The ultra light caravan build continued

Submitted: Friday, Dec 04, 2015 at 23:50
ThreadID: 131040 Views:3904 Replies:12 FollowUps:4
This Thread has been Archived
Some pics of my build so far. Waiting on some light 13x13mm tube to complete the frame. Single bed frames are done.

You'll notice the axle is really close to centre of the van body overall. This is to minimise the towball weight which cant go over 30kgms for the tow vehicle a VW style trike powered by Subaru.

More pics as the build progresses.

Cheers Tony

The aim is for the caravan to have a 200kgm tare weight. I think I'll go just over it but we will see.
Back Expand Un-Read 1 Moderator

Reply By: eaglefree - Saturday, Dec 05, 2015 at 00:01

Saturday, Dec 05, 2015 at 00:01
The tow vehicle

AnswerID: 593396

Follow Up By: Bobjl - Monday, Dec 07, 2015 at 21:15

Monday, Dec 07, 2015 at 21:15
Having towed a heavily laden small box type trailer behind a large two wheeled touring bike for more than 50,000 k's, I am mindful of some of the issues involved with the towing set ups on Motor Bikes.
My mates Harley trikes suspension and the rigid coupling mounting method caused the very strong custom built tow bar to fail, fortunately the failure was identified just minutes before it would have completely collapsed whilst being towed at high speeds in NT, the potential consequences can be imagined.

I may have missed it but I have not noted comment as to the tow hitch set up you will use.
Based on your apparent attention to detail, I suspect you have considered the matter carefully. I wondered however if perhaps you are still working on that aspect that a small air bag tow bar shock absording system at the tow ball/coupling would assist to share the loads and the double bounce caused when riding on rough surfaces.
FollowupID: 861739

Reply By: gbc - Saturday, Dec 05, 2015 at 08:19

Saturday, Dec 05, 2015 at 08:19

Have a squizz at section 21 with regards dimensions.

You look like you are going to be borderline for the rear overhang vs front load space dimension. If you've done the calcs I'll pull my head in. Just trying to help. I'd hate to see the finished product unable to be registered.
AnswerID: 593402

Reply By: Trevor&Verna - Saturday, Dec 05, 2015 at 16:22

Saturday, Dec 05, 2015 at 16:22
Where are you getting your 13x13 tube from?
It seems to be as rare as the stuff the rocking horse leaves behind.
Trevor&Verna, Kal WA

Lifetime Member
My Profile  My Position  Send Message

AnswerID: 593416

Reply By: eaglefree - Saturday, Dec 05, 2015 at 17:55

Saturday, Dec 05, 2015 at 17:55

You are indeed correct. The relevant section states "Rear overhang must not exceed the lesser of 3.7 metres or the front load space." - "or the front load space"

So I just went out to my shed and remeasured. From the axle to the rear of the van is 1650mm. From the axle to the front "load space" eg the rounded front of the van is 1700mm.

This I recall happened when constructing the front curved angle iron. I left more drawbar length than required so left the curve as it was made....about 80mm longer than the plan giving us longer bed length to a full 2 metres. Up to your post I was under the impression the rear was about 50-80mm longer than the front meaning illegal.

Thankyou for bringing it to my attention. I have two bumperettes that I was going to mount to the rear. I will still mount them but will ensure they don't exceed 50mm beyond the mount already there and taken into consideration when measured.

Much appreciated.

I bought the 13x13x1.6 from "Blackwoods" in Shepparton. They in turn get it from a supplier.

It weighs only 550gms/metre. When screwing tech screws into it you have to make sure they aren't too long. It is great to make the frame for windows and access hatches, the rear door etc. I've also found that when making a frame for a large area eg a section of the roof say 1850 x 1400mm its best to make the frame a # design and cut out half grooves so the lengths fit into each other then weld. This means that 13mm polystyrene is the perfect thickness for fill between the roof ply and the ceiling ply.

Some of the areas saved by using light weight items/materials are-

acrylic sink 1.7 kgms
acrylic shower tray 1.6 kgms
Odyssey windows- light compared to some others
Ripping 20x20x2mm gal tube to make light angle
13x13 tube
10" wheels
acrylic/plastic wheel guards
2.7mm ply under roof cladding likely normal
bumperettes made from demolishing my wife ABDOER exercise machine :)
Curved front of van curves under beds to lower weight
Drawbar is 50x25x3mm with 10mm rod under for good strength and less weight
6mm marine ply only for floor and bed tops.
Carpet for ceiling- foamback from Bunnings 500 gms/sq metre

Items not sacrificed for weight-

Door - Camec weighs 15kgms
Floor frame is strong
Strong centre "rollbar" design.
Water capacity 44 litres.


AnswerID: 593419

Reply By: Ron N - Sunday, Dec 06, 2015 at 00:32

Sunday, Dec 06, 2015 at 00:32
Tony - As someone who has built numerous trailers, and towed many, over lengthy distances, I am not impressed with your axle location - and I believe you are making a classic mistake of worrying too much about towball weight and not enough about handling at speed - particularly, the potential for tail-wag.

