Vibrations at 80+

dsrtjeeper · Wednesday at 1:24 PM

The dog bone uses mass and absorption to cancel out harmonics. Manufacturers have used strategically placed, heavy boxes (mass) for decades. They often hang from the chassis and are easily identifiable. Then we have rubber suspension bushings, exhaust hangers, motor and tranny mounts, etc... to cancel out and absorb noise, vibration and harmonics. (NVH).
The dog bone is just one of many components aimed at delivering an enjoyable driving experience. Nothing more, nothing less.
All that being said, a properly setup drive line should not vibrate at speed after a rear gear change. Going up numerically in gear ratio often amplifies an existing drive line vibration that previously went unnoticed. Higher numerically gear ratios result in increased drive line rotations at a given speed. At higher speeds, minute vibrations are now enhanced/increased.
We then modify our suspensions with stiffer poly bushings and solid bushings which transfer more NVH from the rear diff.

OldZeuski · Wednesday at 1:34 PM

Forgive me, I've seen it said many times elsewhere that the damper mass is tuned for the gear ratio. I suppose it makes sense that the ratio doesn't effect the actual resonance of the driveshaft or the axle housing, so a single damper could be used for every mustang.

I had the same vibration with and without the mass damper though. If the damper was already shot maybe it wasn't doing its job to begin with, but I've measured my vibrations and they aren't resonance, they're 1:1 with the rpm of the driveshaft.

dsrtjeeper · Wednesday at 1:36 PM

OldZeuski said:
Forgive me, I've seen it said many times elsewhere that the damper mass is tuned for the gear ratio. I suppose it makes sense that the ratio doesn't effect the actual resonance of the driveshaft or the axle housing, so a single damper could be used for every mustang.

I had the same vibration with and without the mass damper though. If the damper was already shot maybe it wasn't doing its job to begin with, but I've measured my vibrations and they aren't resonance, they're 1:1 with the rpm of the driveshaft.

I'm right there with you.

dsrtjeeper · Wednesday at 2:00 PM

I'm trying something new. I've already had the rear on jack stands and ran it up to 90mph and the vibes are present. Next I'm going to remove the wheels/tires and use open lug notes to hold the brake rotors on. Then I'll run it up to speed again. This will rule out the tires and wheels which have been balanced multiple times. I'm actually hoping one of the wheels is bad.

cobrajeff96 · Wednesday at 2:12 PM

The red/yellow dots on new tires.....they serve balancing purposes but most tire shops don't know this because they're full of summer hires and stoners. You can certainly have a shop balance wheels only (with no tires installed) and that's a good starting point. OEM wheels are usually pretty good, most aftermarket not so much (especially anything from American Muscle).