lgodom

I asked in another post and the OP said the vibration definitely wasn't there when the car was new, so I don't know how the rotors could be causing it to begin with.

stevebz06

I don't either, but the rotors are cast in two pieces from different types of iron. I don't know how they are held together but if it is an interference fit then something could have moved. Or, maybe they are balanced and the weight was slung out. Or, maybe some gravel ended up lodged between rotors.

Whatever the case, if they changed rotors and the vibration changed (in this case it got worse), that has to mean that the new rotors are not balanced if the wheels and tires are ok. I would have a machine shop check the rotors for balance.

travisnd

Been racing/tracking C5s for a decade... changed many rotors... never balanced one ever and never had a vibration related to one. The only time I've ever had a bad vibration in my car was when I mounted some race scrubs and didn't balance them. Bad vibration over 100... balanced them and they were find the next race weekend.

stevebz06

But these rotors are probably different from any rotors you put on your C5. Wouldn't you at least try to eliminate this as a problem before making assumptions?

travisnd

I run two piece floating rotors, but that doesn't really matter. If GM had a bad batch of unbalanced rotors you'd probably have more chatter about vibration issues on the forum. My money is on the torque tube couplers being defective.

VetteDrmr

I can't see how a rotor could possibly be out of balance enough to be annoying, the radius is just too small.

To the OP, did you ever see what happened when you shifted into neutral and disengaged the clutch? That would isolate the driveshaft.

HTH, and have a good one,
Mike

stevebz06

You may very well be right that the rotors are not the problem, but does it make sense to not eliminate them as the cause before you start removing the drivetrain?

VetteDrmr

It's just that shotgunning parts gets very expensive.

stevebz06

Oh, I didn't know that. I thought they were free.

OK, I'm being sarcastic. Sorry. I was just going on the report that when they changed rotors, it got worse. If that is really the case, how else can you interpret it?

Since the rotors are manufactured differently than previous rotors, I don't see how previous experience is useable here.

If it were my car and nobody could figure this out, I would take the time to eliminate this as a problem. You could probably do it yourself for only a time investment, but even if you bought new rotors, if you drive the car hard or track it, you're going to need rotors eventually anyway.

Since these rotors are cast as two separate pieces and then joined together, is it completely unreasonable that a problem with the orientation between the hat and the rotor could develop?

VetteDrmr

Previous experience still counts, but has to be considered with the new process, sure. BUT, the physics of a rotor are the same: you have a 12-14" diameter disc of metal, spinning at around 1200 rpm (just guessing based on tire diameter), it just doesn't generate that much energy. Only way would be if the rotor was seriously out of round (i.e. your manufacturing defect), and if it's more than just a couple of tenths it's going to hit the caliper. Since the frequency of the vibration measured is *close* to my 1200 rpm back of the hand estimate (1200/60 is 20 Hz), I'm beginning to wonder if it's something to do with a half shaft or CV joint.

HTH, and have a good one,
Mike

Shaka

That's not exactly how it works which makes it so difficult to trace the vibration. If you bang a tuning fork on top of the wooden piano case, the amplitude will stay low on the tuning fork and the strings of the piano won't vibrate. If you bang the fork on the cast iron frame, the tuning fork will hum as it should as will all the strings within the frame. If you hit one of the keys with the tuning fork on top of the piano and try to trace the source of the slight vibration of the fork, measured at the bridge, which was transmitted thru the wooden cover, it would be a formidable task. Rotating shafts out of alignment or unbalanced is far more complicated to trace. So the 20g htz measured at the driver's seat really means nothing at all. The frequency of the tuning fork vibration was caused by the wood which can't vibrate at the frequency of the tuning fork and has nothing to do with the piano players finger, the actual source.
We can also be talking about harmonics where a few natural material frequencies align with certain operational frequencies which could explain the 90 mph thing. Without very expensive equipment, it could take months to find the problem otherwise.
Soichiro Honda was the first to consider acoustics and Reynolds in engine design and Fred Lanchester was the inventor of the harmonic balancer for his engines in 1889. These problems have existed for 300 years. You still learn 300 year old formulas at engineering school. Fred's cars were also the first with interchangeable parts, first production line, first dampers, first epi-cyclic or planetary transmissions, long before Olds in 1902.

VetteDrmr

Oh, I'm not proposing to use the sensing equipment to isolate the problem, only that the measured frequency is somewhere close to wheel rpm. Not a guarantee, but something to consider when the mechanics start doing a physical inspection.

Have a good one,
Mike

marktigerz

Yes I tried that and the vibration persisted but not with the same intensity and it will quickly fade as the speed drops.

Further updates. The car right now is sitting at a performance auto shop which has a dyno. Dealer is still going back and forth with GM with what looks like a trial and error game.

The latest I got today was the dealer provided two new wheels and tires from a new car (although we tested this already on a road test) to reconfirm that the vibration is not due to my wheels and the vibration again persisted.
the car was then put up on a car hoist, wheels were removed, and the car was sped up in the air and again, vibration persisted (although the shop owner said this was a very unreliable way of testing because the car is in the air).
I am losing patience where I feel GM is not doing enough in trying to help me out even though it is still under a bumper to bumper warranty!! I am losing my patience

VetteDrmr

So, that would put it towards the rear driveline, wheels, axles, diff, or back half of transmission (the part that's driven off the diff). It *could* still be in the front suspension, although I doubt it.

Patience is one of those skills learned through painful experience, hang in there!

Mike

travisnd

My money is still on the drive shaft couplers... her's the guts of my C5 race car... The C7 is similar... there are rubber couplers at each end. If one fails or if the bolts came loose...

Entire torque tube assembly, drive shaft is inside


Front end that slides into back of the crank


Back side

Shaka

That was my thought also, but OP claims he pops it out of gear and it still does it. That would be my focus particularly because of the 90mph thing.

travisnd

At high speed the prop shaft is still turning when out of gear. I'd test the following:

1. Stopped in neutral - Engine and drive shaft are turning

2. Stopped clutch in - Only engine turning

3. At speed push in clutch - Engine disconnected from drive shaft and trans

4. At speed put in Neutral - Engine and drive shaft disconnected from trans (sort of)

5. At speed put in neutral and push in clutch - Drive shaft now free-wheeling

If the vibration dissipates more quickly in scenario 5 then it's the shaft bushings.

Shaka

At speed clutch out in neutral, drive shaft will be turning at about 300rpm because of friction. Clutch in, the drive shaft will be turning at engine RPM.

VetteDrmr

And running at engine speed. According to the OP when he tested this the vibration continued until the speed decreased, then it dissipated.

travisnd

You've got it backward.