Flexing chassis

fostertom · Mar 12, 2017

Does anyone know about caravan chassis technology c1997?

We have Elddis Genesis 3 with (I think) Al-ko chassis, the front and back ends of which have drooped about 50mm each, leaving the floor humped from front to rear - and therefore also crosswise amidships. The strain of the latter had caused the body sides to detach from the floor.

Because it's such a lovely caravan inside, I started engineering works to cure the hump and reattach the sides. We loosened the 3 sets of bolts that join the 4 sections (lengthwise) of the chassis, jacked the various bits up until laser-straight, tightened the bolts and welded the joints for luck, removed the jacks - and it went right back to sagged shape!

Further experimental jacking seems to confirm that the chassis is in fact incredibly flexible. Beginning to realise it's probably made of very high tensile spring steel, intended to flex like this?

Can anyone confirm?

I recall a caravan mechanic saying that these vans are intended to move/flex/bend at the bolted joints, because manufacturers found chassis fracturing if made rigid. I couldn't believe that, but now suspect he was half-right -
not that the bolted joints move/flex/bend, but if the chassis is high-tensie flexible, welding invites fracture, not because it make the chassis rigid (it doesn't) but because it affects the metallurgy.

So we have welded it - will it now fracture? (not so far).
How were these chassis set to start with - ends high so they'd flex to level under load?
How were sides supposed to stay connected to floor, when the flex causes the floor to strain up and down at the wall/edge?
Do these spring-steel chassis yield, to a permanent sag, over time? That's what's happened here.

Any wisdom much appreciated.

GStockton · Mar 12, 2017

I would think that the structural integrity of a modern caravan does not fully rely on the Chassis which is basically just 2 members held apart by the axle. Then bolted to this is the Laminated plywood floor, to which a laminated aluminium/ polystyrene/ thin plywood walls are fixed to with a front and rear end of GRP supported by some bits of 2x1 cheap rough sawn timber. Also the caravan is held together by all the internal cupboards etc.

Failure or weakness in any of the above and the thing is going to fall to pieces. The Floor is a major part in all this. How Strong and dry is your Floor?

fostertom · Mar 12, 2017

Right - yes the walls/roof/floor would form a rigid monocoque box if they all hold together, and might then keep a springy chassis flat, provided the floor didn't 'dish' in the middle (but it has, bigtime). Even if that floor was still a factory-fresh bonded sandwich, I don't believe it capable of staying that flat, as 'outrigger' from the chassis rails, under the weight of roof and walls bouncing up and down on the end of that 'outrigger' - I doubt the shear strength of the middle 25mm of expanded polystyrene is anywhere near sufficient.

So I do think there's more to it than meets the eye, than 'the construction strength in a UK van is the van not the chassis'.

Thanks anyway - any further thoughts?

Maybe I will contact AlKo.

GStockton · Mar 12, 2017

Yes Don't forget the older pre sandwich constructed caravans had a far more substantial box section and outrigger chassis

ProfJohnL · Mar 12, 2017

I suspect your consideration of the whole caravan body being a monocoque is probably correct.

A cardboard box when fully closed is much stronger than individual sides, so whilst if the floor of a caravan had weights equivalent to the weight of the walls just hung of the floor panel, it may not be able to adequately support it, but each wall and the floor and roof all add have certain strengths that mutually add to strength of individual panels when they are correctly bonded.

Inside a caravan there are also certain furniture elements that can also provide some additional rigidity to the structure.

The chassis is more of a space frame, and yes it will provide some strength its principally carrying the body. If the body is not rigid (i;e joints have worked loose) then I not surprised the chassis has sagged.

fostertom · Mar 13, 2017

Thanks.
It turned out that the chassis sag was not because of loose chassis bolts/joints, but just bending - 35 to 50mm down at each end - without buckling. That being the case, I'm now guesing it's high tensile ('spring') steel.
Not the stuff you should weld, like mild steel. But we have welded.

So if anyone knows whether these chassis were in fact high tensile, and whether welding it makes fracture likely ...

ProfJohnL · Mar 13, 2017

fostertom said:
...So if anyone knows whether these chassis were in fact high tensile, and whether welding it makes fracture likely ...

