Vibration 85 K20

[72c20customcamper]

Been dealing with a rhythmic vibration. I can definitely say it’s in the front as the vibration comes up through the steering wheel . I’ve changed the tires had them road force balanced still the same . I just changed the front shocks from Gabriel gas to KYB the ride is much nicer and the vibration is faded but still there after the test ride put new KYBs on the rear still the same . Like I said it comes and goes in a rhythmic cycle . Be smooth and the vibrate then smooth out again and repeats . I don’t think it’s a balance issue and everything is tight in the steering parts ,could the springs be shot causing it to bounce at times . Almost feels like an undulation.

The vibration is mostly felt above 60 mph but is present at lower speeds too . It’s not a bad vibration but it’s very annoying.

Anyone deal with this ?

[Richard]

Maybe check the hubs and make sure they are fully unlocking?

[Keith Seymore]

The "undulating" is called a "Boom/Beat" in the biz.

What that means is that you have two disturbances going on, very close in frequency, such that the sine waves gradually add and subtract, giving you this variation.

Anyone who has been in high school band has likely experienced this as they have attempted to tune their instruments.

In a vehicular context normally it is something like engine firing frequency coupled with a driveline vibration.

The good new is if you can fix one vibration then the boom/beat goes away; the bad news is then you have one vibration remaining - and - if it is engine firing you can't very well make that go away.

K

[72c20customcamper]

Originally Posted by Keith Seymore

The "undulating" is called a "Boom/Beat" in the biz.

What that means is that you have two disturbances going on, very close in frequency, such that the sine waves gradually add and subtract, giving you this variation.

Anyone who has been in high school band has likely experienced this as they have attempted to tune their instruments.

In a vehicular context normally it is something like engine firing frequency coupled with a driveline vibration.

The good new is if you can fix one vibration then the boom/beat goes away; the bad news is then you have one vibration remaining - and - if it is engine firing you can't very well make that go away.

K

Guess it’s time to check all the rotating assemblies. Tires are out since it didn’t change so I need to look at the rear driveshaft and rear end front assembly doesn’t rotate with hubs unlocked . Hat made me think it’s the front is that I feel the wheel starting faintly before it gets a little more . It’s not a bad vibration just annoying.

[72c20customcamper]

Originally Posted by Richard

Maybe check the hubs and make sure they are fully unlocking?

I’ve done that

[400/400]

Is it road speed or engine speed related?

I’ve had an odd front end vibration the would come and go when a harmonic balancer came apart. The weight was free spinning around the “hub” when the rubber came apart.

[72c20customcamper]

Originally Posted by 400/400

Is it road speed or engine speed related?

I’ve had an odd front end vibration the would come and go when a harmonic balancer came apart. The weight was free spinning around the “hub” when the rubber came apart.

Road speed . The balancer was replaced when the engine was rebuilt

[Shifty One]

My front driveshaft did this to me.

[Accelo]

Originally Posted by Shifty One

My front driveshaft did this to me.

Exactly where I would look. Pull the driveline and drive it without it installed. Then get back to the group with the results. A picture of the driveline would help. Is the truck lifted?

[72c20customcamper]

Originally Posted by Shifty One

My front driveshaft did this to me.

Originally Posted by Accelo

Exactly where I would look. Pull the driveline and drive it without it installed. Then get back to the group with the results. A picture of the driveline would help. Is the truck lifted?

Front shaft doesn’t rotate when it’s not in 4 wheel drive ,truck has lockouts . Truck is bone stock

[57taskforce]

Lock your hubs in but don’t put the transfer case in 4x4 and see if it goes away…. Report back what you find.

I had one that did something similar few years ago that needed hub bearings. The vibration would go away(or maybe be offset?) when the hubs were locked in.

[72c20customcamper]

Originally Posted by 57taskforce

Lock your hubs in but don’t put the transfer case in 4x4 and see if it goes away…. Report back what you find.

I had one that did something similar few years ago that needed hub bearings. The vibration would go away(or maybe be offset?) when the hubs were locked in.

I’ll give it a shot

[cadillac_al]

Are your slip yokes tight or sloppy?

[72c20customcamper]

Originally Posted by cadillac_al

Are your slip yokes tight or sloppy?

Going to pull the shaft have it balanced . I’ll Check the slip yokes too

[72c20customcamper]

Pulled the front shaft at the diff this morning . Slip yoke is free no slop ujoints in both shafts have no play . I check to see if a hub may be hung up front driveshaft spins real easy in park . This is a light 3/4 not sure if the front springs should have a reverse arch or not but the truck is level

[57taskforce]

Yeah the front springs should have a negative arch from the factory.

[cadillac_al]

Seems like a lot of slip yoke showing on that rear driveshaft. That would bother my OCD too much. How much engagement does it have?

[72c20customcamper]

Originally Posted by cadillac_al

Seems like a lot of slip yoke showing on that rear driveshaft. That would bother my OCD too much. How much engagement does it have?

I’m not sure never been apart while I’ve owned it . I have every receipt for work on this truck since new . No mention of a new driveshaft or yoke . The log book the original owner kept had everything from gas, oil changes to the engine rebuild and repaint . He also would jot down when it snowed and he used the 4 wheel drive . So I’m sure the shaft and yoke are original to the truck

[72c20customcamper]

Thinking it’s in the left front . I checked the tires and the left front tire is starting to feather. I’m going to rotate the tires but I don’t think it’s weight related to the tire . Since it’s done it before with regular balancing and with the road force. Going to get new rotors and try that too might have thrown a weight on the left one . I looked at the rotors right side has two barrel pin looking things pressed into the rotor vent the drivers side has none . They are originals and are getting thin anyway

[JJH Jimmy]

Originally Posted by cadillac_al

Seems like a lot of slip yoke showing on that rear driveshaft. That would bother my OCD too much. How much engagement does it have?

That is normal. Mine was the exact same.

[JJH Jimmy]

The front spindles can get bent on these fairly easily. Maybe have the alignment checked? The only thing you would be able to do is get a new spindle or possibly shim it.

[72c20customcamper]

Found the front passenger wheel was 3/8+ inches back from the center eyelet of the front spring shackle bolt as apposed to the drivers side. Took it apart the dowel pin on the leaf spring wasn’t in there . Put a new one in and 99% of the vibration is gone . When th PO redid the truck they must have neglected to put it back in can’t for the life of me figure why it was taken out . Now the truck drifts to the left so I need to get it aligned.

[72c20customcamper]

After alignment truck started vibrating again . I took the front tires off my 72 and put them on the 85 . Vibration is completely gone . Had the takeoffs spinbalanced again and the left front had a radial hop in it . This is the second set (Michelin) tires on the truck so the assumption it’s not the tires . Sure enough the rim has a good amount of radial movement . We put that tire on the spare rim and balanced again . The truck is vibration free . Weird that a factory rim would have so much radial run out .