Northeastconfederate

I'll start with what I know:

• Universal joints need enough of an angle to operate smoothly
• Lowering (or lifting, for that matter) can adveraely affect pinion/driveshaft angle either by reducing it (lowering) or increasing (lifting) out of its operating range of tolerable angles
• Wedges are available to correct pinion angle as needed when lowering or lifting

Now for what I have:
2011 RCSB 2WD with 4.8 and 4L60E, rear lowered 3" using 2" shackles and removal of factory 1" block.

It has a shake beginning at 60 when accelerating and as low as 55 when decelerating which seems to be worsening. I bought a pair of 2.5° wedges to hopefully increase pinion angle enough to eliminate this condition.

My question is: What exactly causes the shake? It doesn't feel like a bad U-joint bearing. And this isn't fast enough for a 'critical speed' deformation of the driveshaft. So what is it that is happening?

I was a Benz tech for many years. Benz uses rubber 'flex discs' at the trans and pinion with a U-joint in the middle and a carrier bearing (slip joint is also here; trans tailshaft flange is fixed) and shims between diff housing and frame to control pinion angle. I am familiar with the need for and process of checking and adjusting driveshaft angles to eliminate shaking/vibration, I just never understood the physics involved in what's causing it. Can anyone help me understand this?

Also, what are the consequences of it besides discomfort? Will this destroy the rear U-joint? Both U-joints? What other components are at risk?

Given the 3" drop, is 2.5° likely to be enough to cure the shake?

ss4chad

iTrader: (19)

This happened last year:

"So, vibes went bezerk while "following" a Pontiac G8 at about 125.

Took out the transmission, tail shaft completely broke off, broke off a bolt hole tab on the rear end pinion yoke, beat the **** out of the metal rock guard on the side of the gas tank. warped the drive shaft and broke off a bolt hole tab as well, gouged the gas tank but luckily it didn't puncture it."

Yeah, more than discomfort!!

Chad

Northeastconfederate

Holy ****! Was this due to inadequate driveshaft angles? Or driveshaft critical speed?

FFDP

iTrader: (2)

I'd measure the pinion angle to be sure. This way you know if the wedge is too much or too little.

Then if the pinion angle is not the issue. Start looking at the u-joints and or a out of round driveshaft.

Northeastconfederate

The issue is definitely pinion angle, no doubt. It was smooth before I lowered it; the shake was induced with the 3" drop. The only question is whether or not 2.5° will be enough to correct it.

Measuring the driveshaft angles is definitely a good idea but honestly, I'm going to start by tossing these wedges in and see what happens. If I still have an issue after that I'll get to measuring. But right now the way I'm looking at it is: I can only improve the situation with the wedges I have. It ain't gonna get worse.

Northeastconfederate

I have just installed the pinion wedges and road tested the truck: the shake/vibration is much improved but not completely gone. I will measure driveshaft angles and see what's what.

I also have very large, heavy tires in back: 315/70/17. I'm wondering if rebalancing the tires might help me too. I'll give that a shot as well.

One thing I was not real thrilled about with these pinion angle wedges: the leaf spring center bolt no longer positively locates the axle tube because the head of the bolt doesn't protrude through the wedge far enough to lock in on the spring pad. I actually had to disassemble the passenger side a second time and use a ratchet strap wrapped around the axle tube and hooked to the rear spring hangar to pull the axle tube back into its proper location. I noticed during the road test the steering wheel was off center and realized I hadn't gotten the axle back where it belonged on the right side causing misalignment front to rear. I can imagine if I ever put any real power to the ground this could be an issue.

I may make little alignment dowels on my lathe to resolve this. What does anyone else do about this? Am I concerned about nothing?

FFDP

iTrader: (2)

I bought the wedges with new bolts that are suppose to work with the increased gap, this way is locates on the rear end better. Their DJM's from summit racing.

Northeastconfederate

Wish I found those when I was looking.

I'm going to have to go through it carefully. I'm starting to think it probably has a bad U-joint after driving it quite a bit today.

Northeastconfederate

Pulled the driveshaft today and checked U-joints. They are both good. I pulled the 2 caps on the rear that are held in by the bolted straps, and the grease still looked red and all the rollers are there. Felt all the other bearings for looseness or grinding, etc, and all are fine. I was pretty well convinced that was going to be my problem but definitely not.

Measured driveline angles also. Here's what I got:

Pinion: 2° "down", meaning tilted down toward the front.
D/S: 7° "up" (front higher than rear)
Trans: 2° "up" (front higher than rear)

According to my book on Chevy Differentials (an SA Design book) which has a section on driveshafts, I think I put my wedges in backwards. It says the pinion should be 1 to 2° "up" and should be the same as the trans. If I reverse the wedges, that's about what I will have.

That's going to be my next move unless someone here can tell me I have this all wrong.

madmann26

iTrader: (7)

Here is a good video that shows what happens when the angle changes.


I've got a smidge of noise in mine around 70 but that's because it has a little negative angle.