Yea, I have the same concern. I'd like to see that photo of basinranger's truck from a higher perspective to see where the majority of the twist is. In front of the step, in the step, or in the straight section of the rails?
I doubt that there is much flex in front of the step up, and if it's in the curve itself you could add the frame braces for the dump option as shown in the body builder's papers. If the twist is in the straight section I wouldn't worry much about a steel structure that controls the twist.
On the other hand, box bodies and utility beds keep the straight section of the frame from twisting and they don't seem to be breaking frames. I wish there was a simple answer.
Firstly I must say this is a great pic of FG chassis twist . No matter how often body mounting systems get discussed in the FG section, new points and ideas come out every time. A good thing about ExPo.....
Not only does an FG frame twist at crazy angles without restraint but as you can make out in this pic , the section above the "step" appears to be moving to the right (relative to the lower section anyway). Well you can easily see this in reality but the twisting is really going on through the low section.
Whatever is going on in the pic changes completely when you mount a body to it though so don't let it mess with your head too much. A bare chassis mainly twists towards the back and very easily too. Notice in the pic that the springs have hardly moved. Put a rigid body on and go offroading >>> the part that easily twisted before is now easily held rigid. It's still that bit up the front that's trying to tear sideways that's the culprit.
Sorry for all the theory. From experience, if you hard mount a rigid body on an FG frame say with 4 or 5 mounts spaced along the length of the frame, the most damage sustained will be at the front mount, less at the next one back and virtually none by the time you get to halfway. Damage usually in the form of bolt holes tearing. Of course, this also depends on the stresses applied to the truck off road too. It's not likely to happen on road. Well certainly not to the same extent and probably not enough to ever be any problem.
BTW As I saw this written somewhere else about why landscapers work trucks don't fail >>>Work trays twist more than you imagine and so are not really rigid. They still need to be well mounted. A work body/tray with separate camper body attached to that should have plenty of leeway and give too. Especially if it's correctly U-bolted down (Kerry's going "Phew!!" Lol). A pantec (big box) usually skinned in alloy sheet (or a bus / camper body) is a different story. They are inherently rigid and that's when we see damage. A high CoG offroad seems to definitely magnify things too.
BTW Basinranger, I can't believe I haven't see your build thread before tonight. I rarely go to any of the other sections other than this one and the FG section. Peter, amazing work. I'll be going back over it for ages. A real credit to you. Congratulations on everything to date. BTW If it makes you feel better, I have the business brain of a Lemur too.
