pivoting frames and mounting campers

[whatcharterboat]

iandraz

I'm not saying the 4 point mounting is the only way to do it, and it's not ideal for all situations. But if I had to choose someone to design a mounting for trucks traveling rough terrain, my money would be on Mercedes Benz due to their great experience and engineering resources in this field.

Hahaha!!! Jacob, that's not fair. Yeah, you're right and my money would be on Mercedes too. But likewise my money would also be on Mercedes to put together a chassis that could support that mounting design. If Mercedes were designing an FG chassis I don't believe it would be with the same 4.5mm steel that Fuso used.

One thing I haven't commented on is how the strength at rear of the chassis drops away drastically. Dhackney has made available a Section Modulus diagram for the FG chassis on his website that you can refer to. The diagram shows exactly how much "strength" (for want of a better term) the chassis has at any given point along it's length. Look closely at the tapered rear end.

Now when you do a 4 point pivot design on an FG I guess the middle mounts are around the cross member at the forward rear spring hangers and the rear pivot mount goes close to the very rear of the chassis. So those two areas have considerable down force while the area that is receiving considerable up force is around the rearward rear spring hangers. Approximately halfway between the two down forces.

This is another reason why I don't believe Fuso ever intended their chassis to be mounted this way and why they recommend the load should be distributed evenly along the chassis. I've seen others express this view already. Maybe I've just stated it differently but when you want to break a stick over your knee you grab it at either end, put your knee halfway along the stick and pull back. I see this as what is happening to the FG frames in question, only your knee is the diff pushing up through the springs. Resulting in bent or twisted chassis rails.

The pivot designs that have incorporated a separate frame over the top of the original chassis seem to have escaped this fate but at the cost of extra weight and height. Still at the end of the day they have isolated the camper which was the main idea to begin with.

Kerry

I think Doug Hackney has posted in the past that MAN expedition campers have had problems with frame failures. If I'm right in remembering that, I wonder if these failures are related to pivot mounts in the same way that the recent FG failures are related to pivot mounts.

The MAN chassis seem thin too however they are very high tensile and maybe thats how they get away with the it. As we are now in the process of constructing MAN based expedition vehicles, MAN here in Oz have definitely stated that we not use pivot mount designs on their trucks in favor of their own mounting system which also supports the load evenly along the entire length of the chassis. Sounds like Fuso heh? To me the MAN mounts don't look as though they would effectively isolate a lightweight composite camper body but I'm sure they are fine for military style bodies and trays. See the pic in the "Brisbane Truck Show "thread of the MAN Kat.

 

[DiploStrat]

Bingo!

"The pivot designs that have incorporated a separate frame over the top of the original chassis seem to have escaped this fate but at the cost of extra weight and height. Still at the end of the day they have isolated the camper which was the main idea to begin with."

Have to remember why we are doing this - we are trying to protect a large volume, lightly constructed camper. MB, Mitsubishi, et al, normally could care less about this market, it is too small. The closest parallels may be buses (which lack interior partitions) and ambulances (which are usually smaller). Big cargo vans and trailers are made to flex and have no partitions.

All of this means that the market is very small, and thus, with the exception of a few specialty manufacturers in Europe, Australia, and the US, not many people are really worrying about this.

A pivoting frame is an excellent, if complicated, way to isolate a camper from frame movement. The only trick is that it is a bad idea if it raises the stress points on the frame and thus damages it. (Probably cheaper to replace/repair/reinforce the camper than the frame.) Don't overlook the benefits of a more compliant suspension as well.

 

[FusoFG]

iandraz

If Mercedes were designing an FG chassis I don't believe it would be with the same 4.5mm steel that Fuso used.

Just an FYI, Mercedes Benz owns Fuso.

 

[haven]

This blog entry by Michel Szulc Krzyzanowski clearly shows Eric Ferguson's design for a subframe that rests full-length on the Fuso chassis, and that provides flexible bushings to attach the subframe to the camper body.

http://michelszulckrzyzanowski.blogspot.com/2009/05/rebuilding-fuso-szulc-part-4.html

I assume that the subframe will be attached to the truck chassis using conventional U-bolts. The photos don't show any wood or rubber strips separating the subframe from the camper, but I assume (a second time) that the strips will be added during final assembly.

Chip Haven

 

[Terrainist]

The subframe has welded plates bolted to the truck frame for mounting, for clarity's sake (per photos).

Looks like the metal strips seen in previous photos were used between the subframe and frame.

Will be good to see the camper mounted. Hope that goes easy enough.

 

[Jacob7963]

Seems this is a hot subject...I don't mean to critique any one camper, I think there are many different ways to design an expedition vehicle and there are well designed vehicles both with and without pivot frames.

I found the MAN body builder guides here:

http://manted.mn.man.de/manted/aufbaurichtlinien/gb_all.html

There is an interesting statement in 5.4.2 of the document for TGL/TGM trucks:

"Closed bodies in particular, such as box bodies, are designed to be relatively torsionally stiff with respect to the chassis frame. So that the desired twisting of the frame is not hindered by the body, the body fixtures should be flexible at the front. For an example, see Chapter 5.3.6., ‘Flexible connection’ (see also Fig. 41). This is not sufficient for off-road vehicles. For this application, we recommend a front body mount with a three-point or diamond-shaped mounting (see Fig. 46 for mounting principle)."


