Chassis twist - an issue for motorhomes?

Hi all,

I have a 516 CDI dual cab 4x4 which we are putting a foam sandwich unitary construction motorhome body onto. These are inherently stiff, and don't like being twisted.


Thus we normally (ie, on Japanese 4x4s and light trucks like Mitsu Canter 4x4) fit a centre pivot beam at one end to allow the chassis to twist underneath. You can see one of our pivot beams on a Navara, part way down this page: http://www.insideoutbackcampers.com.au/vehicles/2018-micro-motorhome.html

The Sprinter Body Building Guide, which is excellent in all other respects, makes no mention of chassis twist. It actually instructs bodies to be rigidly mounted to each chassis mount AND optionally to the back of the cab.

The chassis LOOKS like it has been designed to resist twist, but our test induced about 8mm from rear of chassis to back of cab. So it is there, albeit less than any other chassis we've ever done.

So - how have others dealt with this? Anyone had ay long term experience with an expedition type vehicle re cracking of body or chassis?

Thanks in anticipation...

"Soft mount" the motorhome body.

Just put a boot seal between the cab and the box. Hard mount it to the rest of the chassis. Also, Understand that if you rigidly mount a stiff sandwich construction box to the chassis, that you will be adding an order of magnitude or more of torsional stiffness to the system and will not experience anywhere near the twist you get with the bare chassis alone.

Plenty of companies rigidly mount composite motorhome bodies to sprinter chassis, seemingly without issue.

Build one and test it. If it doesnt meet your requirements, adjust your mounting to an isolation mount.

Why do you suggest soft mount to cab? Wouldn't a rigid connection there share the load a bit more uniformly?

I am aware that most Euro motorhome builders rigid mount chassis back of cab and over the top of cab too. But most Euro motorhomes don't cross 1100 sand dunes on the Simpson Desert.

InsideOutBack said:

Why do you suggest soft mount to cab? Wouldn't a rigid connection there share the load a bit more uniformly?

I am aware that most Euro motor home builders rigid mount chassis back of cab and over the top of cab too. But most Euro motor homes don't cross 1100 sand dunes on the Simpson Desert.

Click to expand...

On second thought now that I have had more time to think about this:

As a mechanical engineer with nearly 40 years of mechanical design experience fundamentally it is not good practice to use "add on's" to stiffen the primary structure.

Some twist is good. Look at the wings of a 737 during take off and flight. The wings flex, why, because if they were infinitely stiff they would fracture and potentially fail.

Now you have to ask yourself whether that amount of twist would do harm to what you are bolting to it.

8mm is about .320" that is a very small number and the greater the distance it is over the smaller the bend angle.

Over 20ft for example you are talking about an extremely small bend angle (this assumes everything takes place linearly).

In this case I wouldn't worry about it. Hard mount it, you'll be fine.

Yes that's what Mercedes Benz have recommended also. Just feels a bit weird when for the last 37 years (I'm three years behind you!) we have worked with floppy chassis and pivot mounts. There's always something new...

Assuming you have a strong composite box, I would hard mount the box to both the front cab and the "frame". I don't think you need to use a boot for the cab, because the cab on a sprinter cab-chassis is part of the unibody, and will not move the way a separate frame/cab does on a big truck or E series.

You will want to use some rubber, or proper thickness urethane adhesive between the composite floor and the frame. Thermal expansion differences between the composite and metal will produce undesirable forces otherwise.

Thanks Midwestdrifter, that is the system we have arrived at. Fibre reinforced high density rubber between floor and chassis, and urethane to cab. We will have some clearance between cab roof and peak overhang mainly because the shape of the roof is too complex to fit too. The overhang is only 1.2m anyway.

Are you going to put rubber mounts along the full length, or just at the front of the box mounting (i.e with a torsion free mount the rear is solid mounted and front has room to move).

And how much flex do you think you can allow for when urethane mounting/bonding the box to the front cab

Im in a similar dillema with a T1N 4x4 chassis crew cab

I eventually had a conversation with the techies at MB Australia, who confirmed the text of their bodybuilding guide: rigid mount all the way. I will be mounting on dense rubber 6mm thick on each point, but effectively rigid. Plus I will be bolting the body to the back of the cab, but not the overhang - the geometry is too complex to be practical, and balsa-core composite has plenty of strength to carry the cantilever.

To compensate for the extra stress applied to the body, I will add 100g to the outside skin (500g flax/bio-epoxy) and stick to 400g on the inside skin, but with another 400g carbon layer 250 wide each way to all corners, in and out. Plus the 'wasteband' - the stiffening weatherproofing ledge around the top of the lower body (what the pop top roof sits down on) will be stiffened a bit.

Then finally the rear-hinged roof will be allowed to 'slide' a bit laterally at the front so that any residual twist (which will manifest as lateral movement there) is not resisted at all, thus the roof and its tie-downs will not be subjected to any loads from twist.

Thanks for that.
I did a quick test using my wheel alignment gauge (on My T1N - your NCV3 is probably stiffer) . Max flex is 12.5mm per side/25mm total. So I think I will bolt box to cab and rigid mount first couple of bearers then towards the rear put in some machine dampers or rubber to allow for +- 12.5mm movement.