High freq. vibe above 70mph

[Spridal]

Hello all,

How common is it to experience high frequency vibs above 70mph in a 70z? The entire drive train has been rebuilt but I can't seem to get rid of the vibe.

[MikeW]

I'd temporarily swap the front and rear wheels to see if that changes the vibration. It could be a tire balance issue.

[Walter Moore]

The entire drive train has been replaced? Does that include all 6 universal joints?

When U-joints fail they typically produce a vibration. I have had bad drive shaft U-joints on a couple of different cars, and the basic characteristic is a vibration that is too high of a frequency to be tire balance. (3 times the road wheel speed or more)

It it likely to be a tire problem. It doesn't cost much to have your tires balanced usually, in fact if they are fairly new, say 3 months or less I would take them back and have them checked. Most reputable tire dealers will re-balance them for free after 500 miles or so, because it is a proven fact that the tires do not actually seat fully on the wheel until then.

(my employer sells a machine to car companies that seats the tires in the factory...)

It could also be a bad tire. Normally tire uniformity problems show up at low speed, but sometimes a tire will have a defect that only shows up at high speed. (We sell machines to check for that as well.)

(We also make tire balancers for the automotive industry.)

A good clue to see if it is a balance problem is to check all of the wheels for balance weights. Sometimes you get lucky and end up with a tire and wheel combination that doesn't need weights, but not very often. If there is a wheel without a weight, particularly if you think that wheel used to have a weight, then that is likely to be the one with a problem.

Also, if you had to make a panic stop in the first 1000 miles or so after buying new tires, it is very likely that several of the wheels will be out of balance. Several sources have documented that heavy acceleration, either starting or stopping on newly mounted tires can shift the tires on the wheel by several degrees. (I have heard of examples where tires moved 45 degrees, but I haven't seen photographs to prove it.) Anyway, car tire and wheel assemblies are balanced as an assembly, not individual parts. So shifting the tire usually produces a balance problem.

[280~Master]

Don't forget that you may have problems with your bushings or your rack and pinon. I have seen this before. So now you have tires, U joints, Bushings and rack & pinion to check out. Good luck and hope you fix it.

[Spridal]

Thanks all,

I don't believe the problem is caused by the rims and tires since this is the second new set that I have replaced in two months (trying different syles of wheels.)

I suspect that the u-joints or bushings might be a good place to start as suggested here. I had a mechanic at one of the shops that specializes in Zs tell me that the problem is caused by the drive shaft being too short from the factory and that the fix is to replace it with a 73 shaft. That seems a bit far fetched to me. I don't think the Z would have been such a success if they came from the factory with the type of vibration mine has.

[DaveN]

The driveshaft isn't the problem, but that mechanic may be half remembering a real issue with the 70's and 71's.

They had a high half shaft U-joint failure rate and a mild high freq. vibration at freeway speeds which fit your description. The root cause was half shaft accelleration/decelleration (twice per revolution) caused by large U-joint deflection angles.

In the 70's and 71's the inboard ends of the half shafts were 3/4 inch forward of the outboard ends as well as being higher. Starting in 72 the differential was moved back that 3/4 inch to reduce the U-joint angles. That's when and why the driveshaft got longer. It seemed to work, half shaft U-joint life was much improved in 72 and the high speed vibration was reduced so much that it wasn't noticeable.

[Spridal]

The driveshaft isn't the problem, but that mechanic may be half remembering a real issue with the 70's and 71's.

They had a high half shaft U-joint failure rate and a mild high freq. vibration at freeway speeds which fit your description. The root cause was half shaft accelleration/decelleration (twice per revolution) caused by large U-joint deflection angles.

In the 70's and 71's the inboard ends of the half shafts were 3/4 inch forward of the outboard ends as well as being higher. Starting in 72 the differential was moved back that 3/4 inch to reduce the U-joint angles. That's when and why the driveshaft got longer. It seemed to work, half shaft U-joint life was much improved in 72 and the high speed vibration was reduced so much that it wasn't noticeable.

Thanks for your input. I am willing to try anything at this point. The car runs like a well oiled machine as long as you keep her below 70mph.

[MikeW]

Perhaps you can describe the symptoms in more detail. Does the whole car shake, does it happen in 3rd gear as well as 4th, do you hear any noises, does it come on gradually, etc.? With more information I'll bet that some of the experts here can give you a better diagnosis.

[beandip]

Here is one thing not mentioned. I found on my '73 that at speed 70+ with a little head wind or faster on a calm day. At about 90 the front end becomes light and the geometry of alignment changes as the front lifts. I was getting a vibration and some shimmy in the steering. I lowered the front by removing some of the front springs , giving the car a slight rake. It eliminated all the vibration and shimmy. Now she is good and solid to 100 and could be faster but I haven't taken her any higher. I now have euro springs and new struts and all is great. On your tires , try jacking up the front end and loosening the lugs and re tighten them with the wheel off the ground. The lower the car and tighten the lugs down and see it this helps. Gary