"3 Point" vs "4 Point" Pivot Frame

[iandraz]

It seems sometimes the terms "3 point" and "4 point" are used interchangeably to refer to the torsion free mounting system used by Unicat, Mercedes Unimog, and others over the years. I think there is an important distinction between the two. I'm not sure if there is a good reference for this but let me try to articulate :) what I'm thinking:

The 4 point system is the one used by Unicat on MAN and Unimog trucks, and I think is officially recommended by Mercedes for use on Unimogs. It first involves a pivot at the front (near the cab) and a pivot at the back. These pivots (bushings) allow rotation about an axis parallel to the truck centerline but restrict all other motion. Second is a mounting point near the center of the truck frame which is either rigid or only allows rotation about an axis perpendicular to the truck centerline and parallel to the ground. As I understand it, this system has been used for at least 30-40 years on many different types of vehicles with great success. This mounting system distributes the load by applying it at three separate points along the truck frame. But the middle point is actually attached to the truck frame at 2 points, hence the "4 point" definition.

The "3 Point" system is the one used on the Hackney truck and the Fuso Szulc. I do not think it is used by the European expedition vehicle manufacturers on any trucks of this size. The 3 point system consists of a rigid mounting either at the front or back of the truck, and a pivot at the opposite end which allows rotation about an axis parallel to the truck centerline. While this is the simplest way to acheive a torsion free mounting, this mounting system has recently resulted in the failure/fatigue of 2 Fuso FG frames. It appears that this mounting system greatly amplifies the stresses in the frame, primarily because it is only contacting the frame at 2 points (front and back). In fact the difference in frame stress is similar to the difference in mounting 33% heavier camper. Also, I think having the rigid mounting at the extreme end of the camper vs. in the middle further amplifies the stresses on the frame in the dynamic condition when driving over rough terrain where the truck rocks side to side.

While the 4 point mounting has been "tried and tested" on many different truck chassis over the years and on extremely rough terrain, it seems the 3 point mounting system has generally resulted in frame cracking and/or failure when the truck weight approaches the rated GVW. I think it's important to distinguish between the two, and not write off the 4 point design because of failures in the 3 point design. The 4 point design is simple, effective, and has been used with great success for many years on all types of vehicles. The 3 point design appears to be okay for smaller, light weight vehicles, but inadequate for a near-GVW Fuso.

If anyone has any diagrams or references to add to this I'd greatly appreciate it. Or if I'm totally off track please correct me!

- Jacob

[FusoFG]

I do think the 4 point mount is tried and tested but I think a properly designed and attached 3 point mount is adequate for a light ( 3,000 - 4,000 lb ) payload.

Carl Hunter used a 3 point mount like you describe for mounting a 'Bigfoot' trailer on his 2002 Fuso FG.

And he drove it from London to China via the Silk route, and to the tip of South America and back with no trouble, not even a flat tire.

I have a 3 point mount just like Carl's and haven't had any trouble after 3 years and 36,000 miles.

I think it's important to note that Carl's Fuso was probably 2,000 lbs under GVW.

My first FG was 1,000 lbs under GVW and my current FG is 4,000 lbs under GVW.

Marvin Patchen, who built the first FG camper I heard about was 3,000 lbs under GVW.

RUF Inc's first FG camper uses a 3 point mount but it is attached to a sub frame that makes full contact with the FG frame rails. I believe this camper was right at if not slightly over the GVW.

I think there is a lot more to the problem than just GVW - including maximum axle weight, maximum wheel weight and weight distribution across the 3 or 4 points of the mounting system.

There is a lot of important information in the Factory Body Builder Manuals - especially the ones that describe mounting a dump body. This is the mounting problem most like a 3 point mount. They usually call for a reduction in payload depending on how far behind the rear axle the dump body pivot is located.

Lightening the load and slowing down will make everything last longer.

Someone else said just because a person can weld doesn't make him a fabricator. Well just because a person can fabricate doesn't make him an engineer and / or a designer either.

[Tennmogger]

I suggest the 3 point mounting is the basic torsion-isolated setup and a 4 point (or 5 point, etc) is simply an addition to the basis setup. Your descriptions are good! There will always be only one non-rotating mount to both frame rails, and there can be numerous center-line pivot points.

A truck bed might require the extra support of a 4 point/5 point, but Mercedes uses the basic 3 point isolation on the cabs, and on the engine and transmission combination (except on divorced engine-tranny combinations).

Tennmogger


It seems sometimes the terms "3 point" and "4 point" are used interchangeably to refer to the torsion free mounting system used by Unicat, Mercedes Unimog, and others over the years. I think there is an important distinction between the two. I'm not sure if there is a good reference for this but let me try to articulate :) what I'm thinking:
...chop....
- Jacob

[dzzz]

...............
I think it's important to note that Carl's Fuso was probably 2,000 lbs under GVW.

................

It seems the metal's structure is first changed, then broken. If the "hammer" isn't big enough, the metalic structure doesn't change.
At least this is my understanding from reading.
If I remember Carl's rig, he didn't spend time building up the frame. He welded some light pivot points.

[ntsqd]

3 points in space define a plane. A fourth point can not always be co-planar, so I do not see a 4 point mount as being torsion or bending moment free 100% of the time. Simply isn't possible. How often it is co-planar, then, is what becomes important.

If we assume even force distribution and the fourth point is co-planar 98% of the time and the vehicle is significantly under GVW, then that 2% may not be terribly significant since the force behind the torsion or bending is within the design limits of the frame. Were the vehicle over GVW, then that 2% is likely very significant since it has exceeded the frame's design limits. If the force distribution isn't fairly uniform then because of the localized forces the frame's design limits can easily be exceeded by an under GVW loading, which could be easily overlooked.

I can see the appeal of 4 or more points, but I think that the better design will be one that uses three points and effectively distributes the load evenly along the frame rails.

I'm reminded of an old race car design saying "Simplicate, and add Light-ness."

[dzzz]

In practice one of problems with three points may be the sole pivot between rails. It's modifying the rail design with a strong cross-connections. It also isn't a true point, and probably transfers torques in unusual ways. A better connection might be an X shape with four connection points sitting on hardwood. A less stiff shape than a tube in the cross connection might also help.
The three point system as I know it is designed to preserve the cargo - the camper box. With Unimog and Man the camper is apparently a pretty wimpy load on the frame. With unimog in particular the frame needs to be built well in excess of the GVW to allow for machinery point loads.