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Old 02-11-2020, 11:18 AM   #1
InsideOutBack
 
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Default 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....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... :-)
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Old 02-11-2020, 11:30 AM   #2
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Default Re: Chassis twist - an issue for motorhomes?

"Soft mount" the motorhome body.
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Old 02-11-2020, 06:57 PM   #3
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Default Re: Chassis twist - an issue for motorhomes?

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.
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Old 02-12-2020, 09:26 AM   #4
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Default Re: Chassis twist - an issue for motorhomes?

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.
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Old 02-12-2020, 11:53 AM   #5
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Default Re: Chassis twist - an issue for motorhomes?

Quote:
Originally Posted by InsideOutBack View Post
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.
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.

Last edited by eralimited57; 02-12-2020 at 12:13 PM.
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Old 02-13-2020, 09:30 AM   #6
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Default Re: Chassis twist - an issue for motorhomes?

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...
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Old 02-13-2020, 02:36 PM   #7
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Default Re: Chassis twist - an issue for motorhomes?

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.
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Old 02-17-2020, 10:04 AM   #8
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Default Re: Chassis twist - an issue for motorhomes?

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.
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