Frame Stiffing to reduce Porpoising?

mkirsch wrote:
I'm betting facebook guy thought nothing of his 2" of flex too...

I think Facebook guy's frame was flexing a lot more than 2". From what I have read, he was overloaded at least 1500lbs, maybe closer to 2000lbs and he had a huge moment arm created by the bikes hanging off the back like they where. EBikes weigh close to 100lbs, he had two of them so he's got 200lbs that was nearly 8 feet from the center of his rear axle (rough guesstimate). That is 1600lbs of moment arm pulling on the frame. That coupled with rough roads and lots of miles seems to be what did him in. Other's have studied and argued his situation way more than I but from the little I have seen, it doesn't surprise me what happened to him.

The center of gravity of our campers is generally very close to right over the top of the rear axles or just a bit in front. This means the center of the frame isn't supporting our four to six thousand pound campers. Most of that weight is transmitted directly into the rear wheels. However, the frame is having to "balance" or keep in place a heavy engine up front over the front wheels and the heavy camper on the rear wheels. The frame flex we observe is more due to the frame keeping these two heavy pieces together going down the road. Having the camper weight slightly forward of the rear axles puts the center of mass between the front and rear wheels such that the frame is dealing with the frame flex occurring consistently between the front and rear wheels of the truck. This is an ideal situation and what the engineers design for.

If you add a huge moment arm off the back with a heavy trailer along with a hitch extension you are now adding a rearward twisting motion to the frame. So now you have competing forces that the frame is dealing with, almost like a teeter-saw. I would venture that the amount of frame flex Facebook guy's truck endured was significantly more than is common for a properly balanced truck.

Finally, rigidity isn't everything. I know I started this post asking if frame stiffening was something others had done. However when you make something rigid, you lose the ability to flex without causing permanent deformation. As others have said, this deformation causes fatigue over time and leads to frame failure. The engineers that designed the truck obviously know this but what we might be seeing is that the increased rigidity of the frames is causing overloaded trucks to deform and fatigue the frame material versus just flexing like the older and more flexible truck frames of the past. As campers continue to get heavier and truck frames continue to get stiffer, we may see a lot more Facebook Guy's in the future.