Rear shocks twisting frame?

[Keeper]

Yep, mine has that vibration at 45mph regardless of tire size. And it is worse on certain types of roads than others. Weight in the bed is the only way to get it to calm down. I always figured it was the oddball wheelbase combined with the 3500 springs. I went down the rabbit hole of balancing/rebalancing every moving part before finally accepting the reality of it. Good to know that when I tackle the build on it next year that there are some options out there to resolve the issue. Nice.

 

[Hipster]

That's the general trend. Newer stuff is stronger and stiffer, but actually lighter on a unit length basis.

How do they do it? Cross sections are generally thinner, but deeper to make stuff more efficient.

I imagine everything is optimized (using powerful analysis programs and manufacturing technology not available in the 70s-80s,) whereas the older stuff was more-or-less a constant cross-section.

New trucks use a considerable amount of High Strength Steel in the frame. Some have hydroformed sections etc. The stuff is brutally strong and you can't put heat on it to straighten it etc. It has it's own rules for dealing for dealing with it..

 

[Hipster]

I've heard of that vibration thing. I would imagine it affects a certain cab/bed configuration only? Or more so than others?

From my limited knowledge on vibrations in static systems, the major players affecting vibrations are stiffness and mass (weight). There's also excitation cycles, but I don't think you can control that unless you avoid driving a specific speed over a known road conditions cause a specific frequency, LOL.

Think old Harley shovelhead. Vibrations can eventually crack the frame and every bracket welded to the frame will eventually fail and fall off at some point. lol

Burbs and the others that have the unitized upper structure that works in conjuction with the frame are much stiffer.

 

[Schurkey]

Burbs and the others that have the unitized upper structure that works in conjuction with the frame are much stiffer.

Not directly related...but...I was AMAZED at how thin and flexy the frame under my '68 'Camino was/is. The whole car rattled and creaked, I could watch the bumper and body separate when I jacked it up.

And then I installed fresh body-to-frame bushings, with new hardware...

Those vehicles are practically "unibodies" with a front-to-back "suspension bracket" (the frame.) The body is enormously more rigid than the "frame".

Body Bushing Replacement GM “A” Body

www.chevelles.com

 

[sewlow]

Some forethought has to be put into it and a singular reinforcement can transfer loads and particularly vibrations to a different point on the frame and can cause cracks somewhere other than where the reinforcement was installed. Given the choice if I got in one that deep I would box it and go from there.

Yea, I could see that happening.

Boxing the frame? Pix! I got pix!
A former member's truck. JurgenS'. From Belgium!
Posted Oct. 2/15.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[sewlow]

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[Frank Enstein]

Ain't that the truth.
In fact, these truck's frames have a problem with what's called 'beam walk'.
An inherent vibration that occurs between 30-40mph (slight) & again between 65-75+mph (pronounced)
GM's aware. ...and they have a 'cure'.
Ballast. Yea. Extra weight.
Their recommendation is to carry a couple of hundred pounds in the box. All the time. WTF?
A more permanent fix is to box the frame.
From the rear cab mount to the shackle mount. Top plates welded to the cross members help, too.
Makes accessing any lines running inside the frame difficult, though.
I've seen some frames that have had trusses added. Alternating vertical & angled steel bars welded inside the leading edges of the frame's C-channel shape.
Easier access to those lines. Less weight than boxing. Easier to fab.

I wondered what that was! The frequency of the vertical vibration is far lower that wheel/tire or driveshaft would have.
Oh Geez! I gotta box the frame now!

 

[Caman96]

I've never seen a full tank of gas change ride height 2.5 inches

This may be possible with gas being worth it’s weight in gold!

 

[Erik the Awful]

Yep, mine has that vibration at 45mph regardless of tire size. And it is worse on certain types of roads than others. Weight in the bed is the only way to get it to calm down. I always figured it was the oddball wheelbase combined with the 3500 springs. I went down the rabbit hole of balancing/rebalancing every moving part

Does it get quieter when you turn left or right? If it's quieter when you turn left, check your left front wheel bearing. Quieter right, check the right front.

 

[Keeper]

Does it get quieter when you turn left or right? If it's quieter when you turn left, check your left front wheel bearing. Quieter right, check the right front.

Thanx, but it has been going on for 23 years. I never noticed it on previous 400's, which is why I thought it was unique to the truck.

It's not a noise per se, more like a frequency bounce in the middle of the truck. Not front, not rear. I chased it forever. Still have it. I just avoid going 45 on smooth roads.... 65 also has it but it's less bouncy.