Frame Stiffing to reduce Porpoising?

[adamis]

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?

-------------------------Update-------------------------------
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.

[BigfootBill]

BigToe wrote:

Bedlam wrote:
For frame strengthening, I was thinking along the lines of L-stock clamped with U-bolts to the rails. This would prevent the original frame from being drilled or welded. It would be the automotive version of a medical splint or brace. However, I still am not convinced that this is needed.

Not only are frame reinforcements not needed, they can cause more harm than good, by creating sudden transitions in stiffness between the reinforced and un-reinforced portions of the frame.

It is at these transitions where the frame suddenly encounters additional resistance to flexing due to the reinforcement, so the stress is then concentrated in the areas of the frame just prior to encountering the reinforcement.

The engineering of frame reinforcements to minimize transitional stress concentrations is an art and science unto itself... sufficient to challenge even the most competent repair facility, as they lack the means to mathematically model and materially test the outcomes of frame reinforcement schemes that are further hampered by the packaging constraints dictated by retrofitment to an existing frame.

Earlier I mentioned that for this same model year (1999), Ford produced L channel frame reinforcements for the Chassis Cab frames. Yet even these reinforcements are not retrofittable after the fact of production, even to identical chassis cabs built without the reinforcements.

To achieve a smoother transition between un-reinforced and reinforced sections of the frame, Ford ran the horizontal flanges of the L channel longer than the webs, and Ford tied the attachment of the flanges to the attachment of the crossmember that ties the forward rear leaf spring hangars together laterally across the frame.

The web of the reinforcement is then tapered forward, and then tapered and flanged out again on the forward end, intersecting with inverted reinforcement under the back of cab wall at a forward rake angle.

The OP talked about adding several hundred more pounds of weight in the form of a front drive axle, new transmission, and transfer case, in order to convert his truck to 4WD. But he then added the twist that he also wants the coil sprung version that Ford offered for 2005 and up, even though his frame is a 1999 that does not have the boxing behind the engine cross member that will keep the frame from cracking, as Ford found when testing the coil spring / long radius arm 4WD suspension when it was being developed in the early aughts.

Even that frame reinforcement is designed to minimize sudden stress risers. The web of the boxing is fish mouthed, and the attachment to the crossmember is tabbed. The reach back of the flanges stretches toward the existing service frame splice, spreading the strain throughout the frame, rather than attempting to shore up just the area of demonstrated frame failure from the design change in front suspension.

U Bolts not only concentrates stress/strain at attachment points, they also crush the flanges of the frame when torqued properly. Ford strongly recommends that blocks be placed between the upper and lower flanges at every U bolt.

The most logical course of action is to determine how much actual weight (by weighing the wet and loaded camper and truck on a physical scale) is being borne by the existing truck. The data derived from this physical weighing can then be entered into the mental weighing of options to solve the porpoising, as well as options to cowboy the truck into doing something more than it was originally designed too do.

I know I'm late to the party, but all of this should not be ignored. If you aren't consistently reinforcing through the whole frame you are just moving around stress loads.


Also, porpoising is a function of suspension (including frame flex) and wheel base. You may fix it for the road that you are concerned about, but then it will show up on a different road.

[Grit_dog]

^Agreed. But it’s time for this thread to wander off to the pasture. It’s a half baked idea that I was fueled by emotional attachment and financial upside downness with the truck in question.

[TxGearhead]

My last: I'm sure all of us with a TC have met an 18 wheeler on a 2 lane road at speed and have seen the front overhang lift. That is not necessarily a function of flex, although flex might be a factor.
I think we all, well almost all, are making valid points.

[HMS_Beagle]

mbloof wrote:

HMS Beagle wrote:

mbloof wrote:

TxGearhead wrote:

OP: are you sure you're not just seeing the camper rock back and forth, the spring loaded Fastguns doing their job?

I always get a kick out of these sorts of comments. 🙂

So a flat hard object sitting flat on another flat hard object is "rocking"??!?!??! hahahahahahahaha
.

Clearly you have little knowledge of material science. "Hard" and "flat" are relative terms. Steel is quite flexible, fiberglass very flexible, and any rubber map between them extremely flexible. Neither the bottom of the camper, nor the bed are flat, nor do they stay flat running down the road.

Granted nothing is hard/flat like a rock or chunk of concrete and almost everything has 'flex'.

