Mine had excessive(I thought) flex between the bed and cab. Even with an empty trailer. The plating with FRAME STEEL stopped that. I would NEVER box a frame. All it does is trap dirt and making mounting things VERY difficult

 

The bracing should help a lot. Plating may be the way to go. I know my truck flexes BAD, and the frame is still solid from the factory. The flexing has caused some of my bed mounts to go bad prematurely. The terrain is not very kind in Kentucky. I would expect that a little reinforcement of any sort would be a good idea even if its just to stop the shimmy.

 

Bed mounts?

 

As odd as it seams yes. I parked it over night some time ago where the front right tire was about 2 foot higher than the rear, and about 2.5 foot higher than the left side. When I came back out the next day bed mounts had failed. I thought the frame had actually bent at first. The frame is still true though. The only thing I can figure is it was the flex of the frame.

 

Subscribing to thread. interested to see if any root cause is found here. my CCLB had a moderate vibration between at 35 mph, then a small one at 70 before my rebuild. the vibration has reduced, but not completely gone away since rebuild. my cab bushings are now, but spring and swaybar bushings are all old - they're waiting for the RSK, rebuild D60 and new springs that are in the plans.

i'll have to try 'actuating' my bumper like yours, Leo, but in your video from the front, you can see even the front of your front springs moving up and down. its like a standing wave is set up in the frame. if you keep that same frequency, but increase the amplitude of input, you could probably crack the frame!!! that would require alot of input force, but we've all seen the Tacoma Narrows video - same idea.

anyway, i'll stay tuned and good luck solving it. plating it would obviously chance the stiffness of the frame and drive the vibration frequency up and amplitude down - as Brad has shown. interesting stuff.

 

The bed bolts directly to the frame and don't use Mounts

 

Exactly, the vibration at 35 mph was probably the first mode of vibration. The frame needs stiffened to raise the natural frequency.

 

I have what I've called a nervousness about the truck at a similar speed. Feels low in the truck, makes the bottom of my door panels rattle sometimes. And at the higher speeds it feels low, just gets more of the truck involved.



Just our of frame at the bottom right is my safety sandaled foot palpitating a receiver hitch that is an option on the PMF RSK. It is tricky to get your foot quivering fast enough, and you have to speed up and down until you find the right frequency.


Yes they do start moving, but not at first. Agreed, I believe my foot is flexing the cab which in turn start to excite the frame.

The rear springs are moving in the side video too. But in both cases I'm not convinced that the springs are compressing, just their mounting points are being moved.


Kenwood or similar had a TV ad that used some of that footage...


Agreed that the natural frequency needs to change.

Awhile ago I removing my trans case skid plate. I reinstalled it thinking it might tie the two frames together. No noticeable improvement. Later, I installed the same style skid plate (from a different truck) just in front of the front fuel tank, with no noticeable difference. But, I recently removed the trans case kid plate to swap the front yoke in preparation to install the D60 drive shaft. It being removed made a noticeable increase in the nervousness at slow speed. That is a long winded way to say that changing the natural frequency may rather easy if I can find the right to dampen the frame movement.

 

It looks as if your core support is damaged near mounts. That is literally the only reason the front end could move like that.

 

Perhaps I am mistaken, but given the fact that my bed let go from the frame unexpectedly over night I should think that the fact that the frame was under stress, that it equally put on the bed was the cause for it to pull free. Whether or not it has bushings is aside from the point.

From what I can tell there is a design flaw in the bracing of these trucks. I believe the trucks have a 15 foot wheel base if memory serves me right. That is 6" shy of the length of my dodge grand caravan from bumper to bumper.

I do understand that the frame can not be so rigid that it would shear instead of flexing. My opinion sways to the idea of brace supports. That would still enforce the frame to make it slightly more rigid, but allow for flex.

 

How does one properly stiffen a frame? Metal type, location and fasteners.

My 1997 f250 CCSB suffers from excessive flex & twisting with a Travel Lite camper mounted.
When rocking the camper, the cab almost seems disconnected. A friend's 2001 F350 CCLB cab stays in line when doing the same rocking motion.

Note: Looked under the latest F150 today and its frame is a massive boxed structure.

Great post, I thought I was suffering alone. Thanks for the tips,

Bob

 

Frame plating......

I got a crewcab long bed frame from the junk yard. I them cut it down to a little over 7' long. I then trimmed the top and bottom off just leaving the side of the frame. That was then sandblasted bare, painted with POR15 and then top coated with chassis black. 2 coats of cosmoline was put on the side where it would meet the Inside of the original frame.

Then I removed the front tank crossmember and support brace along with the rear cab crossmember brace. The inside of the frame also received 2 coats of cosmoline. Note the frame was already sandblasted and painted 2 years prior.

The "plate" was then installed using existing holes and BOLTING it in place using grade 8 bolts(overload pad, spring hangers, e-brake cable mounts, step holes, etc). It goes from the trans crossmember back to the front overload leaf pad. If you look close you can see the factory oval tie down holes do not line up. This is because the donor frame I used was a Long bed.


Please excuse the dirt. I just got it back together in the pics. Also I used FRAME steel because it will FLEX and also FLEX BACK. Regular steel will NOT

 

Excellent details and photos, you should be proud of your work. Glad it's an "Easy Fix". I will assume it solved your flexing.

This is the last truck I will own, so I just found my next project.

Thanks again, Bob

 

Nice mod brad.
Thats a hell of a frame overhaul without anything crazy.


From what I can tell knowing how my truck rattles.
I'm going with body mount issues. The front clip is moving really odd compared to the rest of the truck and probably floating the vibration backwards.

I'm with the other fella here about the core support mounts.
Mine are both gone, my front clip actually rests on the bumper and bounces.

 

New poly bushings a year or so ago.

The frame movement is moving the body. If they were sloppy, the frame could move some before the body was moved. If the front core supports were broken off, wouldn't the front clip just rise and fall instead of getting bowed?

Lot's of good ideas so far...