Rear shocks twisting frame?

[Pinger]

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!

Can you take a guess at what frequency it is at?

 

[evilunclegrimace]

Can you take a guess at what frequency it is at?

Instead of guessing the frequency get a reed tachometer and find out exactly what the frequency is and then it will be much easier to diagnose the problem.

 

[evilunclegrimace]

Here is a demo on how a reed tach works. Jump to the 10 minute time stamp.

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[Frank Enstein]

Can you take a guess at what frequency it is at?

About 2 Hz @35 mph and 4 Hz @ 70 feels like a bent rim or eccentric tire.

 

[Pinger]

About 2 Hz @35 mph and 4 Hz @ 70 feels like a bent rim or eccentric tire.

What does it feel like? Is it just a vibration throughout the truck or can motion be detected - like a bending up and down behind the cab - or something like that?
I have a Suburban and have never noticed anything like that but Suburbans are much heavier at the rear and as Schurkey alludes, they much better tie the frame corners together.

 

[Pinger]

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

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

Kind of confirms what I'd been thinking for a while (for the long-roofs at least). Unitary body with rubber mounted subframes that just happen to be connected front to rear.

FWIW, I think torsional rigidity is over-rated and that is because of its importance in mid engined race cars where there's a need to transfer roll at the rear to the front anti-roll (sway) bar. With the required roll stiffness for each axle provided for by that axle there's no real need for torsional rigidity between them.

 

[Frank Enstein]

The vibration on my truck is vertical and feels like it's at the back of the cab.

You need torsional stiffness so your suspension is predictable. My Firebird rides better after the roll cage install than before.

 

[Hipster]

FWIW, I think torsional rigidity is over-rated and that is because of its importance in mid engined race cars where there's a need to transfer roll at the rear to the front anti-roll (sway) bar. With the required roll stiffness for each axle provided for by that axle there's no real need for torsional rigidity between them.

That goes against everything I've read in chassis manuals etc. It might not make a tremendous difference on a pick up but I've seen my fair share of rear end collisions on pick-ups and almost always one or both upper bedside corners smack the cab corners below the rear window in a collision. Flexi-flyers.

Race cars and hot rods can use all the stiffness they can get. As already mentioned it makes for more predictable suspension action. I do alot of Mopar stuff and following the Mopar chassis manual of just additional welding to what's already there makes a night and day difference in chassis stiffness without adding a single piece. Add sub-frame connects and torque boxes and really make progress.

 

[Pinger]

That goes against everything I've read in chassis manuals etc.

Which is why it's come to be 'over-rated'.

It might not make a tremendous difference on a pick up but I've seen my fair share of rear end collisions on pick-ups and almost always one or both upper bedside corners smack the cab corners below the rear window in a collision. Flexi-flyers.

That's different though, viewing it in terms of an accident - not in the driving.

Race cars and hot rods can use all the stiffness they can get. As already mentioned it makes for more predictable suspension action. I do alot of Mopar stuff and following the Mopar chassis manual of just additional welding to what's already there makes a night and day difference in chassis stiffness without adding a single piece. Add sub-frame connects and torque boxes and really make progress.

Race cars will always be chassis tuning on the anti-roll (sway) bars so torsional rigidity is vital there (and I don't dispute the predictability aspect. Some of the shift in geometry can possibly be catered for in a road car/truck but the motion will always be undamped - never ideal). But if torsional rigidity was as important as some imply then my Suburban (can't talk for the pick-ups as I've not driven one) would not handle as well as it does.

 

[Hipster]

Which is why it's come to be 'over-rated'.

That's different though, viewing it in terms of an accident - not in the driving.

Race cars will always be chassis tuning on the anti-roll (sway) bars so torsional rigidity is vital there (and I don't dispute the predictability aspect. Some of the shift in geometry can possibly be catered for in a road car/truck but the motion will always be undamped - never ideal). But if torsional rigidity was as important as some imply then my Suburban (can't talk for the pick-ups as I've not driven one) would not handle as well as it does.

Both are body over frame construction. The Burb has a unitized upper structure along a greater length of it's frame. It's no doubt the stiffer chassis setup. The additional torsional rigidity is exactly why it handles like it does.

I always felt Burbs handled better and have a better road feel then pick ups.