Vibration

[Bill E.]

Hello All:

My 1974 has developed a vibration between 50 and 60 mph. At 70 TO 75 mph it is much less severe but still there. Below 50 mph it is not noticeable. It feels like I'm driving over a washboard. I don't hear any excessive noise.

The tires have been balanced, new shocks, and a recent alignment. The Service manual states to check for rough rear wheel bearings, unbalanced or damaged propeller shaft, worn universal joint in propeller shaft, mis-indexed propeller shaft at companion flange, and excessive companion flange runout. I'm taking the car into the shop to have it checked out. Any thoughts and is there anything else should I have checked out?

I did notice that the rear wheels don't turn as easily as I remember. Last Fall I had the car serviced before storing it for the Winter, I'm wondering if the wheel bearings were tightened up. I checked the invoice and it doesn't say anything and I don't recall the mechanic mentioning it. I noticed the problem the first time I took the car out on the highway after the work was done.

Thanks, Bill

[Dave F.]

Re: Vibration

Bill,
Beginning with the most simple of things sometimes will do it. Check for a thrown wheel weight. Also, check all lug nuts. Tire condition may be a factor. Make sure your parking brake hasn't begun to sieze. beyond that gets into the elements you have read in your manual. When you do find the problem, please post here so we can all learn.

[Jack H.]

Re: Vibration

When all the 'ususal suspects' have been rounded up, examined and released, there's ONE MORE source to investigate...

If one or more of your wheels are out of round and you've logged any appreciable milage on the tire(s) mounted to that wheel, you could well have generated a non-uniform tread surface (taken a 'set') and you're fighting intrinsic tire squirm. If this is the case, about the only solution is to either replace the offending wheel(s) OR have them 'trued', discard the tires that've taken a set and mount fresh/new rubber.

A professional wheel shop can tell you pretty quickly whether or not the wheel(s) are a problem. They de-mount the wheels, remove the tires and re-mount the wheels on a stub axle fixture that's been outfitted with dial micrometers (one measuring the horizontal rim surface, the other measuring the radial rim surface). The wheel is rotated slowly on the test fixture and the wheel's concentricity profile is measured and compared to spec.

I've seen guys literally pull their hair out and squander LOTS of $$$ fighting vibration problems that wound up being out-of-round, original wheels...

[mike cobine]

Re: Vibration

Jack could be right on with this. I had a BF Goodrich TA suddenly become lopsided with a huge cup and bulge in the tread just a couple fo months ago. I am unaware of any road damage (pothole, object, etc.) but this tire suddenly got a lump in the outer thrid of the tread about a half inch higher when off the car than the rest of the tread. The center section next to it dipped in about half an inch compared to the "normal" third on the other side.

Besides the vibration at 50-60 mph, it also had a kick sideways at very low speed, like 5 mph, kind of a waddle that I first though was u-joints locking up.

Changed tire, all cured.

[Bill E.]

Re: Vibration

Well at least I've eliminated the tires and rims as the source of the vibration. I have a brand new balanced spare or a straight rim so one-by-one I swaped the spare with the tires on the car and took it on test drives. No difference in the vibration. Looks like I'm headed for the shop next week. Bill

[Dave F.]

Re: Vibration

Were your half shafts or drive shaft removed or replaced last year when serviced?

[Jack H.]

Re: Vibration

Well, that 'test' was conclusive provided only ONE of the four mounted wheels was out of round...

[Bill E.]

Re: Vibration

I thouhgt about that too but I don't want to rush out and buy new tires if I don't have to. My mechanic told me to do some other tests which I did this morning. I brought the car up to speed where the vibration starts (55-60 mph)and then shifted into neutral. The vibration remained. I then shifted into 2nd gear and and brought the car back up to speed, the vibration came back at the 55-60 mph speed. All this seems to indicate that the problem is speed related rather than rpm related.

[Michael H.]

Re: Vibration

Bill,

Sounds like you have already looked into some of the typical causes of a vibration of this type but I still feel that your proplem may be in the wheel/tire/rotor area. Typically, driveline (engine, drive shaft etc) vibrations will be most obvious at around 30 MPH and become less severe as speed increases. (unless there is a very serious unbalance condition) Wheel/tire imbalance conditions generally felt most in the 60 MPH range. (plus/minus 10 MPH) If your condition is near 60 MPH, as described, it would indicate the wheel/tire/rotor area.

Because you have tested with the car at speed in neutral, engine idling, and noticed no change in the level of vibration, I would agree that this eliminates everything forward of the transmission.

There is still one strong possibility though. Tires that have become severely flat spotted from sitting in one position for several months will give you the exact problem that you describe. Although not nearly as common with radial tires as it is with bias, it's still quite possible that this is the case. If all four have become flat spotted, replacing one at a time with your spare would not make much difference.

GM recommendations for this problem, as described in several service bulletins in the 70's, was to drive the car at moderate speed and swerve from side to side repetedly after the tires are at operating temp. (don't over do it) They described the exact problem that you have and reported that driving normally in a straight line, even at speed, would not cure the problem.

If you have changed nothing on the car since last season, yet have a new vibration the first time you drive the car this season, I would have to guess the tire flat spot issue is a strong possibility and it's an easy/free repair if indeed that is the problem.