So, I've been chasing a vibration in my truck since late June.

After several tries with rebalancing the wheels I discovered that my left rear axle flange had excessive lateral runout. I replaced it and it helped a bit, but I still had some vibrations left.

After watching a video online of a repair shop trying to track down a mysterious vibration on a truck using vibration meters, I thought I would have to use that. So when talking with the owner of the transmission shop working on my 82 Prelude I asked him about this and his response with quite a bit of confidence was "U-joint or driveshaft."

I trust this shop but the nerd in me couldn't and wouldn't be satisfied until I could do some more diagnosis on my own. So I got hold of an accelerometer and taped it to the on the steering wheel on top of the center part that holds the air bag where I could feel vibrations. I knew I wouldn't be able to check the relative amplitude of the vibrations as a function of location of the meter, but I figured I might be able to find the frequency and match it to the expected frequency of the driveshaft or tires.

Below are the results. Sure enough, the fundamental harmonic is at the frequency I calculated it would be if it was the driveshaft that was the culprit. For comparison, due to reduction in the differential, the wheel frequency would be much smaller, for me by a factor of 3.73.

Looking online, that there are harmonics of the fundamental frequency can mean there is some misalignment between the rotating components in addition to just being unbalanced. I'll be having the shop replace the output bushing and seal on the transfer case sometime soon so hopefully the shop will find out what the origin of the vibration is, e.g., the driveshaft, the bushing, or the u-joint. I'll keep post an update then.

The raw data from the accelerometer showing the vibrations.



The frequency of the vibrations. The frequencies are in terms of the expected frequency of vibration of the driveshaft. The main peaks line up with these indicating that the vibrations are not with the wheels.

 

interesting,,hard to believe a drive shaft could look like this...lol...

 

This is fascinating Christine, nice work. I can confirm that this is good for nerds and engineers, which are often one and the same.

George

 

And don't forget how it can be fun to analyze data!

 

Right?? LOL

 

Have you checked all your universal joints? Did you replace your rear axle bearings on both sides?

 

Oops its not a blazer, I would recommend checking cv axles, and all 4 wheel bearings to start. Do you get a clicking noise in the front when you turn the wheel all the way either way and exellerate hard? That indicates bad cv axles.

 

Is it a second gen blazer?

 

So, I checked all the wheel bearng and axle flanges before I did the vibration measurements. What I found was three were fine but the left rear axle flange had excessive lateral runnout of 7 mils where the max allowed is 2 mils. All the others where 1.5 mils or less.

I regret not doing a frequency analysis earlier.

 

Very interesting work there Christine! I'm really curious to see your results. Thanks for sharing.