You could easily move the axle rearwards by 75mm - which would have very little effect on towball weight - and doing so will ensure you have minimised the dreaded tail-wag problem.

I have towed numerous trailers that have had serious handling problems due to poor axle positioning, and I have had to deal with numerous severe tail-wag "events" - one of which resulted in total destruction of the trailer when it overturned. It didn't help that it was late at night when the trailer went out of control, and it was difficult to see what it was doing in the mirrors.

I can assure you, ensuring you have a reasonable setback in the axle positioning is vital for comfortable and safe caravan handling.

Cheers, Ron.
AnswerID: 593433

Reply By: eaglefree - Sunday, Dec 06, 2015 at 02:20

Sunday, Dec 06, 2015 at 02:20
Hi Ron,
I respect your experience in this matter.

Two things come to mind with this particular project that is a lot different to every other caravan project, relevant to your concerns.

Firstly, it is going to be towed by a VW style trike. These trikes have the engine in the rear and have a maximum towing GVM of 250/300 kgms. My trike has added weight due to a roof and other added items for a GVM of 660 kgms far from the 448 kgms when new and bare. Add the rider/s and we have a tow vehicle of more than 760 kgms. So towball weight is important in that if it exceeds 30 kgms it is more dangerous than less that figure, due to the front wheel of the trike becoming lighter with less ground contact with a worse case scenario of loss of steering.

The second consideration is caravan weight. Being around 200 kgms by using lightweight building materials there is zero caravans to compare to for reference. Small teardrop campers are minimum 270 kgms with most of them around 440-550 kgms. And they are heavy at the tow coupling due to their rearward axle design. All teardrop campers being towed by VW trikes are likely illegal for this reason.

I've had several experiences towing campers and vans with this trike. I towed a ChevronRV "Guppy" for several months (yes it was illegal). That weighed 540 kgms tare. There was no issue with the steering or control issues but I wasn't happy with the legal side. My last camper trailer, a Detour motorcycle camper trailer weighed tare 190kgms, 300 kgms GVM with brakes. It had a trunk on the drawbar and no matter all attempts to lower the towball weight I couldn't get it down to a reasonable level. Battery, water jerry can, tent pegs all at the far rear and the towball weight wouldn't budge under 40 kgms. Its a fickle process.

This led me to the current design in regard to the axle positioning. By getting the van balanced slightly forward (eg currently it has a towball weight of only 6 kgms) half built. I can, eventually when build is completed, add the spare wheel to the front of the drawbar if required (or toolbox), raising the towball weight to what I want. In terms of safety you can see my direction here.

I take your concerns seriously. However moving the axle back now would require the front spring mounts to be repositioned. The rear spring guides could remain where they are. I'm not keen to do it but it remains an option. I think the fishtail likelihood would be far less with a 250kgms GVM van. Think lots of testing post registration at low speeds will be on the cards in the first instance with our Hyundai i30.
AnswerID: 593435

Follow Up By: Ron N - Sunday, Dec 06, 2015 at 10:39

Sunday, Dec 06, 2015 at 10:39
Tony, I understand your concerns - and it's great, the amount of serious thought you have put into the construction of your van - but weight is only one of the factors to be considered.
Another important consideration is the amount of overhang behind the axle, in relation to the length of body forward of the axle.
The maximum overhang requirements mentioned in VSB1 are not only important for the prevention of turning problems at intersections - the overhang restrictions are also important for satisfactory handling.
A trailer or caravan with a sizeable amount of overhang, in relation to the length of body forward of the axle - even with good weight positioning, forward of the axle - is a trailer or 'van with a high potential for poor handling, in the form of developing constant or easily-induced tailwag.
It all comes back to the leverage exerted by that overhang, on the rest of the rig, when a mild steering correction is carried out.
A substantial amount of overhang will encourage the rapid development of tailwagging, even when the majority of the weight is forward of the axle.
This problem is not properly understood, even by a lot of so-called "professional" caravan builders.
There was a South Australian engineers dissertation online at one time, involving a fatal accident in SA, caused by loss of control of a caravan, which was initiated by tailwagging.
The engineer was scathing of the so-called "professionally built" caravan design, which had an unacceptable amount of overhang behind the axle, which the caravan builder tried to counter by placing more weight forward of the axle.
The additional weight forward of the axle did nothing to counter the poor design of the 'van that led it to have serious tailwagging problems, that showed up rapidly, even with small inputs such as steering corrections, hitting a pothole, or getting hit with windblast from a large truck.
There is also the relationship between the physical size of the body, and the weight of the 'van. A very light 'van with a large body has the potential to be buffeted more easily, and thrown around by wind blast and wind gusts, than a bigger 'van which is substantially heavier, and which has a bigger "weight-to-surface-area" ratio, which provides better stability.
You have no doubt experienced the major wind blast effects from large trucks - which becomes worse with narrower highways - and which is particularly bad from some particular makes and models of trucks. The trucks with a huge flat frontal area seem to produce the worst air displacement effects.

