Quick Q maybe.
If building a teardrop trailer chassis, what would be the weight advantage of aluminium over steel given that the aluminium one would have to be built
from heavier gauge material to maintain the same strength? 5%, 10%, 50%? Would the heavy gauge strength requirement completely negate the lighter
weight material advantage?
Thanks
First, I'm not a structural engineeer, I do have a few years of building stuff from different materials behind me though.
My guess would be around 50kg weight saving in just the chassis frame, in return for a reduced service life expectancy due to eventual chassis cracks
in the aluminium from the inevitable twisting.
While looking at caravan interior materials for a refurb, I found it interesting that 10mm pine T&G is actually lighter (including fixings), has
greater sound deadening, far superior insulation properties and greater twisting capacity before cracks than 3mm ally bonded plate.........and it's
heaps cheaper too!
Anyway, light reading found in the Library of Google.........
http://www.kastenmarine.com/alumVSsteel.htm
http://xl.com.au/assets/img/brochures/1866-XL-WHITEPAPER-STEEL-VS-ALUMINIUM-2...
https://books.google.com.au/books?id=js3tCAAAQBAJ&pg=PA105&lpg=PA105&...
Thought you'd be a 'Steel' man, Chris...never mind the weight. 
Hi amazeer
I'm a mech eng and my last role was in a fleet section of a very large Aus company operating a significant number of small plant trailers (2.5T). I
designed a steel trailer in order to replace an Off The Shelf aluminium trailer. These ali trailers had issues after approx 2-3yrs of service
(including light offroad duties) where structural components (such as drawbars) started to shown signs of fatigue, and cracks developed. My design
ended up being a same weight as the aluminium version and in the same 2yr period has had no issues reported. In regards to fatigue, steel can be used
for "infinite" life whereas aluminium will fail eventually due to fatigue.
A good simple article which explains the difference in materials is shown here (ignore the marketing type stuff at the end; incidentally i know these
people and agree with their assessment):
http://xl.com.au/assets/img/brochures/1866-XL-WHITEPAPER-STEEL-VS-ALUMINIUM-2...
It depends on what your usage is (duration and harshness), fabrication abilities, budget etc. My advice: build it in steel but be smart about how you
design it and you'll be able to keep the weight down
OK. Written off as a bad idea. I was thinking about weight as the main consideration due to using a beetle as a tow car.
I dunno how one goes about designing a caravan chassis when you dont know what the end weight will be. I guess there's and ADR that tells of minimum
materials.
I like the wood idea.
But only so you can call it the 'spruce caboose'.
Dreadful stuff, wood...but I suppose you could use balsa, that's not very heavy.
What's wrong with sleeping in a big pine box?
Chris,
I had a customer that used to build aluminum boat trailers and eventually built himself a toy hauler/caravan with an aluminum frame. The boat
trailers were because they don't rust, and the toy hauler was just because he could.
There are weight savings to be had, but hardly warranted on a small trailer like a teardrop given the difficulty involved in welding aluminum well.
There's a hot rodder near me who builds a few teardrops with light steel frames, like a garden trailer. As you know, I'm no engineer either, but I
sell trailer parts and I'm not aware of any guidelines regarding structure or materials, but there are requirements for lighting, brakes and
couplings. I was able to look them up at Vicroads online so I'm guessing NSW would be similar.
Aluminium has a finite fatigue life, meaning eventually, regardless of the load it will eventually fail (crack). Greater load, sooner it will crack.
Heck... it doesn't stop us making aircraft out of it though and running em' for 40 years. Or rims!
Ferrous materials don't have this finite fatigue life properties 
If you can find forged alum materials as opposed to extruded it will give you more strength (avoid cast if there is such a thing for alum beams
available). You might not have anything other than extruded readily available when it comes channel.
