Rear axle shim install?

I've been fighting an annoying vibration and the next step is to change my pinion angle.
Is it as simple as loosening the u-bolts and putting the wedge between the springs and hanger?

It depends on the shim, if it has a hole for the centering bolt you will need to lift the housing up enough to slide them over that bolt head or nut

Yep.

What angles are you showing? full or part time vibration?

Seems to be part time, hard to tell, but it gets really bad above 65mph.
I’ve had the the tires and driveshaft balanced twice, with no difference.
Here’s where I started, but have since shimmed tranny up so it’s now 2 degrees down.
Again no change, so next step is to shim rear end down 3 degrees.

I've slept since this, but wouldn't you want to nose the diff up 2° so that each yoke is in a parallel plane?

You want the driveline angle and the pinion angle on parallel planes ON ACCELERATION, a lot of variables determine how much the pinion rotates upward on acceleration, can be upwards of 5 degrees

That’s my thinking at this point, it rotates up on accel, so I’ll move it down and see what happens.
Because the vibration does significantly reduce under decel.
Under load at 70mph vibration is bad, when I let off gas and coast the vibs go away.

& you want the angles to be minimal. Yes the pinion will rise & how much depends if it is a DD or driven hard. I had a part time vibration & it turned out the yoke was not far enough into the trans. A slightly longer shaft fixed it.

at what speed was the driveshaft balanced ?
steep gears and OD transmissions don't play well with driveshafts that are not high speed balanced.

Overdrive transmissions have no impact on the prop shaft speed.

In third or overdrive at any given vehicle speed the prop shaft turns at the same speed in either gear.
Engine speed is lower in overdrive than in third, that’s the benefit. Not shaft speed.

In a way they do... most production driveshafts are generally balanced for 3000-3500 rpm, when you add an overdrive transmission along with let's say a 4:10 rear gear, .... a .64 overdrive is now capable of moving the vehicle to a speed of 140-150 mph with engine rpm being the limiting factor, prop speed is now extremely high, one of the reasons most aftermarket overdrive suppliers balance the shaft to 10,000 rpm

Please read my post again - “at a given vehicle speed”.

No arguement that going faster makes the prop turn faster.

Agreed, I was making a case for the possible need to look at the driveshaft balance, to rule out possible harmonics


Mike

This is what works for me, every time light load at highway speeds. It would not be the best for a drag car. I am suprised that with the rear at zero and trans 2 down you have an issue, it should be fine, like almost perfect under light load. Is the dana offset left or right, and is engine/trans straight in the car? Just wondering if you may be getting an offset induced vibration.

agreed, no matter what speed, the tire/rear gear combo will be turning xxxxrpm.
however, more than once, i have found deep gears and a short tire has caused driveshaft vibrations because the shaft was only balanced at a somewhat low speed.
balancing the shaft at a high speed cured the vibration issue.
just putting this out there as something else to look at if the vibration continues after fixing the driveline angle[s].

Update for others who may experience this issue.
I tried shimming the rear end down and it made no difference.
I took the shims out and tried a different yoke.

Success!
I can’t believe it, just got back from a vibe free drive and it felt great.
I changed out the yoke with a new u-joint and it worked.
Luckily I had the original yoke from when I pulled the 518 from the junk yard. It was rusted and scarred up, but I dressed it up.
Worst case scenario is it may leak, but I’ll worry about that later.
Another member gave me a tip about the difference between the 2 yokes. I had never seen that mentioned elsewhere.
Apparently the 518 yoke has a slightly different pitch on the splines compared to the 727 yoke,