4-Point Pivoting Subframe Design

[Krangmang]

There will always be stresses, a 3-point subframe would reduce the stress even more, but reducing stress really isn't needed. The truck frame is built to twist, the habitat isn't. The habitats subframe should be as rigid as possible for this reason. Also, a point on the side pivots is that they must be able to move in and out of the pivots, unlike the end pivots. This is because as the frame twists the center dimension shrinks slightly.

Loads are something you could bother calculating, but I usually don't. Using 5/8 grade 8 hardware the shear strength is 75k lbs of ONE bolt. Tensile strength is even higher, and I'm using 2x per side for the spring mounts. The frame would rip itself apart before the bolts gave way. In my case, the weak links are likely my welds and then the truck frame itself.

I'm only trying to provide flex over 100" or so of truck frame. I backed the truck up onto a 12" block and put some 2x4 across the frame rails to measure deflection, which was slightly more than an inch. I doubled this and specd the springs accordingly - they have spec sheets for how much they will fully compress, and are linear spring rates so that's easy to calculate with a load.

Are we all winging it? Yeah. I'm also not concerned with designing the system on the bleeding edge of its engineered capacities, I just overbuild it 4x and send it. The truck frame is the weak link and before that the leaf springs will be. If I were more concerned about weight then I would probably consult an engineer and spec the material a bit closer to its limits, reducing weight of the system.

 

[rruff]

If the Unimog has a 4-point subframe, which most of them do, then it is very unlikely that the movement is coming from there. The mechanics of a 4-point subframe would not allow that kind of movement.

With 3 or 4 point I think you'd need to twist the frame under the fixed points before you'd get lateral motion like that due to the mounting. Probably it's just suspension and tires. So strapping down the suspension and airing up bags (if you have them) like Idasho mentioned, and also airing up your tires, and pointing into the wind should work.

There are also stabilizers for RVs... or you could use jacks.

 

[rruff]

I'm only trying to provide flex over 100" or so of truck frame. I backed the truck up onto a 12" block and put some 2x4 across the frame rails to measure deflection, which was slightly more than an inch.

An inch is nothing. And that is over the whole width of the habitat rather than at the rails? What truck is this?

I measured mine by driving across a ditch at an angle so that all the weight was on two opposite corners, which would be the worst case.

 

[yamaha225]

I regularly watch the YouTube channel with the Unimog so I’ll chime in to explain a bit. Their habitat subframe is spring mounted I believe. It’s not a 3 or 4 point.

I would guess at least some of the movement was coming from the spring mounting as I’m not sure if they have any fixed mounts at either end which would certainly make it more floppy. A 3 or 4 point would reduce that issue greatly due to its mechanics as was stated previously.

I’m really not sure why they didn’t just move the truck or at least turn the nose into the wind. They have always seemed a bit nervous to me and in the video seemed worried about driving in the wind. A commenter in the YouTube video aptly pointed out that if the truck didn’t tip over in the wind sitting still it was likely to tip over while driving.

They also could have easily ratchet strapped the subframe to the chassis and stopped the spring movement. Lastly, given the tall tires and soft suspension of the mog I’m guessing that was contributing too.

I’m planning a 4 point mounting system for my flatbed on my F-350 somewhat similar to what Idasho has done. His has been well proven to work well.

 

[SkiFreak]

Their habitat subframe is spring mounted I believe. It’s not a 3 or 4 point.

I'm sorry, but you are wrong. It is a 4 point mount.

 

[rruff]

A commenter in the YouTube video aptly pointed out that if the truck didn’t tip over in the wind sitting still it was likely to tip over while driving.

That was the 1st thing that occurred to me also, but... a lot of trails have quite a lot of side tilt which would definitely make a tip over with a gust more likely.

 

[yamaha225]

I'm sorry, but you are wrong. It is a 4 point mount.

I stand corrected! I thought I remembered seeing springs in a previous video but the human memory is a fickle thing. Thanks for the correction!

 

[yamaha225]

That was the 1st thing that occurred to me also, but... a lot of trails have quite a lot of side tilt which would definitely make a tip over with a gust more likely.

That’s certainly true and there’s nothing wrong with playing it safe!