Torsion-Free Sub-Frame

[Gold Boy]

Avi Meyers has a short movie on the Unicat Americas website that illustrates the motion of a torque-free subframe. The movie shows Avi driving his International 7400 Unicat over rocky, rutted, and sandy terrain in North Africa.

http://www.unicatamericas.com/video/international.mov

Like FusoFG says, you can clearly see the camper stay parallel to the rear axle as the cab of the truck stays parallel with the front axle. The camper doesn't flop around on the frame of the truck, it just moves in unison with the rear axle.

Chip Haven

this suggests to me, that two pint linkage is over the rear axle and and there maybe a front single pivot at the front of the sub-frame.
the front single [dual pivot on a frame cross member] pivot could be located at the optimum point twist on the frame?

 

[Bajaroad]

Brent,

No the camper doesn't flex outward in relation to the truck chassis during a turn.

There maybe some lean relative to the road, but the camper and chassis lean together in the same amount so there is no movement of the camper relative to the truck chassis.

Assuming the camper is rigid enough.

The camper doesn't twist relative to the truck chassis when you move around in it when parked either. It can't because at some point it's rigidly attached to the truck chassis. Either at one end in a 3 point system or in the middle using Unimog's 4 point system.

Movement between the camper and truck should only occur when the front axle twists in relation to the rear axle and causes the truck chassis to twist. That's part of the articulation that keeps the wheels on the ground on uneven surfaces.

I think any 'movement' reported by owners when the truck chassis isn't twisting is caused by a) cab movement - the FG cab is flexibily mounted, b) suspension movement, or c) flex in the camper itself.

It sounds like you are describing the kind of air ride suspension system used on 18 wheelers to mount the cab and sleeper box. Especially the ones with custom 13' long sleeper boxes with bed, bathroom and kitchen.

They mount them on air bags and shocks for comfort while they drive, not to accomodate chassis flex off road. They appear to move quite a bit when you seem them hit bumps on the expressway.

I would be concerned about mounting a 4000 - 8000 pound camper using just rubber 'motor mounts' without some type of addition metal fastening system that would keep the camper connected in the event of a motor mount failure.

Good luck,

Thanks for the response Tom.

I'm affraid I am not clearly communicating my ideas - sorry for that.

The airbags I'm describing are only supplemental, distributing the load more evenly - not like a truck air spring sys.

The engine mounts I am referring to are rated up to 4500lbs (EACH) depending on the model (I am looking at the series 22000) and they are fail safe, meaning they have a bolt that runs through them. Even if the polymer was to fail, the bolt would not allow a disconnect. If you look at Rob Gray's rig it's one of the biggest/heaviest I've seen, he got by fine with just 3 of these mounts - years of use.
Some of these rubber mounting products are rated to 10,000lbs, so its only a matter of applying them correctly. From my experience, I am more concerned with metal to metal connections on vibrating systems. Vibration and shock are killers. We develop DC Contactors for aerospace and ground vehicles (Bradley), and the first qualification test they want to do is shock and vibration. I'm sure I could get by without the shock mounts, but I believe they are a good benefit.
I hope to start cutting material in a week or two - I'll keep you posted on the progress.

 

[Robthebrit]

Mounting the cab on shocks is a really good idea but you have to keep some sort of tension otherwise it'll be boat. My mog was factory modified for this and the difference in ride quality is huge. The cab is 4 point mounted, there are two flexible rubber mounts at the rear and two shocks at the front - typically there is a solid mount at the front and sligtly different rubber mounts at the rear. This saves everything in the cab from taking a beating when on dirt. Air ride seats save the people in the cab even more, I can barely tell I am on dirt unless I go fast.

Rob

 

[Bajaroad]

Mounting the cab on shocks is a really good idea but you have to keep some sort of tension otherwise it'll be boat. My mog was factory modified for this and the difference in ride quality is huge. The cab is 4 point mounted, there are two flexible rubber mounts at the rear and two shocks at the front - typically there is a solid mount at the front and sligtly different rubber mounts at the rear. This saves everything in the cab from taking a beating when on dirt. Air ride seats save the people in the cab even more, I can barely tell I am on dirt unless I go fast.

Rob

Rob, I think that would be difficult on the Fuso because the cab tilts on a rigid pivot. That would be a nice improvement though. It's impossible for me to evaluate the ride of the FG right now, because I have nothing on the chassis yet - very harsh ride.

 

[Robthebrit]

A mog typically has a single connection at the front but its not used for tipping the cab. There is a set of flanges on the front of the cab which are used for tipping and in general its a royal pita as you have to pry bar the front of the cab up a few inches. Lots of people don't do this and just tip using the front hinge.

