I've spent an inordinate amount of time in recent months wading through the murky waters of torsion free subframe design (specifically for an FG). One thing that keeps gnawing at me is the fact that there are a few examples on here of boxes that are rigidly mounted with u-bolts, yet have experienced zero problems after years of use off road. Furthermore, it occurs to me that the only cases I've read about where a frame actually failed was when some sort of torsion free type design was implemented. Forgive me for proposing such overlanding heresy, but isn't it at least possible that we're overthinking this subject? Can someone point me to a case where a standard u-bolt installation was used off road and eventually resulted in either a frame or box failure?
Rethinking the need for a torsion-free subframe...
- Thread starter glennm01
- Start date
I'll preface this by saying that i have zero experience with real heavy duty trucks.
However, My Lance 855 on my Ram 3500 weighed close to 13000# total. I'm sure you guys are building things significantly heavier than that. Are you really taking those trucks on "trails" that are rough enough to flex that much? I highly doubt it. I crawled my truck/camper combo on anything remotely close to requiring 4low. Like, super slow. It's pretty hard to tear stuff up at 1-3mph.
However, My Lance 855 on my Ram 3500 weighed close to 13000# total. I'm sure you guys are building things significantly heavier than that. Are you really taking those trucks on "trails" that are rough enough to flex that much? I highly doubt it. I crawled my truck/camper combo on anything remotely close to requiring 4low. Like, super slow. It's pretty hard to tear stuff up at 1-3mph.
Aussie Iron
Explorer
Then you haven't seen the videos of where I take my Canter.
My steel tray is bolted to the chassis with angle iron plates, sitting on the chassis and only bolted on the sides. Four along each side - the front mounts are no longer bolted there as the flex has finally taken the bolts out but it still rests on the chassis. The front plates are on top of the step at the rear of the cab.
My alloy Camp Box is rubber mounted to the tray and is showing a few cracks at the top of the doors so flex is certainly being transmitted to the box.
I probably take my Canter where others Dare not go,
Dan.
Come on man! You can’t post that without links to these videos.
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SkiFreak
Crazy Person
When I was initially looking for a truck I inspected over a dozen used FGs. On more than half of those trucks there were signs of cracking on the chassis in front of the step section.
Quite a few of these trucks had been configured as service vehicles and had hard mounted setups on the back.
You should also bare in mind that the FG does not have a strong chassis, being only a 4.5mm thick C section. These chassis flex like a wet noodle!
When you restrict that flex to a small area, by hard mounting a box to the rear, you will get focused stress loading. How that affects the flexing will be determined by the weight you have on the back and the terrain you take the truck into.
As has been mentioned already, that flex will be transferred into the box and may affect your internal fitout, or the actual box itself. A kinematic mounting system will limit that transfer of flex.
At the end of the day, it's your truck, so build it the way you want...
Quite a few of these trucks had been configured as service vehicles and had hard mounted setups on the back.
You should also bare in mind that the FG does not have a strong chassis, being only a 4.5mm thick C section. These chassis flex like a wet noodle!
When you restrict that flex to a small area, by hard mounting a box to the rear, you will get focused stress loading. How that affects the flexing will be determined by the weight you have on the back and the terrain you take the truck into.
As has been mentioned already, that flex will be transferred into the box and may affect your internal fitout, or the actual box itself. A kinematic mounting system will limit that transfer of flex.
At the end of the day, it's your truck, so build it the way you want...
SkiFreak -- thanks for the insights. Interesting to hear of the evidence of stress you saw in those service trucks, but without the benefit of knowing their history, it's hard to know how meaningful of a datapoint this is. Commercial non-owner/drivers very likely treat these trucks with a lot more disregard than any of us would, for example.
What I'm after is a case where an FG-based camper -- rigidly mounted, and according to the guidelines in the bodybuilders guide -- suffered a frame or box failure from off-road use. Better still if the owner is around to elaborate on the general extremeness of conditions to which the truck was exposed. My working theory being that for the sorts of terrain most of us are likely to ever want to expose our houses to, along with the level of driving care a fully invested owner is likely to implement, the standard Mitsubishi mounting method might be perfectly fine.
