dmruschell

I have a 79 Corvette that has been causing me lots of grief lately, but the issue I'm trying to tackle now is a driveline vibration that starts around 70MPH. This year, I've had the suspension rebuilt (all new bushings and ball joints front and rear and new U-joints on all the drive shafts), the transmission replaced (new Richmond Super T-10), and new clutch installed (and various other costly things that shouldn't be related to this issue). The car would vibrate when I reached around 75-80MPH. The shifter would rattle like crazy, and it was a generally unpleasant experience. This is after all of the work listed above was completed. However, there were fewer vibrations in the car than before the work was done. The vibration is still there (and in fact, increases slightly) at that speed with the clutch pushed in and/or the transmission in neutral, so the vibration is somewhere between the rear of the transmission and the wheels. It doesn't feel like a wheel is out of balance, as that is usually a slower vibration. This is a very fast vibration.

I was told that the driveshaft was slightly bent. I think the mechanic said that it was a few thousandth's of an inch away from being perfectly straight. So, I waited for Eckler's to get a driveshaft in stock and ordered a new driveshaft. I installed it, and now the vibration occurs starting at 70MPH.

Does anyone know if new driveshafts are balanced? I didn't see any weights on it when I looked at it, and the vibration almost seems worse with the new driveshaft installed. (Unfortunately, one of the U-joint end caps on my old shaft took a dive as I was removing the shaft and spilled its bearings all over the place, so I need to get a new U-joint if I'm going to put it back in.)

The only thing that hasn't been replaced or rebuilt at this point is the transmission yoke. The outside of the yoke is round to within about 0.001 inches, and the inside measurement across is equal at various points to about 0.004 inches. There is some free play when it's inserted into the transmission, but I can't say if it's excessive or not, as I don't know.

I know there are several threads about driveline vibrations, I'm just out of ideas since just about everything was rebuilt or replaced already.

rdroket

I see no mention of having tires balanced. I would have the tires balanced before going any farther.

learje

u mention everything being replaced already...unfortunately its possible some new component has caused your vibration problem. some of the vib threads by ignatz i think and a couple others show in great detail how to measure and calculate the correct alignment....

my approach was a bit different. my car was silky smooth until i replaced my well worn center tranny mount w a new rubber one. after replacing the mount i had an issue similar to yours. fortunately i had the old one, and when i measured the height of the new vs the old there was 1/8" or so difference, if i remember right. i had to dig through some parts books at my local napa, but the found a new one w the same bolt spacing that was the same height as my old worn one. changed to this mount and vibration problem solved. anyways, an easy fix you could try would be to remove the center exhaust bracket that on my 73 is also bolted up w the tranny mount. that effectively makes the mount a bit shorter...see if the vib gets worse or better. ive read where guys make shims from 1/8" steel to raise the tranny tail as well. w all the stuff you have replaced, probably including motor & tranny mounts, rear end snubber etc u can rry my trial and error approach, or the measure and correct method. my problem w the measure method was i couldnt find anywhere i was happy measuring from, and it was far easier for me to simply find the correct height mount since it was the only component i changed.

good luck, many have chased vibrations for quite awhile...hope yours works out easy!

lear

dmruschell

Tires were installed and balanced less than a year ago. I did mention that it didn't feel like a tire vibration because it was a fast vibration.

dmruschell

Motor mounts were replaced when. The engine was built 3.5 years ago. I don't know which ones were used. Transmission mount was replaced with one from zip last year when I discovered that the old one had completely separated. I don't have the old one, but I'm not sure the old one would be much help anyways.

The center exhaust bracket was installed for the first time last week when the new exhaust was installed. It made no difference in the high speed vibration, unfortunately.

I found a place near me (thanks to the forum) that balanced driveshafts, so I'm going to make sure at least one of the ones I have is properly balanced with good u joints. I may also replace the yoke if they think it's worn. After that, I'm out of ideas and would get a corvette mechanic to verify the angles of my driveline, probably the mechanic that rebuilt it lol. Any other ideas are welcome in the mean time

Big2Bird

Take the yoke with you. They balance them as a unit, and mark the index how they go back it. Have them change the u-joints to solid spicers so you are done with it.
Check the differential snubber bushing. If it is crushed, your pinion angle may be off.

learje

yea its entirely possible its something other than the driveline angles. ive never had any drive shaft problems, or other vibration issues so i probably cant help

i have replaced a worn yoke and the tailshaft bushing in my muncie though...w ur new tranny i guess its possible wear on the old yoke could be a contributor..


all the best!

dmruschell

I figure I'll probably order a new yoke since I think mine is original. If I don't need it, I can return it, but I think it would be good to have it when I get the driveshaft balanced if they're balanced as a unit.

It might be the driveline angles, but I want to get all of the easier stuff taken care of first.

7T1vette

Is the vibration still there if you put the car in Neutral and let engine idle while at 70mph? If not, it's not the driveshaft, as it is still being turned by the rear axle.

Also, at 70 mph, your engine is probably turning around 2000 rpm (if you are in overdrive gear). Do you feel the vibration at 2000 rpm in lower gear ranges?

What about any vibration at 2000 rpm (or above) with the car in neutral in your driveway?

If the answer to question #1 above is "Yes" and you don't get the vibration under any other conditions in other gear ranges, you most likely have a tire/wheel/rotor imbalance problem.

dmruschell

From the original post: "The vibration is still there (and in fact, increases slightly) at that speed with the clutch pushed in and/or the transmission in neutral, so the vibration is somewhere between the rear of the transmission and the wheels. It doesn't feel like a wheel is out of balance, as that is usually a slower vibration. This is a very fast vibration."

I have 3.73 gears and no overdrive. The rebuilt engine has its own vibration issues and is being replaced soon, but that is a whole other saga.

I've also downshifted into third gear at 70-80 mph and the vibration is still there, unchanged.

7T1vette

If all the components behind the transmission are the same as what you had that did not have the vibration, then your problem lies with the 'phasing' of the driveshaft or with any yokes and/or U-joints that you had replaced. While it is possible that something at the output end of the transmission is causing the problem, that is unlikely, IMO, unless it is with the rear transmission mount (allowing some movement).

If you changed any U-joints, that's a likely source for the vibration. New joints can be defective and they certainly can be damaged to the point of binding during assenmbly.

If your front and rear wheels/tires are the same size, it might be useful to swap the rears with the fronts, just to see if there is any change in the vibration. If you find NO CHANGE, I would think you could rule out the wheel/tires as the primary cause.

The kind of vibration you have can be frustrating to find. Try to think up some simple tests that might pare down or eliminate portions of that rear drivetrain as causes. Eventually, you may have to remove the driveshaft just to inspect the freedom of joints and/or have it checked from balance and runout.

mbunch

Just a thought. I was in the retail parts business for over twenty years. There are a lot of loose crappy "u-joints" out there. Before you buy one, take it out of the box, remove the caps, remove the seals, and then put the caps back on. I shouldn't have a whole lot of play. If it does, try a different brand. Some brands just have a closer tolerance. Good luck.

dmruschell

I finally have a driveline that is free of vibration! I bought a new yoke, and that helped a little, as it had less play in it than my original (I'm not sure the original is completely worn out, but every little helps). I then took the two driveshafts and yoke to a machine shop here to be balanced. They were able to balance my original driveshaft with the new yoke, so I can return the new driveshaft and I am rid of the vibration! Thanks everyone for your input. It was through another post on this forum that I found the machine shop, so the forum came through for me once again.