Severe Vibration// Problem and cure
 

Hello all, I post this to potentially help others who may run across this issue. I bought this 79 Rx7 2 years ago or so, and during the initial ride, I noticed a strong vibration. I was going to change out wheels and tires anyway, so I hoped that may solve the issue. No joy, same strong vibration. I then decided to convert it from Auto to Manual, and thought I would check components at that time.
I bought a nice RB flywheel and a good quality clutch and PP. While it was out, I checked the drive shaft, and decided to change the front joint.
Conversion finished, the car idles smooth as glass but still a very strong vibration. I took the car on the highway, and regardless of speed, the vibration existed. I was wondering if a wheel or tire was bad? Usually if a tire is out of balance, there will be a speed where it goes away. Usually tire balance issues can find a sweet spot that they will act normal in, and they usually don't start vibration until 35 mph or more.
I jacked the car, and spun the wheels. Nothing abnormal. I checked the wheel bearings for play or roughness, and nothing unusual was found. Then I took the car thru some twisting roads, usually when you have a bad wheel bearing, the abrupt turns will change the vibration to a degree, as you load and unload the side with the issue. No change.

Then went back to the highway. I ran it to 70 mph, and on level ground, I put it in neutral to see if the motor was contributing to this. In neutral, motor at idle, the car continued this strong vibration. No speed difference mattered?

Back on the lift, and I removed the drive shaft. There were 2 magnets taped on the shaft for the cruise control sensor. I was surprised that the tape was so strong after almost 40 years. Anyway, I decided to remove the magnets ( I wanted to remove the aftermarket cruise anyway) and I sent it to a driveline specialist. I asked them to balance it. They replaced the rear joint, although they weren't certain anything was really wrong with it. I left it when I checked it being it seemed ok.

Then they welded in 2 oz of weight ( which is a fair amount) and lo and behold, no more vibration. I only post up this story, in case someone ever comes across a similar issue. It became a process of elimination, but at first glance, the drive shaft seemed just fine. Lesson learned!!

Be well

Rich

I had the same issue, hard vibration while driving at high speeds. I switched my motor mounts, tranny mount and tranny brace bushings. Still had vibration until I installed a new driveshaft. After that the car was back to riding smooth.

Yeah, I have a vibration at all speeds kind of like that but its minimal. I attribute it to using the
82 flywheel on my 80 SA engine. I had the flywheel resurfaced as well and figure that probably
made it more noticeable as well. Using the 82 flywheel on an 80 12A is an old racer trick to get
a 7 lbs lighter flywheel, 30 vs 23 lbs.

Good info on the driveshaft. Did those little magnets weigh 2 oz. by any chance?

Nice find - thanks for the share!!

Stu Aull
80GS
Alaska