Frame Stiffing to reduce Porpoising?

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.