What I'm after is a case where an FG-based camper -- rigidly mounted, and according to the guidelines in the bodybuilders guide -- suffered a frame or box failure from off-road use. Better still if the owner is around to elaborate on the general extremeness of conditions to which the truck was exposed. My working theory being that for the sorts of terrain most of us are likely to ever want to expose our houses to, along with the level of driving care a fully invested owner is likely to implement, the standard Mitsubishi mounting method might be perfectly fine.
I'm currently designing the subframe for my canter build on an FG637.
Have been considering the same thing but decided to go with spring mounted to be safe, don't want to invest $10k+ into a body build where an extra say $500 for spring mounts would save the body is worth the cost in my books.
I was wondering if you could spring mount along the frame rails and just have it rest on the front upper section? Would mean I don't need to worry about extra spring mounts up there and a different plane of movement...?
Have been considering the same thing but decided to go with spring mounted to be safe, don't want to invest $10k+ into a body build where an extra say $500 for spring mounts would save the body is worth the cost in my books.
I was wondering if you could spring mount along the frame rails and just have it rest on the front upper section? Would mean I don't need to worry about extra spring mounts up there and a different plane of movement...?
SkiFreak
Crazy Person
I have 2 spring mounts at the front, but they offer no fore/aft/lateral restraint, so do pretty much what you are describing.
One suggestion... if you change the front uprights to match the angle of the step you will have less of an unsupported area at the front.
I'm guessing you have a shorty. Even so, that is a really short subframe. My MWB has a 4300mm long subframe.
Yeah its a short wheel base FG637, however I'm planning to store two spares between the cab and the body and keep it overall relatively short and not have the frame extend beyond the chassis rails at the rear.
I'll have a look at that, was about to start doing the moment analysis for weight ditribution etc and this will help. I'd probably add an angle brace rather than angle the vertical support to keep the vertical strength there.. I should start a build thread...
Aussie Iron
Explorer
Try looking in here. https://www.youtube.com/user/AussieIron1
They are from a few years ago, I have a lot more footage that I need to post up when I finally get to it.
Dan.
yabanja
Explorer
My short wheelbase FG flexes very little. This is largely due to the massive subframe I built according to the FUSO factory builders guide out of 3/16" box 10"x3". It is U-bolted every 18" along it's entire length. This combined with the reduced leverage of having a short wheelbase and a very soft suspension control the torsional flex. That having been said, I still have a three point mount and feel it has increased the longevity of the camper. There are literally hundreds of pages in these forums about the benefits of different designs-4 point, 3 point, spring mount, etc...... Not really worthwhile re-hashing it all. I chose 3 point because it has Zero torsional transfer to the camper. Spring mount by it's very nature has significant transfer directly related to the rate of the springs used. I found four point overly complex. So much of durability is directly related to how abusive you are going to be. FIDO(our truck) gets used almost exclusively off road and needed all the help it could get. See video. So far so good!
Allan
Fido in the dunes
Allan
Fido in the dunes
Have you searched on "Water Tank Truck Body Mounts"?
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The Artisan
Adventurer
I have a 07 and I plan on converting to 4x4 at some point.
For my flatbed I plan on using the spring mount idea but using body mounts stacked. Mount, angle, mount, angle and mount all bolted, 4 on each side.
The framerail will have them under the mounting tubes in place of traditional 2x4s and it will get ubolted like a traditional box. It will take some of the stress off the box.
Kevin
For my flatbed I plan on using the spring mount idea but using body mounts stacked. Mount, angle, mount, angle and mount all bolted, 4 on each side.
The framerail will have them under the mounting tubes in place of traditional 2x4s and it will get ubolted like a traditional box. It will take some of the stress off the box.
Kevin
billiebob
Well-known member
The only way to be torsion free is with a 3 point mounting system. If you bolt to the frame with 4 or more points, regardless of how stiff you try to make it, when the truck frame twists it will pressure the box to twist too. And truck frames are ALL engineered to twist.