Frame resonance.

[thinger2]

This has been discussed before (the resonance/vibration from the middle of a GMT400 frame) though I can't remember the proper name for it. My C2500 Suburban had it mild at around 60-65mph - but it's got much worse and now occurs at 70mph - a useful cruise speed.
What has changed is that I rotated the tyres (as per handbook for four - spare not included) and a change of tyre pressures from 45/50psi F/R to 40/45psi F/R. I'm pretty certain it is the frame resonance thing - it is at a frequency much higher than wheel related. There's a remote chance that I got some underseal on the propshaft but I don't think I did. The vibration now is quite intrusive and I'd like to be shot of it.
Which if either of the above two changes has most likely caused the changes (in speed that it occurs and severity?
If tyre rotation - will it right itself as the tyre bed into their new positions?
If tyre pressures - is front or rear the bigger contributor? (If the fronts then I have scope to raise the pressure there but not at the rear (psi limit is 44psi on sidewall)).
All advice welcome! TIA.

I have read about the frame resonance you mention and I believe they called it "Ladder Wracking" or "Ladder Twist"
That might even have been from a GM TSB.
The frame on these trucks is pretty much built like a ladder and it can twist.
Have you had someone re-balance the tires?
Truck tires can be difficult to balance properly.
The correct procedure if you have a tire that is way out of balance is to unseat the tire, rotate it 90 degrees and try again.
A decent offroad shop will do this.
The average tire shop will just stick a bunch of wheel weights on it and call it good enough.
On alloy rims, they might have half the inside of the rim covered in tape weights and not really prep the surface.
Those weights get flung off one after another as you drive.
Is that vibration actually at a higher frequency or are you hearing the higher resonance tones of a set of vibrations that have now moved to the front of the truck?
Imagine if we built some monster sound system and attached it your rear axle in the back of the truck and behind your ears and played War Pigs untill the truck shook.
Then imagine if we took that same system and installed it under the front axle and played War pigs.
Same song through the same system but a very different experiance.
It would not surprise me at all for that percieved vibration to change when rotating tires.
I would assume that it was a tire problem or a rim problem way before I would worry about a ladder jacked frame.
I think you have a tire problem or a motor mount transmission mount problem.
Swap the fronts side to side temporarily and see if the vibration changes.
And obviously, make sure you arent chasing an out of round brake drum or a sticky caliper.
Swapping a poorly weighted rim onto an already sticking brake can change how that brake contacts the surface.
And, just in general I highly recommend to everyone that you spend the money to buy a good quality digital tire pressure gauge with a bleed valve feature on it.
The 3 dollar chinese stick gauges are wildly innacurate.
And difficult to use.
The quality digitals have a clip that locks onto the valve stem and gives instant readout and it becomes a one handed process.
Lock the hose on the valve stem, hold the gauge in your hand, push the bleed with your thumb.
Every tire ends up at exactly the same pressure.
Never trust the pressure gauge at the gas station.
Our tires are so friggen expensive that buying a good tire gauge is just protecting your investmant.

 

[1998_K1500_Sub]

This has been discussed before (the resonance/vibration from the middle of a GMT400 frame) though I can't remember the proper name for it. My C2500 Suburban had it mild at around 60-65mph - but it's got much worse and now occurs at 70mph - a useful cruise speed.

I once had a similar problem.

Out of curiousity, I jacked the vehicle up and supported it at THREE corners with jackstands, and I left one corner airborne BUT... with the fourth jackstand placed about 1" below that corner, so there was an air gap. I'll call the fourth jackstand the "free" jackstand.

I then took a long pry bar (5' maybe, one of those long muthers), stuck it between the "free" jackstand and the frame, and simply banged the bar against the frame so as to excite the frame resonance. Some might call this action "applying an impulse".

I then watched the vehicle wiggle. Man, what a set of weird vibrations. Watching the frame move up and down in the air gap over the "free" jackstand, it was evident there were multiple frequencies involved. The wiggle wasn't simply akin to a decaying sinusoid, rather, it's amplitude would beat, suggesting there were two or more sinusoids of differing frequency acting within the vehicle. My take away was there were multiple modes of vibration... which didn't surprise me, but detered me from trying to identify them.

With that said, I never acted on this information; I didn't try "box" the frame of the vehicle (it was a unibody 1991 Chevy Beretta) or stiffen it...

But I haven't forgotten how illustrative it was.

My K1500 Suburban has weird vibrations too, which I made worse by putting on a set of late model GM K1500 take-off 18" wheels and tires which are much heavier then my Sub's original equipment (16"). I haven't tried the "jackstand" experiment with it although some day I might. I may consider boxing the frame, but for now I'm just living with the vibration.

Edit: Back to the 1991 Beretta, I found that if I ran the rear tires at 20 psi instead of their recommended pressure (30-32), it really quieted the vehicle vibration. It was a light vehicle on the back anyway, even at 20psi the rear tires showed little squat.

 

[Pinger]

Have you tried returning the tires to their original positions to remove them from the equation? Maybe when you put one of the wheels back on it didn't seat on the hub properly.

No, not yet, and if I do, I intend swapping the rears L to R, R to L to reinstate their rotational direction. But not before I've checked the propshaft more thoroughly - weather here isn't conducive to that right now.
I'm super diligent mounting wheels and they've been re-torqued to at around 80 miles.

