Pivoting frame

[wytzor]

Hi , I read the forum and get more and more confuse about if I need a pivoting frame/ 3 point pivot frames.
I'm planning on buying a camper truck et flatted bed truck Soon.

It seems when people buy their truck like a F350 or HD2500/3500 they don't bother to build or install a 3 point pivot frame.

So why if I buy a Chassis Cab truck or flatbed truck I would need one?

Here the truck I'm planning on buying.
I'll mount a Lance 845 on it.
I'll do dirt road but I don't think I'll do crazy off road!
I hope you understand

Thanks

William

 

[rruff]

That's older than 2017, correct? Is it a chassis-cab? It looks like the flatbed is hard mounted which will greatly stiffen the frame where it's mounted. When you get crossed up, that will transfer more stress/twist to the front of the frame under the cab. The transition (near the front attachment point for the flatbed) is probably the critical area. No way to tell for sure if it will be a problem. You could probably find a good welder to beef up the chassis in that area to get a little piece of mind.

I'd suggest going through a ditch at an angle so you are teetering on two diagonal wheels, and get an idea of what happens.

 

[wytzor]

Thanks you very much. I understand a little bit better

 

[IdaSHO]

Anything Superduty with an open C-channel frame (pre-2017) has considerable flex.
If its a pavement pounder, no worries. Anything else, you need to account for flex IMO.

Our truck is a 2011, same chassis as superduty in the photos above, though shorter (extended cab vs crew)
And the flex is still more than you would want for any hard mount.

This is with the front right stuffed into a bank, in 4wd and rear locker engaged to lift the right rear.
This was nothing more than a flex test to determine just how much flex I needed to account for, when designing the 3-point subframe.

Notice the angle introduced between the cab and the blue level.

So, similar flex test but in the shop, exploring how well the 3-point works.
Notice the angle introduced between the chassis and the bed.

The bed is remaining flat, no matter the flex/twist in the chassis.

 

[billiebob]

Also depends on what you are mounting, a flat deck likely will flex when the frome flexes, less than the frame but it will flex. A welding deck or service body is structurally much stronger than a Lance camper body, subject a complex camper body to flex will often weaken or damage the camper as the frame flexes.... and the camper twists. But regardless, a 3 point mount is always better if there will be any articulation.

 

[rruff]

But regardless, a 3 point mount is always better if there will be any articulation.

Or the spring mounts if it's a straight railed chassis-cab.

 

[billiebob]

Or the spring mounts if it's a straight railed chassis-cab.

yes, agreed, in many ways I prefer a flexible 4 point mount.
with a cab/chassis and isolated camper flex behind the separate cab is not a factor.
but if the camper/cab is a solid connection like a Class C Motorhome, there is little if any room for twist between the 2.

No matter what you do.... you need to allow for movement. You can never eliminate it all.
In the 1960s Ford shelved the UniBody pickup for just that reason.

 

[rruff]

No matter what you do.... you need to allow for movement. You can never eliminate it all.

The trucks with stiff fully boxed frames seem to do fine while ignoring movement (camper is hard mounted). Torsional stiffness on those frames looks like at least an order of magnitude higher than open Cs. You run into problems when your frame is very flexible and your camper isn't... then the camper needs to be strong enough to withstand a few thousand pounds of force, and >10,000 ft-lbs of torque when you get crossed up.

 

[BeNimble]

The 2006-2014 Honda Ridgeline truck was a unibody,
I looked at the frame rails of my F550, like 5 inches high compare to 10 inches on my GMC500 2-ton, maybe 8inches on my e450
.