(Note, this letter originally appeared in Tony’s Garage in the Dec, ’13 issue. – Ed.)
Back in the day, I bought a new ’78 Z28 with a 350 and a four-speed. Since the day I drove it off the lot it has had a bad vibration starting around 75 mph and gets worse all the way up to 90 mph, where the vibration doesn’t get any worse or better. The dealer changed the harmonic damper, rebalanced the tires, and may have changed out the flywheel.
None of these fixes worked. Over the years, I have replaced tires and rims, had the engine rebuilt (balanced and blueprinted) with 9:1 pistons and a mild Isky hydraulic cam. I have also had a new driveline built with a larger diameter tube. The engine runs like a sewing machine up to 5,500 rpm in Neutral. No vibrations. The only way I can describe the vibration is that it is related to the speed of the car and is a very tight or short-wave type of vibration. If the shifter is against the rear stop, the vibrations are transmitted to the shifter. If I move the shifter off the rear stop, the vibration goes away from the shifter, but is still present. There’s no vibration in the steering mechanism. I have not done anything with the rearend as far as troubleshooting. The car has never been wrecked, raced, or excessively abused. Any suggestions as what I may try next to troubleshoot this problem.
Thanks for your help.
WOW! It sounds like you’ve been pulling your hair out for a while trying to find this vibration issue.
One of my first thoughts was that the driveshaft may be out of phase (universal joints clocked 90 degrees off of each other). Properly phased universal joints are in the same clocked position in the front and the rear. Universal joints out of phase are not a common occurrence, but it has happened. Since you said that you’ve replaced it already, that turned into a fleeting thought. It truly sounds like it has to be in your rearend. Not the one you sit on. I’m referring to the one mounted to your leaf springs.
Have you attempted to check how “true” your axles are by using a dial indicator with the brake drums removed? An axle could have been machined improperly from the factory. An axle could be slightly bent. I used to work at GM, and I’ve seen some cars get seriously abused before they end up on the lot of a dealership. A Teamster or a truck driver could have taken your car for a joyride and hit a curb before you received the keys to your brand-new pride and joy back in 1978. Most likely you would have noticed the brake shoes on one side wearing more than the other. It doesn’t take much to cause a vibration like the one you’re experiencing.
If you haven’t already, check the pinion yoke on the rearend to see if it may have been machined slightly off-center. If the pinion yoke universal joint cups are improperly machined, the driveshaft could be whipping around and cause the car to shake at higher speeds. You would probably need to measure the runout of the rear of the driveshaft with a dial indicator with the car up on jackstands. This would allow you to rotate the driveshaft.
If the problem increases or decreases while accelerating or decelerating in the trouble range, you may have pinion angle problems. Also, check the angles of the driveshaft. Changing the ride height of your car can change your pinion angles and cause unwanted drivetrain vibrations. Since you’ve had this problem since you purchased the car, pinion angles are probably not your issue but still worth a look. You can measure the angle of your crankshaft/transmission output shaft as well as your differential pinion angles with a basic angle gauge. For best results, the car should be level (easiest to accomplish on a drive-on rack). A production leaf spring car should have about 3 degrees down on the transmission output shaft and about 3 degrees up on the differential pinion angle. These add up to 6 degrees of difference. Seven degrees is about the maximum for long universal joint life. Three down and three up should get you a pretty smooth ride. If you’re way off of these measurements, you can get some angle shims to adjust the pinion angle or you’ll have to figure out a way to shim the transmission mount to get it within spec. If your car was a moderate- to-high-performance Pro Touring leaf spring car, you can adjust the differential pinion angle to as much as 3 degrees down before you may start noticing chassis vibrations. The variation from stock and high-performance angles has to do with combating the amount of leaf spring wrap-up from applied power/torque.
Mystery Vibration, Part 2 …
I had the driveline shop technician take the Camaro for testdrive yesterday. He thinks there is a slight vibration from the engine, which could be felt up through the shifter. He recommended a fluid-type harmonic damper. He also mentioned about having a new pressure plate/clutch/flywheel assembly that has been balanced as a unit. While the car was on the lift, we ran it in First gear. The driveline and pinion yoke looked good. No noticeable problems. However, the passenger rear brake drum did have some noticeable vertical (up and down) motion. Possibly a bent axleshaft. He also said to keep my hand off the shifter while driving. I wouldn’t notice the vibration as much and, more importantly, holding the shifter back against the stop puts pressure on the shifting forks, wearing them out. Who knew?
I’m not familiar with the fluid-type damper. Any comments?
I’ve never personally had experience with fluid-type dampers. The few engine builders I trust have all steered me to non-fluid-type dampers from reputable manufacturers. For that reason, I’ve only used conventional dampers from the factory on my stock engines, and from TCI or BHJ Dynamics on my performance engines.
Your mechanic was on the ball about keeping your hand off the shifter. He’s correct that you will prematurely wear out shift forks and sliders. I would have mentioned it, but when it comes to manual transmissions, I assumed everyone knew to keep their hand off the shifter and their foot off the clutch pedal unless they were shifting.
Have that questionable brake drum and axle checked. I really hope you find the culprit. Good luck!
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