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# Half-shaft torsional damping

## Half-shaft torsional damping

(OP)
Hi,
FWD cars often seem to use a clamp-on or spline-on torsional damper. Some seem to be simple masses, others have an elastomer component too.

Can anyone point me to a calculation of how these dampers work, and what their capabilities are? Google just throws up countless references to parts manuals & 'auto 101' type references.

Thanks, Ian

### RE: Half-shaft torsional damping

(OP)
Thanks Greg,

I can understand where the damping comes from in the elastomer versions, but not the bolt-on masses.

Maybe they are trying to create two stiffer shafts connecting an inertia?

Regards, Ian

### RE: Half-shaft torsional damping

Adding inertia to suppress vibrations is the oldest trick in the book. They could be detuning some local resonance or they could be increasing the input impedance of the system so that some fixed excitation is insufficient to create a problem.

Adding inertia is an inelegant solution, but it is quick, robust and cheap.

Cheers

Greg Locock

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### RE: Half-shaft torsional damping

They are not typically torsional dampers (with exceptions). The usual problem is bending vibration caused by resonance with one of the engine firing frequencies. If the problematic frequency occurs near idle, a simple mass will lower the natural frequency. ωn = √(k/m). If the problem frequency is near the engine redline, stiffening the axle with a tubular bar is the most frequent solution. If the problem is in the middle of the power band, a tuned absorber is what is called for. These are metal rings molded into rubber mounts. The rubber acts as a spring, and the whole thing acts like adding a second-degree-of-freedom absorber. Look up 2 degree of freedom vibration solutions. Since the equations of motion are coupled, you can trade energy between the two. Essentially the absorber vibrates like crazy and the axle stays stationary.

The only real torsional dampers I'm aware of were in the late 80's early 90's upper end GM FWD cars had torsional dampers to lessen the clunk in the transmission while shifting into drive from reverse.

### RE: Half-shaft torsional damping

kozlok-

Your post is very interesting. I can imagine coupled modes being a problem for auto drivetrain components when it comes to minimizing every potential source of noise or vibration in modern production vehicles. But I also can see how difficult it would be to design an elastomeric mass dampener that is effective at mitigating a low-frequency (and energetic?) bending mode in a driveshaft without creating problems at other operating conditions. The huge combination of gear ratios and engine firing frequencies possible between the engine flywheel and drive wheel would seem to make the design task extremely difficult.

### RE: Half-shaft torsional damping

I had one of those on my old VW. It eventually came off the axle ...
Decouplers are much more useful.
Litens Automotive is the leader, but does not make much readily available for public study. http://www.litens.com/vibprod2.cfm

Cheers,

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