Been away for a while as I just moved from Tucson to San Angelo TX.

I had the Firebird shipped, which turned out to be a good idea as I discovered something while testing her out on the highway to see if she could endure the drive.

I have never had her over 40ish MPH until now, so I have no idea how long this may have been going on. I have an "idea" as to what it may be, but I would like some opinions from more experienced car guys/gals if you have the time.

My car is a '69 Firebird convertible, modded out a lot to include transmission (TREMEC TK-500) and ('71 455 450hp aprox.)engine. I doubt anything in the drive-line is stock, I bought the car with the engine and trans. I still have my cocktail shakers.

Here is the vibration description:

No vibration under 40mph
Vibration starts around 40-45 and is "scary" at 65mph.
Vibration is less severe if I have more "tension" on the drive-line... (going down hill it is worse, braking & accelerating it lets up a little).
My alignment "seems" good, if I take my hands off the wheel it is steady and does not pull to either side. There is also no odd wear on the tires.


Thoughts? I lean towards the drive shaft balance... suggestions?

As always, thank you!

Do you have vibration dampeners installed up front and in the rear?

Check your universal joints on the front and rear of the drive shaft.

Where precisely do you "feel" the vibration?

Steering wheel?

Shifter?

Pedals?

Seat/floorboards?

Assuming you are cruising steadily at 45, does it change:
If you accelerate slowly to 50?

If you decelerate slowly to 40?

If you turn right at 45? (freeway ramp)

If you turn left at 45? (traffic circle or left-to-enter freeway ramp)

Depending on where the issue is felt, it could be steering, wheel bearing, driveline, or wheel issue.

Check the tires for rot and check the lug nuts

What's the situation on the rims and tires? I would recommend having them balanced on a machine that checks "road force variation" and rim runout. I've seen new tires that will balance, but have excessive road force which still gives you a vibration. I would recommend anyone that's going to get tires balanced find a shop that has this newer machine. Like this-- Hunter And go here to find a shop that has one... Locator Just for the record I don't work for Hunter or get a kick back . Good luck

In addition to the above, also check your pinion angle.

Wow, thanks guys! Ok:

Vibration dampeners: those are the cocktail shakers right? I have the rears for sure, not 100% sure about front.

Tires are very new. Less than 1 year old. Rims are original but in great shape. I don't know about balance though, I will have the tire balance checked.

The U-joints looked good a few months ago when I gave the car a once-over, but I will add those to my checklist when I get her up on a rack this week. The drive shaft had some odd wear on it now that you are making me think about when I had her up on the rack. The exhaust clamps (not sure about terms: the "U" bolt holding two tubes end-to-end together) ends had worn into the drive shaft a bit. Maybe that caused a balance issue?

The vibration is a suspension/vehicle vibration. The car needs new springs (shocks are good actually lol), but I did not think that would cause vibrations, just cause the car to "float" and ride low.

The steering wheel is steady and has no vibration. Actually, I would say the vibration is coming from the rear of the vehicle if anything, not the front or steering.

I feel it most when cruising. Maneuvering, changing lanes, turns do not have an effect.

No particular gear seems to matter much at all. if I am doing 45 in 4th gear or 5th it is the same.

This car has been the strangest experience for me. The body looked great. Good paint job, nice trim, nice interior. However, whoever had this car before me did NO upkeep. Every wearable part is worn out.

A bent rim, wheel out of balance, bent driveshaft, excessive pinion angle, or bad u-joint can also do this. A balancing machine will bring to light bent rims and balance issues. If you have someone who can ride alongside and behind you in another car, they can look for wheel hop, wobble, out of round or other issues.

Is there any noise associated with the vibration? I had bad spider gears in one diff and it growled with a vibration. Bad axle bearings will also cause a rumble or vibration, you can find these with the rear axle on jackstands.

You can isolate driveshaft/u-joint from engine/trans simply by pushing the clutch to the floor to see if the vibration stops. From the sounds of it, it will make no difference.

No, there is no sound.. it is actually kind-a oddly quiet! I feel the shake, but hear nothing lol.

Ok, so here is the plan of attack:

First, replace the front/rear springs. (then check angles)

Second, get the tires balanced and checked out at a local tire shop.

Third, remove the drive shaft and have it+U joints inspected/replaced if needed.

Then I will take her up on the highway and see how she does.

Sound like a good plan?

Thanks for the quick replies guys/gals!

Start with the second step. If you are going to do the first step anyhow, go ahead...but I am a firm believer in fixing what's broken before adding more parts. Third step if second step does not solve the issue, but not until you've resorted to spotter and self-inspection.

Just a note from recent personal experience, when you take the wheels to the tire shop, take them somewhere that will let you watch them work and check the wheels yourself carefully while on the balancer. I just had to replace one of the factory mags on my wife's car that had just a slight bend to it. Hard to notice even on the balancer if you weren't looking for one (tires had been spin balanced dozens of times in the past) but it would cause a vibration in the car anytime it went over 65 mph. New wheel - no more vibration.

Thanks, yeah I need the springs, they are worn out bad. She is riding low and even bottoming out from time to time. I figure that will also ensure that the pinion gear angle is right.

The tires will probably be cheep. I think most places do an inspection free and charge only a few bux for the balancing if the tires are good, and I am sure they are.

Thanks Lee, I will keep an eye on it. Frankly, I don't like to let anyone work on my car, I have had bad experiences (I saw my 98 T/A getting air through a red light when she was in the shop, I nearly killed the mechanic)

Since you think the vibration is in the back, rotate the tires from front to back and see if the vibration follows the tires.. If it moves to the front then the tires/rims might need some attention, if not then must be something else. It would be a quick way to check before throwing a bunch of money at it.

--Kyle

Hey Lee,
A balancer with Road Force Variation would if picked up that bent rim and saved you a lot of time. Just because a wheel is "balanced" doesn't mean it won't vibrate. That statement might be a little confusing. These balancers spin the wheel ( just like any other balancer) but have a roller that pushes on the tread while the wheel is spinning. This simulates the road and finds road force which could cause a vibration.

I know I've been going on and on about this but bottom line is.. If you go to a tire shop and they DON'T have a machine that checks road force, Your only half balancing your tires..

Id check tires, balance etc first, see if wheels are straight or bent...then U joints, drive shaft...do the cheaper stuff first! In other words dont buy stuff ,esp if expensive ,that you may not have to.
check control arm bushings also

Yea, probably but I don't think those machines have been out for too many years. They are hard to find in this area, and the only place I know that has one I refuse to do business with. I had that vibration for years and honestly thought it was the cheap WalMart tires.

I had a vibration, it was my drive shaft...

don't forget the bushing in the trans. it could be worn out also.

I've seen bent axle act that same exact way. Not only do they bend but they twist also. $300 or so for a new set from Summit.

i have the same thing. feels like the front end though. everything is new but the wheels. new ones on the way