60 mph shake, 65+ mph vibration

[bumpside1968]

hey bump fans,

I've got a bad shake at mid-to-high speeds. pronounced shake at 60-65 mph, and higher frequency vibration at 65+ in my 68' long box

wheels and tires have been balanced
not engine related, vibration is the same when clutch disengaged, throttle released
fresh wheel bearings in front
can feel vibration in pedal and steering wheel (more pronounced in steering wheel)
poly bushings on radius arms
worn rubber bushings in steering assembly
needs shocks, possibly replacement coil springs

I'm wondering if there is a common prognosis for these types of pickups or if I should start throwing money at it and see what works...

[basketcase0302]

"bumpside1968"]hey bump fans,
I've got a bad shake at mid-to-high speeds. pronounced shake at 60-65 mph, and higher frequency vibration at 65+ in my 68' long box
wheels and tires have been balanced
not engine related, vibration is the same when clutch disengaged, throttle released
fresh wheel bearings in front
can feel vibration in pedal and steering wheel (more pronounced in steering wheel)
poly bushings on radius arms
worn rubber bushings in steering assembly
needs shocks, possibly replacement coil springs
I'm wondering if there is a common prognosis for these types of pickups or if I should start throwing money at it and see what works...

More commonly referred to on our twin I-beam trucks by chebby guys as "the death wobble"!
My money is on the king pin bushings being worn out as you've given the typical description of it, (look at part/kit #18124 in this image). And it's a very common thing on our trucks as guys were just too lazy to keep the king pins properly greased as it means laying in the dirt with a grease gun.
Jack the truck off the ground one side at a time, (with the jack centered under the king pin bolt) one side at a time leaving the wheels on, (this is best done with two people). Put one hand on the top of the wheel and one hand on the bottom of the wheel and pull back and forth, (from top to bottom-not back to front of truck).
Is there any movement? If so, it's time for new king pins to be installed in the I-beams.
Here's the schematic on what your looking at. And yes it is a very hard job to properly replace the bearings as most the time the beams have to be reamed out to accept the new bushings / bearings.

[MadMike]

Death wobble is a bit more violent and makes the vehicle harder to control, as well as can be induced at lower constant speeds, an annoying vibration is more likely the wheel bearings. ~60MPH I would think is more bearing related, especially if the problem was not there before the wheel bearings were replaced. Death wobble can sometimes be induced by hitting the catseyes in the road, nail a few at a lower speed, if the wobble starts up and continues on it's own then that is more likely death wobble. But these trucks do have some pretty bad roll/bump steer, sloppy components can make them pretty darty at higher speeds.

Being that the wheel bearings have been replaced, I would think they may not be snugged up enough. What I have found over the years is the typical tapered wheel bearing adjustment is just too loose or purely by 'feel'. 'Tighten nut to ~12lbft while turning wheel, back off half turn, tighten by hand til snug.' Or something inane. My hand tight is not the same as the next guys. New bearings tend to need to be readjusted after being driven on as they break in a bit. They may feel fine initially but after a few miles they'll need a final adjustment. Older bearings that just get re-greased don't seem to have as bad a shake/vibration and are more forgiving as they will already be set.

Similar suggestion as for checking the king pins. Jack up each side, remove hub/center cap/dust cap(leave wheel on). Remove cotter pin, check if there is any movement in at 12 and 6 positions. Snug up the castle nut until you no longer feel play. No need to crank the begesus out of them, you just want to get rid of the play. Slide in cotter, pop on caps, go for a drive.

[basketcase0302]

"MadMike"]Death wobble is a bit more violent and makes the vehicle harder to control, as well as can be induced at lower constant speeds, an annoying vibration is more likely the wheel bearings. ~60MPH I would think is more bearing related, especially if the problem was not there before the wheel bearings were replaced. Death wobble can sometimes be induced by hitting the catseyes in the road, nail a few at a lower speed, if the wobble starts up and continues on it's own then that is more likely death wobble. But these trucks do have some pretty bad roll/bump steer, sloppy components can make them pretty darty at higher speeds.

Being that the wheel bearings have been replaced, I would think they may not be snugged up enough. What I have found over the years is the typical tapered wheel bearing adjustment is just too loose or purely by 'feel'. 'Tighten nut to ~12lbft while turning wheel, back off half turn, tighten by hand til snug.' Or something inane. My hand tight is not the same as the next guys. New bearings tend to need to be readjusted after being driven on as they break in a bit. They may feel fine initially but after a few miles they'll need a final adjustment. Older bearings that just get re-greased don't seem to have as bad a shake/vibration and are more forgiving as they will already be set.

Similar suggestion as for checking the king pins. Jack up each side, remove hub/center cap/dust cap(leave wheel on). Remove cotter pin, check if there is any movement in at 12 and 6 positions. Snug up the castle nut until you no longer feel play. No need to crank the begesus out of them, you just want to get rid of the play. Slide in cotter, pop on caps, go for a drive.

Yeah I missed the wheel bearings replacement above but then we could go on about the issues with possible poly bushing replacement also.
Most common thing you'll find wrong will be the king pin bushing are worn out due to some lazy dude who did not want to crawl in the dirt to grease them properly.

[Ohiotinkerer]

Had one come into the garage I worked at in the early 90's that the king pins were so seized up that I had to yank the steering wheel left or right and wait for the pop and turn it some more..... took me almost half an hour to get it onto the lift.............never done a king pin replacement and after that swore I would NEVER do another if it hadn't been greased regularly..........and fortunately mine have been over the years.....

Grandpa always said a little lube goes a long way...........