My turn...
!
So, I pondered how best to fit our hab box to our truck for a long time. Rail on rail vs 3-5 point mount. I settled on the 5 point mount for several reasons. Firstly, having spent several years driving Mercedes 1617's and 2421's on some pretty average African roads. The thump and bang from the hab box flexing on the rails was one thing, but welding a box up where it was cracking was another. These trucks did get used off road. Not a huge amount, but off road none the less.
Now I'd settled on using a system with mounts I then looked into that a lot deeper. I came to the conclusion that most who use this system run a completely seperate subframe with their pivots built into the subframe... I soon worked out that this has a flaw in that all the 'pivot points' sit above the chassis (approx 300mm above the chassis twist axis). This in its own right builds in huge shear loads on the pins, chassis and subframe, which is why most designs are over engineered and require the chassis to twist along a different plane. It also adds complexity and weight to the build into which it is deployed. I have a hab box which was originaly rail on rail mounted and is already over engineered and built to be hammered over corrugated dirt roads in WA. It survived, the Isuzu that was under it fell apart after 5 years! Keep this in mind as you read on. Soooo, I have a heavy well built 12 tonne truck with a very flexible chassis (which I used as a crane truck for several years before starting this build). It loves to twist, making it surprisingly good off road. I have a solid steel contruction hab box. Why then do I need a subframe? I don't believe I do. The chassis and the hab box form the two halves of the 'subframe' required. All I need is the pivot points. By building like this, I can put the pivot points exactly where they need to be without inducing load. I have my two fixed points about a meter from the front of the hab box, utilising two existing holes in the chassis no more than 100mm from the rear hanger on the front axle springs.... right where the load is carried (this is to help keep the crawl thru doors in line with each other when under mild twist). I have the first central pivot mounted to the X member that links the two front hangers of the rear axle springs. The second pivot is mounted to the second x member which happens to be between the two rear spring hangers. This means that all 3-4 tonnes (max) of our hab box is being carried close too or at the spring points. The chassis is just there to hold it all together. Finally I mounted a third center pivot right at the back just for a little support and to stop any chassis chatter. I have built into the base of the hab box 2 bump stops along it's length on either side, allowing the chassis to twist no more than 100mm over it's usable length of 6m (which equates to 200+mm of twist at the wheels before the springs do anything. This allows the chassis to twist a little when needed but not excessively. The hab box can handle some load. The springs then do the rest. I was able to jack the rear left wheel off the ground 450mm (with blocks still in between the front springs/axle and chassis... which I remembered were still in there later on that day). While under twist, all three pivot pins could be moved easily with a light tap of a hammer (no excessive load).
Now to the anti roll bar... I looked at hydraulics, I looked at air, I looked at locking pins. Hydraulics (2 rams with a tap in between) is the way to go if $$$ was in abundance. Both hydraulic and air require hefty mounts, air bags or rams and hoses and space. Locking pins require crawling under the truck and hammering several pins home if you have managed to park in an un twisted position first up! No thanks. That left me looking at what was left of my donor truck, and it's front anti roll bar (not back). This anti roll bar is about 35mm thick, and designed to stop the cab and front of the truck from rolling about. Small loads... With this in mind, I tacked the anti roll bar mounts in place where I wanted it to be, and undertook a chassis twist test again to see that I could still get the travel I required. I stopped at 500mm purely because I ran out of jack and blocks! Everything (including hab box mounts) were still tack welded at this point, and nothing let go. Happy days.
In closing, all I'm after is to allow movement between the hab box and chassis to assist with axle travel when off road, whilst still hoping to retain an upright hab box when cornering on roads, while not having anything complicated or difficult to replace under the truck in the way of air bags, hydraulic rams, hoses, taps etc should they decide to let go. I have built in a 'get out of jail' card in that I have an accurate 100mm gap between the hab box and the chassis so if something does go drastically wrong, I can very easily slide in a pair of 100mm rails and make it rail on rail. At this point though, I don't see there being an issue. The hab box allows chassis twist giving me an off road advantage which we will use occasionally, whilst still feeling rigid and sitting firm when on the road. Wish me luck!
