I've got that leak in my differential repaired so my mind is at ease a little more when it comes to pushing my car's limits. Last night I pushed the speedo further than ever before on this car and now I know something else isn't right. At about 110mph I felt a severe rotating vibration. I stayed in it until 120mph, but the vibes were only getting worse. I checked air pressure in the tires, they were a little off from one to the next, but by no more than a pound or two. I've heard driveshaft troubles thrown around before, does that sound like what I'm experiencing? Could it be something else? How do I need to go about checking these things?

That speed sounds high for the driveshaft vibration. Could be a tire out of balance.

A tire or a wheel? Tire out of balance would be air pressure and wheel out of balance would be weight, right? I gonna need new tread soon anyway, so maybe that will fix it?!?! I guess if the tires are worn unevenly that could cause something like this as well... the saga begins

I'm talking about the wheel with the weights. If one of them is out of balance for some reason, at high speeds they can cause quite a vibration.

Tire out of balance could be air pressure, uneven wear, bad spot in the rubber. Wheel is usually the weight but it can also be a problem with the rim itself.

when u get the vibes do u feel it in the steering wheel? If so i would also had an alignment done when u have your tires balanced. Sometimes when the front end is off a little yound wont feel it in the wheel till u r up at speed i had the same prob not too long ago and my alignment was off. i would chec the balance first its a lot cheaper

I guess I may feel it a little in the wheel, but mostly SOTP. I'll get on it tonight and pay real close attention to where I feel it. I'm always looking for a good reason to go fast, thanks buddy !

From "Wheel Imbalance"

a technical paper written by Charles O'Toole, published and copywrite 1998, Charles O'Toole

There are 3 types of tire/wheel imbalance to be concerned with on any tire/wheel combination:

1) Force Imbalance
2) Mass Imbalance
3) Out of Round Imbalance

Force Imbalance is a complex interaction between the tread, sidewalls, out of round wheel shoulder and tire carcass in both the sidewall and tread area, usually it is caused by overlapping the carcass threads and stiffners. It can also be caused by thick and thin sections of rubber and varying stiffness or density/elasticity of the rubber in any part of the tire. The only solution to this problem is to buy a tire that has these problems minimized--in short, you get what you pay for.

Mass Imbalance is the out of balance caused by heavy and light areas within the tire and wheel combination. The fix to this is a spin balancing of the tire/wheel and the addition of weights.

Out of Round Imbalance is how much the tire surface is out of round. On any typical size tire for a midsize car, a mere .003 out of round is the equivalant of 16 ounces (454 grams) out of balance at just 60 mph (@98 Kph). That means that the high spot on the tire will slap the pavement with a force of 16 ounces (454 grams)!. The solution to this problem is to shave the tread until the tire is perfectly round. The tire is mounted on the rim, inflated to normal pressure and on the location where it should remain for the life of the tire--don't rotate tires.

Air pressure has nothing to do with tire imbalance or vibration, but the design, construction and quality of the tire have everything to do with all of the above. Remember, you get what you pay for.

When balancing a tire, if the "technician" doesn't balance with the weights spread roughly in a triangle, the balance is no good and is not balancing the tire in all 3 planes of movement. The avg computer wheel balance should be avoided for any high speed driving.

This excerpt may not be copied or distributed in whole or part without express written permission from ME.

Tried it today and it is felt throught the steering wheel more than I thought. In fact, it seems the majority of it is felt through the wheel. Looks like I need to go ahead, burn this set of tires all the way off, get some more and get an alignment while I'm there.

charles, thanks for the description of three common tire balancing problems. I believe in and have stated before that tires are a big item concerning you get what you pay for. I believe in buying brand name tread and the technician that installs for me is way over qualified for such a job, but I get him to do it because I know he does it right. As for who put the current ones on and balanced, I have no idea so that may very well be part of the problem. I believe that with the new tread and an alignment job(done by the same guy) I will be riding silky smooth when I attempt to see how fast this LS1 and 2.73 gear combo will take me. I plan on trying it shortly after I get new tread. Speaking of high speeds, I read about aerodynamics being the biggest limiting factor above 150mph. How do our cars rate in this category, pretty slick or what?

Hello Charles,

One item to add to the mix of the high speed imbalance issue of the rotating parts of the chassis is the brake rotors and /or drums being out of balance or even sometimes warped. Every time a brake rotor or drum is turned in a brake lathe enough material is removed from them to affect their overall original "as manufactured /adjusted at the factory zero balance".
Think of them as little flywheels.Most brake shops will laugh at you if you ask them to perform the re-balancing service to these components (as will most transmission shops when you ask them to re-balance a flywheel they just resurfaced.) and tell you it is not required .Truth is most of them are not aware of it and don't have the equipment on site to perform the work even if they were awatre.They would have to send it out to a shop that does balancing.
All these sloppy procedures "stack up" causing vibrations at higher road speeds.
Most customers will never drive those speeds so the problems seem imperceptable at normal highway speeds and the shops get away with it for the most part unless the conditions are very gross.
Most people want work done cheaply and quickly and are not willing to wait or pay for these extra steps.
Even the lug nuts not all being the same weight can cause trouble.For instance, those special security lugs that take the special socket to get them off.You have one on each wheel ,does the security lug weigh the same as the standard lugs?Best to check to be sure.If you replace a broken wheel stud with one that is shorter or longer than the rest on a particular hub or flange an imbalance condition will exist at that location.
This may sound extreme but if you are going extremely fast all these little details need to be addressed.

What about if all the lugnuts aren't torqued the same, could this cause some vibes as well? Even if this is so, I don't think it would cause the severe shaking that I feel. The reason I ask is because I did lose a lugnut not long after buying the car, so I put the four way on them and tightened them all the best I could, but didn't torque wrench them.

The posting from Skinned Knuckles is exactly correct. It is especially important when considering the age that Firebirds are reaching today--some were made quite awhile ago, changes add up, imbalances add up.

Also, for some that read this today, on Jan 9, 2010. I just resurected this, but if anyone has a vibration in their car, this 6 year old thread is as important now as it was back then. That was me back then: "Charles" .

If new members have never used the archives for research, check it out. Just type in some key words and start checking the old posts. There is tons of info available on just about any question.