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2003 chevy trailblazer_ltz
134 Posts
Discussion Starter #1
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.

556 Posts
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.


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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