I am restoring my 1973 and have been left with an unusual vibration that seems to be coming from the rear.
The vibration comes between 55-60 mph and smooths out at higher speeds. I recently had the main drive-shaft and
half-shafts checked and balanced, unfortunately the vibration is still there. I have new steel rims with the Cooper
Cobra tires that have been check and balanced.

 

Moved thread to C3 Tech.

 

Maybe C3 folks can help you.

 

In the case of a speed dependent resonance, It was stated that the vibration was 55-60 and then goes away. Does it come back again at higher speeds - 80 - 90 - 100?

When you say the rims and tires were checked - they could be balanced, but are they round and true? If you put a dial indicator on the edge of the rim - what is the run out? I assume all the lugs are tight and the wheels are fully on the hub.

And - I have experienced vibration due to a bad u-joint, but it did not go away as I went faster, the frequency just went up.

Do you feel the vibration in the steering wheel - If that bushing on the end of the power cylinder piston rod has developed any play where it connects to the frame, it can get interesting???

That is all I can think of to ask at this point.

A

 

could be a bad wheel bearing did you put it on a lift and pull the tires left and right ,up and down for play did you change the diff oil

 

You say that tires/ wheels have been checked and balanced , do you mean by doing road force ? Road force not only balances the tires but it matches the tires to the rims . Most tire places can perform this way of balancing . Are all bushings OK and don`t have play in them ?
Steven

 

How old are the tires ..rotate the wheels

 

spend the money and road force balance.
get it below 15…
maybe in the old days they made better tires as far as balance goes…?

 

Tires are the first and probably the most likely cause of certain speed vibrations. Flat spots can make specific vibrations but more often they "talk" from 0-100 mph.

Half Shafts can cause that type of vibration so make sure that the u-joints used on the two half shafts are not rusting up. If you have the grease-able u-joints try adding grease to them. If you see any rust dust it is time to service u-joints.

Wheel bearings might do it so put the car on a jack and check for any play of the wheel bearings. Wheel bearings on the rear can really damage a Corvette if they fail.

Sometimes have another person drive the car while you move around and listen for the source of the Noise/Vibration.

 

Do you feel the vibrations during hard acceleration through that speed range or just cruising on the highway at that speed/rpm range?

Does your 73 have an automatic or manual transmission?
There have been reports and threads regarding vibrations at certain speeds/rpm’s caused from Center Force brand clutches.
When the Center Force clutches were replaced the vibrations went away.

During your restoration, did you change anything that could effect your drive shaft angle and did you check your drive shaft angle between the transmission and rear differential after your reassembly.
Adjusting the ride height, changing rear springs, new bushings, or installing new engine and transmission mounts can effect the drive shaft angle, putting it out of spec.

Did you rebuild or replace the engine?
If so, was it balanced?
Did you replace the flywheel, flex plate or harmonic balancer?

All these things can cause vibrations.

Please give us more information about your 73 and your restoration project.

 

The vibration goes away at higher speeds,70-80-90.
No vibration in the steering wheel, feel it more in the seats like it is coming from the rear.
U-Joints are all new.
We have not checked the true-ness of the tires or rims. Back in the day at my father's service station (and later garage) we used to true tires and now you can't find anyone that even knows of that technique, much less do it. We been looking for someone in the Charlotte area and surrounding area that has the truing machine with out any luck. As far as the rims they too have not been checked (tires off) for turing. I have had two different tire companies check the balancing and watched as they balanced the tires (off car). I would have thought that if the rims were not true that it would have showed up on the machines as sophisticated as they are these days.
I am having the guy that overhauled the differential to recheck the pinion gear to make sure it is in line in a couple of weeks.

 

Had the wheel bearings replaced when I upgraded to Wilwood brakes (complete system). Differential rebuilt but having it checked again in two weeks.

 

New Cooper Cobra tires, on front P235/60/r15 and on rear P255/60r15. The last time I had it balanced the driver side rear took 10oz of weight so the store manager said that was too much weight for a new tire and ordered a replacement under warranty. Unfortunately, that still did not make much difference in the vibration.

