Bad vibe

[Prairie Dog]

I've been chasing a vibration that feels like an out of balance tire for years. It comes on at 41 and 74 mph and happens whether it's under power or coasting (clutch in or out). The speed at which this happens was raised a few MPH after installing the bigger (285/75) tires.
As you can see from the list below, I have addressed almost every rotating thing under the truck with the exception of the front rotors. I have read of situations where a rotor is out of balance and causes this kind of problem but it sounds like it's extremely rare and I couldn't find a single case where it happened in a HD Dodge.
So here's my question, is it worth it to blow $200 on new rotors just to weed them out or is there something else I'm missing?

Thanks!
Scott

What we've tried:
Tires balanced many times.
Replaced rear drive shaft with Dodge 1 piece (had it balanced). No more launch shudder!
Removed front DS and drove - no change. (had it balanced anyway)
Replaced Front U-Joints.
Front wheel bearings tight.
Replaced tires (twice - once with stock size)
Replaced all wheels with stock aluminum type. (Checked for out of round)
Replaced rear rotors..
Ran in 2WD and 4WD - no different.
Ran truck at speed on a lift with chassis supported at axles - no vibe evident. (???) Shop was stumped.

 

[Ozymandias]

Is it only adressed to speed, doesn't matter which gear nor engine rpm?

 

[Prairie Dog]

Is it only adressed to speed, doesn't matter which gear nor engine rpm?

Exactly.
No relation to engine speed, gear or whether it's even in gear.

 

[sag2]

Time to stop changing things and find a dealer or shop with a vibration analyzer. It will pinpoint the vibration without guessing. The Dodge dealers had one back quite a few years and was called an EVA for electronic vibration analyzer. Some GM dealers also had it as a required tool.

 

[Prairie Dog]

Sag, I should clarify that none of the parts in the above list were replaced to fix the vibe problem. They were replaced for other issues.
I realize I could take it to a dealer but there's only one left in the area and they have proven themselves to be less than capable.

 

[mfurrh55]

I just fixed a vibration on mine, turned out it was front right tire, was wearing funny, had high and low spots all over, the other 5 were wearing like they were suppose to, changed front 2 out and ride is smooth again, good luck finding yours too, Monte

 

[leaky gasket]

Somewhere here on TDR I believe one member posted a whole list of third generation vibration solutions. For the life of me I can't find it right now. But one remedy I do remember was loosing up the motor mounts and transmission mount then jacking up the engine and then letting it settle.

 

[cerberusiam]

Might try front rotors, ot, have them checked for runout. They wull throw vides into the drive train that are impossible to track. Other point if issues could be the clutch. You may just be feeling the torsional harmonics coming thru the drive train.

 

[Prairie Dog]

Thanks LG!
I can see where if the eng/trans was set in pushed to one side there could be some phasing issues.

FWIW, I had the tires balanced and rotated today and it seems to be better - no vibe till about 85. But they also set all my tire pressures to 50 psi so we'll see if it's bad again when i run them up to 75 psi for the RVing season.

Scott

 

[SnoKing]

Fronts should need about 60 for the weight on them stock. I run my rears at 44 empty. Chris

 

[Prairie Dog]

Might try front rotors, ot, have them checked for runout. They wull throw vides into the drive train that are impossible to track. Other point if issues could be the clutch. You may just be feeling the torsional harmonics coming thru the drive train.

I might go ahead and do the rotors because they're rusty enough that I don't think I could measure run-out. Funny that the tire balance today effected it so much. Tells me I'm in the right area.
I thought about the clutch - if thats it then I guess I'm stuck! But wouldn't that be more eng speed specific?

Chris - I agree, I usually run 60 in the front no matter what.

 

[cerberusiam]

The harmonics can show up at any point and any engine speed. The unfortunate result of the 1 piece DS and the better clutch is more vibes get thru. The front and rear rotors on mine caused definite vibrations thta diappeared when I put new ones on. I have a resnonsce problem now that shows up over a certain speeds from the exhaust. Crazy things hrmonics in the engine and sound can do.