Scain33

Need some help! Have vibration/shimmy in rear. Feels like it's in the a$$ of your seat. It's a 2006 GMC Sierra 1500 ecsb. Lifted 6in has rough country anti axle wrap blocks in rear and 1 1/2" add-a-leaf. Riding on 36x15.50x20 Mickey mtz's. I've had the driveshaft checked, new transmission mount, new u joints, 2nd set of tires, all new body mounts, the entire front end has been replaced and inspected and aligned about 10 times now. Torsion bars have been adjusted just to see if it would change anything it didn't, there cranked almost 3/4 and was aligned here. Tire pressure is at 32psi right now but I've ran it from 55psi all way to 28psi. Rides best at 35psi.
If it's cold out I get in for the first time of the day it will ride smooth as silk for the first 5 miles then start vibrating.
Tires ride very smooth down the road even when vibrating, but you can see the bed shaking, and feel the cab shake. Starts at 70mph-85mph
Any input/brainstorming would be great this has been on going for 2 years and tired of changing parts that there's nothing wrong with.

truefire04

i would believe your problem might be axle wrap. do you have anything on your rear springs? even though you say you have the axle anti wrap blocks... go down to autozone or like auto parts store and pick up some leaf spring clamps, and install them 6" in front of your axle and see is that works. or you can try track bars on the rear.

Scain33

Yea that's what I was leaning towards, trac bars. Is it possible that sitting still the pinion angle is good but once i hit about 70 mph it's lifting the rear axle and throwing the angle off? And it doesn't always do it, it's intermittent. Usually the first drive of the day it will ride smooth for about 5 miles then will start kicking in, and when it's a really cold morning it don't seem to do it much.

TXsilverado

iTrader: (19)

I hate to direct you to another forum, and I'm sure that someone here has the answer...but there is a lot of experience with big lifted trucks over at GMfullsize. most trucks here are a little (a lot) lower to the earth than your setup. we're more of your lowered powerful race scene where they have more lifted slow show/offroad style trucks.


if you ever have a performance question, do not click on their performance link LOL.

MileHighSierra

Is this entirely speed dependant? Meaning, if you stay under 60mph, does it never happen?

Underbody aerodynamics are significant above 55mph, where lift is equal to the square of the Cd times your frontal area. IE, at 110mph, you don't have double the drag, you have 4 times the drag.

So at 70mph, you could very likely be lifting the cab to an angle where the pinion is causing it to bind.

madmann26

iTrader: (7)

I would have your tires road force balanced using a Hunter GSP 9700 balance machine.