The easy solution would be to have a steel trailer base for housing axles and hitch and alum frame for rest of the teardrop as the loading on this
aluminium will be minimal, meaning it will last (or just use timber). Thats the 'no brainer' approach, but I believe there is still validity in
using aluminium for a light trailer with a bit of engineering 
.
Alum is just over a quarter of the weight of steel per cc. Alum has a GREATER tensile strength till it yields then steel (AMAZING!), yield meaning
deformation / elongation. however, it has a far lesser 'ultimate' tensile strength, so naturally would break sooner.
Some general alum alloy and steel properties to compare:
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A
Ways to reduce fatigue is simply load distribution, over engineering and dampening of shocks (so suspension, generous welding, gussets).
some more food for thought.
I have a dead box trailer but that is way too heavy duty. 100x50 I think. I'm guessing that it would hold at least 5 times the 300kg random
guesstimated weight of a basic baby caravan with just a bed, overhead cupboard and kitchen. I'd have to go 50x50 RHS to allow for something to weld
the spring mounts to and also attach the sides. But what would be minimum wall thickness?
As Kol said, there doesnt appear to be any statutory specs for a chassis apart from the drawbar:
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(I've looked at product data from onesteel etc but it isnt written for hackers)
dont worry about the weight of the trailer, just add more boost
50 x 50 x 3mm should be plenty for something of that weight. You could go even lighter if you add fish plates where the springs mounts attach.
Check out this idea... https://youtu.be/IH5h0qU4EQ4
Don't skimp on the draw-bar & chssis rails to save weight...do that on the less important bits.
Drag is also important, so look at the 'C of D', too, 
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these trailers were specifically designed for the company i used to work for so they hold the intellectual property. I can't share that info sorry.
no they aren't available for sale to the public. you may get lucky at vehicle auctions
I bought "The Big Lap" DVD series and it has only added to my paranoia. The boat rack on their trailer has failed twice (fatigue cracks) and I'm only 1/2 way through the series!
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G’day Amazeer
I just have gone through the same thought process as yourself in regards to designing a teardrop chassis to be towed behind a beetle 1600cc. Here are
the main points I picked up:
• Go with steel
• Go with an Australian supplier (Onesteel not Stratco etc) as their steel is guaranteed to conform to AS standard (there’s a lot of junk steel
out there)
• Use RHS not square section as the stiffness is much greater for a comparable weight e.g. use 50mmx25mm in place of 35mmx35mm.
• When you do your design ensure that you triangulate the 90 degree joints, you can do this with another section of tube say 200mm from the joint or
just triangle plate steel (top and bottom)
• Consider using a torsion beam suspension as the axle tube can be used as a stressed member of the chassis thus reducing the requirement for
reinforcement around the spring hanger area (cost benefit analysis will have to be done on this to ensure that it is worth the extra expense and
weight of the axle in the first place)
• Build the teardrop as light as possible thus reducing the strength requirement of the chassis
• Consider adding breaks to the trailer to reduce demand from the beetles breaks
• Really take some time to consider the actual requirements of the camper, if you’ll be using it mostly in camp sites do you really need a full
kitchen with large water storage?
• Get yourself a pad of graph paper and pencil to work the whole thing out and then transfer to a 2D CAD (libreCAD) package to finalise the
design.
• Currently I have a design for a chassis weighing approximately 75Kg ex suspension for a large teardrop 2+2. My total weight target is 350-400kg, I
haven't started building yet as my the first priority is getting my beetle back on the road.
Hope this helps
Dicko.
Yeah I'm sold on steel now. The gidget looks like a nice practical tool but its more weight too. All I'm looking for inside is a bed, a small clothes cupboard and a 24V TV/CD/radio. In the kitchen a gas burner/bottle, fridge just big enough to hold a beer and a steak, and the battery. And externally I want it to look like it is straight out of the 50s with the exception of the solar charger on the roof
Sounds good, keep me/us updated with progress.
You may have found this already: http://www.angib.pwp.blueyonder.co.uk/teardrop/tear00.htm