Vibration over 50 mph

[Ted K.]

Took 64 convertible for a rare spin, usually around town. This time went on a highway and at 55mph started feeling a vibration from the rear of the vehicle. It definitely felt like rear wheel balance. I had the rear wheels balanced (bolt on knock offs) one was off about 4 ozs. I was sure that was the problem but no improvement. Thinking the wheel balance was done incorrectly I rotated the wheels to the front and then test drove the car. No change. The vibration is definitely not coming from the front of the car.
It appears that work has recently been done to the rear drive line ( half shafts, rear spring, possible rear wheel bearings, etc.)
I've read about the drive shaft being installed 180 degrees off and causing a vibration. How does one tell if the drive shaft is installed incorrectly. The vibration only starts above 50-55mph. Would not an improperly installed drive shaft vibrate all the time.
Any other suggestions?

[Mark F.]

Re: Vibration over 50 mph

...Would not an improperly installed drive shaft vibrate all the time.
Any other suggestions?

Ted,

Yes, it will (and does) VIBRATE all the time; but even if the driveshaft is out of balance, it probably won't vibrate to the point it makes the noticeable NOISE you are hearing all the time at 50-55mph.

Think of a church bell or any other bell that "rings" when you strike it. OR, think of the old Memorex commercial with Ella Fitzgerald when she sings a particular HIGH note and it breaks a wine glass. That ring on a bell and the vibration that breaks the wine glass are called the bell's and wine glass' resonant frequencies. Meaning they will both vibrate at a "natural frequency" determined by the physical parameters of each vibrating object.

Your drive shaft also has a resonant frequency...we don't know what it is, but the energy put into it at 50-55mph must be matching the energy input it needs to start vibrating ("ringing") and causing noise (probably a "rumbling" sound).

Do you hear any of the same vibration noise in lower gears? You'd have to do the math to see what the rpm of the driveshaft is for each gear ratio to match it to the 1:1 drive ratio when you're in high gear.

All that being said, I guess the easiest way to prove it to yourself if the driveshaft is the culprit would be to rotate it 180 degrees and see if it goes away...

good luck...

[Frank D.]

Re: Vibration over 50 mph

It is important to note where you feel the vibration, in the shifter handle ? under your seat ? etc.. Here are two articles with some troubleshooting tips.

Note the distinction in the one article between speed-related vibrations and RPM-related vibrations - a key troubleshooting factor.

I will also add that the last vibration I resolved on my 63 before its sale was a sticking parking brake cable. It held the right rear shoes partially engaged, all went well until things heated up and expanded the the brakes started dragging and caused a bad vibration at anything above 35 mph.

[Edward J.]