Now, if an aluminium frame lasts 100 years, then who cares if it cracks. But if it cracks after one year, that's a problem. The difficulty for the amateur frame builder is getting this difference right, as the 100-year frame may be only 50% stronger than the 1-year frame. For commercial builders, this isn't a big issue, as they have experience of making many frames over a long period, so they know what works.

For the amateur, the only way that I can see to make a safe aluminium frame that will last for years is to make it really strong - but then it won't weigh much less than a steel one and will probably cost more. So why bother?

Oh, and incidentally, it is the lack of a fatigue limit that means modern aluminium aircraft can't last forever - higher-strength aluminium alloys are particularly susceptible to fatigue cracking and so eventually the whole airframe has to be withdrawn from service. DC3s were made from earlier weaker alloys that didn't have such a fatigue problem, so they can fly pretty much forever - but they are heavy and fuel-inefficient as a result. I mention this just in case anyone wanted to say "they don't build them like the used to".......

[droid_ca]

Wow thanks for that answer Andrew I think I'm leaning towards steel now as the frame I was looking into is around 208 pounds and I thought it was going to be a lot higher then that plus with a little paint no-one will know

[Shadow Catcher]

CR has a Dexter axle and quite frankly I wish it were a Flex ride http://www.ucfamerica.com/flexiride.html
Yes aluminum has a limited modulus of elasticity but if designed with that in mind it can out live you. Witness DC3 still flying after 70 years.

[Bogo]

OK, cool. That is good to know. From my experience with cattle trailers, I know aluminum really doesn't save that much in weight. I can see why now. I'm looking at using an aluminum frame because I've been looking at aluminum clad foam walls and floor, but I'm looking again at cold molding. With cold molding I can also build a stressed skin I-beam structures with integral insulation. They could rest on a steel frame. I may make my lifting top in cold molded wood for aerodynamics, and the bottom in aluminum over foam.

[mezmo]

Here are a couple frame ideas I came up with earlier on for lowering
the floor to gain headroom. Lowering the floor fore and aft of
the wheels wouldn't really present that much of a clearance
problem in my view.

[Click on pics for a larger view.]



and



Cheers,
Norm/mezmo

[droid_ca]

Hmmm those are interesting ideas I might have to consider something like that since I have lots of clearance might be a good way to bring me down to earth some what

[doug hodder]

For commercial builders, this isn't a big issue, as they have experience of making many frames over a long period, so they know what works.
For the amateur, the only way that I can see to make a safe aluminium frame that will last for years is to make it really strong - but then it won't weigh much less than a steel one and will probably cost more. So why bother?

Bingo....perfectly said Andrew. Then there is also the issue of "galvanic action" between the dissimilar metals to deal with. It can be dealt with, but it's just more work. I'd imagine that after all is said and done, it would be much less expensive, less labor intensive, less material acquisition headaches to just build in steel. To each their own....