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 > Frame Stiffing to reduce Porpoising?

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Northern California

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Posted: 01/21/23 11:35am Link  |  Quote  |  Print  |  Notify Moderator

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?

Because several people have focused in on the camper weight and that has taken this question off from what I asked, I am adding this here for anyone new reading this thread that thinks to jump in and comment about weight. Later in this discussion thread I post a response where I provide the factory weight as measured was 2733lbs and with AC and Genset just under 3000lbs. That is way lower than the 4500lb to 5000lb I stated in my original question above. I have also weighed my truck and it is 7600lbs and so the payload capacity is 3600lbs and the axle capacity of 4870lbs. I am not grossly overweight as some thought based off my original and inaccurate guesstimate in my question above.

Secondly, I also overstated the amount of "porpoising". It is not excessive in that I am concerned about the truck breaking in half. What I have observed others mainly in the overhead movement of the camper above the driver seems to be common with others.

Finally, my question in this thread is asking if someone has ever added additional bracing to the frame of the truck. I'm asking the question if this has been done before. The consensus in this thread is that it is very rare and it is very complicated to do and probably not worth the effort.

* This post was edited 01/30/23 10:14am by adamis *

1999 F350 Dually with 7.3 Diesel
2000 Bigfoot 10.6 Camper



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Posted: 01/21/23 02:59pm Link  |  Quote  |  Print  |  Notify Moderator

For my 97 air bags did the trick. Dampens it out.



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Posted: 01/21/23 03:08pm Link  |  Quote  |  Print  |  Notify Moderator

You might have more load and less CCC than you think. I would weight the loaded rig and check the door sticker for the CCC.

I am not sure why you "probably could move the camper an inch or two forward". The camper should be loaded all the way forward and touching the front bed rail. Also be sure to load your gear to also help with a forward COG.

If you are within specs and your COG is acceptably forward of the rear axle, then some flexing on rough roads should not be a concern. A small amount of flexing on rough roads is normal when carrying a heavy load.



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Posted: 01/21/23 04:34pm Link  |  Quote  |  Print  |  Notify Moderator

How do you know the frame is flexing vs just the springs compressing?
Your truck is as old as mine, maybe the springs are getting tired?

Grit dog

Black Diamond, WA

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Posted: 01/21/23 06:15pm Link  |  Quote  |  Print  |  Notify Moderator

Contrary to what you think, your truck is not rated for that load I’m pretty sure. Thought them old Fords had like a 11,300gvw oe somethin like that?
Not saying I wouldn’t and haven’t hauled big weight on the old C channel super dutys but this is not something that seems even remotely worthwhile to do.
I know you have an inordinate amount of $ tied up in that good old truck and have turned it into an even more capable machine than it was, new. But at some point you gotta either live with what ya got or upgrade and dunno why Ford was so backasswards here as the competition got better in the frame dept literally within a couple years of your trucks year.
Ford didn’t until 2017.
But as was mentioned you may be able to make the suspension work better and reduce some of the flex when it’s porpoising. That seems worthwhile maybe. Not rebuilding the frame essentially.
Are these broken TC hauler pics starting to keep you up at night?

2016 Ram 2500, EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold


ozark mountains

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Posted: 01/22/23 05:34am Link  |  Quote  |  Print  |  Notify Moderator

Your center of gravity needs to be forward of the rear axle to place weight on the steer axle. Like a poster said earlier, move the camper as far forward in the bed as possible. When you add water, your porpoising goes away, that is a dead give away that you don't have enough weight on the front axle.

2021 Ram Limited, 3500, Crew Cab, 1075FTPD of Torque!, Max Tow, Long bed, 4 x 4, Dually,
2006 40' Landmark Mt. Rushmore



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Posted: 01/22/23 07:26am Link  |  Quote  |  Print  |  Notify Moderator

I will not weigh in on how good idea it is to stiffen the frame.
Working on heavy trucks, if I suspect excess frame flex,* the first thing I do is look where crossmembers mount. Any sign of movement between the parts? Rust around the fasteners? Then spaces between the members. Any concentrated areas where paint is loose? And are there any places where it looks like the steel has been hot?
Now if you decide you have a frame issue I have had pretty good luck using L shaped piece of frame steel, cut so near as tall as rail, and as wide as the flange. I try to end at crossmembers, and make ends angle, so flange end is longest. Use hard bolts, in holes drilled for tight fit, and flat washers. (NO SPLIT WASHERS!) Drill in pattern so no 2 holes are in the same vertical line, and spaced between half of, and the width of vertical frame section. Most important, NO HOLES IN THE FLANGE

*(Never seen a case on Ford, 500 to 9000, and have extended the wheelbase on more than a few 9000s)


No paticular place.

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Posted: 01/22/23 09:47am Link  |  Quote  |  Print  |  Notify Moderator

I couldn't find the 1999 ratings but the 2004 ratings (truck camper rating) are at best 3800lb and could be less than 2000lb depending on the exact configuration. Since the ratings tend to get better each year, I would be surprised if your older 1999 truck has a rating 25% higher than the 2004. Most likely it's lower. This would imply your 4500-5000lb is well above the ratings.

If you are correct and the frame is bending an inch or two just rolling down the road, that's also a good indication that you are over the ratings.

First step would be to take it to a scale and get some real weights not guestimates. Then I would head on down to the Ford Dealership and see if they can pull the correct figures for your specific truck.

Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV



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Posted: 01/22/23 09:53am Link  |  Quote  |  Print  |  Notify Moderator

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.

HMS Beagle

Napa, California

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Posted: 01/22/23 10:13am Link  |  Quote  |  Print  |  Notify Moderator

A crew cab truck has a long frame, and it is going to be more flexible. I've had a 1999 Supercab, followed by the current 2015 Supercab which is identical in almost all respects except the powertrain. One thing I found on both of them was the porpoising was greatly reduced by increasing the shock resistance on the front. Even though the front carries very little of the weight of the camper. With the camper onboard, when cyclical pitching starts it contains a lot more energy, not so much from the longitudinal position of the CG, but the vertical position.

I had (have) Rancho 9000 shocks, when I loaded the camper I'd leave the front ones on 4 or 5 and dial the back ones up to 9. I found that putting the front ones up to 8 significantly helped the porpoising. If it is caused (or aggravated) by suspension movement, then the shocks are the only way that movement is dampened.

Bigfoot 10.4E, 2015 F350 6.7L DRW 2WD, Autoflex Ultra Air Ride rear suspension, Hellwig Bigwig sway bars front and rear

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