Is the vibration consistent at speed regardless of rpm?

I think so - but on the day it appeared I had more going on than to be probing at that - and limited opportunity.

Like does the vibration go away if you shift down a gear but stay at the same speed?

I intend checking that at next opportunity - more to eliminate the drivetrain than expecting to find the cause though.

 

[Pinger]

I have read about the frame resonance you mention and I believe they called it "Ladder Wracking" or "Ladder Twist"
That might even have been from a GM TSB.
The frame on these trucks is pretty much built like a ladder and it can twist.
Have you had someone re-balance the tires?
Truck tires can be difficult to balance properly.
The correct procedure if you have a tire that is way out of balance is to unseat the tire, rotate it 90 degrees and try again.
A decent offroad shop will do this.
The average tire shop will just stick a bunch of wheel weights on it and call it good enough.
On alloy rims, they might have half the inside of the rim covered in tape weights and not really prep the surface.
Those weights get flung off one after another as you drive.
Is that vibration actually at a higher frequency or are you hearing the higher resonance tones of a set of vibrations that have now moved to the front of the truck?
Imagine if we built some monster sound system and attached it your rear axle in the back of the truck and behind your ears and played War Pigs untill the truck shook.
Then imagine if we took that same system and installed it under the front axle and played War pigs.
Same song through the same system but a very different experiance.
It would not surprise me at all for that percieved vibration to change when rotating tires.
I would assume that it was a tire problem or a rim problem way before I would worry about a ladder jacked frame.
I think you have a tire problem or a motor mount transmission mount problem.
Swap the fronts side to side temporarily and see if the vibration changes.
And obviously, make sure you arent chasing an out of round brake drum or a sticky caliper.
Swapping a poorly weighted rim onto an already sticking brake can change how that brake contacts the surface.
And, just in general I highly recommend to everyone that you spend the money to buy a good quality digital tire pressure gauge with a bleed valve feature on it.
The 3 dollar chinese stick gauges are wildly innacurate.
And difficult to use.
The quality digitals have a clip that locks onto the valve stem and gives instant readout and it becomes a one handed process.
Lock the hose on the valve stem, hold the gauge in your hand, push the bleed with your thumb.
Every tire ends up at exactly the same pressure.
Never trust the pressure gauge at the gas station.
Our tires are so friggen expensive that buying a good tire gauge is just protecting your investmant.

All points noted.
The vibration when it occurs is at 70 mph and is as if 'switched on' and permeates the entire cabin - impossible to say from front or rear. If I had to say from where I'd say the middle. New front calipers this year. I'll check for missing wheel balance weights but I don't think the problem is that. Or any rim damage or wheel run-out.

 

[thinger2]

All points noted.
The vibration when it occurs is at 70 mph and is as if 'switched on' and permeates the entire cabin - impossible to say from front or rear. If I had to say from where I'd say the middle. New front calipers this year. I'll check for missing wheel balance weights but I don't think the problem is that. Or any rim damage or wheel run-out.

Are you absolutely sure that the pinion bearing is good?
And for that matter the cv shafts?
I can imagine a scenario where swapping tires front to back might cause the vibration to happen at a different rpm due to the change of diameter and wear.
That it happens at 70 sounds to me like something that should be spinning in a linear way is now running out of straight and starting to bind at that speed.
At this point I think you have to beyond any doubt eliminate the rear diff and the cv shafts and the driveshatfs from the possibilities.
Only because a failure of any of those components at speed can be deadly.
I would pull the diff cover and drop the driveshaft and check for pinion wear and chunks and broken gear teeth in the differential first thing.
And rear wheel bearings and axles etc..
Just to check it off of the list.
You must make sure that your diff is not starting to disintegrate.
I ignored a little strange vibration on my 64 T-bird and went around a corner and the entire right axle came out of the car and we spun twice in the gravel and tore the gas tank open.
Twas ugly, Another first date gone all to hell.
Then I would take a close look at the driveshafts and cv shafts.
Eliminate the things that could kill you first.
It is a lot of work but on a vehicle that old you need to do it anyway.
And by doing it you no longer need to worry about it.
Best of luck.
Please let us know what you find.

 

[Pinger]

Update time. I think I've banished it. Not exactly sure how as I did two things at the same time. I checked the propshaft for underseal and removed a few very small blobs. Very small - no one of them could have weighed more than a gram and mainly on the couplings ie, smallest radius - and not convinced that they were the cause I swapped the rears L to R, R to L to reinstate their rotational direction. Whichever of those two made the difference I'll never know, but the vibration appears to have gone.

 

[SUBURBAN5]

Update time. I think I've banished it. Not exactly sure how as I did two things at the same time. I checked the propshaft for underseal and removed a few very small blobs. Very small - no one of them could have weighed more than a gram and mainly on the couplings ie, smallest radius - and not convinced that they were the cause I swapped the rears L to R, R to L to reinstate their rotational direction. Whichever of those two made the difference I'll never know, but the vibration appears to have gone.

I think tires. Would of never believed it. This past Saturday I was driving to the tire shop to get my wife new tires. It was vibrating hard all the way there , worse when you take your foot off the gas. 4 new tires later. Like magic ..smooth all the home.. looked down, I was creeping on 90mph..lol Anyways point is. her issue was tires the whole time I guess... I blame it on there sticker weights. That's one thing I love about the older rims with the lip. At least there weights were less likely to fall off..