I believe the chassis is bolted together mainly because the chassis manufacturer produces a catalog of parts that can be to some extent be mixed and matched to suit each customers requirements. It also means if the chassis becomes damaged parts can be more easily changed.

Certainly ALKO don't like additional holes being drilled into their sections, as doing so voids any warranty, claiming they're a stressed member and additional holes will weaken the integrity of the structure. How true that is in my mind open to debate, but it is the reason that caravan movers only use existing holes or use clamping to secure to the chassis.

I suspect that welding the chassis together will have locally changed the nature of steel, generally welding tends to harden or reduce the ductile nature of steel, and in high stress areas this could render it more likely to fail. It will depend on how extensive the area of the weld, but If the welds did fail, if the chassis is still bolted, the failure may have little effect on the integrity.

I don't think anyone on the forum can predict how likely it is you will suffer a failure, as it depends on how close the chassis is loaded to its limits, how much stress it will be subject to when being used. Such things are mainly down to the user.

fostertom · Mar 13, 2017

Thanks especially to ProfJohnL for detailed replies - giving me additional thoughts. Also posdted same on a Aus forum, getting their perspective - like 'weld a new chassis under it'! so my own engineering doesn't look so excessive.

I had first tried pulling the bulged sides flat by a kind of aluminium exoskeleton but they refused to flatten - I realised because the half-height (kitchen) furniture was maintaining a right angle between floor and wall, and the 'outrigger' part of the floor in the area of the axle was sloping sharply downward. Hence discovery of the end-to-end sag of the chassis. Trying to flatten the chassis then failed, as described.

The Aussie vans' problems seem due to failed steel outriggers to the chassis, but ours has no such; the floor sandwich seems to be the outrigger which is supposed to not bend and so support wall and roof weight, assisted yes by similar non-bend wall sandwiches right-angle connected to floor by furniture! Though ours is not rotten, apparently the sandwich-bonding has failed, leaving floor and walls free to bend. Subsequent 'working' up and down ripped the walls' connection from the floor, forward of the axle. That is now all safely reconnected via the aluminium exoskeleton, but walls remain bulged, floor humped and chassis bent.

ProfJohnL · Mar 13, 2017

fostertom said:
Aussie Caravaners Forum

And your purpose for posting this link?

fostertom · Mar 13, 2017

To help me understand (or confirm) what's happening - which I'm getting - thanks. Also, worry about having welded steel that maybe turns out to be high tensile.

chrisn7 · Mar 13, 2017

A caravan engineer once pointed out to me many years ago, that caravans of that time could suffer a 'jacking' (his word) effect as they get older. That is, the action of suspension bumps causes the floor to bend downwards in the section between the chassis rail and the outer wall, since this section carries the furniture and other heavy units such as fridge, cooker, boiler and so on.
This did appear to be true since out old van suffered from this with, work surfaces that sloped slightly away from you and also a slight end to end bend as described by the OP, though that not to his extent.
It seemed to me that the areas affected were where the furniture was not full height , and therefore not fully bracing the wall to floor joint.

ProfJohnL · Mar 13, 2017

It sound like the caravan is more seriously damaged than you would like.

Certainly if the floor panel has effectively lost its internal bond between is its layers it will have lost a lot of its rigidity as a compound construction. Sponginess is a well known problem and there are well documented solutions to repair small area's in situ. But the larger problem you seem to have is a much more serious issue. And its not just related to the usability of the caravan, but also to its on road safety. If there is any freedom for the cabin to lift or sway when being towed, it might be construed as an unsafe load.

Please when posting bear in mind we are not mind readers, and matters that you might take as being common knowledge may not be so by readers. It helps if postings are clear about there intention or reasons.

fostertom · Mar 13, 2017

chrisn7 I think that's right.

Prof, you guys are confirming most of what I've come to realise about the deteriporation.

But the floor and the 2 walls' exoskeletons are now strongly reconnected, albeit still distorted as described.
To correct the floor-edge droop, thus bringing the furniture upright, thus allowing the walls to come flat, we have a plan to sling 3 50x25 steel RHSs crosswise through existing cutout holes in the chassis rails, the ends of which forming outriggers to rigidly jack up and support the bottom rail of the 2 walls' exoskeletons.