The $64 question is how much? So flat camper sitting on flat bed on top of flat side of frame rails.

Is the camper compressing/expanding? Not likely.
Is the bed compressing/expanding? Again no. However given the support is in the center of it surely if enough force were applied to ether far side or single corner it might 'twist' or 'rock' side to side.
Is the camper 'rocking in the bed'? Again no. (unless it is on a thick soft rubber mat (horse stall mat comes to mind here)
Is the bed moving around on the mounts to the frame? Maybe, many have rubber mounts (to account for frame flex/twisting, see the side2side above).

Is the frame flexing/twisting? Sure, they are known to do that.

IMHO: Given the pivot point is the center of the frame and the camper nose is ~6-8' forward of that what the OP is seeing is minor frame flex amplified by the distance between pivot point and tip.



- Mark0.

Does the camper flex? Absolutely. A Bigfoot probably more than some other brands due to the construction method. You can sit one on rigid supports or concrete, and get the nose bouncing a bit just with your hands. Does the bed flex? Again, absolutely. I carry machine tools in the bed, about the same weight at a camper, CG much lower, the tools themselves are VERY rigid being cast iron and designed specifically for rigidity, yet you will see them visibly rocking forward and backward as you drive. That will be true even when placed on 6x6 sleepers directly over the frame. The bed is after all just thin sheet metal, supported by sheet metal hats of low section. On an F350, no rubber, bolted directly to the frame. The cab supports have rubber.

I'd agree the question is, how much? It is easy enough to test, just tape a piece of wire onto the overhang so that it comes close to the windshield or mirror or some other point of reference, mark that with some tape. Movement is pretty easy to see, how much is difficult to estimate without a close reference.

[Grit_dog]

Or to sum up 8 pages, it’s a big camper on a pickup truck. One that arguably flexes more than new trucks and it’s ok.
If you wanna find a shop to disassemble frame cross braces, probably pull the trans, move/re-rout brake lines, fuel lines, electrical, drop the tank etc and then try to fit/cab it all up to fit back in there like it was after doing idk what to the frame, have at it. It’s a bad idea on all fronts.
That’s all. Easy peasy…..
On a similar note I had a new 89 Dodge 1 ton truck at work that the guys would constantly overload so bad it would sit hard on the bump stops. (Not my truck and wasn’t the boss, just a high school kid turning wrenches.).
They broke the frame on one side right over the axle. We paid a shop to repair it. (In 1990).
Then the other side broke the same way. I copied the first repair. And the truck frame was still in one piece years later. But that was FAR easier than what is being proposed here….
And it was a work truck that got the tar beat out of it daily. Whole diffenrrt scenario

[JIMNLIN]

Is the bed moving around on the mounts to the frame? Maybe, many have rubber mounts (to account for frame flex/twisting, see the side2side above).

I've had 8' and 9' 6" and 10' 6" truck campers on several different GM and Ford one ton drw service trucks. All three TCs were held in the bed with stake pocket tie downs or TC bolted thru TC floor to the bed frame.
At speed the TC tied solid to the bed had lots of lateral and vertical movement. I used truck camper shocks (camper to the cab mounts) to control the harsh vertical movement on rough roads. I've watched the shock extend a good 2" to 2 1/2" going into a strong head wind at highway speeds...so yeah lots of bed movement in this scenario.
Granted all my LDTs in service were not the long WB crew cabs which don't work the best pulling heavy GN flatdecks in and out of close shipping yards/worksites/etc but IMO there probably would be more TC/bed/frame movement with the longer wheel base.

[mbloof]

HMS Beagle wrote:

mbloof wrote:

TxGearhead wrote:

OP: are you sure you're not just seeing the camper rock back and forth, the spring loaded Fastguns doing their job?

I always get a kick out of these sorts of comments. 🙂

So a flat hard object sitting flat on another flat hard object is "rocking"??!?!??! hahahahahahahaha
.

Clearly you have little knowledge of material science. "Hard" and "flat" are relative terms. Steel is quite flexible, fiberglass very flexible, and any rubber map between them extremely flexible. Neither the bottom of the camper, nor the bed are flat, nor do they stay flat running down the road.

Granted nothing is hard/flat like a rock or chunk of concrete and almost everything has 'flex'.

The $64 question is how much? So flat camper sitting on flat bed on top of flat side of frame rails.

