mbloof wrote:

Interesting abet comical thread - frame flexing.

If one were to do some Youtube searches you'll likely find videos where trucks from "the big three" are all driven on/over grossly uneven terrain and the frame flexing/twisting (or NOT!) of each OEM was shown and documented.

Generally speaking some OEM's are worse than others and some years are better (and worse!) than others.

- Mark0.

valhalla360 wrote:



Yes, frames will flex but big difference between a low speed off road once in a while vs repeated 2inch flex for hundreds/thousands of miles cruising down the freeway

Look up fatigue loading failures on google to get an understanding. In a fatigue failure, the item typically looks and operates perfectly fine...until it doesn't. Most of the damage is microscopic and internal to the metal until it finally gives way.

adamis wrote:

Got a question for you seasoned drivers with older rigs. I have a 1999 F350 Dually Quad Cab Long Bed. I've been driving it for thousands of miles and for the most part, it's been fine. However, while driving on rougher roads, I do get a porpoising effect happening. I can look in my rear view mirror and see the frame flexing at least an inch or two.

My camper is the Bigfoot 2500 and with everything loaded up is probably in the 4500lb to 5000lb range. According to the sticker on the camper the center of gravity is almost exactly over the rear wheels. Now the center of gravity does change with water in the tank as the fresh tank is in the front of the camper which puts it at the front of the bed.

The porpoising isn't a "problem" in the since that I am concerned about it causing damage. I'm well within the carrying capacity of the truck. That being said, it is an annoyance for comfort and drivability. I have already replaced the shocks on the truck and that has helped somewhat. I am also looking and seeing that I probably could move the camper another inch or two forward when I load it. I don't think this will make much of a difference though.

So, my next thought was to look at having my frame reinforced. I know there are all sorts of complications when going this route though. Ford does fully boxed frames for a reason because it gives so much more rigidity. I no there is at least one person that has done this on their 7.3 from a posting on the Ford forums. This is obviously a huge expense and extreme measure.

Has anyone else gone through this or found a way to handle excessive porpoising on older C channel frame trucks?

JRscooby wrote:


Do you think we should be a little more accurate about how much the frame is actually flexing. How far from the frame is the cabover section of camper? Guess 4 feet or more. 1/8 inch of frame flex would make for 6 inches of movement. And that does not include any flex in the camper itself.

adamis wrote:


That contrasts with the 1999 with Max Payload for the Crew Cab Dually Long Bed 2x4 appears to be 5355lbs and the 4x4 is 4910lbs.

https://www.xr793.com/wp-content/uploads/2016/10/1999-Ford-Super-Duty.pdf

So, with basically the same frame, how did they manage to get another 1500+ pounds of payload capacity out of it?

HMS Beagle wrote:



You should go by the Ford bodybuilders manual that I referenced, not by that brochure. The bodybuilders manual gives the specific configurations, in California - less, diesel engine - less. Because the empty truck weighs more. The bodybuilders manual does not give the diesel 4x2 DRW crewcab, but does give the 6.8L, and the diesels are less by about 500 lbs in all models. You could just weigh the empty truck, the GVWR is 11,200 for all models of DRW pickups in those years.

The frame configuration and layout is nearly identical between 1999 and 2015, but the material may be thicker. That is also listed somewhere in the bodybuilders manuals, haven't bothered to look. You probably can't go from 11,200 to 14,000 without doing something.

Marcela wrote:

I had the same rig for a little while and here's what I found out.

The bed opening width is minimal. The curved part at the bottom necessitated that I use two pieces of plywood to raise it above the curvature.

Make sure your bed is flat. On my new GMC truck the bed is crowned front to back, which necessitated different pieces of plywood of varying thickness.

Re the center of gravity. I had race car scales so I put my camper on them. If you look at some of my old posts you can find out what I found out. There guesstimate is best case, what I found out was different. Though the cog was behind the wheels a bit, I didn't find it to effect the driving.

Also on my F250 I had the biggest spring pack installed and largest sway bar. I still didn't like the way it drove. The cog is just so high.

I removed and reinstalled my jacks on the camper. When they installed them the holes in the brackets were too small for the fasteners used there is no way they would pull up tight against the camper. So I enlarged the holes in the brackets and reinstalled.

Also if you have a gooseneck ball in the bed, you can install the plywood on the bed with a hole that matches this and install a stub into and flush with the plywood to hold it in place, or two pieces if that is what you do.

Good luck.

Grit dog wrote:

But a very important point that maybe scooby just brought up is the amount of movement in the overhead portion that we ALL see from time to time with big TCs is not necessarily indicative of excessive frame flex, nor is porpoising. Unless it’s literally a rocking horse most or all of the time. Which is more likely suspension and shocks. Given the camper weight and configuration changing is not an option.