After installing a T-56 tranny, shortened aluminum driveshaft and 4.56 gears (all done at the same time), my vibration went from annoying to being quite bad - as in the rear view mirror being about to vibrate off the windshield above about 60 mph.
My first thought was that the aluminum driveshaft was not balanced properly. So, I had it check for run-out and balanced. I was told that it was very well balanced. I then went through the field balancing procedure again. My vibration was still far worse than it had ever been.
Indexing the driveshaft would take it from horrible to better. The vibration was very dependent on indexing. So, it seemed that maybe I just was not able to add enough weight or maybe the guys who balance my aluminum D/S missed something. So, I tossed in the OEM-balanced steel IRS-specific D/S and was able to index it to a position that further reduced the vibration.
The problem now was that I picked up a harmonic vibration at around 40 mph, even though my vibration above 70 mph was dramatically reduced. With the aluminum, I did not notice any vibration below about 60-70 mph. Little did I know how much vibration I had below 60 with both driveshafts.
While trying to sort out this vibration problem, I came to the concusion that 4.56's were too much gear for me. So, I swapped in 4.30's. I still had the essentially same vibration with the 4.30's (same peak magnitude, just at a slightly different road speed).
Based on a hunch from a friend that the problem was pinion angle/driveshaft angle related, I thought I'd check out the driveline component angles. Sure enough the tranny-driveshaft angle and the driveshaft-pinion angles were way off. The tranny-driveshaft angle was about -3 degrees and the driveshaft-pinion angle was about 1 degree (both relative to the longitudinal axis of the driveshaft).
FIG. 1 shows the tools used to determine the various angles referred to in this document.
FIG. 2 below depicts POSITIVE tranny-driveshaft and POSITIVE driveshaft-pinion angles. Make note of these POSITIVE angle orientations.
1. Both the tranny-driveshaft angle and the driveshaft-pinion angle between about 0 deg to about +3 deg; and
2. The tranny-driveshaft angle and the driveshaft-pinion angle be the same.
This arrangement puts the driveshaft in a stable mode and limits instability associated with negative angles as the nose of the pinion rotates upward when delivering power to the ground. Having one angle positive and the other negative is known to cause driveline vibrations. As the pumpkin is mounted in the IRS subframe in a relatively fixed position, the tranny-driveshaft angle and the driveshaft-pinion angle should not change to any great degree asthe pumpkin rotates when putting power down (unlike with a Mustang's OE 4-link suspension).
The relationship of the transmission output shaft angle and the driveshaft angle are depicted in FIG. 3. To determine the tranny-driveshaft angle, the driveshaft angle and the transmission output shaft angle must be determined (relative to horizonatal). The sum of these to angles is the tranny-driveshaft angle. The transmission output shaft angle can be measured off the front of the waterpump pulley or crank pulley. The driveshaft angle is taken off the face of the driveshaft. Be sure to measure the angles relative to horizonatal and to note whether each angle is positive or negative relative to the horizonatal axis. A positve angle and a negative angle are additive under subtraction (e.g., 2-(-1)=3).
The longitudinal alignment of the pinion to the driveshaft and the driveshaft to the output shaft were also checked and found to be off by a substantial amount.
FIG. 5 depicts (as viewed from looking up at the driveshaft) a measurement location for determining the tranny-driveshaft longitudinal alignment. Using a tape measure, I measured the tranny-driveshaft longitudinal alignment dimension. This dimension is from the rear face of the tranny tail to the front edge of the bore that holds the u-joint cap. Space was too limited to accurately use a more precise tool such as a digital micrometer.
Using this measurement technique, it is important to meaure with the driveshaft in a first measurement orientation (i.e., the driveshaft-side center axis ofthe u-joint pointing down) and then rotate the driveshaft 180 degrees and repeat the measurement technique. This is done to get a feel for any difference in manufactured dimensions. In my case, upon initial measurement, I could not get insert the steel rule on the passenger side in either measurement orientation of the driveshaft while it fit with slop on the drive side. This indicated that the longitudinal axes were misalighned, rather a nominal difference in the cast and/or machined surface.
With the steel driveshaft back in the car, I was able to index the driveshaft to a point where my vibration was less than it has ever been (without any added weight). Just for grins and giggles, I thought I'd toss my balanced aluminum DS back in, even though I had worse peak vibration with it previously.
I installed the aluminum driveshaft and, without any field balancing, the car runs "Perfectly" smooth. I mean 'Lincoln Towncar smooth'. It now runs smooth at all speeds...well up to about 140 so far. One nice result is how smooth the car runs at speeds below 40 mph. I guess I had vibrations at lower road speed, but was attributing them to road noise from suspension mods.
Please let me know how this procedure works for you.
