I have been reading a little bit about this too. I have a Mech Eng degree, and while I spend all my time in manufacturing and plastics now, a good engineering problem always gets the mind spinning. I read an article this morning about controlling vibrations in mechanical systems.
ARTICLE
Its a pretty nice primer on mechanical system vibration. We've been talking a lot about the effects of the vibrations. Generally speaking, when these types of vibrations happen, you blame driveline angle. However, in some cases, it seems that the pinion angles are good and vibrations still happen. What to do in that case? I think the first thing to figure out is what is actually causing the disturbance. There is a formula to calculate the frequency of "power pulses" from a 4 stroke engine.
Fd = (
N x
S)/ 2 x 60 where N is number of cylinders and S is the RPM.
I have a 2004 Rubicon with a 5 speed, 241 T-case, stock gears (4.11), 285/75R17 tires and the 4.0L.
The Formula for me at 60 is (using Grimm Jeeper to calculate RPM at road speed)
(6x1788)/120 = 89.4 Hz. This is the disturbing frequency. That is pretty close to the 55 Hz
@psrivats was talking about with respect to the T-case dampner. If I were to do the same calculation on a factory tire...
(6x1985)/120 = 99.25 Hz which gets me further away from the 55 Hz...Unless that 55 Hz is designed to cancel on 2x Frequency. I gotta admit, I don't know much about harmonic balancers.
If I were to go to 4.88 gears with my current tire size...
(6X2123)/120 = 106.15 Hz...
What am I saying...I think there is something to this harmonic balancer thing. Deeper gearing will increase the RPM of the engine which in turn pushes the Disturbing Frequency higher. In @tworleys case...He had vibs, but going to 35s fixed it. Well, going to 35's lowered the Engine RPM with a corresponding decrease to the disturbing frequency.
I dunno though, random musings on a Saturday morning. Kinda feel like I'm creating a theory based on incomplete knowledge of the system at hand. In other words, Grasping at straws. I do know I want to regear my rig to 4.88 someday...and I don't want to fight vibrations, so I have a bit of vested interest.