Torsion-Free Sub-Frame

Bajaroad

Adventurer
A question for those of you that have torsion-free sub-frames, whether 3-point or 4-point.

I understand the idea that the truck frame twists independently of the camper frame on uneven ground. But I was wondering if there is an affect on the stability of the camper while driving on the road, especially in the case of the 3-point frame.

In other words, if I take a hard turn on the road, does the camper flex outward on the axis of the sub-frame pivot?

Or maybe the shear strength of the camper walls keeps everything rigid?
 

dhackney

Expedition Leader
You will experience roll / flex of the camper / frame assembly in response to turns, road tilt, off-camber 4x4 use, etc.

We installed 50/50 Fox racing shocks on the frame to quiet its motion down and it made a huge improvement in handling and feel of the entire rig.
 

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Bajaroad

Adventurer
Michel Szulc Krzyzanowski said:
Also the Fuso Szulc has shock absorbers next to the pivotal system and the independent movements of the camperbox are not noticed when driving even in uneven terrain.
Thanks Doug and Michel.

Michel, if you are not seeing independent movement on uneven terrain, is it possible your shock absorbers are too stiff?
 

Tom_D

Observer
TFS input

On a typical three point system, if the two points of contact are on the middle of the frame and the single contact is on the rear end, the camper stays quite level while the rear wheels can twist with the ground. My camper moves over rough and rutted dirt it is very stable on the highway with no shocks. I know of no Unimogs that need shocks either.

Tom
 

egn

Adventurer
Tom_D said:
On a typical three point system, if the two points of contact are on the middle of the frame and the single contact is on the rear end, the camper stays quite level while the rear wheels can twist with the ground.
The disadvantage of this setup is that you have to provide a lot of space above the rear wheels compared to a setup where the two points of contact are exactly at the rear axle.

You may also have to look into the instructions of the vehicle manufacturer regarding platform mounting. It may not be allowed to use some setups to prevent the break of the frame under heavy load and maximum twist.
 

Robthebrit

Explorer
Tom_D said:
On a typical three point system, if the two points of contact are on the middle of the frame and the single contact is on the rear end, the camper stays quite level while the rear wheels can twist with the ground. My camper moves over rough and rutted dirt it is very stable on the highway with no shocks. I know of no Unimogs that need shocks either.

Tom
This is true, my mog camper is a 4 point mount and it has no shocks. Also you can't move it by being inside. Could it be the pivots are not tight enough if you need shocks? In mine you can get it to sway by jumping around on the inside but its the suspension moving not the mount. I don't see any sway at all when driving down the road, the camper very gently bobs about even doing 40mph over severe washboard heading to the racetrack the camper was pretty rigid. The only time I get a noticable shift in the camper is when going over uneven ground and rocks.

Rob
 

Bajaroad

Adventurer
Tom, I'm glad to hear your rig is stable.

So, in the simplest analysis, a 4500lb camper will be more stable than a 8000lb camper. The 8000lb camper will need additional suspension work and camper damping.

My ever-evolving plans calls for two outside rubber engine mounts to go as close to the cab as possible - minimizing the mismatch between the body and the cab (pass-thru), and a centered rubber engine mount at the back - allowing angular deflection. I also plan to add an 3-5 pt air spring system which will distribute the subframe load on the truck frame, and also add stability and vibration damping.

Rear tire clearance: My present subframe design height doesn't seem to have an issue - even with the larger SRW there is about 9-10 inches.
 
A 3 or 4 point mounting system is not a "suspension". It doesn't matter if my camper weighs 20 or 100 tons, it wouldn't lean outwards on turns (relative to the frame) unless the frame rails flexed assymmetrically. It should merely allow the frame to flex without putting torsion or stress on the floor.
My advice: don't do what you mentioned in the last post, designing a true suspension for the camper. You will be asking for instability. Copy Unicat's system.

Charlie
 

Bajaroad

Adventurer
charlieaarons said:
A 3 or 4 point mounting system is not a "suspension". It doesn't matter if my camper weighs 20 or 100 tons, it wouldn't lean outwards on turns (relative to the frame) unless the frame rails flexed assymmetrically. It should merely allow the frame to flex without putting torsion or stress on the floor.
My advice: don't do what you mentioned in the last post, designing a true suspension for the camper. You will be asking for instability. Copy Unicat's system.

Charlie

Charlie, I hear what you are saying, but what I am proposing is no more a suspension than an engine mount is a suspension. An engine mount is more a shock absorber/vibration mount.


Anyhow, what I am designing is a 3-point system in which the three points are fail safe rubber engine mounts. The 3rd point center mount will pivot much like a pivot point on a Unicat, except it will absorb shock as well. Rob Gray in Oz took this same approach with his first big rig. Barry Controls makes these particular engine mounts and you can order them straight from McMaster Carr.

I agree with you that a strong enough box will not sway or twist on a 3-point frame, but that is because the walls of the box/or bulkheads are extremely strong in shear. I can't say for sure, but I'm guessing DougH doesn't have the shear strength necessary to control the flex on his subframe as much as he would like, so adding dampers stabilizes reduces flex when making sudden turns or hitting bumps.

There are two reasons I am probably adding airbags in addition to the 3point system.
First is I am building a convertible/modular camper system that enables the truck to be used as a flatbed or an expedition camper. While the camper is attached, it will be seamlessly and rigidly attached to the subframe/flatbed, adding all the shear strength a permanently built camper box would. But while the camper is off, it won't have nearly the shear strength (no walls) - Just like a rectangular tube is torsionally stiffer than a plate of the same wall thickness. Air springs placed on opposite sides of the pivot point will reduce the need for the subframe to be as strong in shear. While offroad the air pressure will can be reduced to allow the truck frame to flex without transferring stress to the subframe. A group of us ride the White Rim trail in Canyonlands NP on mtn bikes every year or so. I hope to have the subframe/flatbed finished so I can use it as the support vehicle and test the frame.
Second reason for adding air springs, I am buiding the entire subframe and camper frame out of 6061 aluminum. Aluminum is a great material, but considerations for cyclic loading -i.e. vibration, must be made. Using air springs to support and dampen the frame between the mounting points will greatly improve its resistance to fatigue and spread the load more evenly over the truck frame.
Like I mentioned, the design is ever evolving, so who knows . . .

I bet you have you the best built strongest camper box and subframe money can buy, but it too has limitations and with 100 tons, your subframe and box will twist.
 

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FusoFG

Adventurer
Bajaroad said:
A question for those of you that have torsion-free sub-frames, whether 3-point or 4-point.

I understand the idea that the truck frame twists independently of the camper frame on uneven ground. But I was wondering if there is an affect on the stability of the camper while driving on the road, especially in the case of the 3-point frame.

In other words, if I take a hard turn on the road, does the camper flex outward on the axis of the sub-frame pivot?

Or maybe the shear strength of the camper walls keeps everything rigid?

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,
 

haven

Expedition Leader
movie of torque-free mount

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
 
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