WoodlandsCamper · Mar 13, 2017

Fostertom - may I ask which country you are in, as you quoted an Aussie website

fostertom · Mar 13, 2017

UK - interestring to get the Aus perspective tho

BryanTheSnail · Mar 14, 2017

I saw a caravan at my dealers a few years ago with the sides detached from the front of the chassis. They had taken on the insurance job to reattach the chassis. The owner had hit a speed bump rather too fast. Could the problem be damage rather than flexing. Even twin axle caravans are sort of triangular in their support points with respect to wheels and hitch so should not need to flex greatly..

fostertom · Mar 15, 2017

Before I discovered that the sides had detached from the front half of the floor, I would look in the mirror and see the van gently nodding up and down as we drove along. Bumping around would have caused the progressive tearing-out of the connecting wood-screws. Now no more nodding - really solid, safe - but still sagged/bulged as described.

DrZhivago · Mar 15, 2017

fostertom said:
Further experimental jacking seems to confirm that the chassis is in fact incredibly flexible. Beginning to realise it's probably made of very high tensile spring steel, intended to flex like this?

No way. HT steel would be far too expensive, hard to work with at the factory, and unnecessary. It will be made of normal structural steel, which is relatively mild stuff.

ProfJohnL · Mar 15, 2017

Hello Bryan.
What you say is correct to a point, provided the forces involved all lie in the same plane as the chassis, But in reality the forces move all over the place and in all sorts of directions. The most obvious forces re those associated with the vertical movement of the caravan. Then there are those associated with side loads from tuning corners which will apply a twisting load as the centre of gravity is above the plane of the chassis.

A company I worked for undertook some testing on a car and caravan outfit by having MIRA fit accelerometers and recorders to the caravan, and then towing it around one of their surface proving tracks. The recordings showed some quite alarming motions in all sorts of directions, which could be reproduced on a multi axis orthogonal vibration rig so that components could be endurance tested to destruction. The rig looked like a space shuttle simulator.

DrZhivago · Mar 16, 2017

You have obviously put a lot of work, money and effort into this van which sounds to me like it still has very serious structural issues. You need to ask if it is worth it. You could probably buy a second hand example of the same type in much better shape for a relatively low price.

I would guess that the sides of the van are no longer firmly attached to the floor, as otherwise the shear strength (in the technical sense) of the sides would be supporting the ends of the floor as they should.

Dustydog · Mar 18, 2017

This is a very interesting problem that I am afraid is without resolution Imo .
A few on here will recall a case a few years ago involving an Avondale .
New a year before they went bust.
The side wall on the nearside pulled away from the floor leaving a gaping hole and rendering the caravan too dangerous to tow.
At the time I assumed , and still believe, it was a faulty manufacturing problem whereby the wall to floor screws were incorrectly installed.
We couldn't prove it and the owner dumped a three year old caravan, literally.
Other BPW and Al-ko chassis have been around for years and on the face of it done the job. I doubt the problem here is the chassis but more probably the caravan structure itself failing. No amount of welding will fix this.
Note the chassis are galvanised needing grinding to ensure a good weld.
So for me in 39 years this is the 2nd case I have heard of .Is it more common than we think?
Are the chassis inadequate or on the limit?
No idea but I'd be gutted if my caravan had this problem!

BryanTheSnail · Mar 19, 2017

Hello John,
I agree there are a range of dynamic forces involved with a moving unit, my point is that a substantially rigid "box" is possible in a caravan, but more problematic with a 4 (or 6) point wheel layout as found in a motorcaravan..

ProfJohnL · Mar 19, 2017

BryanTheSnail said:
Hello John,
I agree there are a range of dynamic forces involved with a moving unit, my point is that a substantially rigid "box" is possible in a caravan, but more problematic with a 4 (or 6) point wheel layout as found in a motorcaravan..

I haven't looked that closely at motor home chassis so I cant make an informed comment about them. But without any doubt caravan bodies and caravan chasis mutually supply strength and rigidity to each other.

20170602yuanyuan · Jun 2, 2017

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