Do I need rear axle shims?

[Yondu]

A while back I installed a 2" rear block, for a 3/4" rear lift.  I always meant to measure my driveline angles, but never did.  

 

I acquired a digital inclometer and set out to do this.  I put the inclometer on the flat spot/flange of the transfer case output and zerod out the unit.  I then put it on the same flat spot of the rear axle pinion, using a socket on the U joint to insure I was measuring a flat machined location and I get 3*.  

 

Ideally, it should have been 0, correct? Then the two pieces would be parallel to each other.  Since it will move some while driving under load, I've read I want a 1 degree difference.  To achieve this I would need 2 degree shims.  

 

Any input?

[flyingfool]

you can always call GM dealer and ask to talk to the service writer , he has the shop manuals and specs on his PC.  otherwise I would stop in at a u-haul , and ask to crawl under and measure thier silverado.

 

[Yondu]

The service manual calls for it to be parallel.  Or the tsb on vibration analysis does at least.  I have a feeling it was a little off even before I lifted the front 1.3" and the rear .75.  

 

I am thinking either 1.5 or 2 degree shims. 

[dieselfan1]

Is it vibrating?

[Donstar]

10 hours ago, Yondu said:

A while back I installed a 2" rear block, for a 3/4" rear lift.  I always meant to measure my driveline angles, but never did.  

 

I acquired a digital inclometer and set out to do this.  I put the inclometer on the flat spot/flange of the transfer case output and zerod out the unit.  I then put it on the same flat spot of the rear axle pinion, using a socket on the U joint to insure I was measuring a flat machined location and I get 3*.  

 

Ideally, it should have been 0, correct? Then the two pieces would be parallel to each other.  Since it will move some while driving under load, I've read I want a 1 degree difference.  To achieve this I would need 2 degree shims.  

 

Any input?

 

1 hour ago, dieselfan1 said:

Is it vibrating?

Good questions!  We all appreciate that even a small level lift will alter factory specifications. However,  does this modification fit within a range of tolerances?  Until I read this,  my understanding was that such a small change resulted in insignificant consequences.  I was planning to add a little height with a 1.5" front level and an additional leaf in the rear.  My guess is that your 3/4" of extra height in the rear is well within the design tolerances of your truck and look forward to hearing opinions/experiences from other members!

[Yondu]

Some days I can't notice any vib, some days it has the tiniest amount of vib.  It mostly started after my new wheels which I have had road forced 2 times.  Vibs can be a compunding issue So I want to make sure everything else is right. 

 

My wife thinks it's very smooth on the highway and has commented how easy it is to fall asleep in. I'm pretty ocd though. 

Edited December 9, 2019 by Yondu

[sonerick]

Were the lift blocks you installed tapered, and if so is the taper facing the right direction?

[dieselfan1]

Wheels go on at the same time?

Even though it's been roadforced maybe you got a bad wheel???

[Yondu]

8 hours ago, sonerick said:

Were the lift blocks you installed tapered, and if so is the taper facing the right direction?

No they were flat.  Tapered would have worked better depending what the taper angle is.  

 

I ordered 2 degree shims.  

[Grumpy Bear]

http://www.4xshaft.com/driveline101.html

 

This particular link is lifted 4X4 specific. Growing up we set them for cars in the 3* to 6* range. When we did the 3.5" drop on Pepper we were at a natural 5* and O'l girl is smooth as a kitten. Fact is, it only moved 1* from stock doing that drop.

 

Personally I never go below 3*. I had a 65 Ford I drag raced that ate tail shaft housings like pop corn. Drive line angle was near zero. When the car would launch the angle would go positive/negative/positive when is squatted then relaxed. U-joints evidently don't like this. Increasing that angle over 3* stopped the transmission housing breakage. 5* is where that one ran.

 

For what that's worth.