Frame flex fundamental question

SkiFreak

Expedition Leader
I was out today and came across some of our larger rural fire fighting vehicles. There was a Volvo and two Isuzu 750 trucks, all of which used some form of spring mount to connect the subframe to the chassis. There setup also had a secondary flex system for items mounted on the subframe. On all of these trucks there was a piece of polyurethane between the chassis and subrame along the full length of the chassis.
I took the happy snap below...

01.jpg

02.jpg

03.jpg

04.jpg
 

pugslyyy

Expedition Vehicle Engineer Guy
I was out today and came across some of our larger rural fire fighting vehicles. There was a Volvo and two Isuzu 750 trucks, all of which used some form of spring mount to connect the subframe to the chassis. There setup also had a secondary flex system for items mounted on the subframe. On all of these trucks there was a piece of polyurethane between the chassis and subrame along the full length of the chassis.
I took the happy snap below...
Thanks I will add these photos to my collection!
 

Recommended books for Overlanding

First Overland: London-Singapore by Land Rover
by Tim Slessor
From $13.4
Motorcycle Messengers: Tales from the Road by Writers who...
by Lois Pryce, Mark Richardson, Carla King, Sam Manic...
From $9.99
Deep Survival: Who Lives, Who Dies, and Why
by Laurence Gonzales
From $9.99
4WD Driving Skills: A Manual for On- and Off-Road Travel
by Vic Widman
From $17.27

whatcharterboat

Supporting Sponsor, Overland Certified OC0018
Hi Owen,

That FTS 800 crewcab is a beast. I guess you noticed the emergency sprinkler system over the cab. Those guys don't get paid enough. What am I saying.....many of them don't get paid at all.

Regards John.
 
Does how the box is attached to the frame, say a Unicell or Supreme Cargo box matter if the intended use is moderate off road? I mean it's still a road, but say really bad forest service roads that need clearance and/or lots of washboard? No articulated rock crawling.

Thanks
JM
 

SkiFreak

Expedition Leader
Millions of cargo bodies have been hard mounted to chassis over the years, so their can be no denying that this is a valid mounting method. But is this the best mounting method for an expedition camper; I would argue no.
As you have correctly indicated, roughness of the road/track, along with frequency and the weight of the vehicle should all be taken into account.
Fractures of the chassis can occur because of fatigue or if the steel or over stressed. The occasional large pothole, deep wheel rut or washout will definitely stress a chassis with a hard mounted box, but will this cause a frame failure... probably not.
However, continually putting the vehicle in these kinds of situations will arguably increase your risk of encountering issues.

In my opinion, how you mount the camper body to the chassis really comes down to risk management, which is determined by how YOU plan on using the vehicle.
 

SkiFreak

Expedition Leader
Another thing to take into account is what happens inside your hard mounted box when the truck travels over uneven ground and things begin to twist.

On a goods truck, flexing of the box is not really an issue, as nothing is normally mounted to the walls, but in a camper scenario this flexing could potentially damage your internal fitout.
I have heard of numerous cases where cupboard doors/drawers start jamming, which is definitely not ideal.

By including some form of kinematic mounting system for the camper box you can greatly reduce the amount of twisting that gets transferred during moments of articulation, but if you don't, you just have to be a bit more cautious about the roads and tracks you travel on.
 

kerry

Expedition Leader
Utility body u-bolted to frame here. Wood strip between frame and body. 225k miles, no frame problems yet. Utility body does twist. There are broken welds between front wall and bottom of the utility body. It was only tack welded in 2 or 3 places. Those broken welds permit even more twist. My guess is those welds broke the first time the truck did any serious twisting offroad. The tailgate on the utility body forms no real structural connection to the boxes at the back end. So it's really only the floor connected firmly to the frame. Camper is attached to utility body floor and the back bumper(50k miles) and moves independently of the utility body a little bit. I have a couple of 3" bolt connections at the very front of the camper on top of the utility boxes and to the jack brakets on the camper and the bolts have broken a few times. Those bolts are the only things that complete any 'box' structure above the floor apart from the sheet steel connecting the side boxes at the front of the utility body. I consider those bolts sacrificial. A lot of dirt roads but not a lot of off roading. No mathematical calculations involved in the camper system, just educated guesswork. I bought the truck with the utility body on it. I might consider alternatives if starting from scratch. I'm happy so far.
 
Last edited:

yabanja

Explorer
WOW! This thread never ends. Can't remember if I threw in my 2 cents yet so here goes.

We have a short wheelbase 2000 FG. This means that to begin with there is a lot less stress torsionally on the frame than the long wheelbase models because the wheels have less leverage from front to rear.

When I was building Fido I read every thread on this forum and then made my own decision as to how to proceed. We have had outstanding results with our setup. Our truck gets used off road more than many. The truck flexes very little even when a wheel is off the ground. The truck frame shows no signs of damage or fatigue and miraculously the camper continues to function as intended despite significant abuse. I will point out that it is really important to address the suspension as well to reduce the strain on the whole truck which we have done.

I had two primary concerns when building the camper.

First concern: control frame flex of the stock chassis. I did this by following the Fuso factory builder's guide and building a very robust subframe with 14 large Ubolts holding it place with oak between the two. The subframe has fish mouths cut out of both ends per factory recommendation to lower stress points.

Second concern: protect my habitat space from torsional fatigue. I accomplished this by mounting the camper to the subframe using a 3 point mount which does not transfer any torsional strain to the box. A spring mount system will ultimately transfer significant energy into the box directly related to the tension of the springs used or when the springs bottom out. It is amazing how much the chassis is capable of flexing so spring bottoming is very likely. This was not acceptable to me as the camper we used is a complete noodle and could never survive these kinds of strains!

So far so good.

Allan
 
Top