The system on my cab is actually better and I can tip by cab anywhere without any special hardware. The front hinge is not even there, the shocks on the front of the cab don't go to the frame, they go to a flange which is bolted to the frame with some rubber torsion pivots. To tip the cab all I do pull out a few blots, disconnect the steering and lift the back. The previous owner welded an extendable bar under the back of the cab and with that a few strong people can lift it or if you are not feeling strong you can use a high lift.

I am sure a similar system would work on the fuso but it will require some modifications. I can send pictures if you are interested.

Rob

 

[charlieaarons]

I was speaking "metaphorically" to point out that the Unicat mounting system only responds to frame rail assymmetric motion, not inertial or gravitational loads on the camper. Incidentally it is hinged in the front and the rear with the "fixed" tubular mounting just above the rear axle. There is no subframe except for steel bars embedded in the camper floor tapped for mounting bolts for the brackets.
Definitely stronger camper boxes exist, like globalexpeditionvehicles, with internal framing and aluminum skin. They are likely heavier as well.

Charlie

 

[Robthebrit]

What Charlie says is true for all properly configured multipoint/pivot mounts, nothing special about the Unicat system. The pickup beds on my other unimogs have a 3 point mounts too.

As Unimogs go the U500 is not that bendy, its designed as a semi rigid frame. The UHNs (U3000,U4000 and U5000) still use the ladder type frames as used in the older 1300s and 406/416s. You can get a huge amount of twist across the frame, multi point mounts are essential and on these mogs everything is mounted in a 3 point system including the cab, engine and tranny.

Rob

 

[Bajaroad]

You Mog guys are preaching to the choir - I'm on board with the properly designed 3-pt system.
My point is that a properly designed 3-pt system is not necessarily adequate for a weight bearing flatbed and the bed will respond significantly to inertial or gravitational loads depending on the load and shear strength of the flatbed. I've done the FEA analysis on my flatbed frame sitting on 3pts - a camper box on top makes all the difference.

A 4-pt system that you describe should reduce that response by about half if the two outsides mounts are in the middle. That's probably the best approach for my flatbed, but I want the subframe to follow the cab as much as possible, so I'm going to stick with 3pts and a supplimental air springs.

Just to argue . . . I will say your subframe does respond to inertial loads, it's just that the response is too small to notice. Continue to increase the inertial load and the response increases more or less linearly.

Thanks for the discussion.

 

[Robthebrit]

Just to argue . . . I will say your subframe does respond to inertial loads, it's just that the response is too small to notice. Continue to increase the inertial load and the response increases more or less linearly.

Thanks for the discussion.

I don't think thats much of an argument, in theory you are 100% correct.

Rob

 

[Bajaroad]

Vertical location of pivot point on 3PT subframe

Has anyone thought about the vertical location of the pivot point in relation to the twisting frame?

I would assume when the frame twists the axis of that rotation is centered between the frame rails, and this location would be the ideal location of the pivot axis.

 

[Gold Boy]

Has anyone thought about the vertical location of the pivot point in relation to the twisting frame?

I would assume when the frame twists the axis of that rotation is centered between the frame rails, and this location would be the ideal location of the pivot axis.

yes, i have. post#16

i suspect that the optimum twist will be at where the frame steps up... theoretically speaking.

however, if the new fg has a more rigid frame i wonder how this would change the the 3 point pivot frame

 

[Bajaroad]

?

Why would the optimum twist be at the step-up. The modulus is highest at the step-up.

My question was about the location of the axis of rotation of the twist - see attached. If the 3pt pivot point is not on this axis then it will have a laterial component (sideways movement) when the frame twists. I worry this will place stress on the two stationary points.

 

[egn]

My question was about the location of the axis of rotation of the twist - see attached. If the 3pt pivot point is not on this axis then it will have a laterial component (sideways movement) when the frame twists. I worry this will place stress on the two stationary points.

In reality you will never have a rotation exactly at this point because the twist is never fully symmetric. And you also get some bending. So you have to give some room for lateral movement in the bearings anyway.

 

[Robthebrit]

My camper has lower pivots and the box is directly mounted to them to get it as low as possible. I don't think it could go any lower without the frame and box contacting at max flex. I posted pictures elsewhere but I am not sure where.

Rob

 

[sprale]

Adjustable shocks on sub-frame

Has anyone thought of/tried using adjustable shocks on the sub-frame? You could tighten it up for use on smoother maintained roads, then soften them up for use on unmaintained surfaces. This could help control body roll on more heavily used roads where traveling with traffic would mean higher safety needs, while still providing off-road flexibility with a simple adjustment. I'm sure someone could be creative enough to run a remote adjuster inside the cab.