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2003 chevy trailblazer_ltz
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Just solved my vibration problem...again.

2003 Trailblazer 4.2, 4WD. NOT extended version. Currently 247K miles.

Several years ago, I had a vibration and noise from 68--78 mph. No* shake at 65 or at 80 mph. Worst about 74 mph. Not dependent on power or trans gear--would do it on throttle, or coasting; in OD or D. 2WD or 4WD. Made no difference except for vehicle speed.

This was eventually traced to a faulty rear U-Joint on the rear drive shaft. I had the shaft serviced by the local specialist--they slapped in both U-joints then put the shaft on their lathe. They told me the shaft was slightly bent, but they straightened it--got "almost" all the bend out of it--and balanced it. Reinstalling the shaft, the vibration and noise was G-O-N-E. In fact, the vehicle was so smooth that It was actually better from 25 mph and faster--*I did not realize there was vibration at that low speed until it wasn't there any more!

So several years go by, and my vibration comes back. Same story--68--78 mph, floor shakes, seat shakes, steering wheel is smooth. Vibration is accompanied with a "booming" noise inside the vehicle. Just like before, it's worst about 74 mph. Everything was exactly like the first time.

Except the rear U-joint is fine. So is the front U-joint of the rear driveshaft.

Balancing the wheels doesn't change the vibration. Changing wheels entirely doesn't change the vibration. Rear axle does not seem to have wheel-bearing or differential problems.

I grabbed a rear driveshaft from a 2008 Trailblazer being parted-out. The rear U-joint of that shaft was predictably wiped-out, so I squeezed a new one in myself. Front U-joint is fine. Installed the replacement driveshaft.

Smooth as glass at all speeds. AGAIN, I didn't know I had vibration from 25 mph-faster until it was gone. And the horrible vibration and noise at 74 mph is gone, too.

APPARENTLY, the original driveshaft either went out-of-balance; or the bend came back.

APPARENTLY, the 4WD short-wheelbase GM360 is sensitive to rear driveshaft problems. The extended-wheelbase 4WD rear driveshaft is longer, of course--but it's also MUCH larger diameter than the short-wheelbase shaft. IF (big IF) this vibration ever comes back, I'm having an extended-wheelbase, large-diameter rear driveshaft cut down to the shorter length.

SWMBO is very happy that her ass and eardrums aren't getting massaged at highway speed any more.
 

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Interesting....... I know that "moment-arms" can cause primary and secondary harmonics.....never expected tertiary harmonics though. .

Fundamental harmonic resonances can be tricky....they are typically sinusoidal and can .... where the period and amplitude concur ... create damaging harmonics that can destroy metals and other conducive materials.

55145


I made a study of waveforms during my musical theory classes and although it might be kinda hard to correlate musical sound with a vibrating driveshaft.... physically, they are both on the same wave length [I don't believe I said that!].

So... yeah.... although I don't honestly believe your driveshaft spontaneously re-bent itself.... I CAN believe it could toss a balancing weight that wasn't welded on correctly. I've seen that a few times.

Another causative factor could be "high centering" your vehicle on some object..... a boulder, a fence post, an unfortunate pedestrian, and I've even caused it myself with an old Dodge Van (or two --- or three) on a center post hoist in the first shop I owned.

ALL ACTIONS AND REACTIONS ARE CAUSED...... you can copy/paste that and sig it or print it out if you like.... no royalties needed nor requested.

BTW.... a larger diameter driveshaft tube would exacerbate an imbalanced condition ... because any out of balanced component that existed on it would have a larger, mechanical, moment-arm advantage ... and would actually be much more susceptible to amplification of any vibration than a smaller diameter/shorter moment arm tube would have.

There's something to be said about a larger cross sectional diameter verses a smaller one.... but since the smaller tubes are normally steel and the larger one of aluminum.... there's the modulus-factor that then comes into the computation too.

That, however, having been said: a larger diameter aluminum driveshaft is a "duller/harmonic" metal, and therefore less likely to "ring" in sympathetic harmony unlike a steel, smaller cross section tube, with it's "bright/harmonic" wave generator .... waveform-wise.

FUN FACT: steel driveshaft tubes are lined with a dense cardboard to attempt to dampen any audible (human range) pinging or ringing sounds from the driveshaft... either self-generated or in sympathetic harmony with any other resonant frequency generator.
 
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