I have a persistent vibration when my 1980 corvette reaches about 55mph and above. The vibration can both be heard and felt, it is quite violent. Felt in the seat, not in the steering wheel, that tells me the vibration is coming from the driveline/rear end. Vibration is not the type of vibration you would feel from an unbalanced tire, more of a long pulse type vibration. mmmmmmm......mmmmmmmm........mmmmmmmm... .....mmmmmmmm...(hope that make sense, lol)

Background information.

3.73 gears, unknown mileage on gears
Transmission just rebuilt, like a few miles ago (200 miles)
Good tires, just balanced......and double checked after continued vibration.
New motor (100 miles)
All 6 u-joint just replaced
Good brakes
Rear wheel bearings feel good with no play
Brakes feel fine, no pulse felt in brake pedal
Rear diff appears ok, rotates smoothly (see below)
Trailing arm bushings just redone (used same shims so alignment should be close assuming it was before I purchased the car)

Any ideas on where this vibration may be coming from? What am I missing.

Now the only thing I can see that may be off is....The output yokes on the differential have a slight amount of play. If I grab them I can feel (not see) a slight amount of up and down movement (no in and out movement) The amount of up and down movement is so minor I doubt that it could cause the vibration but I could be wrong. Can any one comment of this, is a small amount of up and down (left and right) movement normal?

Thank you in advance everybody.

 

The seat usually means rear and drivetrain but check the fan and water pump for any movement. The mmmmmm mmmmm sounds a bit harmonic. Whatever the RPMs are at 55 run the engine in neutral with hood up watching the fan,see if it looks clear or the fan blades go fuzzy. If they go fuzzy the fan could be setting up a harmonic vibration.

Next I would look at the differential snubber upper and lower bushing. The upper is usually the one that falls apart.

 

its almost a game of ruling things out......so lets rule out your fan clutch......look real closely at the shaft going into you fan clutch and see if there is any looseness or wobble when you move it by hand.....some times you can see the packing material breaking away......also look really close at the tires for seperation of the layers of belts of the tire. good luck and keep us posted

 

I had same type of seat of the pants vibration that started about 60mph and up. Did not go away with clutch pushed in.

I changed my tailshaft bushing(M21), front yoke and had the driveshaft balanced(they said it had some ruout).
Vibration is now gone, no more shifter rattle either

 

Two more things to check, at no cost-
1.All belt pulleys to make sure they have been properly torqued into place.
2.All bolts connecting trans to motor for the same thing.

I'm not thrilled to admit that I've had nasty vibrations caused by both of these things. :bb

 

If the cause is something attached to the engine, you should be able to make it vibrate just sitting in your driveway by revving the engine to the same rpm as 55 mph would produce. Now, vibration from the engine, tranny, etc. may be different due to the added load when actually driving. But accessory vibrations are not dependent on engine loading or wheel speed.

 

U-joints probably. I had a big vibration from 45 mph on up on my 93K mile C3, 4-spd. Drove it from Raliegh NC to Northern VA like that. Had the U-joints replaced and all rear bushings. Result: vibe is gone.

BTW: I could not make the ujoint feel loose by twisting the D/S with my hands.

 

Don't rule out the tires yet. I've been chasing a vibration on my 64, had the tires balanced on a Hunter gsp9700. Still vibrated. Moved rear tires to front. Found my problem. Probably radial run out.

 

If Your pinion starts to point upward under load it will cause this to happen. If Your tranny was installed incorectly and the angle changed it could cause this. Was there a traction control part installed? I had a 70 Z28 that I put southside traction bars on and it did the same thing, took forever to diagnose. It may have had one less "m" in the harmonics.LOL. good luck.

 

Thanks but all ujoint are brand new. Changed them this week thinking this was the cause, found 2 bad joints...if anything the vibration just got worse.

 

OK lets recap.

can't be the clutch fan as the car does not have one, the car came with a flex type solid fan, no clutch.

OK all 3 drive shaft are being dropped off at a driveline shop later today for testing, Perhaps one of the shafts in bent, twisted, slightly out of phase, or not balanced properly.

While I feel this vibration is speed related not RPM related I will start it up tonight and see if I can replicate the vibration while the car is sitting in the garage. The vibration at highway speed starts at about 3000rpm, the neighbors are going to hate me, this thing is loud.

Once I get the shafts all back and if they check out ok, I will try rotating rear tires to the front, see if this changes anything.

Also someone please explain to me how I can test these differential mounts, what should I do, what should I be looking for?

Thanks again everyone for the suggestions.

 

Driveline shop just called...

Drive shaft is straight but slightly out of balance, they tell me not enough to cause vibration but they are going to balance it anyway

Drivers side 1/2 shaft, few small dents and out of balance, they claim the dents can thow out the balance. This side not out enough to cause vibration but they are going to re-tube and balance to be sure.

Passenger 1/2 shaft, a few decent side dents out of balance. They say this side would be able to cause a vibration, dents are too big to even try to balance this shaft so they are re-tubing and balancing this one as well.

Hope this solves the vibration problem.

Still waiting on someone to tell me what I should be looking for in reference to possible bad bushings on the differential truss. How can I tell if they need replacing?

 

Installed new balanced shafts, vibration is much less not gone. Rubber bushings on differential truss appear to be very dried out and cracked. I will be replacing these next. I will also be replacing he trans mount and center diff mount while I am at it. Tomorrow I will try rotating the tires to see if this does anything to the vibration.

 

what kind of exhaust do you have? can it be touching the frame?

 

I think Your on the right track with the diff mount.

 

if you still have the vibration after all this go somewhere that has bit of a straight-away and remove all of the engine belts on your car and go for a very quick drive up to speed. This will eliminate every engine accessory..fan,power steer,ac,water pump,alternator etc. I have seen a power steering pump with a bad bearing send a horriffic vibration right through the shifter on an automatic.

 

Hey Bogdon, I have the "EXACT VIBRATION" problem with my '78 L82 4spd and have pretty much chased the problem as you have. New tires balanced and rebalanced, all new u joints, had all shafts balanced, trailing arm bushings replaced and had front and rear allignment done and had transmission rebuilt, all to no avail. Am looking now at the angle of degree from the transmission to the rear end. It appears to be right about 3 degrees which is what I hear is the maximum that it should be. I'm definitely interested in what you find. I'll keep chasing my problem and let you know what I find (if I live that long) and you let me know. OK? thanks, Mike

 

Hmmmm - consider having your rear wheels looked at for concentricity. I had a 911 Porsche once that had a 'repaired' rear aluminum wheel. It had been dinged and was no longer perfectly round, but vibed like nuts over 45 or so. Got a new wheel and my vibe over 45 went immediately away.

 

If you have a local GM dealer they "should" have an EVA tool (electronic vibration analyzer). The EVA reads vibrations in Hertz and will get you into the ballpark of where the vibration is coming from. Low 20-30 hertz is a wheel speed vibration (something you can "feel"), 40-60 hertz is typically driveshaft balance (something you can "hear"), and 80-120 hertz is driveshaft angles.

We rigged up a van with wheel weights on the driveshaft and it sounded like an airplane roaring down the highway but there was no noticeable vibration, just a loud roar.

 

Half shafts will always show 'out of balance', dents or no dents. They were not balanced by the factory and don't need to be balanced now. They only turn at wheel speed, not transmission speed, which is not fast enough to cause vibration.