Different types of caravan chassis!

Hi Puppy,
What brand of van did you end up purchasing?

Sorry John, I got your name wrong, didn't read enough at the end.

Being such a big manufacturer, I would think they would use engineers to design the stress bearing sections of the chassis.
If something required a 'stress spreader plate' one would think it would cost minimal to put it in place.
When you get into bending moments on beams, compression and tension forces and then apply centre of gravity compensations.
It takes a huge amount of calculation, just adding more steel adds more weight and inertia forces.
I would be much happier buying a lighter engineered van than a heavier 'because it needs it' van.

Hi Ross and 08crd,

I have to declare that I am the owner of a 23 foot Jayco.

I often think of a couple of local fellows that were into off road racing in this area and were getting fatigue fractures on suspension mounts more often than not causing them to pull out of races. Their solution was to build in greater and greater strength by beefing up the mountings and supports. A mate came along and advised them to head in the other direction and lighten everything to reduce inertia. Long story short, they won their class that year because they did not have a breakdown.

Therefore, I would suggest that if the Jayco chassis had a problem with a lack of support structure, given the sheer number of units out there circulating around there probably would have been plenty of talk about it by now.

Over engineering, something I am guilty of often as my engineer mate used to tell me, can be as much a sin as under engineering, according to him.

I originally thought the same as you Ross in regard to the suspension supports on the swinger on the Jayco but also point out that the weight is actually bearing on the 150mm vertical face as opposed to the 75mm horizontal plane of the member. Also the welds are along the latteral or horizontal face of the 150 vertical face of the member and I cannot see any vertical welds at all in that area. Current models may be different as mine is a 2006 model and I have had no problems so far.

The only problem I did have was a broken centre bolt in one of the spring packs which was caused by loose U bolts. Not a factory problem as the axle has been underslung and it would have been carried out by the previous owner I imagine.

Cheers, Bruce.

Check out the ROGUES GALLERY or HORROR STORY on the caravanersforum.com for some real shocker stories from a repairer. Or dirtvan.wordpress.com for the low down on the type of van I've got.
Basic design is OK but some real crap things (badly buckled and unbalanced from new cheap wheels, and no fusing on fridge, just two things but plenty more) and don't try a warranty claim as they won't even respond to e-mails.
We had to get consumer affairs (WA) and take action in the small claims court to get any where with them or the retailer.
Good luck with any thing they claim is "to industry standard" as there ain't no standards! :-)
AlanH.

First thing I would be asking is the grade of steel used in manfacture and the carbon and silica content.
Unfortunately I don't think any of the Australian manufacturers are large enough to use robotic welding as in the USA which results in a much better finished product in weld consistency.

Bruce C NSW
The welds I referred to are either side of the vertical plate which is the suspension mounts.
The weld runs directly vertical, both sides, on the inner and outer faces of the 150RHS.

Previously, ie a few years ago, I had seen engineering drawings we had to follow when building special trailers for government use and it showed specifically what not to do. These welds are in total contradiction to the engineering advice given at that time.

Ross, your followup got me looking under my 2006 Sterling and what I found does not quite relate to your observations.

What I found is that there is, as you say a weld up the vertical edge either side of the rocker mounting plate but the weld is so light, that is why I missed it earlier, that in my opinion does not compromise the integrity of the structural strength of the chassis rail which is 150 X 50. The inside vertical plate does not extend up on to the chassis rail.

I have drawn it up so that you can get a better idea of what I am talking about. You have raised some interesting issues none the less.



Hope this helps the discussion.

Cheers, Bruce.

G'day Bruce
I see what you mean, the one I saw extends both sides and a fair way up the RHS ie, 2/3 or more. The plate being narrow and the welds being not very far apart means all the stresses are localized in a very narrow area of the chassis rail. If you look at trucks, earthmoving vehicles and light commercials they don't do that in load bearing areas.

I know what you mean Ross, you sometimes have to wonder at some of the engineering applied to critical areas don't you.

I am with you re spreading the stress, seems to be a lost art these days. I guess it depends on the engineer designing the structure I. All the older engineers would have been on top of that problem I have no doubt.

Interesting that Jayco have changed their structural design. Sounds like for the worse too.

I suppose when you do the sums it is really only carrying 33.33% of the load on that side of the van. I know all sorts of inertia and side thrust loads have to be dealt with as well as shock loads but it is not like it is carrying 4 or 5 tonne weight so I guess they would be adequate.

As Lyn said, it depends on the thickness of the material being used in the chassis rails mainly.

We obviously would not build it that way, we would have it plated for a foot in all directions with 3/8 plate I guess. LOL

Cheers, Bruce.

Bruce,
Looking at your sketch of your rocker arm assemble I would say that the design is appropriate to transfer the loads in this area into the chassis rails.

The majority of the load in this area would vertical (a combination of dead load, live load and most importantly the dynamic load created as the van travels along the road). You will note in your assemble that one of the vertical plates that are on either side of the rocker is bents at right angles so the 150 X 50 box section of chassis rail sits directly on top. This allows the loads to be transferred directly into the chassis rail.

The welds between the chassis rail and these side plates are their to hold the assemble in position and to take the lateral loads (the horizontal loads) as might be created in a tandem axel when the van is being turned.