Is the camper compressing/expanding? Not likely.
Is the bed compressing/expanding? Again no. However given the support is in the center of it surely if enough force were applied to ether far side or single corner it might 'twist' or 'rock' side to side.
Is the camper 'rocking in the bed'? Again no. (unless it is on a thick soft rubber mat (horse stall mat comes to mind here)
Is the bed moving around on the mounts to the frame? Maybe, many have rubber mounts (to account for frame flex/twisting, see the side2side above).

Is the frame flexing/twisting? Sure, they are known to do that.

IMHO: Given the pivot point is the center of the frame and the camper nose is ~6-8' forward of that what the OP is seeing is minor frame flex amplified by the distance between pivot point and tip.



- Mark0.

[Grit_dog]

^Uhh, yeah they are both “flat” for the purposes being discussed here.
Save for some o them newer trucks with an actual crown in the bed.

[HMS_Beagle]

mbloof wrote:

TxGearhead wrote:

OP: are you sure you're not just seeing the camper rock back and forth, the spring loaded Fastguns doing their job?

I always get a kick out of these sorts of comments. 🙂

So a flat hard object sitting flat on another flat hard object is "rocking"??!?!??! hahahahahahahaha
.

Clearly you have little knowledge of material science. "Hard" and "flat" are relative terms. Steel is quite flexible, fiberglass very flexible, and any rubber map between them extremely flexible. Neither the bottom of the camper, nor the bed are flat, nor do they stay flat running down the road.

[valhalla360]

time2roll wrote:
I am curious if the weight rating is primarily given for a load low in the bed with less front to rear rotational mass. I mean to say a load 4000# of gravel vs 4000# gooseneck vs 4000# truck camper are very different on the dynamic frame stress even if the static downward pressure is the same.

Most certainly it does. That's why they provide a separate LOWER payload for truck camper use.

[adamis]

mkirsch wrote:
To paraphrase, "If you find somebody willing to weld on your frame... you don't want them."

Why not? Thousands of commercial truck frames are welded on a daily basis. They cut 'em behind the cab and splice in a piece to add length. They install frame liners to add strength.

Older pickup trucks are routinely shortened to convert them into the more desirable "short box" versions.

Stock frames are boxed in all the time for offroad applications.

Thanks for the comment. This was along the lines of what I was thinking when I asked the question. I know it isn't common and you don't want just any shop doing it but clearly there are places that do it for the reasons you mentioned.

[Grit_dog]

Meh, the funny math doesn’t really prove anything.
Especially since you’re speculating about 2 out of the 3 weight factors. People and stuff and actual camper weight.
$20 says your 7600lb truck, camper and everything in/on the truck is right around 13klbs.
Maybe 12,500.
I’m not saying I wouldn’t haul it. Quite the opposite in fact, but you’re arguing with yourself again, now trying to prove how light it is I guess? after starting the thread about how heavy it is!
Sometimes these things require one to do a little easy real world information gathering. Like weigh the whole thing and don’t be forgetting anything.
Although what it actually weighs is largely immaterial except to settle the speculation.
And measure the deflection if it’s worrisome (which you also said it’s not, but a definite annoyance). Easiest idea I can think is tape/attach a crude dial indicator to a vertical surface between the camper and the back of the cab at the top of the cab. Like a tire depth gauge maybe. Long enough to touch both surfaces.
See how much it compresses over some bumps at low and high speed.
From that you can do some trig and figure the max rotational angle the frame is flexing and the total amount of droop/flex.
But that is also really just a science experiment…. But better than speculation since it’s bothersome or perplexing.

[mkirsch]

To paraphrase, "If you find somebody willing to weld on your frame... you don't want them."

Why not? Thousands of commercial truck frames are welded on a daily basis. They cut 'em behind the cab and splice in a piece to add length. They install frame liners to add strength.

Older pickup trucks are routinely shortened to convert them into the more desirable "short box" versions.

Stock frames are boxed in all the time for offroad applications.

[JimK-NY]

BigToe wrote:
.........

To be solution oriented, it is strongly suggested to have your camper actually weighed as currently equipped with how you travel with it.

Most of us who have done this are shocked at the results. I know I was. I had the base weight of my specific TC and thought I had an accurate estimate for the weight of everything else. I was off by close to 1000#.