Thread: What is this yo-yo beside the rear diff?

You will find similar vibration dampers are likely to be found in all sorts of places in almost any modern car. Any component which has some degree of freedom of movement will have a natural resonant frequency. If that resonant frequency is anywhere in the range of vibrations that are excited by normal operation of the vehicle, it is likely to cause noise at that frequency.

There are several ways around this. One is to isolate the offending part from the rest of the vehicle (or at least the passengers) by rubber mounts. This is the traditional method, but has become less practical as noise,levels are reduced and previously unexceptional noise become noticeable, and further, increased use of rubber often has adverse effects on handling. A more fundamental approach is to change the resonant frequency so it is either outside the range that will be excited by normal operation, or is less noticeable, but this will require a complete redesign of the part. The third method, is to provide a damper that absorbs energy at the resonant frequency, as is being done here. This has the advantage of being a simple, cheap, add-on, easy to design, and effective.

John

If you lifted an old 2 Dr Rangie 2 inches , the RH front spring would begin to resonate every time you turned a RH high speed curve on the highway .
This was caused by the angles of the front driveshaft UJs not being exactly equal .
It made a godawful racket and scared the beejesus out of you.

The fix back then was to damp the RH spring by wrapping/pushing plastic tube around it . This worked but was a bugger of a job to do involving much cursing and WD40.
The later Rangies have a dongle on the front diff.

Regard sPhilip A

Would re balancing the prop shaft help? Can half shafts be balanced? Come to think of it, can the diff crown wheel assembly be balanced? (I just want the theoretical answer, not the smart "turn up the radio" one).

Originally Posted by Bilbo

Would re balancing the prop shaft help? Can half shafts be balanced? Come to think of it, can the diff crown wheel assembly be balanced? (I just want the theoretical answer, not the smart "turn up the radio" one).

All of these are in fact pretty well balanced, the prop shaft being the one that has most effort put into it, because it turns fastest, but out of balance is not the problem. The axle assembly behaves as if it were a bell, with it being "rung" by not only the vibration of the rotating parts, but by the effects of road surface induced vibrations.

John

not realy a balance problem but a harmonic resonance created in the vehicle or diff.often cars use a double transmission mount with 2 different compounds set up to absorb movement in different directions.

my first disco has the damper on the front axle and the later has the rubber CV joint on the driveshaft.

worked on a few nissan patrols with the nissan engineers and reps.
after measuring rear engine mounts and crossmember dampers we glued a microphone to the bottom of the rear diff and came up with a wieghted bolt for the pinion flange.