I see Unicat is now building a camper on an Iveco Daily chassis, which I believe is similar to or lighter than the Fuso. They have some good pictures showing the motion of their pivot frame on this page:

http://unicat.net/en/pics/EX37-IvecoDaily4x4-2.html

 

[haven]

Thanks for sharing the link to the Unicat on Iveco Daily 4x4 chassis.

The Daily is more like the Mercedes Sprinter than the Fuso FG. It's
a lighter duty vehicle, so you have to be careful with the camper weight.
Here are the specs:

Iveco Daily 4x4
wheelbase 120 in or 134 in
GVWR 7,716 lbs or 12,250 lbs
3.0L turbo 4 cylinder diesel engine
176 hp, 295 ft-lbs
full time 4x4, center differential
center, front and rear diff locks
6 speed ZF manual transmission
front disc, rear drum brakes
9.5R17.5 or 255/100R16 tires

There are several similar cab/chassis trucks available in Europe today.
The Bremach T-Rex, the Mercedes Unimog U20, the Renault Mascott, and
the Nissan Atleon have similar capacity and 4x4 equipment. Isuzu and Fuso have
2WD cab/chassis, but I don't think they are importing 4x4 models in this weight class.

 

[dzzz]

Iveco Daily 4x4
wheelbase 120 in or 134 in
GVWR 7,716 lbs or 12,250 lbs
3.0L turbo 4 cylinder diesel engine
176 hp, 295 ft-lbs
full time 4x4, center differential
center, front and rear diff locks
6 speed ZF manual transmission
front disc, rear drum brakes
9.5R17.5 or 255/100R16 tires

That's a step up from the sprinter. But certainly closer to a sprinter than a man or unimog. The ground clearance and engine are nice.
With the 12K GVWR, I don't think the camper couldn't go over 1000lbs. Any idea what the unicat camper only (dry) weighs?

 

[charlieaarons]

. Any idea what the unicat camper only (dry) weighs?

The military version of the Daily "cargo version" weighs 2900kg empty. That probably has a bed that weighs 3-500kg. So the cab-chassis is ~2500kg. I'd estimate the dry weight of the camper is 1800kg (~4000 lb) with 2X4D batteries. Water and fuel is another 600kg. Tire maximum spec is 134J or 4240kg/axle@84 psi@62 mph.
I think my camper box is ~2700kg dry, empty, minus 2 batteries, and I took 2/3 of that.
The tires won't be overloaded but it might be easy to fill the camper up to GVW, whatever that is.

Charlie

 

[PeterM]

The actual Daily 4x4 is 2700-3000 kg empty - no linkage to the old military WM models

Regards,
Peter

 

[Tom_D]

late reply

I have been catching up with this discussion. I thought I would share my experiences with Darrin Fink's FG.

He used full length boxed rails on top of the frame to which the pivots are welded. The system is a three point system. Two mount points in the front with silent bushings and one (the pivot) in the rear with bushings too. This subframe is attached by several U bolts around the frame and the unit is cushioned with oak. This system is essentially like mounting a rigid service body but adds some protection for the camper by adding pivots and rubber bushings. Also he limits the rotation of the the camper to 4 degrees as advised by FUSO.

The full length rails add stiffness to the frame and spread the load along the length of the frame. I think this is good for strengthening the frame and stiffens it too, but not so much that the camper doesn't ride smooth and quietly over rough terrain.

You can see the box flex thru the pass through from the cab to the camper. It does not flex anywhere near the amount some of the Unicats do in their videos, or even my Unimog, but it is enough to keep the camper quiet and settled. It flexes enough that I have to tape the rubber bellows between the cab and camper or it will pop out (it must be removable to tilt the truck).

The truck has 30K miles, including 10K miles in Alaska and the Yukon on lots of bad washboard including the Dempster Hwy. I just inspected the frame and subframe components in prep for this years trip and everything is tight, straight and sound.

Cheers,
Tom

 

[Jacob7963]

Thanks Tom very much for the info.

It sounds like this mounting system is basically similar to the original system on the Fuso Szulc. The difference being that Darrin mounted to full length rails on top of the Fuso frame, rather than directly to the frame.

Do you notice much side to side sway or oscillation at the rear of the camper on rough terrain? Is there a damping cylinder like on the Hackney and Szulc trucks?

Is the front rigid mounting on the high section of the frame near the cab or the low part farther back?

It's interesting that Fuso recommended limiting the twist to 4 degrees. I'm planning to include some sort of bumpers/stops into my design to avoid excessive frame twist.

 

[Tom_D]

more on the RUF frame

There is no damper system. I have never noticed any sway. I find the ride very controlled and safe at all speeds. The biggest issue is the overall stiffness of the FUSO suspension forcing careful, slow speeds over rough roads. Darrin's subframe is very heavy duty which may add to the stiffness. Of course it does add weight which is another issue for the FG.

Darrin uses stops to limit rotation beyond 4 degrees.

The front mount is back behind the step down.

Check out Darrin's web site, I can't remember but he may have photos.

Tom

 

[Mickldo]

Here are some links to the Code of Practice we need to follow here in Oz.

This first link is for all of the Vehicle Standards Bulletins. http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/index.aspx

From that page check out the Heavy Vehicle section. http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/vsb_06.aspx

And for this thread in particular the one you want to check out is http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/pdf/vsb_06_j.pdf

There is a lot of info on there about body mountings.

 

[ntsqd]

Awesome bunch of links there Mickldo. :victory:
I'm still digesting the body mounting pdf.