Cheers, Ron.
FollowupID: 861660

Reply By: Life Member - Terry 80FTE - Sunday, Dec 06, 2015 at 22:32

Sunday, Dec 06, 2015 at 22:32
A better way to do the "A" frame for the draw bar is to go from the tow ball point, straight to the front of the spring hangers. Then have the deepest part of the under-hanging bridge, under the front cross piece of the box frame.
Having 2 under-hanging bridges (strengtheners) meeting at the point of the most stress will make a weak point there,
Hope I explained it right,
Justify Your Existence
VKS 737 mobile 2458

Lifetime Member
My Profile  Send Message

AnswerID: 593477

Reply By: eaglefree - Sunday, Dec 06, 2015 at 23:11

Sunday, Dec 06, 2015 at 23:11
The drawbar tubes 50x25x3mm go from the ball coupling right through to 600mm from the end of the van. These lengths have one "bend" at the front of the body of the van that is gusseted and strengthened also with a cross tube to the other side corner. Finally that one bend has the two 10mm rod pieces crossing over each other on that bar bend and welded.

Also, from the front spring mount location to the very rear of the van I have welded to that drawbar length 2x 25x25x2mm tube. This is really strong but a lot less weight than had I used 50x50x3mm or similar steel.

Hope I've explained that right also. Here is some pics.

I'd be interested if you still think the drawbar has a weak spot. When I owned my ChevronRV "Guppy" it broke the single 75x75mm drawbar. The experience was disappointing. I was only doing 40kph when it gave way. In this case without the 10mm rods I jumped up and down on the drawbar and it had too much flex, a little is good but not that much. The front rods were installed and the flex from the bend to the spring fronts needed the rods also.

AnswerID: 593478

Follow Up By: Peter_n_Margaret - Sunday, Dec 06, 2015 at 23:48

Sunday, Dec 06, 2015 at 23:48
I agree.
The point of most stress is the weakest point on the A frame.

OKA196 Motorhome
FollowupID: 861688

Reply By: Ron N - Monday, Dec 07, 2015 at 00:11

Monday, Dec 07, 2015 at 00:11
Tony - Terry 80FTE has outlined the best drawbar construction style - two straight sections of tube from the towbar to the front spring perches.

This setup has two major advantages.
1. The narrower angle of the drawbar arms provides sharper turning ability.
2. The drawbar arms can be strengthened with one long truss on the underside, that reaches from the front of the drawbar to the spring perches.

As it is, you have two separate strengthening trusses on the underside of the drawbar and under the front of the frame tubing - but the major stress point (where drawbars always break), is right at the front of the main frame (where the bend in your tubing is).
This point in the drawbar arms is the area in most need of truss support - but it's lacking in your current design.

You are setting yourself up for fractured drawbar arms, right at that bend.

In motion along the road, the drawbar is being hoisted up and down on a regular basis, as your tow vehicle hits bumps, sunken culverts, potholes and general road undulations.
This is the equivalent of bending a piece of wire back and forth until it breaks.
The area where the drawbar bends most, with the constant up-and-down action from the tow vehicle, is right at the point where it meets the front of the main frame.

If you examine any older trailer with a broken drawbar, you will find in 98% of cases, the drawbar has fractured at the point where it meets the main frame or chassis of the trailer.

Cheers, Ron.
AnswerID: 593482

Reply By: eaglefree - Monday, Dec 07, 2015 at 01:27

Monday, Dec 07, 2015 at 01:27
Thanks Ron, that makes sense to me.
t's done now so I'll need to figure out a addition of strength in that "bend" area.

Dare I say it - another 10mm rod piece? I've found that those rod sections are really efficient regarding strength. Positioned in between the current rods I'd imagine, seeing as its low in GVM weight, that it would do the job?
AnswerID: 593484

Follow Up By: Ron N - Monday, Dec 07, 2015 at 11:15

Monday, Dec 07, 2015 at 11:15
Yes, that will assist greatly. Ideally, you need to weld another piece of metal on the flat under the bend, plating the drawbar, to double the metal thickness at that stress point.

Then you need to weld a piece of truss support metal, identical to the ones already in the centre of your current trusses, under the bend area - then bend the 10mm rod to follow the drawbar/chassis bend, and attach the 10mm rod to the high points of the other trusses - and then weld the 10mm rod to the truss support under the bend area.

Straight-line trusses are a far better setup, but a truss with a bend in it is better than no truss at all. You increase the strength of a member substantially with a truss - and the deeper the truss, the greater the support.

Cheers, Ron.
FollowupID: 861704

Reply By: eaglefree - Tuesday, Dec 08, 2015 at 12:43

Tuesday, Dec 08, 2015 at 12:43
Thanks Ron, I'm onto it.
AnswerID: 593556

Reply By: eaglefree - Thursday, Dec 10, 2015 at 21:49

Thursday, Dec 10, 2015 at 21:49
Thanks to Ron on this site I took his advice and added a centre truss making 3 in all each side. I still believe this truss type design with relatively small RHS is stronger and lighter than RHS alone of the size of say 100x50x3mm.

More steel 13x13x1.6 has arrived so onto completing the frame.

AnswerID: 593671

Sponsored Links