Shake at 50 MPH on up

[jim23]

Hi everyone let me tell you what's happening.  At about 50 MPH the truck starts like a side to side shaking you feel in the seat not the steering wheel and get's worse the faster you go so I have not been able to see if it levels out at higher speeds.

Bought a 76 K20 longbed, had a 6 inch lift with 35's on it.  The first thing I did was balance the tires and nothing changed next I removed the 6" lift and replaced it with a new 3" lift and replaced tires and rims to 33's with all new shocks.  did not make any difference so I replaced the u-joints (oh by the way it does not have the front drive shaft in it) and that didn't help.

I also tried to move the driveline back and forth by hand to see f there is any play but seems to be tight at the tranny and rear diff.

I have run out of ideas and was hoping one of you all might have an insight to this because it's really driving me crazy.  Thanks for any help

[FlatBlack77]

I had this exact same problem on both of my trucks. it was the rear tires on both trucks. both had sit for a handful of years and the tires were old to begin with.

having them balanced does not fix flat spots.

but this might not be your problem exactly. so you can swap the tires from the back to the front and see if it puts the shimmmy in the front. or if u have a spare pair of tires put them on the back

[jim23]

Thanks Flatback I had tried that also even tho they are brand new tires and wheels but thank you for the reply

[78BIG-TEN]

It could be the way they balanced the tires.They can set up the machine to balance like static or dynamic.They did that on my 78 and it vibrated really bad at 45 and up to 65.Had them redo them dynamic balance and problem went away,but that means you have weights on both sides of the wheel

[BiggG74]

Check your motor and or tranny mounts. Sometimes they wear out or break and cause that.

[Irish_Alley]

i know when i got new tires on my impala i had to take it back to pep boys to redo the balance and it was better but not good. took it to another shop where my uncle works each tire was off by at least .75 with pep boys weights.

[LTZ C20]

This is why I don't trust pet boys or midass or something like that. Only reputable independent shops that have an upstanding with the community.  Plus the dealer (my work) and my self. I've seen 2 many pet boys type places really mess a car up good.

[Irish_Alley]

yeah first weight fell off less than a 1/4 mile from the store lol. i took an online review of the place when i got my tires told them all that happened and never heard anything back. the second appointment was meant for 1. all they had to do was re-balance the tires think it was around 3 when i got my car back. and the impala has aluminum wheels so they were corroding a little so they leaked bad. i could of took it back to them and had them fix it for free. but i was tired of them and took it to the other shop and paid $40 for each leak. bad part is years ago i would of did it all myself at my dads work. but due to Parkinson he had to quit that so it was the first time i paid someone to put tires on my car/truck

[Mario]

I would try to determine if it's related to the drive train or the suspension by driving it under load or coasting in neutral.  I wouldn't make a habit out of coasting an automatic though.  I've experienced that "death wobble" before, and it was related to the steering gear box and the drag link joints.  In that situation, it was usually a pothole that set if off.  I installed a 4" lift with a raised steering arm.  My drag link is practically level.  I've also experienced drive train noise and vibration that increased with speed.  That was multiple problems, but the main culprit was the rear drive shaft angle with the pinion.  Especially on a lifted truck.  I remedied the situation with a double cardon joint at the yoke, re-arched springs to get 2" of lift, and used 2" blocks angled to provide approximate 1 degree pinion angle to account for torque and weight.  Honestly, it sounds suspension related, but I thought I'd share my woes and remedies.  Good luck Man!

[Stewart G Griffin]

Just because the tires are balanced and there are no flat spots doesn't mean you might still have vibration problems----the tires could be distorted.  This has recently happened to me.

It might be a little more difficult since you have 4X4 but maybe try to jack the truck up so that the wheels can spin freely, BUT DEFINITELY HAVE SOMEONE IN THE DRIVER'S SEAT WHEN YOU DO THIS.   And examine if the tires or rim or both have any wobble.   i guess with 4X4 the engine has to be running and driveline engaged wheareas with 2WD you can put the wheels on the front axle and spin them by hand.  This is why i was stressing to have someone in the driver's seat at all times while you inspect the wheel runout.

Can i ask what brand tires these are?

[Edemich]

Did this ever get resolved?  Was your driveline balanced?

The driveline can throw a weight and this can cause a vibration as well.

[Scott M]

I had this happen in my truck recently once I got up to about 45-55 mph the whole truck would start shaking the transfer case shifter and linkage would be rumbling back and forth so much you couldnt hear inside the cab.  first thing i checked was driveshaft for ujoints and upon further inspection found out that the spline for the slip yoke on the rear driveshaft was gone.  I had about a 1/4" or more play both directions in the drive shaft.  I ordered a new driveshaft and it fixed the problem