Driveline buzz at 65+ mph

I added a new leaf pack that boosted the rear lift to about 4” unloaded. Upon initial test drive no vibes, I was expecting to have them. I did notice a high speed buzz past 65+ mph on hwy. Its not very prone, sounds like a drone through the cab.

No symptoms before the new pack, previously had about 2.75” rear lift from Dakars.

I do have axle shims that correct the angle at the diff, now I’m thinking it has to be from the carrier bearing.

Anyone have experienced similar?

 

what degree are the shims you installed? skinny end pointing forward right?

should be about a degree for every inch of lift. I wouldn’t drop your CSB more than a quarter inch or so, the best working angle for that U joint is with no drop.

 

I don't know for sure what's the degree. They were a carry over from factory installed shims. And no, mine are installed with the wedge side forward. That raises the front of the diff upward to decrease the angle between the driveshaft and input end of the diff.

I will try to shim the carrier bearing a few hundreds of an inch to see if it improves.

 

Wedge side facing forward drops the diff effective angle downward, pointing it towards the ground. Typically this is the opposite of what you want.

 

I had the opposite result when I installed mine. I had to jack it up by the U joint to align the leaf spring centering pins into the axle retainer hole.

 

It’s physics, it’s not really gonna differ from person to person. Don’t just blindly adjust your driveline angles without knowing what you’re actually trying to achieve.

Sorry if it seems like i’m being a dick, but you installed your shims backwards and now you want to drop your CSB. Good luck.

 

How are the shims installed backwards? Having the skinny edge forward drops the diff lower in the front, causing a bigger angle between the diff and the driveshaft.

 

No it does the opposite. I’ll draw a picture for you when I get to work.

 

just think about it, you have two flat surfaces. you then decide to put a tapered shim between them. The fatter end is going to push the axle surface down further. Been stuck in meetings all day

found this thread
https://www.tacomaworld.com/threads/correct-axle-shim-direction.588100/

 

Yeah down, exactly. I don't see why I would want to push the front of the diff down any more, that creates an even greater angle between the diff and driveshaft. My raising the front of the diff upward that angle is less. The shim that I have installed is roughly 1/8" thick at the larger end and it tappers down to an almost of a knife edge.

 

Here is how i'm thinking about it.

 

So your drawing is a representation of a spring under axle (SUA). If you flip the two, where the leaf spring is on top of the axle and has a fixed flat surface, it looks like this:

 

Yeah I'm an idiot. Thanks for pointing that out. I flipped the shim to skinny side forward. I noticed there was also a slight bind when I unbolted the u bolts. Everything has been reset, will go for a test drive tomorrow to see if the buzz is gone.