As an example of what could be consider in redesigning this area if needed to cope with higher dynamic loads such as those imposed in off road use, I would suggest making the two plates that hold the rocker much wider at the point where the chassis rail bears on them, and if the structural calculations show it necessary some localized stiffing in the wall of the box section. How often have you seen a caravan manufacturer just run additional 150 x 50 box sections below the existing ones. Yes, it is stronger, but also a hell of lot heavier than it needs to be.

Regards

Rob

Hi Rob,
Yes I agree with you 100%.

After Ross's original followup I had a cursory look at mine, from the outside only, and thought it was the same as Ross had outlined first up.

However, after his second followup I decided to get under the van and have a really good look. As soon as I saw the bent member which the chassis rail sits on I was very pleased as it is, in my opinion, superior to the one Ross was looking at that show and I instinctively knew it was up to the job required of it.

The other thing to note, as Ross was pointing out, is the placement of welds. You will see from the diagram I drew up, also showing the weld placements, that no two welds are in the same area of the chassis rail, meaning no weakness has been compounded in any one area.
As many of us know, the weakest part of a weld is down either side of it along its perimeter where the weld finishes and the parent metal begins.

Cheers, Bruce.

On the matter of welds.

If you look at the vast majority of trucks and motor vehicles with a seperate chassis, almost without exception the various chassis members and various attachments are not welded, they are either riveted or bolted together.

There may be welding within the individual chassis members, but not butt welds, that predominate in light trailer construction.

OH and those chassis members will be made of steel a few grades better than cheap Chinese mild steel that so many trailer builders use

It is because of the fact that these chassis are welded and the standard of the welding that the various materials have to be so heavy...because every single weld potentially compromises the strength of the beam used

I have built and repaired a few trailers in my time...and...I use similar methods to everybody else.....after all I am a fair back yard welder.

But I am not holding up my work as state of the art and charging as much as a medium sized house for what I have built.

Its sad when to fancy high priced product on the market uses technology no better than is accessable to a reasonably well informed back yarder.

cheers

I am under the assumption that Jayco chasis' are/were manufactured by a specialist company called G & S chasis or similar. You would think they would have to know what they were doing as Jayco are not their only contractors'. Cheers,Bob

Yes... people make a lot of interesting asumptions.

One thing is for certain.
NONE of the caravan manufacturers have the design, testing or manufacturing resources of the major car and truck manufacturers.

I dare say not even as well resourced as a small to medium motor sport fabricator.

The chassis may be competently constructed, but it will still be crude technology in comparison to the vehicle that tows it.

cheers

You have made a real dilemma out of it all now mate!.....LOL.


But your Right!.


Cheers Axle.

Maybe not so good to hold the motor vehicle industry up as a benchmark, when so many dual cabs are suffering chassis failure!
If you look hard enough you will see that both Track Trailer & Vista RV have well a engineered chassis & suspension.
I also question as to whether most people could afford to buy a camper/caravan built to your described standards.

We took delivery of our first caravan in March after more than a year of research.

This research included establishing what we were going to use the van for and then seeing what was in the market that would suit.

My market research included attending a number of caravan show around this country and much to my wife's embarrassment crawling around under the vans inspecting the chassis. My rational for this somewhat unusual behavior was that as a civil engineer, structural steelwork was something I had had some experience with in my career, so if this was executed well then the rest of the van would most likely also be good.

Bantam what I discovered in a lot of cases was as you describe.
• Most chassis were fabricated from mild steel
• Many chassis displayed the absents of proper design or engineering
• Poor workmanship was either covered with galvanizing or just ignored


I guess that mile steel (250MPa) is used because most places can weld this material, but as pointed out here by yourself and others it's challenging to get a lightweight and strong produce with mild steel without a well engineering design.

High tensile steel (350MPa) is being adopted in more caravan chassis as it offers higher strength with less weight. Many examples I looked at showed evidence of proper engineering design. High tensile steel does however require a far more controlled approach with welding.

While exotic material such as chrom molly offer much higher strengths, they are not light compared to some of the magnesium alloys, but unfortunately the cost of these material rules them out of caravan construction.

The caravan industry is represented by the RVMA who state in their objectives

"RVM Australia ensures its Members understand and apply the latest Australian Design Rules and Australian Standards applicable to RV manufacturing in Australia.
Key areas covered by these rules and standards include:
• Gas and Electrical systems.
• Towing and Undercarriage equipment.
• Suspension and Braking systems.
• Permissible masses and ratings.
RVM Australia works with industry and government to keep standards abreast of current technology and usage to maximize the safe enjoyment of the RV experience".

It seems to me if each manufactures had to produce a certificate from an independent engineering consultant to verify that the chassis was designed by a suitably qualified engineer to current best practice it would be a step in the right direction. It would also be worthwhile having the caravans ATM certified by and independent engineering organization.

I think a lot of people would be amazed that there is no independent inspection or varification required for caravan manufacture.

and certainly no requirement for, distructive testing, crash testing, or even any sort of stability test.

As I have said before...those standards...there is more text specifying headlights than caravans.

as for the electrical and gas standards...yeh well.

look at some of those rogues gallery threads...there is no way some of those vans where wired by a qualified electrician.

If you want a chuckle....as a caravan manufacturer for a circuit diagrame for the van.

cheers