4-Point Pivoting Subframe Design

[IdaSHO]

Thanks, I'll give it a read.

its worth a read. And why it isnt pinned (if it isnt) by now is a mystery.

And let me know if you still have questions. Ive hand built a couple of 3-point torsion free pivoting flatbeds.
The latest being the one we are currently running under our custom camper.

Idacamper Custom Flatbed

Ive had quite a few comments and requests for more info on this setup. It is a 10' bed with 8" flat, 2' cross body box. Chassis of the truck (2011 Superduty) was extended 24" Bed rides on a 3-point pivot torsion free subframe Complete with integrated self aligning mounts (for camper) and slide...

forum.expeditionportal.com

 

[SkiFreak]

I still can't see it.

Looks like you will need to break out the Lego.

 

[shortbus4x4]

I'm not doing a 3 or 4-point. I'm doing a spring-mounted setup on my 2024 Chevy 4500 LCF (NPR-HD).

Just something I'm curious about.

Do you have the upfitters guide from Chevy/Isuzu for it? If not get it. Read up on body mounting in it.

 

[Sixinarow]

Looks like you will need to break out the Lego.

Whoa, it just came to me, lol. They allow the subframe to twist. I was under the impression the subframe wouldn't twist at all.

 

[rruff]

Whoa, it just came to me, lol. They allow the subframe to twist. I was under the impression the subframe wouldn't twist at all.

No, the subframe doesn't twist, but it does tilt. The chassis frame twists.

4 point just means your fore-aft tilting pivots (nearly fixed, not much tilting at all) are in the middle (spaced wide) and the side to side tilting pivots are front and back, and in the centerline. The middle of the subframe will match the tilt of the chassis at the middle points, while the forward pivot will tilt less relative to the chassis and the rear pivot more.

Maybe the end of this video will make it more obvious.

 

[rruff]

I'm not doing a 3 or 4-point. I'm doing a spring-mounted setup on my 2024 Chevy 4500 LCF (NPR-HD).

Which end is fixed, and are you doing a pass-through?

 

[gator70]

You're using them on your truck or you have some for sale?

I can sell some. I can post pictures (have CAD drawings for a subframe for the NPR HD)

 

[gator70]

Subframe making people sick - Watch the video

 

[IdaSHO]

Subframe making people sick - Watch the video

Easy enough problem to overcome.

We have weathered many 60+ MPH storms in our camper.
Tip #1 is to point the truck into the wind.
Tip #2 is to anchor the bed to the axles/wheels, eliminating as much suspension and subframe sway as you can.
After strapping it down, I also air up the suspension bags to tighten things further.
Done right the only real sway you have is from tires.

 

[gator70]

Easy enough problem to overcome.

We have weathered many 60+ MPH storms in our camper.
Tip #1 is to point the truck into the wind.
Tip #2 is to anchor the bed to the axles/wheels, eliminating as much suspension and subframe sway as you can.
After strapping it down, I also air up the suspension bags to tighten things further.
Done right the only real sway you have is from tires.

Great tips thank-you

I will installed these strap down points on my subframe

 

[SkiFreak]

Subframe making people sick

Is it really the subframe that is moving? I don't think so.
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.

It is more likely that the movement is coming from the tyres, as they have quite a high sidewall, which would allow lateral movement when buffered by strong winds.
They should have turned the truck into the wind, as that would likely have reduced some of the swaying.

The above being said, these types of posts are not really addressing the initial question, that being what the central mounts on a 4-point subframe do.
That technical question has been answered, I believe.

 

[Krangmang]

Yes, I get that, but the pivots aren't moving in that axis.

They most certainly do move on that axis as a result of the trucks frame being a semi-rigid square.

Take a square piece of cardboard and cut a square out of the center. Draw a line through the center of each side, then twist it. This is why you need the center pivots.

 

[Krangmang]

I'm not doing a 3 or 4-point. I'm doing a spring-mounted setup on my 2024 Chevy 4500 LCF (NPR-HD).

Just something I'm curious about.

This is what I opted for as well, seems much less complicated and easier to execute. Also easier to install / remove the habitat when I want. I have the solid mounts near the cab and the spring mounts slightly behind the rear axle.

The hardware and springs I'm using below if you are curious:

 

[rruff]

Subframe making people sick - Watch the video

I just skimmed through it, but I didn't see them ever showing what was actually moving. The suspension will do that when you are slammed by crosswinds.

This is what I opted for as well, seems much less complicated and easier to execute. Also easier to install / remove the habitat when I want. I have the solid mounts near the cab and the spring mounts slightly behind the rear axle.

I disagree about that being simple or easy. You'll still be transferring stress to the subframe and habitat, but it will be less. And I've seen a few reports of people having a tough time getting that right.

What is the travel on that spring vs how far your chassis articulates?

 

[rruff]

Take a square piece of cardboard and cut a square out of the center. Draw a line through the center of each side, then twist it. This is why you need the center pivots.

Aren't there going to be induced stresses even then? Middle pivots will help some, but there will still be misalignment.

The question is always how much these loads are between the chassis and subframe, and do the mounts adequately spread them out and handle them adequately for both the chassis and subframe? And is the subframe flex enough to cause the habitat to have issues?

Everyone is kinda winging it. The best info is prior experience both for structural integrity long term, and performance in challenging conditions. The later will be known shortly after building, but the first will take a long time.

I can say that pseudo pivots (could do 3 or 4 point) using poly isolators seem to be doing very well for performance. Cheap, simple, have some give in all directions, easy to spread the load, etc. Mine is 3 point and there isn't a bit of lateral "floppiness" that I've noticed.