 

First I have heard of Road Force balancing, I will check on that for sure!

 

All I feel during my hard acceleration is my back pressing against the seat! LOL (such a rush). I had my L48 (190hp) engine rebuilt to include the block with LPC aluminum heads and Edelbrock intake, hydraulic roller cam and lifters, 670 street avenger carburetor, rebuilt my 4sp muncie transmission, flowtech headers and exhaust. 10-1 compression and likely somewhere around 400hp. I had all new suspension front and rear upgrades, wiring harness replaced, new gas tank, brake lines and gas lines. After being parked in my garage for 38 years I needed everything replaced and since replacing things we upgraded where it made sense. I wanted to keep it somewhat original but since I did not have matching numbers then it allowed us to get creative. I had a great guy (and co-workers) that did most of the restoration minus the engine work. All that said, I had pretty much everything replaced and or upgraded. I did away with the vacuum lines and went electronic to include the distributor and headlights. Carpet replaced and seats reupholstered. Other than this annoying vibration the only major thing left is a new paint job!

 

When you rebuilt the engine was it internally or externally balanced?
It makes a big difference in the harmonic balancer, flywheel and clutch set up.
Did you purchase a new harmonic balancer and flywheel or use the originals?
Did you dial in the engine block to the bell housing?

I would also start the engine and watch your accessory pulleys as you rev the engine to make sure there is no wobble with the pulleys.

 

I'm not exactly sure what you mean by internally or externally but it was balanced. I'll have to go back and ask my guy that did the rebuilding.
Everything was replaced, nothing original. Replaced block, new heads, lifters, cam, crankshaft, carburetor, air cleaner, spark plugs, value covers.
This includes harmonic balancer and flywheel. I had one company do all the chassis upgrades and another do all the engine upgrades. Once the engine was complete
me and a friend installed the engine ourselves. All we did was the install and bolt everything down. I then had the vette towed back to the chassis guys and they completed the installation of the wiring harness and hooking up everything else to get it running. Once it got to that point (running) it went back to the engine guy to fine tune everything. Everything is tight under the hood, no wobble. It's a very (very) strong running car as to what it was 30 some years ago, no complaints there, just trying to figure out this shake/vibration.

Thanks everyone for your input and concerns, I will look into all your comments and suggestions and will keep you posted. I am in hopes that having the rear end rechecked (pinion gear in particular) will be the root cause and can be corrected.
Thanks again folks! I'll work on pictures later...

 

Very Basic description:
An internally balanced engine is where the crank and rotating assembly components are balanced to eliminate engine vibrations.
On an internally balanced engine a zero balanced flywheel and harmonic balancer are used because the internal rotating components are balanced.
An externally balanced engine is one where the crank and rotating assembly components have a zero balance and the flywheel and harmonic balancer are balanced by adding weight in certain areas to balance the components and eliminate engine vibration.

Using a zero balanced flywheel or harmonic balancer on an externally balanced engine will cause engine vibrations and the reverse is equally true.
If you have an externally balanced engine you need to insure the harmonic balancer and flywheel are correct for the engine built.

The drive shafts on our C3’s are extremely short and it takes very little to take the drive shaft angle out of factory specs.
If you changed the rear spring, used longer spring bolts, changed the ride height, changed engine mounts, the transmission mount, differential stubber bushings, or anything else, double check the drive shaft angle.

Let us know what you find!

 

Regarding your rear differential,
Contact Gary Ramadei (GTR1999) here on the forum.
Gray is the worlds expert on Corvette rear differentials and loves to help people out.
He can give you a couple things you can check yourself, that could tell you whether your vibrations are coming from your differential or not and if it was set up correctly.

 

I talked with the guy that built my engine and he confirmed that my engine was built as internally balanced and flywheel is neutral.
I am having the differential checked and the position of the pinion gear. Seems it may need adjustment and that could be the cause of my
vibration. Due to pick the car up this week. Hoping for the best.

.